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Controls on Greenhouse Gas Fluxes from Soils: Effects of Nitrogen Fertilizers

Emma HauserDepartment of Environmental Science

Earlham College801 National Road West

Richmond, IN 47374

Advisor: Dr. Jim TangThe Ecosystems Center, Marine Biological Laboratory

7 MBL Street Woods Hole, MA 02543

Semester in Environmental Science 16 December 2013

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Abstract

Microbial transformation of reactive nitrogen in agricultural fertilizers through nitrification and denitrification is responsible for emissions of greenhouse gases like N2O, NO, and CO2. This study investigates the relationship between the form of nitrogen in fertilizer, the transformations that nitrogen undergoes in the soil, and the corresponding release of N2O and CO2, with the ultimate goal of determining which nitrogen fertilizers contribute the most greenhouse gases to the atmosphere. I applied ammonium, nitrate, and organic nitrogen fertilizersto a cornfield in South Deerfield, MA and measured the N2O and CO2 flux from those fields. In addition, I tested the soils from fertilized plots for nitrate and ammonium content. To further clarify the relationship between nitrate and ammonium fertilizers and greenhouse gas emissions, Ialso measured the N2O and CO2 flux from soils fertilized and incubated in a lab. In South Deerfield soils, ammonium fertilizer application correlated with high rates of nitrification and high N2O fluxes. This result indicates that in Massachusetts soils and similar soil types, nitrification is the dominant process that produces N2O, and ammonium-based fertilizers contribute more N2O to the atmosphere on a sub-daily scale. In the field, results of CO2 flux tests were inconclusive, but in the lab nitrogen fertilization suppressed CO2 emissions. Further testing would help establish the long-term effects of nitrogen fertilizers on greenhouse gas emissions andthe effects in different soil types.

Keywords: agriculture, nitrogen fertilizer, nitrogen cascade, greenhouse gases, nitrous oxide

Introduction

Soil treatment is an important component of climate change. As agriculture expands to feed the growing human population, soils become depleted in plant nutrients like nitrogen, which encourages increased use of nitrogen fertilizer in agricultural fields (Lal et al. 1995). Nitrogen fertilization changes soil microbial carbon and nitrogen cycling; processes that also control atmospheric greenhouse gas concentrations (Robertson et al. 2012). Nitrogen is abundant on earth in its molecular form, N2, an inert atmospheric gas. The nitrogen in soil, however, is considered reactive nitrogen (Nr)—nitrogen that is available for biological and chemical processes in the earth’s atmosphere and biosphere (Galloway et al., 2003). In a process known as the nitrogen cascade, Nr transforms between several forms, each of which is used in agricultural fertilizers and contributes to greenhouse gas emissions differently (Galloway et al., 2003). Much of the Nr in soil is organic and is added to soil through crop residue and animal manure. As soil microbes use organic nitrogen for energy, they mineralize the nitrogen to ammonium (NH4), which itself is used in synthetic fertilizers as an inorganic nitrogen source. Soil microbes use NH4

to obtain nitrogen for growth, converting the NH4 to nitrate (NO3) in the process known as nitrification. Nitrate is another form of nitrogen used to make many inorganic fertilizers. Soil microbes use NO3 for denitrification reactions, transforming the NO3 to N2 gas that goes back to the atmosphere (Bartholomew & Clark, 1965, Galloway et al., 2003). As microbes utilize and transform the soil nitrogen through mineralization, nitrification, and denitrification, nitric oxide (NO) and nitrous oxide (N2O) gases are formed as byproducts, which are emitted to the

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atmosphere (Galloway et al., 2003). As soil microbes grow in response to nitrogen addition, they respire and release CO2 to the atmosphere, as well (Raich & Potter, 1995). Thus, soil nitrogen availability and corresponding microbial activity helps control atmospheric greenhouse gas concentrations. Although CO2 is in greater abundance in the atmosphere than N2O or NO, N2O has a longer atmospheric residence time and higher global warming potential than CO2 (Blasing, 2013).Therefore, understanding controls on N2O release is just as important as understanding controls on CO2 emissions when considering climate change. In prior studies, Millar et al. (2010) established that increasing nitrogen fertilizer application rates increases the N2O flux from soil, and Robertson et al. (2012) found that agricultural fertilization is the largest source of atmospheric N2O emissions. One reason so much N2O is produced from agricultural soils is that nitrogen fertilizers are currently overused. In fact, farmers now apply so much nitrogen to fields that plants cannot actually take up much of the Nr applied. Instead, about half of the Nr from agricultural fertilizer gets leached into groundwater or released to the atmosphere as a greenhousegas (Galloway et al., 2003). The amount of nitrogen that is released directly to the atmosphere as N2O compared to the amount of nitrogen added to soil as fertilizer is known as the nitrous oxide emissions factor. A recent IPCC report estimates that the emissions factor for N2O is about 1%, but that number is still quite controversial (Klein et al., 2006). These reports highlight the need tounderstand the relationship between soil nitrogen transformations and the release of greenhouse gases, which could ultimately help reduce a significant portion of current agricultural greenhouse gas emissions.

There are several unknowns about the release of greenhouse gases from soils, however. Prior studies have shown that both nitrification and denitrification release N2O as a byproduct, butit is uncertain which process generates the most N2O (Snyder et al., 2009). As shown in the nitrogen cascade, different forms of nitrogen stimulate nitrification and denitrification, so knowing which process emits the most N2O could also show which fertilizers will cause the greatest N2O release. Additionally, studies on the relationship between nitrogen application and soil CO2 emission are inconclusive. Some studies show no change in CO2 emissions with nitrogen application while others show a CO2 flux increase, suggesting excess nitrogen stimulatesmicrobial respiration (Al-Kaisi et al., 2008). To better understand the relationship between soil nitrogen and greenhouse gas emissions, this study investigates the impacts of nitrate, ammonium,and organic nitrogen fertilizers on soil N2O and CO2 fluxes. By applying high and low-level applications of these fertilizers to a cornfield in Deerfield, MA, I compared the flux of N2O and CO2 emitted from the soil to the soil nitrification rates as well as to soil nitrate and ammonium concentrations. This study thus examines how N2O and CO2 emissions respond to different nitrogen fertilizers used in agriculture. By comparing emissions from nitrate and ammonium applications, I investigate whether nitrification or denitrification is the dominant process in N2O release. Additionally, I explore the relationship between soil nitrogen and the release of CO2 fromfertilized soils. Understanding these processes and relationships is important for making recommendations for future agricultural practices that will minimize greenhouse gas emissions.

