Soil inorganic N availability: Effect on maize residue decomposition

Soil Bid. Biochem. Vol. 27, No. 12, pp. 1529-1538, 1995 Copyright 0 1995 Elsevier Science Ltd

0038-0717(95)00096-8 Printed in Great Britain. All rights reserved 0038-0717/95 $9.50 + 0.00

Pergamon

SOIL INORGANIC N AVAILABILITY: EFFECT ON MAIZE RESIDUE DECOMPOSITION

S. RECOUS,‘* D. ROBIN: D. DARWIS3 and B. MARY’ ‘I.N.R.A., Unit& d’Agronomie, rue F. Christ, 02007 Laon Cedex, France, ‘Grande Paroisse S.A.,

22 Place des Vosges, 92080 Paris La Defense 5, France and ‘Fak.Pertanian Unhalu, Jl. Layjen parman, Kendari-Sul. Tenggara, Indonesia

(Accepted 27 July 1995)

Summary--The effect of soil inorganic N availability on the decomposition of maize residues was tested under aerobic conditions in soil samples incubated for 125 days at 15°C. Carbon residue were ground maize shoots applied at 4 g dry matter kg-’ soil. The C-amended soils contained five initial inorganic N concentrations (IO, 30, 60, 80 and 100 mg N kgg’ soil). Gross N immobilization was calculated with a 15N tracer, using changes in both the inorganic and organic “N pools. Inorganic N remained available in those sclils having the three highest initial N concentrations. In this case the rates of C mineralization and N immobilization were similar. Soil inorganic N completely disappeared at the beginning of C decomposition in the soil samples with the two lowest N contents, resulting in a marked decrease of C mineralization rate compared to the three highest N contents. Gross N immobilization amounted to 39 mg N g-’ added C after 40 days (end of the net immobilization period) for the three highest N concentrations, indicating that there was no luxury N consumption by the soil microflora. N immobilization was much lower in the two lowest-N treatments because decomposition was slow and microbial N immobilization per unit of mineralized C was reduced. The ratio N immobilized:C mineralized also decreased in all treatments during decomposition due to changes in microbial N demand with time or increasing contributions from other sources of N, such as biomass-N recycling, to microbial N assimilation.

INTRODUCTION

Incorporation of straw into soil after harvest reduces nitrate leaching by inducing N immobilization (Darwis et al., 1994; Powlson et al., 1985; Mary et al., 1995). However the amounts of N immobilized are very variable. Published figures for N immobilized per unit of added straw C under laboratory conditions range from 15 (Bakken, 1986) to 35 mg N kg-’ added C (Nommick, 1962). In the intensive cropping systemr; of Northern Europe, 8-10 tonnes of straw ha-’ are often incorporated after harvest, or about 3.24.0 t of C. Using the range of values for the N-to-C ratio indicated above, then between 50 and 140 kg N ha-’ might be immobilized during straw decomposition, but this needs to be clearly established before reliable predictions can be made. The variability in the N-to-C ratio indicated above could arise from differences in the methods using for assess- ing N and C fluxes, from varying experimental conditions (soil type, climatic factors), from differences in the quality and composition of the straw, and from the amount of available N. The aim of our study was to investigate the effect of soil inorganic N on the decomposition of maize residue under controlled conditions. C evolution and soil N immobilization were described to quantify C-to-N relationships during decomposition.

*Author for correspondence.

MATERIALS AND METHODS

Soil and maize residues

The soil was collected several weeks before the experiment started from the surface layer (0-1Ocm) at Mons-en-Chaussee Experimental Station (1.N.R.A) in Northern France. The soil (Orthic Luvi- sol) had silty loam texture (17% clay, 78% silt, 5% sand), 1.0% CaCO,, 1.3% organic C, 0.14% total N and pHcH20) 7.8. The soil had been under intensive crop rotation (sugar beet, wheat, maize) for many years. The soil was air-dried for 4 days to a moisture content of 150 mg g-’ dry soil then immediately sieved to obtain aggregates of 2-3.15 mm dia. Soil finer than 2 mm was discarded. All the visible organic residues were removed by hand after sieving. The water content was adjusted to 160 mg g-i soil and the aggregates were stored at 5°C until the beginning of experiment. The flush of mineralization was mini- mized by a 3 week conditioning (pre-incubation) performed at 15°C and at a water content of 160 mg g-’ soil before amendment with residues. The initial inorganic N content was 10 mg N kg-’ soil. The soil water potential for the experiment was -50 kPa (water content 200 mg g-’ soil), at which soil aggregates were unsaturated. This allowed the decomposition to be almost completely aerobic.

