Influence of lime on soil respiration, leaching of DOC, and C/S relationships in the mor humus of a haplic podsol

Pergamon Environment International, Vol. 20, No. 1, pp. 81-88, 1994

Copyright ©1994 Elsevier Science Ltd Printed in the USA. All rights re.seared

0160-4120/94 $6.00 +.00

INFLUENCE OF LIME ON SOIL RESPIRATION, LEACHING OF DOC, AND C/S RELATIONSHIPS IN THE MOR HUMUS OF A HAPLIC PODSOL

Stefan Andersson and Inger Valeur Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden

Ingvar Nilsson Department of Soil Sciences, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden

E1 9306-152 M (Received 23 June 1993; accepted 30 November 1993)

Mor humus samples from separate plots in a field-liming experiment in southern Sweden were incubated in a leaching experiment. Lime applications of 0.16, 0.35, and 0.88 kg/m 2 dolomite lime were made 7 y before sampling. The leachates from the two highest lime applications had a pH of 7.0 and 5.3, respectively. The control and the lowest lime application had a pH of 3.9. Humus material receiving the largest lime application also showed the highest level of dissolved organic carbon (DOC) leaching (180 mg DOC/L) during the first 100 d of the experiment, probably owing to the increased negative charge caused by the high pH, which increased the solubility. The DOC concentration decreased in all treatments towards the end of the experiment owing to an increase in protonation caused by nitrate formation (nitrification). The C/S ratio in the organic matter of the leachates from the limed humus was lower than that in the control leachates. A positive correlation between DOC leaching and biological activity, measured as soil respiration, was found in humus material that had received the two highest lime applications.

INTRODUCTION

Dissolved organic carbon (DOC) in the soil water is a potential source of C, N, P, and S for soil microorganisms. DOC also contributes substantially to the pH buffering in the soil. The solubility of humus compounds depends on molecular size, acid strength, and charge density. Increasing pH and ionic strength will therefore usually have an important positive and negative regulating function, respectively (Evans et al. 1988; Jardine et al. 1989; Vance and David 1989).

Several experiments have been made to investigate the influence of acidification on the solubility and

transport of DOC in forest soils. In some experiments, DOC decreased (Stroo and Alexander 1986; David et al. 1989; Vance and David 1989, 1991), probably owing to the protonation of weak acid groups, whereas in other experiments acid treatment had no effects (Cronan 1985).

Liming by dolomite is one method for counteract- ing the acidification of forest soils. However, the effects of liming on DOC and the leaching of the organic fraction of elements other than C have been poorly investigated. In a field experiment with an application rate of 0.40 kg/m 2 of dolomite in southern

81

82 S. Andersson et at

Germany, DOC increased, particularly in the O horizon (GOttlein and Pruscha 1991; G0ttlein et al. 1991). Curtin and Smillie (1983) found that liming increased the amount of organic matter in the soil solution from two different Inceptisols, and an Alfisol.

Carbon dioxide (CO2) evolution by microorganisms in experiments on acidification and liming has been monitored by many workers. Acidification decreases CO 2 evolution (Vance and David 1991; Bhhth et al. 1979), whereas it may increase after liming (Ivarson 1977; Persson and Wir6n 1989).

Initial and long-term effects of liming on sulfur mineralization, and in some cases on sulfate desorption as well, have been reported previously (Williams 1967; Haynes and Swift 1988; Valeur and Nilsson 1993). In the latter study, it was shown that the leaching of organic S was higher in the limed treatment (0.88 kg dolomite/m 2) than in the control. By contrast, sulfur mineralization was lower in the limed treatment than in the control.

No one has apparently investigated the effect of liming on DOC leaching and CO 2 release at the same time. The aim of the present paper is to report the long-term effects of dolomite at three different application levels on these two carbon fluxes in a mor humus. Liming-induced changes in dissolved organic sulphur (DOS) and DOC/DOS ratio are also discussed in terms of their usefulness as indicators of a change in the quality of dissolved organic matter.