Methods

Field Study SiteI conducted my study at the University of Massachusetts Agricultural Station in South

Deerfield, MA (Fig. 1). I applied fertilizers and measured gas fluxes from a cornfield on the farm,

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in which the corn stalks had not yet been harvested but had become brown and dry (Fig. 2). Therewere a few other small herbaceous plants growing in between the cornrows. The soil at the site was a homogeneous silt loam soil and I observed no soil horizons after digging a couple feet into the soil. Soil moisture content remained around 20% in all soil plots throughout the experiment. The field where I conducted my experiment had not been fertilized this year, but it is possible thatthe field had been fertilized in past growing seasons.

Field Experiment and Soil TestingOn the first day of the field experiment, I sampled the agricultural field before applying

any experimental treatments. I collected three bulk density soil cores at a depth of ten centimeters, and also used a bulb corer to take three initial soil cores. Most soil microbial activity takes place close to the soil surface, so I took all soil cores from the top ten centimeters of soil.

After sampling the pre-treated soils, I set up the experimental treatment plots by dividing the agricultural field into twenty-one one metre square plots with a two metre buffer area betweeneach (Fig. 3). In each plot, I randomly applied a different nitrogen fertilizer to reduce sampling bias. I used a total of seven different treatments--a high level and low level application of NaNO3

fertilizer, a high level and low level application of NH4Cl fertilizer, a high and low level organic nitrogen fertilizer (Premium Dehydrated Cow Manure, Old Castle Lawn and Garden, USA), and a control with no nitrogen addition. I ran each treatment in triplicate over a period of three days. On average, agricultural fertilizer nitrogen is applied to fields at a rate of 100kgN/ha (Robertson et al., 2012), however, studies have shown that applying nitrogen at 134kgN/ha seems to produce the highest N2O flux (Snyder et al., 2009). Therefore, for the nitrate, ammonium, and manure plots, I applied 150kgN/ha for high-level treatments and 50kgN/ha for low-level treatments. To make each inorganic treatment, I dissolved either NaNO3 or NH4Cl in one liter deionized water per plot. I applied these fertilizers using watering cans according to typical surface level broadcasting techniques (Boyanapalle, 2004). I applied manure treatments by hand-broadcasting and also applied one liter of deionized water to each manure and control plot so that moisture application was equal in all treatments. Application rates and quantities for each fertilizer are summarized in Table 1.

In each plot, I installed a circular collar for use with the greenhouse gas flux chamber. I measured the diameter and height of each collar to calculate the flux area and volume, which I applied to the soil gas flux equation (eqn. 1). I also calculated the volume of the flux measurement chamber to add to the total volume in the flux equation.

After allowing the plots to incubate over night, I ran several tests in each plot. First, I measured the flux of CO2, N2O, and NO gases from each treatment using the assembled mobile greenhouse gas cart. This system includes a LICOR 7000 for measuring CO2 flux, an Los Gatos Research N2O and CO trace gas analyzer to measure N2O, and an LMA 3D/LNC-3D for measuring NO fluxes. I used the listed instruments to measure the gas flux of each treatment plot for five minutes each, twice a day, for three days.

The greenhouse gas cart instruments measure the greenhouse gas flux as a volume concentration in parts per billion for N2O and parts per million for CO2. To calculate the gas flux, I used the following equation:

F =dCvdt

PRT

VA

(eqn. 1)

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Where F = flux (umol m-2 s-1 or nmol m-2 s-1), Cv = volume concentration (ppb/ppm), P = air pressure (Pa), T = absolute temperature (°K), R = gas constant, 8.314 J mol-1 K-1, V = chamber + collar volume (m3) and A = collar area (m2). In addition to measuring the flux in ppb or ppm, I also measured temperature from each chamber, looked up daily air pressure, and used the measured chamber volumes to calculate the gas flux from each soil plot at each time point.

Each day after measuring the gas fluxes, I also took one ten-centimeter deep soil core from each plot. After collecting the cores, I homogenized each one and froze one third of the soil from each core in a labeled plastic bag for initial soil extracts. I put the remainder of each core in a separate plastic bag and kept it at room temperature until return to the lab. In the lab, I placed the soils that had not been frozen in an incubator and incubated them for two and a half weeks at 20°C to simulate mineralization and nitrification in average outside conditions.

I unfroze the initial soil samples and used them to measure wet and dry soil weights. First,I weighed approximately ten grams of each sample and dried the sample for about twenty-four hours. I then compared the initial fresh weight to the final dried weight to determine soil moisture. I ground dried soils from pretreated fields and from the first and third sampling days forCHN analysis to measure bulk carbon and nitrogen content in each core. I also dried and ground three samples of the manure soil treatment for CHN analysis.

I then used the initial soils to make KCL extracts for nitrate and ammonium analysis. Twoand a half weeks later I also made KCL extracts from the incubated soil cores. To sample nutrientconcentrations from the initial and final extracts, I followed the modified Ammonium Protocol of Strickland and Parsons (1972) for soil NH4

concentration and the Flow Injection Analysis for Colorimetry outlined in Diamond (2008) for soil NO3

concentration. I used initial and final ammonium and nitrate concentrations to determine nitrogen mineralization and nitrification rates in each soil treatment. Additionally, I used the initial soils to test soil pH by placing a pH probe in a soil/water slurry made from each sample.