Maize (Zea muys L.) straw residues were collected from a field experiment (INRA Versailles, France). Nodes were removed, and the cortex and pith

1529

1530 S. Recous et al.

Table I. Chemical characteristics of maize residues (cortex:pith = 3: I)

Chemical composition

Total N (%) 0.33 Inorganic N (% total N) 16.4

Nitrate (mg N kg-‘) 465 Ammonium (mg N kg-‘) 105

Total c (%) 43.6 C-to-N ratio 130

Biochemical composition (% in weight) (Van Soest, 1967)

Soluble fraction 40.3 Cellulose 30.0 Hemicelluloses 26.7 Lignin 3.0

were chopped separately to a mean length of 3 mm (2-5 mm). The chemical characteristics of the residues are given in Table 1.

Incubation

Samples of 29 g of soil at a water content of 160 mg g-l soil (25 g dry wt basis) were placed in 60ml wide-mouth bottles and thoroughly mixed with 25 mg pith and 75 mg cortex (the l-to-3 proportion corresponding to the maize straw composition). This addition (1.76 g C kg-’ dry soil) was equivalent to the uniform incorporation of 8 t DM ha-’ into the O-15 cm soil layer. Five initial N concentrations were compared: 10, 30, 60, 80 and 100 mg N kg-’ soil of which 0,20,50,70 and 90 mg N kg-’ soil were added as 15NH4 15N0,, respectively. These amended soil samples at five initial N concentrations will be described as R,,, R3,,, R,, R,, and R,,. Soil samples without residues (control) were compared at three initial N concentrations: 30, 80 and 100mg N kg-’ soil after addition of 20, 70 and 90 mg N kg-’ soil as “NH, “N03. They will be referred to as &,, CBO and C 100’ Labelled NH,NO, solution (1 ml) was added with a micropipette at the soil surface without mixing the soil. The addition of 1 ml N solution brought the soil water content at 200 mg gg’ soil which was the final water content during incubation. The soil inorganic N contents and 15N atom % excesses measured at time 0 after labelled N additions are given in Table 2.

Samples contained in the 60 ml wide-mouth bottles were incubated in 1 1 glass jars with a beaker containing 5ml distilled water to reduce water loss by evaporation. A second beaker containing 10ml

Table 2. Characteristics of the different N treatments compared

Carbon added Initial mineral N* Initial “N* Treatments (mg C kg-’ soil) (mg N kg-’ soil) atom % excess

RIO 1760 IO 0.00 R30 I760 29 4.42 R60 1760 61 6.98 R80 1760 83 7.73 RlOO I760 I01 8.16

c30 0 28 4.95 C80 0 82 8.00 Cl00 0 IO1 8.39

Values measured at time 0 after KCI extraction.

250 mM NaOH was also placed in the jars, to trap the CO, produced. The jars were sealed and stored in the dark for 140 days at 15 + 0.5”C. All the jars were opened periodically, aerated for a few minutes, and soil water content was checked and adjusted by weighing (soil moisture varied by less than f 10 mg H,O gg’ soil). The NaOH beakers were changed at each sampling and two samples (25 g dry wt basis) were removed by treatment, immediately frozen in liquid N, and stored at - 18°C. The sampling times were 0, 2, 3, 6, 11, 15, 18, 29, 40, 69, and 124 days, except for the samples given 100 mg N kg-’ soil, which were sampled at 0, 2, 3, 7, 10, 14, 18, 28, 40, 70 and 117 days in a separate experiment.

Analytical procedures

Determination of trapped CO,. Carbonates trapped in the NaOH were precipitated with excess BaCl,. The remaining NaOH was titrated with 250m~ HCl to the equivalent point (pH = 8.62).

Inorganic N and ‘-‘N determinations. Inorganic N was extracted from freshly-thawed soil samples with 1 M KC1 (soil-to-solution ratio = 25 g dry wt-to-80 g) in centrifuge tubes. Tubes were shaken for 30 min and centrifuged at 3000 rev min-’ for 15 min. The extraction was repeated once and the two supematants were combined for analysis. NH,-N and NOJ-N were measured by continuous flow calorimetry, using cad- mium reduction and the Griess-Ilosvay reaction for nitrate and the indophenol method for ammonium. For 15N measurements, NH,, and NO, were separated by distillation with MgO and Dewarda’s alloy (Brem- ner, 1965). Inorganic 15N was measured using a VG SIRA 9 mass spectrometer after converting the NH: to molecular N, by dry combustion based on the Dumas method using an inline Carlo Erba automatic N analyser (NA1500).

Organic N and j5N determinations. Organic N was determined after a third extraction with 80 ml 1 M

KC1 to completely eliminate inorganic “N. The soil + extract slurries were passed through a 200~~(rn dia screen. The two fractions (>200 pm and ~200 pm) were recovered separately. The coarse mineral fractions (coarse sand) contained in the >200pm fraction were removed by sedimentation. These two fractions were dried at 60°C for 24 h and ground to very fine powders. Organic N and its isotopic composition were determined as described above.