MATERIALS AND METHODS

Mor humus samples were collected in May 1991 from separate plots in a field-liming experiment located at HasslOv in southern Sweden (56°24'N, 13°00'E, 190 m above M.S.L.). In 1984, the plots were treated with dolomite at rates of 0.16 kg/m 2 (D1), 0.35 kg/m 2 (D2), and 0.88 kg/m 2 (D3), with four replicates of each treatment. A description of the site is given in Table 1. After sampling, roots were removed by hand from the humus which was then placed at 4°C in field-moist condition until used in the experiment. The incubation columns were con- structed from PVC cylinders (length 10 cm, inner diam. 6.7 cm). They were equipped with a bottom plate furnished with a 3-ram drainpipe connected to a vacuum-system. A plastic filter plate (approx. pore diam. 15 txm) was placed 2 cm above the bottom plate. The cylinders were filled with 90 g dry weight of acid-washed and ignited quartz sand followed by 19.32 g dry weight of humus.

For CO 2 evolution measurements, an optional cover with a rubber septum was fitted tightly on the top of the cylinder, and gas samples were withdrawn with a syringe after an incubation period of 2 h on each sampling occasion. The columns were otherwise left open, in a dark room with constant temperature (15°C).

The water content in the columns was adjusted each week to keep it at 44% of fresh weight. They were irrigated with a 50-mL solution 1 h before each

Table 1. Description of the Hassl6v site.

SOIL CLASSIFICATION Typic Haplorthod (Soil Survey Staff 1990) Haplic podsol (FAO 1988)

ANNUAL MEAN AIR TEMPERATURE 6.5 °

ANNUAL MEAN PRECIPITATION 1100 mm

TREE STAND

GROUND VEGETATION

40-y-old Norway spruce (Picea abies (L.) Karst)

Almost absent, although Deschampsia flexuosa and various mosses occur where the carnopy is more open.

Table 2. Ion concentrations in the irrigation water in I.tmol/L.

K+ Na+ Ca2+ Mg 2+ NH4 + NOs C1-

77 205 39 29 95 101 423

Liming effect on mor humus 83

percolation event. The solution used had a salt concentration corresponding to the throughfall precipitation at the site but contained no sulfate (Table 2).

Soil leachates were collected under tension each third or fourth week, and the samples were immedi- ately passed through a 0.45-l.tm filter (Millipore HA 0.45 HAWP04700), which was rinsed beforehand with 50 mL distilled water to remove contaminat- ing DOC. After filtering, subsamples were taken for measurements of DOC, pH, NO3-N, SO4-S, total S, and electrolytic conductivity. The concentration of organic sulfur was calculated by subtracting the SO4 concentration from the total S concentration. NH4-N was also measured on a few occasions. During the first 2 weeks of the experiment, CO2 measurements were conducted each second or third day and there- after on a monthly basis. The pH and electrolytic c o n d u c t i v i t y in the l eacha tes were measured within one day. Ionic strength was calculated from the electrolytic conductivity by using the Marion- Babcock equation (Sposito 1989).

Subsamples for DOC analyses were stored in a refr igerator unless the time before analysis ex- ceeded one week, in which case they were frozen. Subsamples used for other purposes were frozen after filtration. Tests to determine the influence of freez- ing showed that DOC values of the frozen samples were slightly below those of refr igerated ones, with a mean difference of -1.3%, sd=5.14%. The difference did not influence the interpretations of the treatment effects, however.

At the beginning and end of the experiment, the humus was weighed, and samples were taken for the measurement of water content, loss on ignition (LOI), total C, and total S (practically equal to organic S). In the quartz sand, the water content and LOI were measured at the end of the experiment. LOI was determined at 550°C. Total C in the ignited residues was then measured to estimate the CO3-C content. This value was then subtracted from the total C content in the humus. The CO3-C content was determined by a dry combustion procedure at 1000°C (Carlo Erba). Total C and total S in the dried humus were determined on a Leco analyser.