Using total carbon and nitrogen obtained from CHN analysis and the measured nitrate andammonium concentrations, I compared soil nitrogen content from fertilized soils to CO2 and N2O fluxes from the field. Based on comparisons of nitrification, mineralization, soil C and N, pH, and CO2 and N2O fluxes, I was able to see if there were correlations between different types of soil fertilization and the flux of greenhouse gases from the soil.

Soil Incubation ChambersAt the Deerfield site, temperature was highly variable, which affected the gas flux

measured from the soils. As a result, it was hard to determine whether fluxes should be attributed to fertilization differences or to temperature differences. To further investigate the differences between nitrate and ammonium application and control for temperature, I set up a soil incubation experiment in the lab.

To run the incubation, I returned to Deerfield and collected three buckets of soil from an unfertilized area of the same cornfield as my initial experiment. Back at the lab, I divided the soilbetween nine cylindrical incubation chambers that were 15cm. in diameter and about 20cm. tall (Fig. 4). To try to maintain field conditions, I packed the soil in the chambers in a similar density as that observed in the field. I stored the chambers at 20°C and checked a barometer daily to determine the room’s air pressure. After soil chamber set up was finished, I took a small subsample of soil from each chamber (about seven grams), weighed the subsamples, and set themin the room for the duration of the experiment. I measured the weight of the subsample in the evening each day to determine the mass and moisture loss per day. The night before fertilization, I covered the chambers tightly to prevent moisture loss.

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The following morning, I uncovered the chambers and used the LICOR and LGR from the greenhouse gas cart to measure the N2O and CO2 flux from each chamber for five minutes each. After taking this pretreatment measurement, I applied fertilizer to each chamber. I applied three replicates of NH4Cl fertilizer and three replicates of NaNO3 fertilizer, each at the high-level application rate of 150kgN/ha. I also left three untreated control chambers. One hour after applying fertilizers, I measured the gas flux again. I left the chambers uncovered for the afternoonand then measured the flux a third time in the evening. I covered the chambers loosely overnight to try to maintain moisture without obstructing gas loss with the goal of simulating conditions in the field.

I continued to take flux measurements for the next two days. I took a morning and evening measurement on the second day and a morning and afternoon measurement the third day.One week later, I took an additional measurement to see how fluxes changed after a longer periodof time. In between measurements I kept chambers loosely covered with plastic wrap. The plasticwrap was also pierced with holes so that gases could continue to flux out while still maintaining moisture similar to field conditions. After measuring the N2O and CO2 concentrations, I used the data to calculate fluxes according to eqn. 1, and compared lab results with field results.

Results

Soil Nitrogen and CarbonSoil nitrogen content varied among soils with different fertilizer inputs. Total nitrogen in

each fertilized plot remained nearly constant throughout the experiment. Plots fertilized with highlevels of ammonium (NH4) and nitrate (NO3) had high total nitrogen content on the first day of soil testing and had similar total nitrogen contents on the last day of testing (Fig. 5). Manure fertilized plots also had high total nitrogen content. The low-level manure plots had the highest total nitrogen content of all the fertilizer types on the first day of experimentation, but the total nitrogen in the low manure plots decreased over the three days of the experiment (Fig. 5). Plots receiving high-level applications of manure initially had a low total nitrogen concentration, but the nitrogen concentration increased over the course of three days such that the high manure plotshad higher total nitrogen than the low manure plots by the end of the experiment.

While changes in total nitrogen in the plots were small, the form of nitrogen in each plot changed more drastically over the three-day experiment period. As expected, ammonium fertilized plots had the highest values for soil NH4 and nitrate fertilized plots had the highest values for soil NO3 throughout the experiment (Figs. 6&7). However, in the ammonium-fertilizedplots, ammonium decreased over the experimental period while nitrate content increased (Figs. 6&7). In the high-level nitrate fertilized plots nitrate content decreased, as did the ammonium concentration, however in low-level nitrate fertilized plots, the nitrate content increased on the second day before decreasing on the third day (Figs. 6&7). In manure-fertilized plots, nitrate and ammonium did not make up much of the total nitrogen content (Fig. 8), indicating that most of the nitrogen in the manure plots was organic. Manure plots started with higher ammonium content than nitrate content on the first day, but ended with more nitrate than ammonium on the third day (Figs. 6&7). In the ammonium and manure-fertilized plots, the decrease in NH4 and increase in NO3 is characteristic of soil nitrification reactions.

Carbon content also varied between fertilizer treatments, but did not change much over the course of the experiment (Fig. 9). The highest carbon content was in the manure-fertilized plots and the lowest carbon content was in the control plots (Fig. 9).

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Fertilized Soil Greenhouse Gas Fluxes

N2O In the field experiment, all N2O fluxes from fertilized plots were equal to or higher than

the N2O flux from the control during the afternoon of the first day and during the second day of the experiment (Fig. 10). The flux from the control was low early in the experiment and rapidly increased on the third day of the experiment when temperatures increased (Fig. 10). The N2O fluxes from manure-treated plots peaked later than the fluxes from the NH4 and NO3 fertilized

plots. Additionally, N2O fluxes were higher from the manure plots than from either of the inorganically fertilized plots (Fig. 10). The low NH4 treatment also had an especially high N2O flux, while the low NO3 treatment had the lowest N2O flux (Fig. 10). Over the course of the three-day experiment, the low NH4 fertilized plot produced the highest cumulative N2O emission and the low NO3 fertilized plot produced the lowest cumulative N2O emission (Fig. 11).

In the lab, the N2O flux from all soils declined rapidly after the first day (Fig. 12). This result correlated with soil moisture loss. Within the first half-day, the soil subsamples lost on average 33% of their mass to moisture evaporation. On the second day, soil subsamples had not lost much more moisture and the wet/dry ratio remained nearly constant (Fig. 13). On the second day of lab testing after most of the soil moisture loss had occurred, the N2O flux was highest fromthe NO3-treated soil. Over the following twenty-four hours, the N2O flux from the NH4-treated soil increased and the flux from the NO3-treated soil decreased (Fig. 12). On the third day of testing, almost all soil treatments in the lab had a similar flux. The cumulative N2O emission overthe three-day incubation period was highest in the NH4 fertilized plots when compared to pre-treated soil N2O fluxes and after correcting fluxes for moisture loss (Fig.14).