Calculation of C and N fluxes

Carbon mineralization from maize residues was calculated as the difference between the samples with residues and the control samples for each N treatment. Net N immobilization through residue addition was calculated as the difference between the amounts of N accumulated in control and amended soils for each N treatment. The mean net C and N mineralization measured in control soils (which was supposed to be independent of the rate of N application) was

Effect of soil N on C decomposition 1531

used for the R,O and R, treatments, which had no controls. Gross N immobilization was calculated according to the model proposed by Mary et al. (1993), using the isotopic measurements of the <: 200 pm and > 200 pm fractions, at each time interval. Immobilization was calculated in two ways:

The first neglected any preferential pathway for N immobilization and assumed that ammonium and nitrate ions were assimilated as a function of their proportions in the total mineral N pool. Immobiliz- ation i during each time interval [t, t,,] was calculated from the change in organic 15N (No*) and the mean isotopic excess of the mineral pool (ammonium + nitrate), e:

i = (No;, , - No:)/e (1)

The second was based on the hypothesis that only ammonium is immobilized when ammonium is available, and that mtrate is immobilized after the ammonium pool is depleted (Rice and Tiedje, 1989; Recous et al., 19!)0). Here, i, was the immobilization from the ammonium pool, i,, the immobilization from the nitrate pool, < the mean isotopic excess from ammonium, and < the mean isotopic excess from nitrate:

i, = (No:, , - No:)/z (2a)

i, = (No;, , - No;)/< (2b)

The mean isotopic excess of the ammonium, nitrate and total mineral N pools during each time interval [t, t,,], were calculated as

(ear + e,, + J/2, (e,, + e, + , )/Z (e, + 6 + J/2.

RESULTS

Carbon mineralization

The cumulative amount of C mineralized in the control was not significantly affected by mineral N during the 140 days of incubation (Fig. 1). After the initial mineralization flush during the first 20 days, the average rate of mineralization was 1.4 f 0.4 mg C-CO2 kg-’ d-i. However the amounts of C were diverging with time with different N additions, more C-CO2 being evolved from the soils containing 30 mg N kg-’ soil (C,,). Although this is not yet statistically significant it may become so with increased incubation time. Figure 2(A) shows the mineralization kinetics for the C-amended soils. The same amount of C was mineralized in all treatments during the first 72 h (10% of the added C). The rate of mineralization for the three higher N treatments was the same throughout the incubation. In these treatments, 25% of the C was mineralized during the first 18 days, and a further 20% was mineralized in the following 106 days. The rate of mineralization declined significantly in the R,, samples between days 3 and 6 and between days 6 and 11 for the R,, samples. The rates of C mineralization for R,, and R,, were similar after 69

0

0 20 40 60 60 100 120 140 Days

250

Fig. 1. Kinetics of C mineralization of unamended soil which contained at time 0, 30 mg N kg-’ soil (O), 80 mg N kg-’ soil (a), 100 mg N kg-’ soil (v), incubated at 15°C. Values are the mean of two replicates. Bar is the mean SD

on replicates.

days and greater than those for R,, R,, and R,,,. However, the cumulative amounts of C mineralized varied widely at the end of the incubation: 3 1.4% for R,,, 41.1% for R30, 47.5 + 1.2% for RO, Rso and R,,.

Organic C in the >ZOOpm fraction

The mean amount of C in the > 200 pm fraction extracted by KC1 from control samples, was 172 f 23 mg C kg-’ soil (Table 3) without significant change for all sampling dates and treatments. From the amended soils, 1316 f 95 mg C kg-’ soil were recovered in the > 200 pm fraction after extraction at time 0. Since 1760mg C kg-’ soil was initially added as maize residue, 610mg C kg-’ soil (1760 + 172 - 1316) has been solubilized in KC1 during extraction (i.e. 35% of C added), assuming that all the insoluble residue-C remained in the coarse fraction. This amount of C soluble in KCl, was compatible with the size of the soluble fraction measured by the proximate analysis (Table 2). The > 200 ,um fraction declined slowly in the R,, and R,, samples and rapidly in the R,, R,, and R,, samples at about the same rates (data not shown). The C in this fraction at day 40 was 35 f 3% of the added C for the three highest N treatments and up to 60% for the lowest N treatment (R,,). The C remaining in the coarse fraction at day 124 accounted for 26 f 4% for the highest N treatments and 45.7% of added C for the lowest N treatments.

The C lost was assumed to be either incorporated by the biomass of decomposer organisms or mineralized or in the ~200 pm fraction. All the soluble C was probably decomposed by day 40 even in R,, and


Fig. 2. Decomposition of maize straw in soil incubated at 15°C. (A) Kinetics of C mineralization. (B) Inorganic N in soil, for different initial inorganic N amendments in soil: 10mg N kg-’ soil (m), 30 mg N kg-’ soil (O), 60 mg N kg-’ soil (O), 80 mg N kg-’ soil (a) and 100 mg N kg-’ soil (v). Maize straw added: 4g kg-’ soil. Values are the mean of two replicates. Bars are the mean SD on replicates.

