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Soil Science Society of America Journal 63:1199-1206 (1999)
© 1999 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Mineralization of Amino Acids Applied to Soils

Impact of Soil Sieving, Storage, and Inorganic Nitrogen Additions

David L. Jonesa and David Shannona

a School of Agricultural and Forest Sciences, Univ. Wales-Bangor, Gwynedd, LL57 2UW, UK

d.jones{at}bangor.ac.uk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
The effect of inorganic N additions on the biodegradation and microbial use of organic N pools in soil is poorly understood. To examine the effects of inorganic N on the mineralization rates of amino acids, four soils under contrasting management regimes were subjected to increasing loadings of NH4NO3, ranging from 0 to 120 kg N ha-1. In addition, the effect of soil sieving and storage temperature and time on amino acid mineralization was also investigated. At times ranging from 1 to 40 d after the addition of the inorganic N, the mineralization kinetics of an equimolar mixture of fifteen 14C-labeled amino acids was followed for a subsequent 24-h period. The rate of 14CO2 evolution was soil dependent, with half-lives ranging from 2 h for topsoils to 25 h for subsoils. For all soils, at all times, and at all inorganic-N loadings, the addition of inorganic N appeared to have little effect on the mineralization kinetics of the amino acids to 14CO2. In addition, the presence of inorganic N also had no major effect on the C use efficiency of the microbial biomass. It is speculated that N release from the amino acids into the soil by the microbial biomass may also be little affected by inorganic-N additions. Sieving and storage of soil at either 4 or 18°C for up to 40 d had little impact on amino acid mineralization rate. Experiments with potential microbial disrupting agents (autoclaving, CHCl3 fumigation, HgCl2, and freeze–thaw) all indicated that the observed mineralization of amino acid C was due to microbial activity. We conclude therefore that inorganic N and soil storage has little effect on the microbial use of readily assimilatable amino acids.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
NITROGEN is frequently the growth-limiting nutrient in many uncultivated soils (Nihlgrd, 1985). In most nonlegume cropping systems, N losses from the previous year in the form of crop offtake, leaching and runoff, denitrification, and volatalization are typically matched by inputs of inorganic fertilizer. However, recently, many agroecosystems have become N saturated due to both anthropogenic N emissions and excessive fertilizer applications, resulting in widescale environmental pollution (gren and Bosatta, 1988). It has been proposed that more integrated farming strategies are required, incorporating treatments such as reduced fertilizer applications, fumigation, and tillage, and enhanced organic residue addition. However, these strategies rely on the premise that a reduced input of inorganic fertilizers can be compensated for by higher rates of organic matter mineralization to release sufficient N and other nutrients to the crop. To reduce N losses, and to facilitate the optimal crop utilization of mineralized N, a detailed knowledge of the effect of inorganic N additions on the activity of the soil biota is required.

The main input of organic material into cropping systems is in the form of root and shoot litter, of which the main organic N components are proteins ({approx}40% plant dry weight) and amino acids ({approx}0.1–1% plant dry weight). It has also been demonstrated that typically more than 90% of a soil's N reserves are present in an organic form, of which 30 to 50% can be recovered as amino acids after organic matter hydrolysis (Stevenson, 1982). Due to the widespread nature of amino acids in soil it is not surprising therefore that all microorganisms appear to have the capacity to take up and assimilate amino acids (Holden, 1962; Anraku, 1980). Amino acids therefore represent one of the most labile and dynamic soil organic N pools. Indeed, this is supported by evidence showing that the soil amino acid pool decreases faster than other organic N pools upon cultivation (Keeney and Bremner, 1964; Meints and Peterson, 1977). While the transport of amino acids into microorganisms grown in pure liquid cultures has been well characterized and shown to be dependent on environmental conditions (e.g., temperature, growth stage, mineral nutrition, and aeration; Glover et al., 1975; Anraku, 1980), there is little information available on the factors that control amino acid mineralization to CO2 and NH+4 in soil. In addition, especially in the rhizosphere, it is not simply the transport of amino acid into the cell that is important, but also the use of the amino acid C for either catabolic or anabolic metabolism (i.e., growth vs. maintenance). It is likely that this may be influenced by the microbe's nutritional status and, in particular, the availability of inorganic N.