CO2

In the field, the CO2 flux from all the fertilized plots was about the same and followed thesame pattern over the course of three days (Fig. 15). The flux of CO2 from all plots was small on the first and second day and increased rapidly on the third day. CO2 emissions in the field showeda positive correlation with air temperature, particularly as the air temperature changed from 5 to 10°C (Fig. 16).

In the lab, CO2 fluxes dropped as the soil moisture content dropped (Figs. 13&17). After soil moisture was stable, the CO2 flux was highest in the control plots throughout the lab experiment. Over the three day period, the CO2 flux from the NH4 fertilized plot increased, while the CO2 emissions from the NO3 fertilized plots increased on the first day, decreased in the middle of the second day, and increased slightly later in the second day (Fig. 17)

NOx In the field, an additional third greenhouse gas, NOx, was measured in each fertilized plot.

The NOx flux from almost all the fertilized plots had the same magnitude and pattern as the control plot (Fig. 18). However, two plots, the high-level nitrate fertilized plots and high-level ammonium fertilized plots, showed a rapid spike in the NOx flux on the first day, which quickly declined on the second day (Fig. 18).

Observed Relationships Between Gas Fluxes, Fertilizer Type, and Soil Nitrogen Content

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Both in the field and in the lab the flux of greenhouse gases from soils varied with fertilizer type and nitrogen content. In general, as the nitrogen content of the soil increased, the N2O flux from the soil also increased for all fertilizer treatments in both the lab and in the field, atleast at the onset of the experiment (Figs. 19&20). Later in the experiment, however, total soil nitrogen content and N2O flux did not correlate well (Figs. 19&20).

In ammonium-fertilized field plots, high N2O peaks occurred in the afternoon of the first day, which correlated with a rapid increase in soil nitrate (Fig. 21). Lab soil incubations achieved similar results (Fig 22). In all ammonium-fertilized plots, the soil NO3 quickly increased over thecourse of only 2.5 days. In manure-fertilized plots, soil NO3 also increased, but the increase was not as large or as rapid (Fig. 23). In high-level manure fertilized plots, soil NH4 increased between the first and second days of the experiment before decreasing, while soil NO3 was slowly and continuously released. The corresponding peak in N2O from the high manure plots was the highest N2O flux of any of the fertilizer treatment plots but occurred later in the experiment than other fertilizer N2O flux peaks (Fig. 23). Unlike the high-level manure plots, the low-level manure plots did not have an increase in soil ammonium in the first day and the corresponding N2O flux peak was earlier in the experiment period (Figs. 6&23).

In nitrate-fertilized plots, correlations between soil NO3 content and soil N2O fluxes were not as clear. In all nitrate-fertilized plots, the total soil nitrate content decreased from the first day of the experiment to the third day (Fig. 24). Similar results were achieved in the lab (Fig. 25). However, in the low nitrate fertilized field plots, soil NO3 first increased before decreasing and the corresponding N2O flux occurred around the same time as the soil NO3 increase. In the high-level nitrate fertilized plots, soil NO3 decreased slightly over time and the N2O flux peak corresponded with the nitrate decline. The decrease in nitrate in the nitrate-fertilized plots was notlarge or rapid over the course of the experiment (Fig. 24).

Additionally, results show that soil carbon content may correlate with N2O flux after fertilization. On the first day of the field experiment, soil carbon content did not correlate with N2O flux, but on the third day of the experiment, plots with the highest soil carbon content had the highest N2O fluxes (Fig. 26). In the lab experiment, soil carbon showed little relationship to N2O fluxes, and some negative correlation with CO2 fluxes (Figs. 27 &28).

Other Factors Correlating with Soil Greenhouse Gas Flux

pHAlthough soil greenhouse gas fluxes show relationships to soil nitrogen content, other

factors also correlated with the obtained greenhouse gas flux results. Different fertilizers changed the pH of the soil and resulting soil pH corresponded with the soil N2O flux. On average, control plots were the most acidic followed by the ammonium, nitrate, and manure plots respectively. Data showed that the higher pH soils (more basic soils) had a high N2O flux and N2O fluxes weresuppressed in lower pH soils (Fig. 29).

TemperatureSoil greenhouse gas fluxes of N2O and CO2 were both temperature dependent in the field

experiment. Temperatures were much warmer on the third day of the experiment than on the first and second days (Fig. 30). As temperature increased, gas fluxes also increased. The effect of temperature was particularly evident in control plots, which had relatively constant gas fluxes until the third day when the temperature increased (Figs. 31&32).

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MoistureAlthough temperature was not a factor in lab soil incubations, soil moisture correlated

with soil N2O and CO2 fluxes from all lab fertilization treatments. Within the first half-day of the soil lab incubation, soil moisture decreased rapidly (Fig. 13). At the same time, N2O and CO2 fluxes also decreased (Fig.12). After the first day of the experiment, soil moisture loss in the lab was slight and the gas fluxes did not continue to decrease (Figs.12&13). In the field, soil moisturedid not show consistent patterns with N2O or CO2 flux. Soil moisture in the field was fairly consistent over the course of the experiment, however, as there was no precipitation during the experiment period.