R,, samples, so that the remaining coarse fraction Y,,, was 0.40 f 0.02 for R,,, R,, and R,,, 0.47 C was considered as an unbiased estimation of C for R,, and 0.50 for R,, on day 40. This yield decomposition. The cumulative yield of C-assimila- of C-assimilation decreased over time. It was tion by microorganisms was calculated as: 0.35 + 0.01 for R,, R,, and R,,, 0.39 for R,, and

Y,,, = (C disappeared - C mineralized)/ 0.42 for R,, for the t&124-day period. This decrease in the yield of assimilation over time has been re-

C disappeared (3) ported by Hart et al. (1994).

Table 3. C and N contents in the cczarse fraction >200/rm of maize straw, extracted from soil during decomposition bv KC1 extraction

Treatments

RIO

Date Carbon Organic N organic “N Immobilized-N (days) (mg kg-‘) (mg kg-‘) C-to-N ratio (atom % excess) (mg kg-‘)

0 1316 (95)

R30

R60

R80

RIO0

40 124

0 40

124 0

40 124

0 40

124 0

39 117

1236 (80) 977 (69)

1316 (95) 1017 (79)

751 (47) 1316 (95) 718 (8) 584 (20)

1316 (95) 829 (49) 605 (28)

1316 (95) 811 (30) 712 (15)

26.0 (6.1) 49 18.9 (3.4) 65 19.0 (2.0) 51 26.0 (6.1) 49 20.0 (1.7) 51 22.0 (I .6) 34 26.0 (6.1) 49 24.8 (1.4) 29 21.5 (0.6) 27 26.0 (6.1) 49 28.0 (1.6) 30 23.1 (1.3) 26 26.0 (6.1) 49 29.9 (2.5) 27 26.1 (1.4) 27

11.5

0.0 0.0 0.0 0.017 0.790 0.848 0.061 2.806 3.358 0.072 3.152 3.080 0.012 2.619 3.162

0.0 0.0 0.0 0.6 9.5

10.2 0.6

14. I 15.2 0.1

13.5 10.5 0.6

18.6 20.7

mean C.V. (%)

c30

6.0

C80

Cl00

0 157 (22) 40 150 (38)

124 154 (61) 0 I57 (22)

40 194 (54) 124 206 (60)

0 155 (15) 39 169 (33)

117 202 (54)

10.5 (1.6) 15 13.1 (6.6) II 12.1 (5.6) 13 10.5 (1.6) 15 13.6 (3.7) 14 14.4 (0.6) 14 9.5 (0.8) 16

10.5 (2.8) 15 13.4 (1.8) 15

Il.8

0.005 0.033 0.017 0.015 0.039 0.070 0.015 0.107 0.09 I

0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.1

mean C.V. (%) 15.1

At time 0. 1760 mg C and 13 mg N kg.-’ soil were added by straw. Amended soils contained 5 initial concentrations of inorganic N. Unamended soils contained 3 initial concentrations of inorganic N. Values at day 0 were measured after KC1 extraction.

Values in parentheses are SD.


Table 4. Inorganic N and lSN and organic “N data for control treatments and C-amended treatments

Treatments Days

N-NH, N-NO,

(mg kg-‘)

Organic N Organic N e, e” <2OOpm >2OOpm

(“N% atom excess) (“N Fg kg-‘)

c30 30 mg PI kg-’ soil

CEO 80 mg N kg-’ soil

Cl00 100 mg N kg-’ soil

R30 30 mg N kg-’ soil + straw



RIO0 100 mg N kg-’ soil +straw

0.0 9.4 19.2 6.344 4.083 47 0 3.0 I.1 27.5 0.455 5.219 37 1

10.8 1.0 28.8 0.230 5.410 56 2 39.7 I.2 33.2 0.131 4.908 73 5

123.7 1.7 37.6 0.540 4.008 50 2

0.0 36.7 46.0 8.932 7.122 139 I 3.0 14.7 68.2 7.448 7.779 221 3

10.8 1.8 82.5 0.348 8.009 192 4 39.7 1.8 86.8 0.267 7.850 196 5

123.7 1.8 87.0 0.469 7.280 187 10

0.0 44.9 58.5 9.047 7.464 239 I 2.9 37.0 64.3 8.770 7.712 282 3

10.3 27.9 73.6 8.294 7.871 313 5 39.0 7.2 loo.0 4.251 8.079 361 10

117.0 2.9 115.8 0.398 7.143 297 I2

0.0 10.1 19.2 5.295 3.777 49 5 1.8 0.8 9.4 0.231 4.203 989 28 3.0 0.9 6.3 0.351 3.757 1148 62 5.8 I.2 2.2 0.521 1.603 1326 109

10.8 1.4 0.2 0.460 0.216 1356 172 39.1 1.3 0.3 0.969 0.414 I394 164

123.7 2.2 2.1 0.243 0.335 1420 190

0.0 27.9 34.2 7.517 6.391 109 14 1.8 6.4 36.0 4.822 7.041 1718 51 3.0 2.7 36.7 2.657 7.208 1886 90 5.8 1.2 31.0 2.490 7.132 2261 226