DOC was analysed on a Shimatdzu TOC-500 TOC- analyser. The amount of CO2 collected in the soil respiration measurements was determined with a gas chromatograph (Hewlett-Packard 5890 A). Soil respiration was expressed as the mass of C evolved per g of organic soil and h according to Persson et al. (1989). Also, NH4-N was analysed by a flow injec- tion technique, and NO3-N and SO4-S were determined by ion chromatography (Dionex-column).

Total S in the leachates was determined by plasma emission spectroscopy (ICP). Statistical analysis was made by analysis of variance followed by pairwise tests using the least significant difference (LSD) method. Statistical significance was set at p < 0.05.

RESULTS AND DISCUSSION

There was a large difference between treatments with respect to initial organic matter content (LOI), with values being lower in the limed plots (68.6- 80.2%, 66.3-70.7%, 57.4-76.0% in D1, D2, and D3, respectively) than in the control plots (77.2-81.7%). This difference could have been due partly to the presence of undissolved lime which contributed to the "mineral soil" content. Earthworms (Dendrobena sp.) which occurred in the lower part of the mor humus, close to the mineral soil, may also have had some influence by mixing mineral soil into the mor humus. The number of earthworms in the field increased over time in the D1, D2, and D3 treatments. In addition, the content of organic matter decreased during the experiment. To reduce the variation within treatments, the values of DOC, NH4-N, and NO3-N in connection with the first five percolation events were arbitrarily divided by the original organic matter content while the values obtained in the following six percolation events were divided by the organic matter content at the end of the experiment. These modified DOC, NH4-N, and NO3-N values were used in all calculations with two exceptions; i.e., the original DOC values were used for calculating the carbon budgets and the C/S ratios in the leachates.

DOC leaching during the first 100 d of the experiment was significantly higher from the soil receiving the largest lime application (D3) (Fig. 1) than from the other treatments, but decreased later to such an extent that it eventually became lower than DOC leaching from the control soil (DO). DOC in all treatments decreased towards the end of the experiment, and there were no significant differences between them after the fifth leachate collection. The ranks in terms of DOC-leaching assigned to the limed treatments were positively correlated with the lime application level throughout the experiment.

Although the pH of the leachates generally showed a positive relation to the lime application rate, D1 had nearly the same pH as the control in the later part of the experiment (Fig. 2). No significant difference in ionic strength was found between treatments. How- ever, in the D 1 leachate it increased slightly until the sixth percolation event, whereafter it decreased to about the same level as in the other treatments (Fig. 3). The variation over time in ionic strength

84 S. Andersson et al,

corresponded largely to the temporal dynamics of nitrate formation. Leachate nitrate concentrations were significantly higher in the limed treatments than the control during the first 100 d (Fig. 4). Later on, the D1 treatment had a lower nitrate concentration than the other limed treatments. Ammonium formation was significantly higher in the control and D1 treatment than in the other treatments; at the end of the

experiment, it was significantly higher in the control than in D1 (Fig. 5).

Nitrogen mineralization was high in all treatments (Figs. 4 and 5). Nitrification was probably enhanced by the pH increase in the limed treatments. That nitrification is an acidifying process can be readily seen in the D1 treatment (Figs. 2 and 4). The relationships between DOC leaching, ionic strength, and pH are presented in Table 3. As expected, the correla-

225 I _ - - ~ . DO ........ 131 ----

,oo I / / .,<-- /--',.-.... 0 I II ~ - - - .~ . . - " " . . "".,..

75 J - / ~ . . . . . . . . . "-.. " ' , . ,

25

t t z 0 ! ) ) 0 30 60 90 120 150 180 210 240 270

Time (days) Fig . l . Temporal variation in leachate DOC concentration

during the course of the experiment.