Discussion

When I applied different types of nitrogen fertilizers to agricultural soils, I observed changes in the amount of N2O fluxing from the soil as well as changes in the timing of N2O flux peaks. Based on soil nitrogen analysis results, I was able to determine that the differences in N2O fluxes between fertilized plots correlate to soil reactive nitrogen (Nr) transformations that are part of the previously described nitrogen cascade (Galloway et al., 2003). In the field experiment, in plots where I applied ammonium (NH4) fertilizers the soil nitrate (NO3) content increased (Fig. 21). The transformation of NH4 to NO3 is known as soil nitrification (Alexander, 1965), and the large increase in NO3 over only 2.5 days in ammonium-fertilized plots indicates that applying ammonium to agricultural fields increases nitrification rates on a sub-daily time scale. My results thus show that after NH4 addition, nitrification rates in soil are high and nitrification occurs almost immediately. When nitrification occurs, the nitrogen from NH4 is not completely transformed to NO3. Some nitrogen is volatilized and lost as ammonia (NH3), nitric oxide (NOx), and nitrous oxide (N2O) (Galloway et al., 2003). My measurements of N2O emissions from NH4-fertilized plots showed that N2O fluxes peaked just before the second day of the experiment. The timing of that high flux of N2O corresponded with the early, rapid increase in soil nitrate, indicating that nitrification reactions were responsible for the N2O fluxes in the NH4-fertilized plots (Fig. 21). I achieved similar results in lab soil incubations, in which both soil N2O flux and soil NO3 content increased in NH4-fertilized soil chambers (Fig. 22). In summary, rapid nitrification occurred where I applied ammonium fertilizer, causing a spike in N2O emissions in a short amount of time.

Manure-fertilized soils showed similar soil nitrogen transformations, but at a slightly different time during the experiment. Although manure does contain some inorganic NH4, most ofthe nitrogen in manure is organic (UMN Extension, 2002). In soils, organic nitrogen is transformed to NH4 in the process known as mineralization (Galloway et al., 1995). After NH4 is produced, nitrification can occur, but the organic nitrogen must go through the additional mineralization step before nitrification. This additional step in the nitrogen cascade could explain why NO3 concentrations in manure fertilized plots increased more slowly than in the NH4-fertilized plots (Fig. 24). Both soil NO3 and N2O flux did increase in the manure-fertilized plots over the sampling period though, indicating that nitrification is still responsible for the N2O flux. In the high-level manure-fertilized plots, the N2O flux peak occurred on the second day instead of the first day of the experiment (Fig. 24) because nitrification did not occur as rapidly with the additional nitrogen mineralization step. High-level manure-fertilized plots ultimately hadthe highest N2O flux peak (Fig. 10), and had a high cumulative N2O emission over the measurement period (Fig.11). Based on these results, it appears that manure fertilizers, although

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they do not seem to emit N2O as rapidly, do still make a significant contribution to N2O emissions, even on a sub-daily time scale. This result might even suggest that N2O emissions from manure fertilizers are sustained over a longer period of time because of the slow release of inorganic nitrogen through mineralization.

Nitrification is not the only process in the nitrogen cascade that produces N2O, though. Denitrification, the transformation of NO3 to N2 gas, also releases N2O as a byproduct, and if denitrification is occurring, soil NO3 concentration is expected to decrease (Schlesinger & Bernhardt, 2013). In my fertilization experiment, NO3 content decreased slightly in the high-nitrate fertilized plots and N2O flux peaked before the second day, shortly after the largest NO3 loss (Fig. 23). In the low nitrate-fertilized plots, though, the soil NO3 content first increased, which was when the largest N2O flux occurred from the low nitrate plots. Therefore, denitrification could be releasing N2O from the plots, but the signal for denitrification occurrence is not that strong. From my results, it is difficult to definitively say that denitrification is causing much of the flux of N2O from the soil.

It is also important to note that the conditions of the Deerfield soils are not optimal for denitrification. Denitrification is an anaerobic process and denitrification reactions are generally attributed to waterlogged soils where oxygen is limited (Schlesinger & Bernhardt, 2013). While denitrification can occur at low oxygen microsites within soil particles, the microbial enzymes fordenitrification are slower at higher oxygen concentrations and lower moisture (Schlesinger & Bernhardt, 2013). Deerfield soils were not unusually moist and had moisture contents ranging from 19.8%-24.7%. There was no precipitation during the experiment that would have increased soil moisture, either. Therefore, it seems unlikely that denitrification is the dominant N2O producing process in these particular soils.

These results indicate that soil N2O emissions on a short time scale are strongly linked to the transformation of NH4 to NO3 through nitrification. Over the course of the three-day experiment, the highest cumulative N2O emissions occurred in ammonium-fertilized and manure-fertilized plots (Fig. 11). In the lab, N2O emissions were also highest in the ammonium-fertilized plots (Fig. 14). These results are consistent with prior soil nitrogen studies, which found that N2O emissions were higher from ammonium-fertilized soils than nitrate-fertilized soils (Eichner, 1990). Overall, my results in combination with other studies suggest that for soils of similar moisture, temperature, pH and carbon content, use of ammonium-rich fertilizer should be limited to produce the greatest reduction in N2O emissions.

While nitrification and denitrification control N2O release from soil, there are other soil properties that can enhance or inhibit nitrification and denitrification and thus influence soil N2O flux. Soil pH, for example, correlated strongly with N2O emissions in this experiment (Fig. 29). The untreated control soils were the most acidic, followed by ammonium, nitrate, and manure plots respectively. Prior studies show that greenhouse gas emissions from soils decrease as soils become more acidic (Slmek & Cooper, 2002), which explains the trend I saw between pH and N2O release. In addition to pH, soil carbon content may also influence nitrification and denitrification, thus changing the N2O flux. The addition of organic carbon in soil stimulates denitrifying bacteria, increasing N2O emissions where NO3 is present (Schlesinger & Bernhardt, 2013). In my field experiment, carbon content correlated with N2O flux on the final day of experimentation, with higher carbon soils releasing the most N2O on the third day (Fig. 26). Sincemuch of the nitrogen would have been converted to NO3 by the third day of the experiment, plots with more carbon may have experienced more denitrification, resulting in the higher N2O fluxes from carbon-rich plots later in the experimental period.