10.8 1.4 22.5 2.050 6.760 2562 482 39.7 1.7 11.3 I .339 4.983 3094 695 123.7 2.1 22.4 0.852 3.899 3086 742

0.0 36.6 46.7 8.553 6.970 136 51 1.8 11.7 46.5 6.810 7.472 1916 55 3.0 4.3 50. I 4.458 7.600 2179 134 5.8 1.4 46.1 I .789 7.597 7.597 2558

10.8 1.9 41.3 2.167 7.494 2719 429 39.7 1.7 24.5 2.384 6.705 3619 875

123.7 2.0 40.5 0.770 5.659 3321 711

0.0 44.8 2.1 19.8 2.9 15.4 6.9 3.8

10.3 2.9 39.0 3.5

117.0 2.7

58.4 8.826 7.291 214 2 57.4 7.711 7.702 2308 50 59.9 7.154 7.781 2523 81 63.9 2.271 7.764 2960 306 62.3 I.145 7.730 3073 467 46.1 I.131 7.264 3441 781 60.2 0.673 6.871 348 I 822

“N data N mineralization

The original data on the amounts of mineral N, “N and organic: 15N are given in Table 4. The i5N balance was established; it indicated that the average recoveries for the 11 sampling dates was 96103%, with no particular trend in the recoveries vs time or vs N added. This indicates that gaseous losses were negligible as expected from the H,O content in soil.

The gross N immobilization could not be calculated for R,,, to which no 15N had been added. Similarly, the N fluxes were not calculated for R,, treatments after day 11, because the inorganic i5N had completely disappeared. Gross immobilization of all other treatments was calculated for the MO day period, during which organic 15N was assumed to be in the biomass and remineralization of organic i5N negligible. We consider this to be invalid after 40 days.

There was a net accumulation of inorganic N in the control samples (Table 4) that was not affected by mineral N. Net mineralization was 80 pg N kg-’ soil d-’ for the &124-day period, and 60 pg N kg-’ soil d-’ for the 40-124-day period. Soil samples containing maize residues showed a very fast decrease in the amount of inorganic N during the first few days after straw addition [Fig. 2(B)]. Inorganic N completely disappeared from the R,, and R,, samples by days 3 and 11. The disappearance coincided with the decrease in the C mineralization rates. No inorganic N reappeared until the end of incubation for samples R,,, while there was a small accumulation in the RX, samples by day 124. Immobilization depleted only a part of the soil mineral N in the other three treatments. There was a net decrease of inorganic N up to day 40, after which net mineralization occurred slowly and reached 11, 16 and 13 mg N kg-’ soil


during the 4s124-day period i.e. a mean mineralization rate of 160 pg N kg-’ soil d-‘. This rate can be compared with the mean mineralization rate for control soils. The rate of net remineralization of N previously immobilized was calculated by difference as 1OOpg N kg’ soil d-‘.

N immobilization

Net immobilization. The cumulative net immobilization calculated by day 40 for &, R,, and R,, were 55, 63 and 57mg N kg-‘, respectively. R,, and R,, treatments showed net immobilization until the end of the experiment (124 days), at which time net immobilization was 23 for R,, and 35 mg N kg-’ soil for R,,.

Gross immobilization in the fraction > 2OOpm. The amount of N extracted from the > 200 pm-fraction of the control samples did not vary significantly, and was 12.0 f 1.7 mg N kg-’ soil for all the sampling dates and treatments (Table 3). The N extracted from the C-amended soils was 26.0 f 6.1 mg N kg-’ soil at time 0. This amount included N from organic matter >200pm (12.0mg N kg-’ in control soils) and N from added maize residues (13.4 mg N kg-’ as initially measured). This implies that no N losses occurred during extraction at time 0, in contrast to the losses of soluble C. There was little or no decrease in total residue-N during decomposition in any of the samples. Residues which were unlabelled at time 0, were gradually labelled with “N as decomposition proceeded (Table 3). This was not observed in the control treatments. This labelled N was attributed to the use of labelled inorganic N by the microorganisms colonizing the coarse fraction of residues. This en- ables us to calculate biomass-N adherent to the residue (using equation 1). The immobilized N in the >200 pm fraction on day 40, was 10-18 mg N kg-’ soil i.e. 4060% of total N recovered in this fraction. Consequently, the decrease in the C-to-N ratio was mainly due to this colonization.

Total N immobilization. The control samples contained 47-240 pg 15N kg-’ soil as organic 15N at time 0, corresponding to 0.5 and 2.3 mg N kg-’ soil. This was attributed to N fixation in clay lattices. The total “N increased thereafter to 1.3-3.8 mg N kg-’ as (fixed + organic) N on day 40.

The use of the mean isotopic excess of either the total mineral N (equation l), the ammonium (equation 2a) or the nitrate pool (equation 2b), resulted in very different estimations of N immobilization during the first 2 or 3 time intervals when both ammonium and nitrate were available (Table 5). As expected, the results obtained with calculations made on the nitrate pool were not significantly different from those made using the total mineral N pool once the ammonium had been depleted.