0.7

0.6

0.5

o

0.4

r= ~ 0.3

._o t--"

_9o 0.2

0.1

ol

I "" I DO . . . . . . . . D1

. / " " \ t7.7-02 - - - - Do , / / \

, ' / / N \

,," - -~ / \ ..~.:'-" .~-~C./. ........ \ / " . / / / ~ . . . . . ,,,~

it.',. / ' . , . . / N . ",.',

I I I I I I f I 30 so 9o 120 150 180 21o 240 270

Time (days)

Fig. 3. Change in the ionic strength in the leachates from the second percolation event to thelast percolation event.

fill . . . . r . . . . . .

o t/F 6.0 1

V - - - - ~1- I___Do

5.0 ~ ~ ~ "~'~

3 .5 H- I I I I I I I I 0 30 SO 90 120 150 180 210 240 270

Time (days)

Fig . 2. Temporal variation in leachate pH during the course of the experiment.

70

6O

.~ 50 r-" o r o o 40 o

........ o, [ . / ' , - - - - 03 1 I:>-:":---',\,, , ,

, \

• " ,' I ":\',,. / / . / , - - ,

/."" / / \ / : .................. ,.

DO D2

lO

0 _ _ l _ _ _ _ _ l t 1 1 0 30 60 90 120 15o 1so 210 240 270

-time (days)

Fig. 4. Temporal variation in leachate NO)" concentration during the course of the experiment.

Liming effect on mor humus 8 5

tion, calculated over all treatments, was positive for DOC and pH but slightly negative between DOC and ionic strength (Table 3) (compare Evans et al. 1988; Vance and David 1989). Correlation coefficients calculated based on mean values within treatments yielded a different pattern (Table 3). There was a negative correlation between DOC and pH in the control and a positive one in D3. Ionic strength and DOC were positively correlated in the control. No other significant correlations were found.

An explanation for the positive correlation between ionic strength and DOC in the control treat-

ment, and possibly also in the D1 treatment, could be that a large fraction of the cations in the leachates consisted of monovalent NH4+ (Figs. 3 and 5). Our hypothesis is that a high degree of NH4+ saturation increases the diffuse double layer of soil colloids and the colloid dispersion, and thereby the DOC, as is the case in soils saturated with the monovalent Na ÷ (Sposito 1989). Dissolved organic acids in these treatments will dissociate and decrease the pH. In the D3 treatment, on the other hand, the pH was buffered by HCO3- to a greater extent, and the high rate of DOC leaching may have been due to the higher nega-

Table 3. Correlations (r) between DOC and pH or ionic strength (I) based on: I. pooled values regardless of treatment. 2. Mean values within each treatment D0,DI,D2,and D3). n.s. = not significant.

pH I

Treatment r p r p

Pooled values =0.38 <0.001 -0.20 <0.01

DO (Control) -0.73 <0.05 +0.78 <0.01

D1 -0.57 n.s. +0.42 n.s.

D2 +0.13 n.s. -0.02 n.s.

D3 +0.84 <0.01 +0.21 n.s.

.ff

c- O

._o

0

Z I

Z

25 DO

- - - - - D2

2O

15

1 0

5

I 0 0 :3O

. . . . . . . . D1

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . %,

l I I I I I I 6o 9o 12o 15o 18o 21o 24o 270

r i m e (days)

Fig. 5. NH4 ° concentration during the course of the experiment.

6OO0

7 5OO0

o

7 4OOO

L ) E~

3ooo

¢- o

2 0 0 0

o" looo L~

J ! \',

. . . . m - - - - m \

i / x ~ . t ' _

' " " : .........................................

I I i I i 0 50 100 150 200 250 3O0

33me (days)

Fig. 6. Temporal variation in soil respiration during the course of the experiment.