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In this study, I also wanted to see if there was a relationship between nitrogen fertilizer application and CO2 emissions from soils. Prior studies provided inconclusive results with some studies suggesting that nitrogen addition increased CO2 emissions, others suggesting that nitrogenaddition decreased CO2 emissions, and still others suggesting that nitrogen did not change CO2 emissions at all (Al-Kaisi et al., 2008). My field experiment did not produce any definitive resultswith respect to CO2 emissions from fertilized soils. CO2 emissions followed the same patterns in fertilized plots as the control plots (Fig. 15). Temperature had more influence over CO2 emissionsin the field experiment, with increased CO2 fluxes as the weather warmed. According to Al-Kaisi et al. (2008), CO2 flux is a result of soil microbial activity, which is likely to increase in warmer temperatures. The same Al-Kaisi et al. study suggests that temperature is more important to CO2 flux than nitrogen fertilizer type, so it seems unlikely that the fertilizers in the field, at least at thistime of year when temperatures are cold, are the cause of soil CO2 emissions.

In the lab, where temperatures were held constant, the CO2 flux from the control chambers was higher than the flux from fertilized chambers (Fig. 17). Although the data set from the lab is not extensive, if similar results were achieved with further testing, the result might suggest that soil nitrogen addition suppresses CO2 release, at least in the short term in average temperature conditions. Such results would support the hypothesis that soils will sequester CO2 with additional nitrogen. Nitrogen is a limiting nutrient, but adding more nitrogen to soil decreases the soil organic matter C:N ratio, which would allow more microbial carbon uptake from soil organic matter (Al-Kaisi et al., 2008). Higher microbial carbon usage would decrease carbon release, thereby decreasing CO2 emissions. A longer study is necessary to determine how CO2 flux relates to soil nitrogen, particularly in the long term.

Conclusions

Based on this investigation, it seems that ammonium-based fertilizers applied to agricultural fields instigate rapid nitrification reactions that release N2O to the atmosphere on a sub-daily time scale. For soils of similar moisture content, carbon content, and pH, I would recommend minimizing ammonium fertilizer use to decrease N2O emissions the most. Ultimately,fertilization recommendations must be made specific to the conditions of the agricultural field. A field that is more waterlogged, for example, might produce more N2O from denitrification and NO3-based fertilizers should be avoided. In order to reduce human greenhouse gas emissions, it isimportant to understand the soil properties of the agricultural site before applying fertilizers. Additionally, further investigation is needed on the long-term effects of nitrogen fertilizers on greenhouse gas emissions. Continued study of the relationship between soil nitrogen and CO2 release will be important to understanding the impacts of agriculture on climate change.

Acknowledgements

I would like to thank Dr. Jim Tang for all of his guidance and inspiration throughout this project. His time and advice have made this project possible. I would also like to thank Sarah Nalven, Rich McHorney, Jennifer Fanzutti, and Mengdi Cui for all of their help with fieldwork, data, and project logistics, and for helping me keep this project running smoothly. Additionally, I would like to extend my thanks to SES TA’s, Fiona Jevon, and Alice Carter for all of their assistance over the course of this project and for making the SES lab a great place to work, as well as to my fellow SES students for helping make this a great semester!

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References

Alexander, M. 1965. Nitrification. Pages 307-343 in W.V. Bartholomew and F.E. Clark, editors. Soil nitrogen. American Society of Agronomy, Madison, Wisconsin, USA.

Al-Kaisi, M.M., M.L. Kruse, and J.E. Sawyer. 2008. Effect of nitrogen fertilizer application on growing season soil carbon dioxide emission in a corn-soybean rotation. Journal of Environmental Quality 37: 325-332.

Bartholomew, W.V., and F. E. Clark, editors. 1965. Soil nitrogen. American Society of Agronomy, Madison, Wisconsin, USA.

Blasing, T.J. 2013. Recent greenhouse gas concentrations. United States Department of Energy, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, TN, USA. DOI: 10.3334/CDIAC/atg.032.

Boyanapalle, D. 2004. Nitrogen fertilizers. Frequently asked questions about organic manures and fertilizers. [Cited October 2013.] Available from http://vasat.icrisat.org/crops/MN/OrganicFAQs/index.html.

Diamond, D. 2008. Determination of Nitrate and/or Nitrite in Brackish or Seawater by Flow Injection Analysis Colorimetry. QuikChem Method 31-107-04-1-C.

Eichner, M.J. 1990. Nitrous oxide emissions from fertilized soils: summary of available data. Journal of Environmental Quality 19: 272-280.

Galloway, J.N., W.H. Schlesinger, H. Levy, A. Michaels, and J.L. Schnoor. 1995. Nitrogen Fixation: anthropogenic enhancement-environmental response. Global BiogeochemicalCycles 9: 235-252.

Galloway, J.N., J.D. Aber, J.W. Erisman, S.P. Seitzinger, R.W. Howarth, E.B. Cowling, and B. J. Cosby. 2003. The nitrogen cascade. BioScience 53: 341-355.

Klein, C.D., R.S.A. Novoa, S. Ogle, K.A. Smith, P. Rochette, T.C. Wirth, B.G. McConkey, A. Mosier, and K. Rypdal. N2O emissions from managed soils and CO2 eimssions from lime and urea application. 2006 IPCC Guidelines for national greenhouse gas inventories. International Panel on Climate Change, Task Force on National Greenhouse Gas Inventories, Hayama, Japan.

Lal, R., J. Kimbal, and B.A. Stewart. 1995. World soils as a source or sink for radiatively-activegases. Pages 1-5 in R. Lal, J. Kimble, E. Levine, and B.A. Stewart, editors. Soilmanagement and greenhouse effect. Lewis, Boca Raton, Florida, USA.

Millar, N., P.G. Robertson, P.R. Grace, R.J. Gehl, and J.P. Hoben. 2010. Nitrogen fertilizer

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management for nitrous oxide mitigation in intensive corn production: an emissions reduction protocol for US Midwest agriculture. Mitigation and Adaptation Strategies for Global Change 15: 185-204.

Raich, W.J., and C.S. Potter. 1995. Global patterns of carbon dioxide emissions from soils. Global Biogeochemical Cycles 9: 23-36.

