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Gross rates of N mineralization associated with the decomposition of plant residues
Soil Bid. Biochem. Vol. 28, No. 2, pp. 169-175. 1996
Pergamon
Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003%0717196 $15.00 t 0.00
00380717(95)00123-9
GROSS RATES OF N MINERALIZATION ASSOCIATED WITH THE DECOMPOSITION OF PLANT RESIDUES NAOMI
WATKINS*
and DECLAN
BARRACLOUGHt
Department of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading RG6 2DW, England (Accepted 27 July 1995)
Summary-Nitrogen-15 isotope dilution was used to determine gross rates of N mineralization immediately following the incorporation of crop residues in soil. Laboratory and field experiments were performed. Two crop residues were used: oil-seed rape stems and winter wheat straw. Incorporation of oil-seed rape residues into the soil resulted in an immediate increase in gross N mineralization rate from 0.87 to 2.2 mg kg-r d-r. After 12 d, gross N mineralization in the presence of the residues had dropped to 0.79 mg kg-’ d-‘, compared to 0.72 mg kgg d-’ in the control. Of the N in the oil-seed rape, 24% was mineralized in 12 d. In contrast, incorporation of winter wheat residues caused only a small immediate increase in gross N mineralization to 0.92 mg kg -I d--l. Of the N in the winter wheat residues, 12% was mineralized in the 12 d following incorporation. Longer-term measurements indicated that the residue incorporation was still influencing N mineralization 54 d after incorporation with 19 and 11% of the mineralization flux resulting from oil-seed rape and winter wheat decomposition, respectively. Field measurements showed that 5 months after incorporation, oil-seed rape residues were contributing 13% of the mineralization flux while winter wheat residues were contributing 9%.
INTRODUCTION of organic matter in soil, and the accompanying mineralization and immobilization of inorganic N, are key processes in the soil-plant N cycle. They are also among the most complex. The substrate is a heterogeneous mix of organic matter ranging from plant residues and litter recently entered the soil, to humic material with mean residence times of thousands of years (Jenkinson, 1988). The soil microbial biomass, the agent of decomposition, is also a varied mix of organisms. Because NH,, the initial product of N mineralization, can be consumed by a number of processes in soil (plant uptake, immobilization and volatilization), nitrification, measurements of changes in NH, (xand Nor) pool sizes over time can, at best, provide estimates of net mineralization, the balance between production and consumption. Despite this complexity, considerable advances have been made in understanding the decomposition process. Studies on CO2 evolution during plant residue decomposition show an initial, more rapid phase, in which about 70% of the C initially in the residue is lost as COz, followed by a slower phase (Jenkinson, 1977; Sauerbeck and Gonzalez, 1977). This two-phase decomposition has been interpreted
The decomposition
*Present address: Department York, York, England. tAuthor for correspondence.
of Biology, University of
as indicating labile (or decomposable) and recalcitrant (or resistant) fractions of the plant residue; a concept exploited in models of soil C dynamics (Jenkinson and Rayner, 1977). A consideration of the chemical composition of plant material supports the idea that residue decomposition is likely to exhibit two phases characterized by different kinetics. The initial, more rapid phase corresponding to the degradation of water-soluble free amino acids, amino sugars and carbohydrates, and cell cytoplasmic and membrane constituents. The slower second phase corresponds to the decomposition of cell walls and structural components, and the decomposition of secondary, more stabilized products of the first phase (Sorensen, 1975). However, results on N mineralization are often contradictory. In some cases there is a clear two-phase mineralization, suggesting a labile and recalcitrant fraction similar to that observed for C (Broadbent and Nakashima, 1974); in other studies this two-phase pattern is not so evident (Amato et al., 1987). Similarly, some work suggests that C-to-N ratios are useful predictors of net N mineralization while in others N-to-polyphenol ratios appear more reliable indicators. Berg and Staaf (1981) reviewed a number of results and concluded that net N mineralization started at a wide range of C-to-N ratios. In part, this confusion arises because only net N mineralization is measured. Thus the relationship being studied is between the C-to-N ratio of the organic substrate and a group of soil processes, not N mineralization alone. Changes in N immobilization or loss may blur or completely obscure any 169
170
Naomi Watkins and Declan Barraclough
postulated relation between N mineralization and a factor such as the C-to-N ratio of the residue. The development of “N pool dilution techniques presents the opportunity to study gross N mineralization unconfounded by the processes that consume NH, (Kirkham and Bartholomew, 1954; Barraclough et al., 1985; Nishio et al., 1985). The work we report here is part of an integrated programme in which 15N pool dilution techniques are being used to study gross N mineralization in terms of the mineralization resulting from the decomposition of specific fractions of the organic matter in soil. It uses as a starting point the hypothesis that, at any one time, the gross rate of N mineralization in a soil is the sum of the N being mineralized from specific fractions of soil organic matter. This part of the work is concerned with that part of the N mineralization flux resulting from the decomposition of plant residues in the period immediately following their incorporation in the soil. Thus, for this work we assume that: m = m, + m,.
AND METHODS
Theory
Nitrogen- 15 isotope dilution techniques, in which rsNH4 is added to soil, enable the gross rate of N mineralization to be determined unconfounded by processes such as plant uptake and nitrification which consume NH,. Thus, for example, the addition of the autotrophic nitrification inhibitor nitrapyrin to a woodland soil reduced nitrification from 2.22 to 0.44 mg kg-’ d-r but gross mineralization, measured by r5NH4isotope dilution, only changed from 1.49 to
‘4:
A: =
(2)
( f! >(‘*io) 1+
(1)
All notation is given in Table 1. To examine the effect of residue N content on mineralization, two contrasting plant residues (oilseed rape and winter wheat) were used. The main experiments were under controlled conditions in the laboratory but two experiments were carried out in the field. MATERIALS
1.59 mg N kg-’ do-’ (Barraclough and Puri, 1995). Isotope dilution offers three ways of determining the gross N mineralization resulting from the decomposition of crop residues: by difference; using a direct method; and a mirror image approach. The di@rence method. The difference method involves determining gross rates of N mineralization in the presence or absence of residues. Unlabelled residues are incorporated in the soil and, at intervals, rSNH4 is injected into the soil. The size and 15N enrichment of the NH4 pool at a minimum of two times after the injection are substituted into Equation 2 to obtain the gross rate of N mineralization (Barraclough et al., 1985). A parallel experiment determines gross rates of N mineralization under the same conditions but without the incorporated residue:
All
If conditions are otherwise identical, differences in N mineralization between the two treatments will be due to the N mineralization resulting from the decomposition of the incorporated residues. This method is straightforward but has two potential drawbacks. The first is the usual one associated with determination by difference; the error in N mineralization attributed to residue decomposition is the square root of the errors sum of squares of the two measurements used in its derivation. This becomes particularly important if there is only a small difference between gross rates of N mineralization with or without residue incorporation. The second concerns the assumption that the basal N mineralization is the same in the presence or absence of residues. If, following residue incorporation, part of the microbial biomass switches from decomposing “soil” organic matter to processing the residue, the N mineralization resulting from residue decomposition, obtained by difference, will be an underestimate as basal mineralization has dropped.