Cumulative immobilization can be calculated using only equation 1, or by assuming that immobilization depletes ammonium first and exclusively when it is available, and then depletes the nitrate pool (Table 5).

The cumulative immobilization calculated from total mineral N (hypothesis 1) was lower during the first few days, than that calculated by assuming assimilation only of ammonium (hypothesis 2). The difference between the two hypotheses was quite large, 9 to 16 mg N kg-’ soil. The cumulative gross immobilization on day 40 calculated by the first method was equivalent to or lower than the cumulative net immobilization obtained by the difference in the mineral N in the control and C-amended treatments. This suggests that gross immobilization was underestimated in this case and that the second method measures gross immobilization more accurately. Using a linear rather than an exponential decrease (as proposed by Barraclough, 1991) of the isotopic excess vs time in the second method, had little effect on the immobilization estimations (data not shown). However, the gross immobilization of unlabelled ammonium which is immobilized as soon as it is mineralized, could not be quantified by either approach.

The immobilization kinetics for the three highest N concentrations tested were quite similar (Fig. 3), except for one time interval (days 611) for treatment R,,. Immobilization was very fast at the beginning of residue decomposition (44-48mg N kg-’ were immobilized during the first 6 days). The immobilization rate decreased: only 20 mg N kg-’ were immobilized during the 6-40-day period. The total amounts of N immobilized were identical for the three N treatments (69 f 0.1 mg N kg-’ soil) on day 40 (end of net immobilization). This represented 39 mg N immobilized g-’ C added. The C loss from the coarse fraction gives a N-to-C ratio of 60 mg N immobilized g-i C decomposed.

C to N relationships

The gross immobilization values obtained previously were used to calculate the relationships between C and N dynamics by the ratio N immobilized-to-C mineralized (called ‘R’ ratio) which corresponds also to:

R = rY/(l - Y) (4)

where r is the N-to-C ratio of newly-formed biomass and Y is the C assimilation yield (ratio of assimilated C-to-decomposed C).

Figure 4(A) shows the change in the R ratio during decomposition for the R30, Ra, RBO and RIM) treatments as a function of percent mineralized C. The R ratios for all the non-limiting N treatments tested were similar, as expected from gross immobilization values. The ratios were stable until 20% of added C was mineralized and decreased rapidly thereafter.

The “N isotope tracing technique could not be used to calculate the gross immobilization for the two limiting N treatments. Consequently, net N immobilization was used to establish the C-to-N ratios in these cases and to compare the limiting and non- limiting N treatments. The ratio between net N


immobilized and C mineralized was calculated for each of the five treatments and is plotted against mineralized C (X) in Fig. 4(B). The ratios for the three non-limiting N treatments were quite similar, decreasing continuously during decomposition. The ratio was much lower for the R,, and R,, treatments.

DISCUSSION

Immobilization of N during C decomposition

Gross immobilization of soil inorganic N was calculated in several ways. The quantification of gross immobilization varied greatly according to whether or not the ammonium ion was assumed to be preferentially assimilated (Table 5). There is strong evidence that ammonium is preferentially immobilized to nitrate, when both ions are available at the same site in the soil (Jansson et al., 1955; Rice and Tiedje, 1989; Recous et al., 1990). The calculations based on the overall mineral N pool are therefore probably incorrect. When N is applied as ammonium nitrate, the proportion of ammonium decreases with time. But there is, as yet, no defined rule for the partitioning of nitrate and ammonium in the immo-

bilization pathway in such situation. We assumed that ammonium immobilization was the only pathway when ammonium was available, which was probably not completely true for the time interval preceding total ammonium depletion. Consequently, the immobilization values obtained are probably slightly overestimated. However the immobilization of unlabelled ammonium would be neglected after *jN-NH, depletion and in experiments with 15N-NO, applications, as pointed out by Recous et al. (1990).

Part of the immobilization flux consisted of soil inorganic N immobilized by the microflora adhering to and growing on the residues (> 200 pm fraction). Much (4&60%) of the N measured as ‘residue-N at the end of incubation in this coarse fraction was, in fact, N assimilated by the biomass formed at the expense of soil inorganic N. This implies furthermore, that C was also assimilated by the adhering microflora, and so that actual decomposition of residue calculated from remaining C in the > 200 pm fraction was underestimated. Assuming a mean C-to-N ratio of 8 for the microbial biomass, the underestimation would be S-10% of added C after 124 days.