Robertson, G.P., T.W. Bruulsema, R.J. Gehl, D. Kanter, D.L. Mauzerall, C.A. Rotz, and C.O. Williams. 2012. Nitrogen-climate interactions in US agriculture. Biogeochemistry 114:41-70.

Schlesinger, W.H., and E.S. Bernhardt. 2013. Biogeochemistry: An analysis of global change. Elsavier, Boston, MA, USA.

Slmek, M. and J.E. Cooper. 2002. The influence of soil pH on denitrification: progress towards the understanding of this interaction over the past 50 years. European Journal of Soil Science 53: 345-354.

Snyder, C.S., T.W. Bruulsema, T.L. Jensen, and P.E. Fixen. 2009. Review of greenhouse gas emissions from crop production systems and fertilizer management effects. Agriculture, Ecosystems, and Environment 133: 247-266.

Strickland, J.D.H. and T.R. Parsons. 1972. A practical handbook of Seawater analysis. 2nd ed. Ottawa Fisheries Research Board of Canada, Ottawa, Canada.

United States Environmental Protection Agency. 2013. Overview of greenhouse gases [Internet]. [Cited 2013 Oct. 28]. Available from http://epa.gov/climatechange/ghgemissions/sources/agriculture.html.

University of Minnesota Extension. 2002. Soil scientist [Internet]. Reagent of the University of Minnesota. [Cited 2013 Nov. 9]. Available from http://www.extension.umn.edu/distribution/cropsystems/components/7401_02.html.

Figures and Tables

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Figure 1. Map of Massachusetts showing the location of the University of Massachusetts Agricultural Station in South Deerfield, MA at point A.

Figure 2. Image of the cornfield at University of Massachusetts Agricultural Station farm, South Deerfield, MA..

Figure 3. Experimental nitrogen treatment plot set up at South Deerfield Agricultural Station, MA.

Figure 4. Images of experimental setup for the lab soil incubations in the MBL lab, Woods Hole, MA.

Figure 5. Total soil nitrogen content in fertilized soil plots at University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 6. Soil nitrate concentration over three days in fertilized and control soil plots at University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 7. Soil ammonium concentration over the course of three days in fertilized soil plots in a cornfield at University of Massachuestts Agricultural Station, South Deerfield, MA.

Figure 8. Total nitrogen content from fertilized and control soil plots on the first day of soil testing after fertilizer was applied to a cornfield in South Deerfield, MA.

Figure 9. Total carbon content from fertilized and control soil plots on the first and last days of soil testing after fertilizer was applied to a cornfield in South Deerfield, MA.

Figure 10. N2O flux measured from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 11. Cumulative N2O emissions over the course of the three-day experiment period from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 12. Soil N2O flux after fertilization of soil chambers in MBL lab. Soil in incubation was collected from the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 13. Change in moisture content of soils incubated in the lab over the period of three days. Moisture content is expressed as soil wet to dry ratio.

Figure 14. Cumulative N2O flux over the course of the three-day experiment period from fertilized soils incubated in the MBL lab.

Figure 15. Soil CO2 flux measured from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

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Figure 16. Soil CO2 flux from fertilized fields at the University of Massachusetts Agricultural Station, South Deerfield, MA compared to air temperature on measurement days.

Figure 17. CO2 flux after fertilization of soil chambers in MBL lab. Incubated soil was collected from a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 18. Soil NO and NO2 flux (NOx) measured from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 19. Total soil nitrogen content compared to nitrous oxide flux from fertilized agricultural soils in South Deerfield, MA on the first and last day of the experiment.

Figure 20. Total soil nitrogen content of soils incubated in the MBL lab compared to the N2O fluxfrom incubated soils on the first and last day of flux measurement.

Figure 21. Soil nitrate content over time compared to soil N2O flux over time in ammonium-fertilized soil plots at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 22. Soil nitrate content compared to N2O flux over time in soils fertilized with ammonium and incubated in the MBL lab for 3 days.

Figure 23. Soil nitrate content over time compared to soil N2O flux over time in nitrate-fertilized soil plots at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 24. Soil nitrate content over time compared to soil N2O flux over time in manure-fertilizedsoil plots at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 25. Soil nitrate content compared to N2O flux over time in soils fertilized with nitrate and incubated in the MBL lab for 3 days.

Figure 26. Soil carbon content compared to soil N2O flux from fertilized soils in a cornfield at theUniversity of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 27. Soil carbon content compared to N2O flux from soils incubated in the MBL lab for three days.

Figure 28. Soil carbon content compared to CO2 flux from soils incubated in the MBL lab for three days.

Figure 29. Soil pH compared to N2O emissions from fertilized soils in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 30. Average air temperature at the University of Massachusetts Farm, South Deerfield, MA from November 12 through November 14.

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Figure 31. CO2 flux from untreated control soil plots in relation to air temperature during the fieldexperiment at University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 32. Soil N2O flux from untreated control soil plots in relation to air temperature during thefield experiment at University of Massachusetts Agricultural Station, South Deerfield, MA.

Table 1. Fertilizer types and application rates for each experimental plot. Application rates are based on studies by Robertson et al. (2012) and Snyder et al. (2009).

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Figure 1. Map of Massachusetts showing the location of the University of Massachusetts Agricultural Station in South Deerfield, MA at point A.

Figure 2. Image of the cornfield at University of Massachusetts Agricultural Station farm, South Deerfield, MA, where I conducted my field study.

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NH4ClHigh

NH4ClLow

Organic High

Control

NaNO3 Low

NaNO3 High

Organic Low

Organic High

NaNO3 High

NH4ClHigh

NaNO3 High

NH4ClLow

Organic High

Control

Control

Organic Low

NaNO3 Low

Organic Low

NH4ClHigh

NaNO3 Low

NH4ClLow

Figure 3. Experimental nitrogen treatment plot set up at South Deerfield Agricultural Station, MA. Each white box represents a 1m x 1m plot and grey boxes represent 2m buffer zones between each plot type. There are 7 treatments in the plots with three replicates of each treatment.Treatments are a high and low level nitrate-based (NaNO3) treatment, high and low level ammonium-based (NH4Cl) treatment, a high and low level organic nitrogen (manure) treatment, and a control treatment with no fertilizer. High-level treatments are 150kgN/ha and low-level are 50kgN/ha.