86 s. Andersson et al

t i re charge density on the humus colloids at the prevail ing higher pH (Fig. 2). Obviously, pH had a much stronger inf luence than the ionic strength in this treatment. In all l imed treatments, nitrate formation increased during the course of the experiment. This was also the case in the control but to a smaller extent (Fig. 4). The increase in nitr if ication implies that protons were being released and, possibly, that there was an increase in the degree of protonation of the humus colloids which eventual ly should have decreased the solubility of the organic matter (Fig. 1).

Liming has been reported to enhance the leaching of DOC or d issolved organic mat ter in o ther inves- t iga t ions , both a f te r 2-3 y in the f ie ld (G6t t l e in and Pruscha 1991; G6ttlein et al. 1991) and in a laboratory incubation of an Alfisol and two Incep- tisols (Curtin and Smillie 1983). These authors also found that the amount of organic mat ter decreased in the soil solution, while nitr if ication increased, accompanied by a decrease in pH. In another field

experiment , however, there was no dif ference between limed and control plots after 11 weeks (Cronan et al. 1992). In cases where the solubility of DOC or organic matter increased, the rise was explained by desorption and higher solubili ty associated with the increase in pH.

Throughout the experiment , soil respirat ion was significantly higher in the highest lime treatment than in the other treatments (Fig. 6). The f luctuations in CO 2 evolution during the first 25 d of the experiment were probably due to f luctuations in soil mois- ture content. In a previous soil column, incubation with unlimed and limed (0.88 kg/m 2) humus from the Hass l0v site l iming was also shown to e n h a n c e soil respiration (Valeur and Nilsson 1993). Similar results have been obtained in other experiments. In mor humus analysed 7-16 y after liming, there was an increase in CO 2 evolu t ion short ly af ter the incubation began (Persson and Wir6n 1989), and also in humus limed in the laboratory (Persson et al. 1990/91).

Table 4. Correlations (r) between DOC and soil respiration (CO~) based on: 1) Pooled values regardless of treatment. 2) Mean values within each treatment (DO, D1, D2, and D3). n.s. = not significant.

Treatment

Pooled values =0.51

DO (Control) =0.20 n.s.

D1 +0.54 n.s.

D2 +0.99 <0.01

D3 +0.60 n.s.

+0.97 D3") ,,,=

o with one extreme value excluded

P

<0.001

<0.05

Table 5. Organic-C budget in the experiment. Start = total amount of organic C at the start; End = total amount of organic C at the end; Resp -- total amount of C respired during the experiment; DOC = total dissolved organic C leached during the experiment; Remaining = D-end + resp-C + DOC; Diff = C-start - Remaining; Values with different letters within each column are significantly different (p < 0.05).

All values in g C.

Treatment

DO

Start

7.86"

End

7.00*

Resp

0.14 ~

DOC

0.05"

Remaining [

7.19'

Diff

0.67"

D1 7.27 b 6.68 "b 0.13 ~ 0.03 b 6.84 "b 0.43"

D2 6"84b 6'36b 0.22b 0.04 *b 6.62 ~ 0.22"

D3 6.30* 5.32 ~ 0.76' 0.05' 6.13 ° 0.17"

Liming effect on mor humus 87

Table 6. Comparison of treatment effects on the ratio of total C to total S in the solid humus at the end of the experiment, the ratio of accumulated DOC to accumulated organic S in the leachates, and accumulated organic S in the leachates. Values with different letters

within each column are significantly different (p <0.05).

Treatment C/S ratio in humus C/s ratio in leachate Organic S in leachato (rag)

DO 199' 103' 0.46 b

D1 195' 59 b 0.45 b

D2 190 ~ 58 ~ 0.62 'b

60 b D3 168 b 0.85"

A peat soil which was limed and incubated showed an increase in CO 2 evolution after 8 months (Ivarson 1977).