‘Table 5. N gross immobilization during maize straw decomposition in the O-40 day period

I”sta”ta”eous gross Cumulative gross immobilization immobilization

Treatments

i 1.4 i. +i ** Cumulative [equation (I)] [equation @a)] [equa& (2b)] [equat:on (I)] [equ;ion”(2a/b)] net

immobilization Days (mg N kg-’ soil) (mg N kg-’ soil) (mg N kg-’ soil)




RIOII 100 mg N kg-’ soil + straw

t&2 23.5 2-3 5.3 3-6 9.8 61 I *

I I-15 * 15-18 * IS-29 * 29-40 *

O-2 2-3 3-6 &I I

1 I-15 15-18 18-29 2940

24.2 3.1 1.4 8.3 3.9 1.0 1.3 0.8

t&2 23.8 2-3 4.7 34 6.5 611 4.8

11-15 6.0 15-18 2.1 18-29 6.6 29-40 4.9

o-2 2-3 3-l 7-10

lo-14 14-18 18-28 28-39

21.4 3.2 8.8 3.7 1.1 2.4 3.9 2.2

34.9 1 * * * * 1: *

25.6 5.0

17.8 * * * * *

23.2 6.1

15.3 * 1 * * *

25.9 3.3

14.0 16.0

* * * *

24. I 23.5 4.9 28.8 8.4 38.6 * ??

* * 1 * * * 1 *

24.5 24.2 2.9 21.2 7.1 34.6 8.0 42.9 3.1 46.8 1.0 47.8 6.7 55.2 0.7 55.9

24.1 23.8 4.5 28.4 6.3 34.9 4.1 39.8 5.8 45.8 2.0 47.9 6.4 54.5 4.7 59.4

28.6 21.4 3.2 30.6 8.5 39.3 3.5 43.0 1.0 44.1 2.3 46.5 3.1 50.4 2.0 52.5

24.1 (2b) 29.0 (2b) 37.4 (2b)

* * * * *

25.6 (2a) 30.6 (2a) 48.5 (2a) 56.5 (2b) 60.2 (2b) 61.2 (2b) 67.9 (2b) 68.6 (2b)

23.2 (2a) 29.3 (2a) 44.6 (2a) 49.3 (2b) 55.2 (2b) 51.2 (2b) 63.6 (2b) 68.3 (2b)

25.9 (2a) 29.2 (2a) 43.3 (Za) 59.3 (2a) 60.3 (2b) 62.6 (2b) 66.3 (2b) 68.4 (2b)

- 22.8

28.8

29.7 -

33.4

22.9

40.7

46.0

55.0

- 29.0 -

42.6

51.2 -

63.0

30.0

39.8

48.3

57.4

Gross immobilization is calculated using the inorganic N pool (i) or the ammonium pool (i.) or the nitrate pool (in) as the reference base and source of immobilized N. Cumulative net immobilization calculated as the difference between amount of inorganic N in amended and control soils. Values are the mean of two replicates.

*No calculation because labelled N is depleted; **(2a), Value obtained by equation 2a; **(Zb), Value obtained by equation 2b.


60

0 10 20 30 40 Days

Fig. 3. Cumulative gross N immobilization during the O-40-day period of decomposition of maize straw in soil containing labelled 15NH, 15N0, for different initial inorganic N contents in soil: 30 mg N kg-’ soil (0) 60 mg N kg-’ soil (O), 80mg N kg-’ (0) and IOOmg N kg-’ soil

(v). Values are the mean of two replicates.

The ratio N-to-C were calculated with “gross immobilization” values at day 40 when N immobilization was at its maximum. The ratio was 39 mg N kg-’ added C for the three unlimiting N treatments at this date. If we expressed this ratio as a function of decomposed C (estimated from the remaining C in the > 200 pm fraction), then the ratio became 60 mg N kg-’ decomposed C. This latter value is close to that obtained with glucose (Mary et al., 1993). This suggests that a fairly uniform value is obtained for substrates as different as straw and glucose, when N gross immobilization data are plotted against decomposed C, rather than added C.

The unlabelled N derived from the residues and The “net immobilization” was 33 mg N kg-’ added assimilated in situ by adhering microflora could not C, which is within the range of published values be quantified. The immobilization of N on the coarse [Nommick, 1962; Powlson et a/., 1985; G. Guiraud fraction accounted for 18% of the total immobiliz- (unpub PhD thesis, UniversitC Pierre and Marie ation (> 200 pm and ~200 pm fractions) on day 40. Curie, Paris, 1984)]. The ratio was much lower when D. Robin (unpub. thesis INA-PG, Paris, 1994) N was limiting (R,, and R,, treatments). The rate of showed that the proportions of newly-formed C decomposition decreased but did not stop, biomass in the coarse and fine fractions (defined by although mineral N was no longer available in a 200 pm dia screen) depended greatly on the nature the soil. There was less immobilized N per unit of

(A) (B)

ot 0 10 20 30

Minerallwl C (% added c4” 50

of the residue: 50% of the total labelled biomass-N was recovered on the coarse fraction of wheat straw, but only 2% was recovered in this fraction for young rye residues, while microbial N assimilation was the same in the two cases.

Gross immobilization of inorganic N after incorporation of residues with low N contents, such as cereal straw, is the major, but not the only part, of the total N assimilation by soil microorganisms (Mary et al., 1995). Robin (lot cit.) has shown that the direct assimilation of wheat straw-N by decomposing microflora was 7mg N kg-’ soil i.e. 55% of straw added N.