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Figure 4. Images of experimental setup for the lab soil incubations in the MBL lab, Woods Hole, MA. Top photo shows the LGR and LICOR 7000 with nine soil incubation chambers. Bottom photo shows a close-up of the soil chamber top connecting to the LGR and LICOR tubing for measuring N2O and CO2.

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Figure 5. Total soil nitrogen content in fertilized soil plots at University of Massachusetts Agricultural Station, South Deerfield, MA. Total nitrogen is shown for the first and last days of soil testing.

A) B)

Figure 6. Soil nitrate concentration over three days in fertilized and control soil plots at University of Massachusetts Agricultural Station, South Deerfield, MA. Graph B is scaled to bottom five lines shown in the box in graph A.

A) B)

Figure 7. Soil ammonium concentration over the course of three days in fertilized soil plots in a cornfield at University of Massachuestts Agricultural Station, South Deerfield, MA. Graph B is scaled to bottom five lines that are in the box in graph A.

Figure 8. Total nitrogen content from fertilized and control soil plots on the first day of soil testing after fertilizer was applied. Soil plots were in a cornfield at University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 9. Total carbon content from fertilized and control soil plots on the first and last days of soil testing after fertilizer was applied. Soil plots were in a cornfield at University of Massachusetts Agricultural Station, South Deerfield, MA.

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Figure 10. N2O flux measured from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA. Measurements were taken over the course of three days in mid-November.

Figure 11. Cumulative N2O emissions from fertilized soil plots over the course of the three-day field experiment in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA. Cumulative fluxes were calculated by adding average daily N2O fluxes from eachplot for three days.

Figure 12. Soil N2O flux after fertilization of soil chambers in MBL lab. Soil in incubation was collected from the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 13. Change in moisture content of soils incubated in the lab over the period of three days. Moisture content is expressed as soil wet to dry ratio.

Figure 14. Cumulative N2O flux over the course of the three-day experiment period from fertilized soils incubated in the MBL lab. Cumulative fluxes were calculated by adding average daily N2O fluxes from each plot for three days.

Figure 15. Soil CO2 flux measured from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA. Measurements were taken over the course of three days in the middle of November.

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Figure 16. Soil CO2 flux from fertilized fields at the University of Massachusetts Agricultural Station, South Deerfield, MA compared to air temperature on measurement days.

Figure 17. CO2 flux after fertilization of soil chambers in MBL lab. Incubated soil was collected from a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 18. Soil NO and NO2 flux (NOx) measured from fertilized soil plots in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA. Measurements were taken over the course of three days in the middle of November.

Figure 19. Total soil nitrogen content compared to nitrous oxide flux from fertilized agricultural soils in South Deerfield, MA on the first and last day of measurement. Blue trend line corresponds to blue points representing soil nitrogen on the first day.

Figure 20. Total soil nitrogen content of soils incubated in the MBL lab compared to the N2O flux from incubated soils on the first and last day of flux measurement.

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Figure 21. Soil nitrate content over time compared to soil N2O flux over time in ammonium-fertilized soil plots at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 22. Soil nitrate content compared to N2O flux over time in soils fertilized with ammoniumand incubated in the MBL lab for 3 days.

Figure 23. Soil nitrate content over time compared to soil N2O flux over time in nitrate-fertilizedsoil plots at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 24. Soil nitrate content over time compared to soil N2O flux over time in manure-fertilized soil plots at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 25. Soil nitrate content compared to N2O flux over time in soils fertilized with nitrate and incubated in the MBL lab for 3 days.

Figure 26. Soil carbon content compared to soil N2O flux from fertilized soils in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA. Carbon content and N2O flux are compared on the first and third days of the experiment. Red trend line corresponds to red data points representing measurements from the third day of measurement.

Figure 27. Soil carbon content compared to N2O flux from soils incubated in the MBL lab for three days. Soil carbon was compared to N2O flux on the first and third days of the incubation period.

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Figure 28. Soil carbon content compared to CO2 flux from soils incubated in the MBL lab for three days. Soil carbon was compared to CO2 flux on the first and third days of the incubation period. Blue trend line corresponds to blue data points representing measurements from first day of testing.

Figure 29. Soil pH compared to N2O emissions from fertilized soils in a cornfield at the University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 30. Average air temperature at the University of Massachusetts Farm, South Deerfield, MA over the time frame of the experiment. Measurements were taken from November 12 to November 14, 2013.

Figure 31. CO2 flux from untreated control soil plots in relation to air temperature during the field experiment at University of Massachusetts Agricultural Station, South Deerfield, MA.

Figure 32. Soil N2O flux from untreated control soil plots in relation to air temperature during thefield experiment at University of Massachusetts Agricultural Station, South Deerfield, MA.

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Table 1. Fertilizer types and application rates for each 1m2 experimental plot. Application ratesare based on studies by Robertson et al. (2012) and Snyder et al. (2009).

TreatmentType

Application rate (kgN/ha)

Application amount (gN/m2)

Fertilizer for 1*1 meter plot (g)

NitrogenNeeded (moles)

Weight of Chemical fertilizer (g)

Nitrogen Fertilizer applied per plot (g)

NaNO3 High

150 15 15.0 1.1 85.0 91.1

NaNO3 Low

50 5 5.0 0.4 85.0 30.4

NH4Cl High

150 15 15.0 1.1 53.5 57.3

NH4Cl Low

50 5 5.0 0.4 53.5 19.1

Approximate %Nitrogen in Manure

Manure applied per plot (g)

Organic (manure) High

150 15 15 1.1 1% 1500

Organic (manure) Low

50 5 5 0.4 1% 500

Control 0 0 0 0 0 0

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