The posi t ive response of the microorganisms to liming was probably due to an increase in the availability of C sources as the pH increased (Persson and Wir6n 1989). This response was partly reflected in the authors' experiment by a positive correlation between DOC and soil respiration in the D2 treatment, and this was also the case in the D3 treatment after one extreme initial value had been excluded (Table 4). The decline in respiration towards the end of the experiment (Fig. 6) could have been a result of a depletion of available C in combination with nitrification. Persson and Wir6n (1989) found that after nitrification had been induced in limed humus, the CO 2 evolution rate decreased.

A tentative carbon budget is shown in Table 5. When the sum of the total amount of leached DOC and the total soil respiration was compared with the difference between the amount of initial organic carbon and the amount of final organic carbon, a discrepancy was found, probably due to the fact that some particulate organic carbon remained on the filter plate and on the millipore filter. Also, CO 2 diffusion difficulties in the humus layer may have contributed to an underestimation of the soil respiration. The magnitudes of the discrepancies did not differ significantly between treatments. Unfortunate- ly, none of the other researchers who used column techniques presented complete carbon budgets, in- cluding pool changes, with which our data could be compared.

Disregarding the discrepancy, the total loss of C connected with respiration varied from 74% in the control columns to 94% in the D3 columns, inferring that losses associated with DOC leaching were 26% and 6%, respectively. In an acidification experiment, Cronan (1985) found that 54-68% of the total loss of

C occurred through respiration, whereas DOC leaching accounted for 32-46%. In a similar experiment (Vance and David 1991), 72%of the total loss of C was accounted for by respiration and 28% by leaching.

At the end of the experiment, the C/S ratio (total-C and total-S) in the solid humus was significantly lower in the D3 treatment (Table 6) than in the other treatments. This can be ascribed mainly to the high rates of respiration in the D3 columns since there was no difference in total S between treatments.

The C/S ratio, based on the accumulated amount of C and S in the dissolved organic matter of the leachates, was higher in the control treatment than in the limed treatments (Table 6). The C/S ratios were not affected by the different lime levels. The accumulated leaching of DOC was the same in D3 and the control (Table 5). The leaching of organic S was significantly higher from the D3 and D2 treatments than from the control (Table 6). A similar result regarding organic sulfur was obtained by Valeur and Nilsson (1993). To explain their results, they sug- gested that the liming-induced immobilization of sulfate increased the amount of organic S dissolved in the soil solution.

The difference in C/S ratios found between the limed and the unlimed humus indicated that they differed qualitatively in their composition of organic substances. G6ttlein and Pruscha (1991) found another difference, i.e., that the DOC of limed humus was less hydrophilic than that of the control humus. The authors also found this to be true in the Hassl6v soil.

CONCLUSIONS

Liming enhanced the leaching of DOC. The pH increase in the limed treatments promoted nitrate formation and nitrogen mineralization in the humus. Nitrification increased the ionic strength, and the

88 S. Andersson et al

protonation of organic matter may have contributed to the lower solubility of organic carbon observed at the end of the experiment.

The contribution of respiration to the total loss of C was relatively more important in the limed treatments than in the control. Respiration and the leaching of DOC were positively correlated in the D2 and D3 treatments.

Liming appeared to cause qualitative changes in the humus, as indicated by the fact that the C/S ratio of the organic material in the leachates from the limed humus (D3, D2, and D1) was lower than that in the control leachates.

A c k n o w l e d g m e n t - - This research was financed by a grant from the National Swedish Environmental Protection Board. Dr. T Persson reviewed the manuscript, and Dr. D. Tilles corrected the language.

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G6ttlein, A.; Pruscha, H. Statistische Auswertung des Einflusses von sourer Beregnung und Kalkung auf die Wasserl6slichkeit organischer Bodeninhaltsstoffe, Forstwiss. Forsch. 39: 221- 228; 1991.

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Persson, T.; Wir6n, A.; Andersson, S. Effects of liming on carbon and nitrogen mineralization in coniferous forests. Water Air Soil Pollut. 54: 351-364; 1990/91.

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Documents

Influence of lime on soil respiration, leaching of DOC, and C/S relationships in the mor humus of a haplic podsol