Table I. Notation A = NH4 pool (mg N kg-‘) 0 = organic N (mg N kg-‘) P = plant residue N (mg N kg-‘) i = gross rate of immobilization (mg N kg-’ d-‘) m = gross rate of mineralization (mg N kg-’ d-‘) n = gross rate of nitrification (mg N kg-’ d-‘) I = rate of N loss (mg N kg-’ d-‘) f = time (d) a = proportion of mineralization flux resulting from residue decomposition 0 = rate at which pool size changes (mg N kg -’ d-9
Subscripts p = attributable to plant residue decomposition s = attributable to soil organic matter decomposition 0 = time 0 I = time f superscripts * = lSN excess (atom%)
Thus, A,* indicates the lJN atom% excess in the soil ammonium pool at time f.
N mineralization during plant decomposition 1.4
The direct method. The direct approach,
avoids these problems, but has some of its own. It involves incorporating 15N-labelled residues into the soil and injecting unlabelled NH, at intervals, in the same way as the previous method. Now, because labelled residues are being decomposed, any involvement of the residue in mineralization will cause the ‘jN abundance in the NH4 pool to change. The greater the proportion of mineralization coming from the residues (as opposed to the remaining soil organic matter, which is unlabelled), the greater the change in the 15NH4. In fact, this situation corresponds to the full form of Equation 2, the isotope dilution equation. As presented above, Equation 2 implicitly assumes the organic matter being decomposed is at natural abundance. If this not the case, the full form must be used as shown in Equation 3: A,* = O* +
(A$ - O*) WV’
1.2
3 g .
1.o
z
0.8
z
L
1:
&-0.4
o.2
-___
0.4
0.1
0.2
0.0
’ 0
o* = LIP*,
‘.O
/I
0.6
(3)
0* in Equation 3 is the excess 15N of the organic matter being decomposed. When this is zero, as normal, Equation 3 reduces to Equation 2. When part of the organic matter being decomposed is labelled (e.g. the crop residue), while the remainder (e.g. “soil” organic matter) is not, 0* is given by
171
1
2
3
4
5
6
7
8
Time (days)
Fig. I. Change in lSNH4resulting from the decomposition of a residue labelled at 1.19 atom%. Lines are calculated from Equation 3; numbers on curves are proportion of mineralization resulting from residue decomposition. Gross mineralization rate is I. 125 mg N kg-’ dm’in all cases.
(4)
where P* is the excess 15Nof the incorporated residue and c( is the proportion of the mineralization flux resulting from the decomposition of the incorporated residues. Figure 1 shows the effect of increasing the proportion of mineralization coming from the decomposition of labelled residues. The example uses data from the labelled residue experiment to be described below. The simulations start at t = 1 d to coincide with the period over which the direct method was used. As the proportion of N mineralization resulting from residue decomposition increases, so, at a given time, does the ‘jN in the soil NH, pool. The gross rate of mineralization in Fig. 1 is 1.125 mg N kg-’ d-l for all the curves; the numbers on the curves are G(, the proportion of the N mineralization flux resulting from the residue decomposition. If m, 0 and c( (and therefore 0*) remain approximately constant over three or more samplings, Equation 3 can be solved for both m and 0* (and therefore c() by fitting it to the experimental points using a least-sum-of-squares approach. This approach is a direct measurement without the problems of differences, but is restricted to intervals over which m, t3 and a are approximately constant. The mirror image approach. This involves paired experiments with unlabelled and labelled, but otherwise identical, residues. Labelled NH, is injected into the soil with the unlabelled residues and the gross rate of N mineralization in the presence of the residues is determined in the same way as that
described for the difference method. In the parallel experiment with labelled residues, unlabelled NH, is injected into the soil and the change in the 15NH4 determined over the same interval. The change in 15NH4 is described by Equation 3. in which the gross rate of mineralization, m, should equal that determined in the mirror image unlabelled residue experiment. This is substituted into Equation 3 together with the NH., pool size and ‘jN enrichments from the labelled residue experiment to determine the only unknown, O*, from which x can be calculated using Equation 4. Our aim was to determine the proportion of mineralization resulting from the decomposition of incorporated crop residues in both laboratory and field experiments, All the three methods described above are used. Thus, laboratory experiments were performed with unlabelled oil-seed rape and winter wheat residues for the difference method. Labelled winter wheat residues were used for the direct method. Field experiments were carried out using labelled and unlabelled residues of both plants for the mirror image approach. Soil
All laboratory experiments were carried out on soil collected from the cl5 cm depth of a sandy loam of the Rowland series (classified as a psamment) growing winter wheat. Selected properties of the soil are: %C, 1.OS; %N, 0.07; pH(H20) 5.98. The soil was
172
Naomi Watkins and Declan Barraclough
sieved <2 mm and stored in sealed containers at 20°C for 2 weeks prior to the start of the experiments. Field incorporation experiments were carried out on the O-5 cm horizon of the same soil. Plant material
For the laboratory incubations, unlabelled winter wheat straw and oil-seed rape stems, and labelled winter wheat straw were obtained from the Jealotts Hill Research Station of Imperial Chemical Industries. The paired labelled and unlabelled plant residues for use in the mirror image experiments were obtained from microplots on the Reading University farm (winter wheat) and a commercial farm field (oil-seed rape), after harvest of the upper plant parts. The winter wheat had received 130 kg N ha-‘; to obtain labelled residues, 70 of the 130 kg N ha-’ was double labelled (21.43 atom%) NH4NOj. The portion of the stalk between S-50 cm above the ground was cut into small (< 5 cm) pieces and subsamples finely ground. Oil-seed rape stems were also collected after harvest from microplots on a commercial farm. The crop had received 220 kg N ha-‘; for the labelled residues, 60 the 220 kg N ha-’ was double labelled (30 atom%) NH4N0,. The portion of straw between 5-30 cm above the ground was prepared in the same manner as the winter wheat. Table 2 gives details of the plant materials after preparation. Laboratory incubations with unlabelled residues: the using unlabelled d@erence method. Incubations
residues were performed at 20°C using 40 g subsamples of fresh soil packed into 2 cm deep plastic cylinders at the bulk density of the field soil (1.0 g cm-j). Cylinders with plant material had winter wheat (235 mg) or oil-seed rape (26.45 mg) incorporated in the soil, which at the final soil moisture content of lo%, gave an equivalent N incorporation of 34 mg N kg-‘. For winter wheat, this rate of incorporation was equivalent to 3 t ha-‘; a typical incorporation rate in the field in this region. The control cylinders, which received no plant residues, were disturbed in the same way as those with plant material. The cylinders were covered with parafilm with 4 pinholes to allow gas exchange. Water loss during the incubations was negligible. To determine gross rates of mineralization, 5 mg N kg-’ as (1SNH4)2S0., at 10.1 atom% was