It is well documented that organic matter inputs can stimulate microbial activity and growth, with the amount of activation related to the protein and carbohydrate content of the organic substrate (Sakamoto and Oba, 1991). In addition, the amendment of soil with organic residues has been shown to stimulate the release of organically bound nutrients such as P (Luo and Sun, 1994). However, the effect of inorganic fertilizer amendments on soil microbial activity, microbial community diversity, and organic nutrient cycling is less clear (Jannson and Persson, 1982). It is likely that this will be dependent on the type and amount of inorganic fertilizer added, the environmental conditions and soil characteristics, and the type and maturity of the vegetation (Meier et al., 1993; Beyer, 1994). Previous studies have shown that inorganic N additions can enhance, suppress, or have no effect on organic matter mineralization (Shields et al., 1974; Kowalenko et al., 1978; Rochette and Gregorich, 1998). The reason for this disparity may be that while inorganic N additions can act directly on the soil microbial population through an obvious transient increase in the labile N pool, they can also act indirectly on the microbial population (activity, biomass, and composition) by modifying soil pH and ionic strength and stimulating plant growth and belowground C inputs (Aleksic et al., 1968; Kissel and Smith, 1978; McAndrew and Malhi, 1992). The cause of the stimulation in N mineralization remains somewhat controversial. Stevenson (1982) concluded from a number of independent 15N tracer studies that the addition of inorganic fertilizer N leads to enhanced organic-N mineralization; however, Jannson and Persson (1982) and Hart et al. (1986) remain extremely skeptical of this "priming" effect due to problems encountered in the interpretation of 15N labelling results. In addition, these conflicting results may have resulted in part from the differing N status of the soils involved, the amount of N applied, the amount of microbial biomass, and the duration for which N was applied. In laboratory incubations, where the problems of interpreting plant C inputs can be removed, Smith et al. (1989) presented evidence suggesting that the availability of soil inorganic N directly regulates the microbial metabolism of organic N–containing substrates. However, the observed stimulation of mineralization under substrate-induced soil N deficiency was small and extremely transitory (<24 h). Because the microbial biomass was not determined under each of their substrate treatments, it remains difficult to assess the exact cause of their observed priming effect. Also, the preparation (e.g., sieving) and storage of soil in the laboratory even at low temperatures is known to influence the availability of inorganic nutrients and soil microbial activity (Nunezregueira et al., 1994; Chapman et al., 1997). Further, it has also been shown that amino acid mineralization to CO2 can occur purely by chemical oxidation on soil minerals without the need for a microbial population (Wang and Lin, 1993).

The objective of this study was therefore to assess the effect of sample sieving and storage, soil type, and increasing inorganic-N additions on the rate of amino acid mineralization. In addition, the degree to which amino acid mineralization is controlled by soil microbial activity was also investigated.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Soil Collection and Characterization
Six surface and subsurface soils under contrasting management practices were collected from within the county of Gwynedd, North Wales. Soil 1 was collected from under ungrazed grassland — velvetgrass (Holcus lanatus L.), Kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and buckhorn plaintain (Plantago lanceolata L.) — that had received no inorganic N-P-K fertilizer for at least the last 40 yr. Soil 2 was collected from under a {approx}2-mo-old pea (Pisum sativum L.) crop that had received regular N-P-K fertilization (150 kg N ha-1 yr-1, 75 kg K ha-1 yr-1, 75 kg P ha-1 yr-1) and has been in grassland–crop rotation for at least 20 yr. Soils 3 (topsoil) and 4 (subsoil) were collected from under sheep (Ovis aries)–grazed grassland — sheep fescue (Festuca ovina L. var. ovina) and colonial bentgrass (Agrostis capillaris L.) — that receives no N-P-K fertilizer. Soils 5 (topsoil) and 6 (topsoil) were collected from under sheep-grazed grassland — perennial ryegrass and crested dogtail (Cynosurus cristatus L.) — that receives regular N fertilizer additions (80 kg N ha-1 yr-1). Routinely, samples of topsoil (0–10 cm) and subsoil (20–40 cm) were coarse sieved to remove stones and then sieved to pass 2 mm; any discernible roots were removed. Soils were subsequently stored field moist in polyethylene bags at 4°C to await analysis (<7 d). The soil's organic C and N content was measured with a CHN analyzer (Leco Corp., St Joseph, MI), while carbohydrate content was determined using the phenol-H2SO4 procedure (Safarik and Santruckova, 1992). Exchangeable NH+4 and NO-3 were determined in 1:10 soil/1 M KCl extracts using methods described in Downes (1978) for NO-3 (hydrazine,–1-napthylethylenediamine assay) and Keeney and Nelson (1982) for NH+4 (indophenol blue assay), while exchangeable cations were determined in 1:5 soil/1 M NH4Cl extracts using a Jobin-Yvon Ultrace ICP-OES (Instruments S.A. Inc., Edison, NJ). Electrical conductivity and pH were measured in 1:1 (v/v) water extracts as described by Smith and Doran (1996). Basal soil respiration was measured at 18°C using a CIRAS-IRGA soil respirometer (PP Systems, Hitchin, UK). A summary of the soil classification, chemical, physical, and biological properties are given in Table 1 .