C-N relationships

Fig. 4. Evolution of the ratio immobilized N:mineralized C (mg g-‘) for different initial N contents in soil: 10 mg N kg-’ soil (W), 30 mg N kg-’ soil (O), 60 mg N kg-’ (O), 80 mg N kg-’ soil (0) and 100 mg N kg-’ soil (v). Time is expressed as mineralized C (% added C) for each treatment. Values are the mean of two replicates. (A) Immobilized N values correspond to gross immobilization calculated by 15N tracer

technique. (B) Immobilized N values correspond to net immobilization.


mineralized at low N availability. This change in the C-N relationships has also been suggested by Parker (1962), Reinertsen #of al. (1984) and Ocio et al. ( 199 1). Several processes may give rise to such results, includ- ing changes in microbial succession, the adaptation of the internal N of fungi (Levi and Cowling, 1966), and changes in C and N metabolism (Bremer et al., 1991). Others (Robin, Zoc. cit. Houot and Chaussod, 1995) have suggested an acceleration of biomass N recycling.

The ratio N immobilized-to-C mineralized decreased with time. This suggests that there was a high N demand during the first stages of decomposition when soluble and easily-degradable C compounds were mineralized, while the N demand was lower when more recalcitrant C compounds were used. In terms of equation 4, this decrease in R may result from a drop in r. The mean N-to-C ratio of the decomposing microflora could change during microbial succession as a function of the nature of the substrate being decomposed. The difference can be explain by the fact that bacteria are the main decomposers of soluble compounds, while fungi are mainly responsible for the decomposition of cellulose and lignin (Swift et al., 1979) and that the mean C-to-N ratio of the bacteria is lower than that of fungi. There may also be a drop in Y. The C assimilation yield could decrease when more recalcitrant C components are decomposed. It has been pointed out that the production and secretion of the depolymerizing en- zymes required for cellulose and lignin degradation imply an additional energy cost for decomposer organisms, so thal: less C would be used for biosyn- thesis (Swift et al., 1989). A third possibility is the participation of N from biomass recycling. This recycling biomass-N could be re-used, in part, as microbial organic N compounds (Payne, 1980; Barak et al., 1990), and so would not be quantified by isotope dilution. This flux probably becomes increas- ingly important as decomposition proceeds. How- ever, these three postulated mechanisms are not mutually exclusive.

Efect of inorganic N availability

C mineralization slowed down and the inorganic N pool become depleted at the same time in treatments R,, and R,,. The subsequent decomposition was reduced in these treatments and was probably driven by N gross mineralization and biomass-N recycling: NH, was immobilized as soon as it was mineralized so that there was no accumulation of nitrogen (neither ammonium nor nitrate) in soil until dav 124.

N in the &20 cm layer, and 32 mg N gg ’ added C in the same, but N-rich, soil (Mary et al., 1995). There- fore, N availability does not only influence the decay rates of the C pools involved, but it also alters the C to N relationships as discussed above. More infor- mation on the processes involved are required in order to quantify this latter aspect.

The kinetics of N immobilization, as well as the total amount of N immobilized, were identical as long as N was not a limiting factor for decomposition (initial N content at 35, 47 and 59 mg soil N g-’ added C). The rate of C decomposition and the associated N dynamics showed a classical pattern, with rapid decomposition at the beginning (when the soluble fraction was available) and decreasing rates thereafter (when cellulose and hemicellulose were decomposing) (Reinertsen et al., 1984; Jawson and Elliott, 1986; Cochran et al., 1988). These results suggest that there was no non-productive (or luxury) N consumption by microbial decomposers, and also implies that the heterotrophic soil microflora is not a sink for excess soil N.

The results also suggest that it is possible to define a critical level of N availability that is associated with C decomposition, below which N will drive C decomposition, and above which C will drive N dynamics. N availability includes all sources of nitrogen for soil microflora, i.e. residue-N content, initial inorganic N in soil, soil organic N mineralized over time. Soil inorganic N probably has little or no influence on decomposition when the residues have a high N content (Bremer et al., 1991). However, it has been shown that soil N can be immobilized in the first stages of soluble fraction decomposition, before any significant residue-N assimilation and mineralization occurs (Robin, lot. cit.). In the case of cereal straw with a high C-to-N ratio and a high content of easily degradable compounds, the soil N content at the time the straw is incorporated has a large effect on the rate of the initial C decomposition.

Acknowledgements-We thank 0. Delfosse, D. Varoteaux and G. Alavoine for technical assistance. We also thank Dr D. Angers for helpful comments on the manuscript and Dr 0. Parkes for editorial assistance. This work was supported by grants from Socitte ‘Grande Paroisse S.A.‘Xontrat I.N.R.A.-S.C.G.P. no 9697B.

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Soil inorganic N availability: Effect on maize residue decomposition