added to each cylinder in 3 ml of deionized water. Underestimation of mineralization rates due to remineralization was minimized by making the measurements over 3-d intervals. Preliminary experiments had shown this to be a satisfactory time interval (data not shown). Thus, label additions were made at t = 0, t = 3, t = 6 and t = 9 to sets of 8 cylinders from each treatment; 4 cylinders were sampled immediately, 4 were sampled after 3 d. All the soil from each cylinder was shaken with 1 M KC1 for 1 h (15 soil-to-solution ratio). The extract was filtered through glass fibre filter paper and stored at 4°C prior to inorganic N and isotope ratio analysis. Laboratory incubations with labelled winter wheat residues: the direct method. The experiments with
labelled residues were performed in the same way as those with unlabelled residues except that 235 mg of labelled winter wheat residue (equivalent to 35 mg N kg-‘) was incorporated into the soil and 5 mg N kg-’ as unlabelled (NH&SO4 was added at t = 0 only. Samples were taken 0, 1, 2, 4, 7, 15 and 21 d after residue incorporation. Field experiments with labelled and unlabelled residues: the mirror image approach. For the field
experiments, labelled and unlabelled oil-seed rape and winter wheat were incorporated uniformly in the O-5 cm layer of the Rowland series soil growing winter wheat in early November 1991. The labelled and unlabelled residues in each case were identical except for the label. The residues were incorporated into an area 30 x 13 cm (slightly narrower than the crop row spacing to minimize damage). Tins of these dimensions were inserted into the soil to 5 cm and all the soil removed, placed in a plastic bag with the residue and well mixed before replacement. Four replicates were employed. On 7 April 1992 and 11 May 1992, mirror image experiments were carried out by injecting 13 mg N kg-’ as (NH&S04, either labelled or unlabelled as described above. Samples were taken after 2 and 9 d by driving a stainless-steel corer 6.5 cm dia to a depth of 5 cm. After mixing, subsamples of soil were taken for NH, and “NH4 analysis. Analyses. NH, in the soil extracts was analysed using flow injection calorimetry. 15N/“‘N isotope ratios of the exchangeable NH, were determined on a VG 622 mass spectrometer coupled to a Europa
Table 2. Plant comoosition details N (%) Laboratory incubations Unlabelled wheat straw Unlabelled oil-seed rape stem Labelled wheat straw
L&in
(%)
Cellulose (%)
Total “free“ amino acids (mg N kg-‘)
“N (atom%)
0.52 4.62 0.54
9.4 5.0 ND
20.3 24.9 ND
203 37.8 ND
0.3933 0.3708 1.19
0.67 0.67 1.77 1.77
ND ND ND ND
ND ND ND ND
ND ND ND ND
ND 6.16 ND 3.75
Field experiment
Unlabelled wheat straw Labelled wheat straw Unlabelled oil-seed rape Labelled oil-seed raw ND = not determined.
N mineralization during plant decomposition
173
residues, gave a gross rate of mineralization of 0.82 mg N kg-’ d-l, 54 d after incorporation, with 19% of the mineralization coming from the incorporated residues (data not shown). The rapid drop in N mineralization in the case of the oil-seed rape treatment suggests the depletion of a very labile N pool in the residue. If this is assumed to equal the cumulative N mineralized from the residue over the first 12 d, it was equivalent to 8.6 mg N kg-’ or 24% of the N in the residue. After 54 d, an estimated 13.2 mg N kg-’ had been mineralized from the oil-seed rape, equivalent to 38% RESULTS of the N in the residue. The proportions of N mineralization coming from Laboratory experiments with unlabelled oil-seed rape the decomposition of the winter wheat straw (a), and wheat residues: the d$erence method determined by the difference method, were 5, 64, 57 Figure 2 shows the gross rates of N mineralization and 23% over the 4 measurement intervals. The in the control, winter wheat and oil-seed rape average value for c( between 1.5-7.5 d was 60%; treatments in the unlabelled residue experiment; rates slightly lower than the 77% determined by the direct were calculated by substituting the size and “N method (see below). A separate mirror image abundance of the NH, pool at the beginning and end experiment on the same soil with winter wheat of each sampling period into Equation 2. No residues with slightly higher %N than those used standard errors are presented for the winter wheat here (0.67%N vs 0.56%N) gave a gross rate of results as marked immobilization meant all 4 mineralization of 0.74 mg N kg-’ d-l, 54 d after replicates had to be bulked to perform an 15N/14N residue incorporation, with 11% of the mineralizisotope ratio. All other results are means of 2 or 4 ation coming from residue decomposition (data not replicates. The gross N mineralization rates for the shown). control treatment at t = 4.5 and 7.5 d appear low. There is no obvious reason for this, but it must be assumed that there was a similar drop in basal N Laboratory experiments with labelled winter wheat mineralization in the residue-treated soil. Over the 4 residues: the direct method measurement intervals following incorporation, the Figure 3(a) shows the change in the soil NH4 pool proportion of the N mineralization flux resulting size following the incorporation of labelled winter from the decomposition of the incorporated oil-seed wheat residues. The NH4 pool in the control soil in residues rose from 60 to 74% and then dropped to 57 which no residues were incorporated is shown for and 9% by t = 7.5 and 10.5 d, respectively. A mirror comparison. The points in Fig. 3(b) show the change image experiment on the same soil, with similar in lSNH4 following the incorporation of the labelled winter wheat residues. Between days 1-7, the rate at 3.0 which the NH, pool in the treated soil changes remained constant, allowing the use of Equation 3 to determine both the rate of mineralization and 2.5 the “N enrichment of the N being mineralized. The line between days l-7 in Fig. 3(b) is the leastsquares best-fit line obtained from Equation 3. It 2.0 \ Oilseed rape gives m = 1.125 + 0.153 mg N kg-’ d-l with 0* = 0.6397 f 0.013 atom%. Substituted in Equation 4 \ with P* = 0.824 atom% (P= 1.19 atom%; see \ 1.5 Table 2) gives c( = 0.77, i.e 77% of the N mineralized over days l-7 results from the decomposition of the wheat residues. The average gross rate of mineraliz1.0 ation obtained with unlabelled winter wheat residues of similar composition to the labelled ones used here, 0.5 gave m = 0.952 mg N kg-’ d-’ (see Fig. 2). This, and the similarity in values for CL obtained by the difference and direct methods, suggests the two methods gave comparable results. 7 6 9 10 11 12 0 1 2 3 4 5 6 The elevated “N at t = 0, immediately following Time (days) incorporation in the experiment with labelled wheat Fig. 2. Gross rates of N mineralization in the difference residues (0.5134 atom%) suggests that 2% of the N experiment. in the wheat was released very rapidly. The data in Scientific RoboPrep combustion analyser after diffusion with MgO onto glass microfibre discs acidified with 2.5 M KH2S04 (Brookes et al., 1989). The mass spectrometer was referenced against IAEA quality control standard 305B. Soil and plant total C and N contents were determined on the same system. The lignin and cellulose contents of the plant material were determined by the acid detergent fibre method (van Soest and Wine, 1986). Soluble amino acids were determined on a dedicated amino acid analyser.
o.oJ
Naomi Watkins and Declan Barraclough Table 3. Proportion of mineralization from crop residues in the field (a)
experiment Crop
residue rate (mg N kg-’ d-l)
Date - Residues
Gross mineralization
7 April 1992 II May 1992
WW OSR WW OSR
2.84 3.34 3.24 I .92
Proportion from crop residue 0.08 0.15 0.09 0.13
WW = winter wheat; OSR = oil-seed rape. Soil was growing winter wheat.
0
N contents from those used in the laboratory incubations, the proportions of mineralization coming from residue decomposition were similar to those obtained in the laboratory experiments: 8-9% for wheat and 13-15% for oil-seed rape. 5
10
25
20
15
DISCUSSION 1.2
Figure 4 shows the laboratory data for the decomposition of oil-seed rape and winter wheat residues obtained by both the difference method and the mirror image approach (result at 54 d). It shows the gross rate of mineralization attributable to residue breakdown at intervals following incorporation. It appears that both the amount and the pattern of N release differed between the oil-seed rape and the wheat. There was very rapid mineralization of the N in the oil-seed rape immediately after incorporation. In contrast, the decomposition of the
(b)
1.1 1.0 0.9 0.8 0.7 0.8 0.5 0.4 0.3 0.2 0.1
1.5 lr
V.”
0
5
10
15
20
Time (days)
Fig. 3. (a) Soil NH4 following incorporation of labelled winter wheat: direct method. (b) Soil 15NH4 following incorporation of labelled winter wheat: direct method. Points are the experimental results: the line between t = 1 and 7 d is the least-squares best-fit using Equation 3 with m = 1.125 mg kg-’ d-’ and 0* = 0.6397 atom%.
1.0 0
Table 2 show this was considerably greater than the water-soluble free amino acid content of the wheat. Christensen (1985) showed that approximately 20% of the N in barley straw could be removed by two cold-water extractions and Hadas et al. (1993) concluded that the decomposition of wheat tops was in three phases, the first of which was water-soluble material amounting to 19% of the N in the residue. Thus, the release of N into the soil immediately following incorporation, observed in our work, is probably the release of water-soluble nitrogenous compounds. Field experiment with labelled and unlabelled residues: the mirror image method
Table 3 gives the results from the field experiment in which, although the mineralization rates are higher (partly because the measurements were on the O-5 not the O-15 cm horizon) and the residues had different
Winter wheat
0.5
0.0 O Time (days)
Fig. 4. Gross rates of N mineralization attributable decomposition of oil-seed rape or wheat.
to
N mineralization during plant decomposition winter wheat exhibited a lag of 3 d before mineralization became significant. If these results are translated into the field, incorporation of 3 t ha-’ of oil-seed rape stems into the soil (note that the equivalent of only 0.3 t ha-’ was incorporated here to equal the N input from the winter wheat), would release 120 kg N ha-’ over a relatively short time, with perhaps 190 kg N ha-’ released over a whole winter. This contrasts with 15 kg N ha-’ mineralized from the winter wheat. The use of finely-ground residues in this work would probably accelerate decomposition, although Ladd’s results with wheat tops and roots (Ladd, 1981) suggest that the use of finely-ground residues has only a secondary effect on decomposition. The proportion of the N in the residues mineralized is in line with other published results. Amato and Ladd (1980) estimated 33% of the N in Me&ago littoralis leaves (C-to-N ratio = 8.7) was mineralized in 34 d. Conclusions
Isotope dilution allows the determination of gross rates of mineralization in the presence of decomposing crop residues, unconfounded by processes consuming NH4. The correspondence between the results obtained by difference and by the direct method for winter wheat is reassuring. At this early stage, however, no method emerges as ideal. Both the difference and the mirror image technique require conditions to be identical, or similar, in two treatments; no easy thing in field experiments. The direct method avoids this restriction but does require that rates of mineralization and the proportion of the mineralization coming from residue decomposition remain constant over 3 or more measurement periods. Our results indicate that incorporation of residues in soil is followed by substantial increases in gross mineralization. The winter wheat results are particularly interesting, illustrating the ability of isotope dilution to disentangle the N cycle. Despite a wide C-to-N ratio (around 80: l), gross mineralization rates do increase following residue incorporation, But this is masked completely by the marked immobilization accompanying residue decomposition [Fig. 3(a)]. The oil-seed rape results confirm its potential to release large quantities of N in a short time, raising questions about its cultivation in areas where water quality is at risk. Acknowledgements-The authors are grateful to Martin Heaps for help with r5N isotope ratio analysis, to Ross Monaghan and Paul Gibbs for lignin and cellulose analysis, and to Dr John Metcalfe and Mike Lomax of the Department of Animal and Microbial Science, University of Reading for amino acid analyses. We are also grateful to ICI Ltd, Jealotts Hill for labelled and unlabelled plant material and to Mr Bill Acworth for access to microplots for the production of labelled oil-seed rape. This work was funded by the Natural Environment Research Council. REFERENCES Amato M. and Ladd J. N. (1980) Studies on nitrogen immobilization and mineralization in calcareous soils: V.
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Formation and distribution of isotope-labelled biomass during decomposition of ‘YZ- and 15N-labelled plant material. Soil Biology & Biochemistry 12, 405-41 I. Amato M., Ladd J. N., Ellington A., Ford G., Mahoney J. E., Taylor A. C. and Walsgott D. (1987) Decomposition of plant material in Australian soils. IV. Decomposition in situ of ‘C- and ‘SN-labelled legume and wheat materials in a range of Southern Australian soils. Australian Journal of Soil Research 25, 95-105.
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GROSS RATES OF N MINERALIZATION ASSOCIATED WITH THE DECOMPOSITION OF PLANT RESIDUES NAOMI
WATKINS*
and DECLAN
BARRACLOUGHt
Department of Soil Science, The University of Reading, Whiteknights, P.O. Box 233, Reading RG6 2DW, England (Accepted 27 July 1995)
Summary-Nitrogen-15 isotope dilution was used to determine gross rates of N mineralization immediately following the incorporation of crop residues in soil. Laboratory and field experiments were performed. Two crop residues were used: oil-seed rape stems and winter wheat straw. Incorporation of oil-seed rape residues into the soil resulted in an immediate increase in gross N mineralization rate from 0.87 to 2.2 mg kg-r d-r. After 12 d, gross N mineralization in the presence of the residues had dropped to 0.79 mg kg-’ d-‘, compared to 0.72 mg kgg d-’ in the control. Of the N in the oil-seed rape, 24% was mineralized in 12 d. In contrast, incorporation of winter wheat residues caused only a small immediate increase in gross N mineralization to 0.92 mg kg -I d--l. Of the N in the winter wheat residues, 12% was mineralized in the 12 d following incorporation. Longer-term measurements indicated that the residue incorporation was still influencing N mineralization 54 d after incorporation with 19 and 11% of the mineralization flux resulting from oil-seed rape and winter wheat decomposition, respectively. Field measurements showed that 5 months after incorporation, oil-seed rape residues were contributing 13% of the mineralization flux while winter wheat residues were contributing 9%.
INTRODUCTION of organic matter in soil, and the accompanying mineralization and immobilization of inorganic N, are key processes in the soil-plant N cycle. They are also among the most complex. The substrate is a heterogeneous mix of organic matter ranging from plant residues and litter recently entered the soil, to humic material with mean residence times of thousands of years (Jenkinson, 1988). The soil microbial biomass, the agent of decomposition, is also a varied mix of organisms. Because NH,, the initial product of N mineralization, can be consumed by a number of processes in soil (plant uptake, immobilization and volatilization), nitrification, measurements of changes in NH, (xand Nor) pool sizes over time can, at best, provide estimates of net mineralization, the balance between production and consumption. Despite this complexity, considerable advances have been made in understanding the decomposition process. Studies on CO2 evolution during plant residue decomposition show an initial, more rapid phase, in which about 70% of the C initially in the residue is lost as COz, followed by a slower phase (Jenkinson, 1977; Sauerbeck and Gonzalez, 1977). This two-phase decomposition has been interpreted
The decomposition
*Present address: Department York, York, England. tAuthor for correspondence.
of Biology, University of
as indicating labile (or decomposable) and recalcitrant (or resistant) fractions of the plant residue; a concept exploited in models of soil C dynamics (Jenkinson and Rayner, 1977). A consideration of the chemical composition of plant material supports the idea that residue decomposition is likely to exhibit two phases characterized by different kinetics. The initial, more rapid phase corresponding to the degradation of water-soluble free amino acids, amino sugars and carbohydrates, and cell cytoplasmic and membrane constituents. The slower second phase corresponds to the decomposition of cell walls and structural components, and the decomposition of secondary, more stabilized products of the first phase (Sorensen, 1975). However, results on N mineralization are often contradictory. In some cases there is a clear two-phase mineralization, suggesting a labile and recalcitrant fraction similar to that observed for C (Broadbent and Nakashima, 1974); in other studies this two-phase pattern is not so evident (Amato et al., 1987). Similarly, some work suggests that C-to-N ratios are useful predictors of net N mineralization while in others N-to-polyphenol ratios appear more reliable indicators. Berg and Staaf (1981) reviewed a number of results and concluded that net N mineralization started at a wide range of C-to-N ratios. In part, this confusion arises because only net N mineralization is measured. Thus the relationship being studied is between the C-to-N ratio of the organic substrate and a group of soil processes, not N mineralization alone. Changes in N immobilization or loss may blur or completely obscure any 169
170
Naomi Watkins and Declan Barraclough
postulated relation between N mineralization and a factor such as the C-to-N ratio of the residue. The development of “N pool dilution techniques presents the opportunity to study gross N mineralization unconfounded by the processes that consume NH, (Kirkham and Bartholomew, 1954; Barraclough et al., 1985; Nishio et al., 1985). The work we report here is part of an integrated programme in which 15N pool dilution techniques are being used to study gross N mineralization in terms of the mineralization resulting from the decomposition of specific fractions of the organic matter in soil. It uses as a starting point the hypothesis that, at any one time, the gross rate of N mineralization in a soil is the sum of the N being mineralized from specific fractions of soil organic matter. This part of the work is concerned with that part of the N mineralization flux resulting from the decomposition of plant residues in the period immediately following their incorporation in the soil. Thus, for this work we assume that: m = m, + m,.
AND METHODS
Theory
Nitrogen- 15 isotope dilution techniques, in which rsNH4 is added to soil, enable the gross rate of N mineralization to be determined unconfounded by processes such as plant uptake and nitrification which consume NH,. Thus, for example, the addition of the autotrophic nitrification inhibitor nitrapyrin to a woodland soil reduced nitrification from 2.22 to 0.44 mg kg-’ d-r but gross mineralization, measured by r5NH4isotope dilution, only changed from 1.49 to
‘4:
A: =
(2)
( f! >(‘*io) 1+
(1)
All notation is given in Table 1. To examine the effect of residue N content on mineralization, two contrasting plant residues (oilseed rape and winter wheat) were used. The main experiments were under controlled conditions in the laboratory but two experiments were carried out in the field. MATERIALS
1.59 mg N kg-’ do-’ (Barraclough and Puri, 1995). Isotope dilution offers three ways of determining the gross N mineralization resulting from the decomposition of crop residues: by difference; using a direct method; and a mirror image approach. The di@rence method. The difference method involves determining gross rates of N mineralization in the presence or absence of residues. Unlabelled residues are incorporated in the soil and, at intervals, rSNH4 is injected into the soil. The size and 15N enrichment of the NH4 pool at a minimum of two times after the injection are substituted into Equation 2 to obtain the gross rate of N mineralization (Barraclough et al., 1985). A parallel experiment determines gross rates of N mineralization under the same conditions but without the incorporated residue:
All
If conditions are otherwise identical, differences in N mineralization between the two treatments will be due to the N mineralization resulting from the decomposition of the incorporated residues. This method is straightforward but has two potential drawbacks. The first is the usual one associated with determination by difference; the error in N mineralization attributed to residue decomposition is the square root of the errors sum of squares of the two measurements used in its derivation. This becomes particularly important if there is only a small difference between gross rates of N mineralization with or without residue incorporation. The second concerns the assumption that the basal N mineralization is the same in the presence or absence of residues. If, following residue incorporation, part of the microbial biomass switches from decomposing “soil” organic matter to processing the residue, the N mineralization resulting from residue decomposition, obtained by difference, will be an underestimate as basal mineralization has dropped.
Table I. Notation A = NH4 pool (mg N kg-‘) 0 = organic N (mg N kg-‘) P = plant residue N (mg N kg-‘) i = gross rate of immobilization (mg N kg-’ d-‘) m = gross rate of mineralization (mg N kg-’ d-‘) n = gross rate of nitrification (mg N kg-’ d-‘) I = rate of N loss (mg N kg-’ d-‘) f = time (d) a = proportion of mineralization flux resulting from residue decomposition 0 = rate at which pool size changes (mg N kg -’ d-9
Subscripts p = attributable to plant residue decomposition s = attributable to soil organic matter decomposition 0 = time 0 I = time f superscripts * = lSN excess (atom%)
Thus, A,* indicates the lJN atom% excess in the soil ammonium pool at time f.
N mineralization during plant decomposition 1.4
The direct method. The direct approach,
avoids these problems, but has some of its own. It involves incorporating 15N-labelled residues into the soil and injecting unlabelled NH, at intervals, in the same way as the previous method. Now, because labelled residues are being decomposed, any involvement of the residue in mineralization will cause the ‘jN abundance in the NH4 pool to change. The greater the proportion of mineralization coming from the residues (as opposed to the remaining soil organic matter, which is unlabelled), the greater the change in the 15NH4. In fact, this situation corresponds to the full form of Equation 2, the isotope dilution equation. As presented above, Equation 2 implicitly assumes the organic matter being decomposed is at natural abundance. If this not the case, the full form must be used as shown in Equation 3: A,* = O* +
(A$ - O*) WV’
1.2
3 g .
1.o
z
0.8
z
L
1:
&-0.4
o.2
-___
0.4
0.1
0.2
0.0
’ 0
o* = LIP*,
‘.O
/I
0.6
(3)
0* in Equation 3 is the excess 15N of the organic matter being decomposed. When this is zero, as normal, Equation 3 reduces to Equation 2. When part of the organic matter being decomposed is labelled (e.g. the crop residue), while the remainder (e.g. “soil” organic matter) is not, 0* is given by
171
1
2
3
4
5
6
7
8
Time (days)
Fig. I. Change in lSNH4resulting from the decomposition of a residue labelled at 1.19 atom%. Lines are calculated from Equation 3; numbers on curves are proportion of mineralization resulting from residue decomposition. Gross mineralization rate is I. 125 mg N kg-’ dm’in all cases.
(4)
where P* is the excess 15Nof the incorporated residue and c( is the proportion of the mineralization flux resulting from the decomposition of the incorporated residues. Figure 1 shows the effect of increasing the proportion of mineralization coming from the decomposition of labelled residues. The example uses data from the labelled residue experiment to be described below. The simulations start at t = 1 d to coincide with the period over which the direct method was used. As the proportion of N mineralization resulting from residue decomposition increases, so, at a given time, does the ‘jN in the soil NH, pool. The gross rate of mineralization in Fig. 1 is 1.125 mg N kg-’ d-l for all the curves; the numbers on the curves are G(, the proportion of the N mineralization flux resulting from the residue decomposition. If m, 0 and c( (and therefore 0*) remain approximately constant over three or more samplings, Equation 3 can be solved for both m and 0* (and therefore c() by fitting it to the experimental points using a least-sum-of-squares approach. This approach is a direct measurement without the problems of differences, but is restricted to intervals over which m, t3 and a are approximately constant. The mirror image approach. This involves paired experiments with unlabelled and labelled, but otherwise identical, residues. Labelled NH, is injected into the soil with the unlabelled residues and the gross rate of N mineralization in the presence of the residues is determined in the same way as that
described for the difference method. In the parallel experiment with labelled residues, unlabelled NH, is injected into the soil and the change in the 15NH4 determined over the same interval. The change in 15NH4 is described by Equation 3. in which the gross rate of mineralization, m, should equal that determined in the mirror image unlabelled residue experiment. This is substituted into Equation 3 together with the NH., pool size and ‘jN enrichments from the labelled residue experiment to determine the only unknown, O*, from which x can be calculated using Equation 4. Our aim was to determine the proportion of mineralization resulting from the decomposition of incorporated crop residues in both laboratory and field experiments, All the three methods described above are used. Thus, laboratory experiments were performed with unlabelled oil-seed rape and winter wheat residues for the difference method. Labelled winter wheat residues were used for the direct method. Field experiments were carried out using labelled and unlabelled residues of both plants for the mirror image approach. Soil
All laboratory experiments were carried out on soil collected from the cl5 cm depth of a sandy loam of the Rowland series (classified as a psamment) growing winter wheat. Selected properties of the soil are: %C, 1.OS; %N, 0.07; pH(H20) 5.98. The soil was
172
Naomi Watkins and Declan Barraclough
sieved <2 mm and stored in sealed containers at 20°C for 2 weeks prior to the start of the experiments. Field incorporation experiments were carried out on the O-5 cm horizon of the same soil. Plant material
For the laboratory incubations, unlabelled winter wheat straw and oil-seed rape stems, and labelled winter wheat straw were obtained from the Jealotts Hill Research Station of Imperial Chemical Industries. The paired labelled and unlabelled plant residues for use in the mirror image experiments were obtained from microplots on the Reading University farm (winter wheat) and a commercial farm field (oil-seed rape), after harvest of the upper plant parts. The winter wheat had received 130 kg N ha-‘; to obtain labelled residues, 70 of the 130 kg N ha-’ was double labelled (21.43 atom%) NH4NOj. The portion of the stalk between S-50 cm above the ground was cut into small (< 5 cm) pieces and subsamples finely ground. Oil-seed rape stems were also collected after harvest from microplots on a commercial farm. The crop had received 220 kg N ha-‘; for the labelled residues, 60 the 220 kg N ha-’ was double labelled (30 atom%) NH4N0,. The portion of straw between 5-30 cm above the ground was prepared in the same manner as the winter wheat. Table 2 gives details of the plant materials after preparation. Laboratory incubations with unlabelled residues: the using unlabelled d@erence method. Incubations
residues were performed at 20°C using 40 g subsamples of fresh soil packed into 2 cm deep plastic cylinders at the bulk density of the field soil (1.0 g cm-j). Cylinders with plant material had winter wheat (235 mg) or oil-seed rape (26.45 mg) incorporated in the soil, which at the final soil moisture content of lo%, gave an equivalent N incorporation of 34 mg N kg-‘. For winter wheat, this rate of incorporation was equivalent to 3 t ha-‘; a typical incorporation rate in the field in this region. The control cylinders, which received no plant residues, were disturbed in the same way as those with plant material. The cylinders were covered with parafilm with 4 pinholes to allow gas exchange. Water loss during the incubations was negligible. To determine gross rates of mineralization, 5 mg N kg-’ as (1SNH4)2S0., at 10.1 atom% was
added to each cylinder in 3 ml of deionized water. Underestimation of mineralization rates due to remineralization was minimized by making the measurements over 3-d intervals. Preliminary experiments had shown this to be a satisfactory time interval (data not shown). Thus, label additions were made at t = 0, t = 3, t = 6 and t = 9 to sets of 8 cylinders from each treatment; 4 cylinders were sampled immediately, 4 were sampled after 3 d. All the soil from each cylinder was shaken with 1 M KC1 for 1 h (15 soil-to-solution ratio). The extract was filtered through glass fibre filter paper and stored at 4°C prior to inorganic N and isotope ratio analysis. Laboratory incubations with labelled winter wheat residues: the direct method. The experiments with
labelled residues were performed in the same way as those with unlabelled residues except that 235 mg of labelled winter wheat residue (equivalent to 35 mg N kg-‘) was incorporated into the soil and 5 mg N kg-’ as unlabelled (NH&SO4 was added at t = 0 only. Samples were taken 0, 1, 2, 4, 7, 15 and 21 d after residue incorporation. Field experiments with labelled and unlabelled residues: the mirror image approach. For the field
experiments, labelled and unlabelled oil-seed rape and winter wheat were incorporated uniformly in the O-5 cm layer of the Rowland series soil growing winter wheat in early November 1991. The labelled and unlabelled residues in each case were identical except for the label. The residues were incorporated into an area 30 x 13 cm (slightly narrower than the crop row spacing to minimize damage). Tins of these dimensions were inserted into the soil to 5 cm and all the soil removed, placed in a plastic bag with the residue and well mixed before replacement. Four replicates were employed. On 7 April 1992 and 11 May 1992, mirror image experiments were carried out by injecting 13 mg N kg-’ as (NH&S04, either labelled or unlabelled as described above. Samples were taken after 2 and 9 d by driving a stainless-steel corer 6.5 cm dia to a depth of 5 cm. After mixing, subsamples of soil were taken for NH, and “NH4 analysis. Analyses. NH, in the soil extracts was analysed using flow injection calorimetry. 15N/“‘N isotope ratios of the exchangeable NH, were determined on a VG 622 mass spectrometer coupled to a Europa
Table 2. Plant comoosition details N (%) Laboratory incubations Unlabelled wheat straw Unlabelled oil-seed rape stem Labelled wheat straw
L&in
(%)
Cellulose (%)
Total “free“ amino acids (mg N kg-‘)
“N (atom%)
0.52 4.62 0.54
9.4 5.0 ND
20.3 24.9 ND
203 37.8 ND
0.3933 0.3708 1.19
0.67 0.67 1.77 1.77
ND ND ND ND
ND ND ND ND
ND ND ND ND
ND 6.16 ND 3.75
Field experiment
Unlabelled wheat straw Labelled wheat straw Unlabelled oil-seed rape Labelled oil-seed raw ND = not determined.
N mineralization during plant decomposition
173
residues, gave a gross rate of mineralization of 0.82 mg N kg-’ d-l, 54 d after incorporation, with 19% of the mineralization coming from the incorporated residues (data not shown). The rapid drop in N mineralization in the case of the oil-seed rape treatment suggests the depletion of a very labile N pool in the residue. If this is assumed to equal the cumulative N mineralized from the residue over the first 12 d, it was equivalent to 8.6 mg N kg-’ or 24% of the N in the residue. After 54 d, an estimated 13.2 mg N kg-’ had been mineralized from the oil-seed rape, equivalent to 38% RESULTS of the N in the residue. The proportions of N mineralization coming from Laboratory experiments with unlabelled oil-seed rape the decomposition of the winter wheat straw (a), and wheat residues: the d$erence method determined by the difference method, were 5, 64, 57 Figure 2 shows the gross rates of N mineralization and 23% over the 4 measurement intervals. The in the control, winter wheat and oil-seed rape average value for c( between 1.5-7.5 d was 60%; treatments in the unlabelled residue experiment; rates slightly lower than the 77% determined by the direct were calculated by substituting the size and “N method (see below). A separate mirror image abundance of the NH, pool at the beginning and end experiment on the same soil with winter wheat of each sampling period into Equation 2. No residues with slightly higher %N than those used standard errors are presented for the winter wheat here (0.67%N vs 0.56%N) gave a gross rate of results as marked immobilization meant all 4 mineralization of 0.74 mg N kg-’ d-l, 54 d after replicates had to be bulked to perform an 15N/14N residue incorporation, with 11% of the mineralizisotope ratio. All other results are means of 2 or 4 ation coming from residue decomposition (data not replicates. The gross N mineralization rates for the shown). control treatment at t = 4.5 and 7.5 d appear low. There is no obvious reason for this, but it must be assumed that there was a similar drop in basal N Laboratory experiments with labelled winter wheat mineralization in the residue-treated soil. Over the 4 residues: the direct method measurement intervals following incorporation, the Figure 3(a) shows the change in the soil NH4 pool proportion of the N mineralization flux resulting size following the incorporation of labelled winter from the decomposition of the incorporated oil-seed wheat residues. The NH4 pool in the control soil in residues rose from 60 to 74% and then dropped to 57 which no residues were incorporated is shown for and 9% by t = 7.5 and 10.5 d, respectively. A mirror comparison. The points in Fig. 3(b) show the change image experiment on the same soil, with similar in lSNH4 following the incorporation of the labelled winter wheat residues. Between days 1-7, the rate at 3.0 which the NH, pool in the treated soil changes remained constant, allowing the use of Equation 3 to determine both the rate of mineralization and 2.5 the “N enrichment of the N being mineralized. The line between days l-7 in Fig. 3(b) is the leastsquares best-fit line obtained from Equation 3. It 2.0 \ Oilseed rape gives m = 1.125 + 0.153 mg N kg-’ d-l with 0* = 0.6397 f 0.013 atom%. Substituted in Equation 4 \ with P* = 0.824 atom% (P= 1.19 atom%; see \ 1.5 Table 2) gives c( = 0.77, i.e 77% of the N mineralized over days l-7 results from the decomposition of the wheat residues. The average gross rate of mineraliz1.0 ation obtained with unlabelled winter wheat residues of similar composition to the labelled ones used here, 0.5 gave m = 0.952 mg N kg-’ d-’ (see Fig. 2). This, and the similarity in values for CL obtained by the difference and direct methods, suggests the two methods gave comparable results. 7 6 9 10 11 12 0 1 2 3 4 5 6 The elevated “N at t = 0, immediately following Time (days) incorporation in the experiment with labelled wheat Fig. 2. Gross rates of N mineralization in the difference residues (0.5134 atom%) suggests that 2% of the N experiment. in the wheat was released very rapidly. The data in Scientific RoboPrep combustion analyser after diffusion with MgO onto glass microfibre discs acidified with 2.5 M KH2S04 (Brookes et al., 1989). The mass spectrometer was referenced against IAEA quality control standard 305B. Soil and plant total C and N contents were determined on the same system. The lignin and cellulose contents of the plant material were determined by the acid detergent fibre method (van Soest and Wine, 1986). Soluble amino acids were determined on a dedicated amino acid analyser.
o.oJ
Naomi Watkins and Declan Barraclough Table 3. Proportion of mineralization from crop residues in the field (a)
experiment Crop
residue rate (mg N kg-’ d-l)
Date - Residues
Gross mineralization
7 April 1992 II May 1992
WW OSR WW OSR
2.84 3.34 3.24 I .92
Proportion from crop residue 0.08 0.15 0.09 0.13
WW = winter wheat; OSR = oil-seed rape. Soil was growing winter wheat.
0
N contents from those used in the laboratory incubations, the proportions of mineralization coming from residue decomposition were similar to those obtained in the laboratory experiments: 8-9% for wheat and 13-15% for oil-seed rape. 5
10
25
20
15
DISCUSSION 1.2
Figure 4 shows the laboratory data for the decomposition of oil-seed rape and winter wheat residues obtained by both the difference method and the mirror image approach (result at 54 d). It shows the gross rate of mineralization attributable to residue breakdown at intervals following incorporation. It appears that both the amount and the pattern of N release differed between the oil-seed rape and the wheat. There was very rapid mineralization of the N in the oil-seed rape immediately after incorporation. In contrast, the decomposition of the
(b)
1.1 1.0 0.9 0.8 0.7 0.8 0.5 0.4 0.3 0.2 0.1
1.5 lr
V.”
0
5
10
15
20
Time (days)
Fig. 3. (a) Soil NH4 following incorporation of labelled winter wheat: direct method. (b) Soil 15NH4 following incorporation of labelled winter wheat: direct method. Points are the experimental results: the line between t = 1 and 7 d is the least-squares best-fit using Equation 3 with m = 1.125 mg kg-’ d-’ and 0* = 0.6397 atom%.
1.0 0
Table 2 show this was considerably greater than the water-soluble free amino acid content of the wheat. Christensen (1985) showed that approximately 20% of the N in barley straw could be removed by two cold-water extractions and Hadas et al. (1993) concluded that the decomposition of wheat tops was in three phases, the first of which was water-soluble material amounting to 19% of the N in the residue. Thus, the release of N into the soil immediately following incorporation, observed in our work, is probably the release of water-soluble nitrogenous compounds. Field experiment with labelled and unlabelled residues: the mirror image method
Table 3 gives the results from the field experiment in which, although the mineralization rates are higher (partly because the measurements were on the O-5 not the O-15 cm horizon) and the residues had different
Winter wheat
0.5
0.0 O Time (days)
Fig. 4. Gross rates of N mineralization attributable decomposition of oil-seed rape or wheat.
to
N mineralization during plant decomposition winter wheat exhibited a lag of 3 d before mineralization became significant. If these results are translated into the field, incorporation of 3 t ha-’ of oil-seed rape stems into the soil (note that the equivalent of only 0.3 t ha-’ was incorporated here to equal the N input from the winter wheat), would release 120 kg N ha-’ over a relatively short time, with perhaps 190 kg N ha-’ released over a whole winter. This contrasts with 15 kg N ha-’ mineralized from the winter wheat. The use of finely-ground residues in this work would probably accelerate decomposition, although Ladd’s results with wheat tops and roots (Ladd, 1981) suggest that the use of finely-ground residues has only a secondary effect on decomposition. The proportion of the N in the residues mineralized is in line with other published results. Amato and Ladd (1980) estimated 33% of the N in Me&ago littoralis leaves (C-to-N ratio = 8.7) was mineralized in 34 d. Conclusions
Isotope dilution allows the determination of gross rates of mineralization in the presence of decomposing crop residues, unconfounded by processes consuming NH4. The correspondence between the results obtained by difference and by the direct method for winter wheat is reassuring. At this early stage, however, no method emerges as ideal. Both the difference and the mirror image technique require conditions to be identical, or similar, in two treatments; no easy thing in field experiments. The direct method avoids this restriction but does require that rates of mineralization and the proportion of the mineralization coming from residue decomposition remain constant over 3 or more measurement periods. Our results indicate that incorporation of residues in soil is followed by substantial increases in gross mineralization. The winter wheat results are particularly interesting, illustrating the ability of isotope dilution to disentangle the N cycle. Despite a wide C-to-N ratio (around 80: l), gross mineralization rates do increase following residue incorporation, But this is masked completely by the marked immobilization accompanying residue decomposition [Fig. 3(a)]. The oil-seed rape results confirm its potential to release large quantities of N in a short time, raising questions about its cultivation in areas where water quality is at risk. Acknowledgements-The authors are grateful to Martin Heaps for help with r5N isotope ratio analysis, to Ross Monaghan and Paul Gibbs for lignin and cellulose analysis, and to Dr John Metcalfe and Mike Lomax of the Department of Animal and Microbial Science, University of Reading for amino acid analyses. We are also grateful to ICI Ltd, Jealotts Hill for labelled and unlabelled plant material and to Mr Bill Acworth for access to microplots for the production of labelled oil-seed rape. This work was funded by the Natural Environment Research Council. REFERENCES Amato M. and Ladd J. N. (1980) Studies on nitrogen immobilization and mineralization in calcareous soils: V.
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