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  		Table 1 Selected management, physical, and biological properties of the soils used in the study

 
Impact of Soil Sieving and Storage Temperature
Intact blocks (10 x 5 x 5 cm) of Soil 1 were collected from a 2- to 10-cm depth and either sieved to pass 2 mm or left intact followed by storage in polyethylene bags at either 4 or 18°C for periods of up to 40 d. For the mineralization assays using nonsieved (intact) soil, small, discrete intact soil crumbs were used (1–3 mm diameter).

Inorganic Nitrogen Amendments
To simulate N loadings of either 0, 30, 60, or 120 kg N ha-1, NH4NO3 was added to each soil at rates of 0, 1.8, 3.6, or 7.2 mmol N kg-1 with application as a liquid at a dose rate of 50 µL g-1 (7.2 mmol N kg-1 = 144 mM solution). Addition rates were calculated assuming a soil bulk density of 1.2 g cm-3 and rooting depth of 10 cm. The soil and N was thoroughly mixed by vortexing. Ionic strength was not normalized in any of the soils. The soils containing different N levels were then incubated in the laboratory with a day/night temperature cycle of 20:10 ± 2°C (10:14 h). The moisture levels in the tubes were checked daily by measuring weight loss, and moisture was added back in the form of deionized water in order to maintain a constant moisture content.

Amino Acid Mineralization Assays
To determine the mineralization rate of organic N–containing compounds in soil, a mixture of fifteen uniformly labeled 14C-labeled L-amino acids were added to soil and their subsequent mineralization to 14CO2 measured for a 24- or 96-h period. The standard amino acid mixture (pH 5.60) was obtained by mixing together the following 14C-labeled amino acids (ICN Pharmaceuticals, Costa Mesa, CA) to give a final concentration for each amino acid of 333 µM and specific activity of 1.3 kBq mL-1: alanine, arginine, aspartic acid, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, phenylalanine, proline, serine, threonine, tyrosine, and valine. The final concentration of amino acids in the mixture was 5 mM. This was chosen based on the total concentration of amino acids present in maize root cells (10–20 mM; Jones and Darrah, 1994) and the probable concentration present in the rhizosphere after the bursting of a root epidermal cell. As the exact amino acid composition of root cells can vary widely, it was decided to use an equal quantity of each amino acid.

To quantify the use of amino acids by the microbial biomass, 50 µL of the standard amino acid mixture was mixed with 0.5 g of soil in a 15-mL polypropylene tube. The tubes were then attached to an air line that had previously been bubbled through two deionized water traps to maintain a high humidity in order to prevent soil drying. The outflow from the sample chamber was then bubbled through a 1 M NaOH trap to catch respired 14CO2. The flow rate through the gas-tight apparatus was 20 mL min-1, and the efficiency of the NaOH trap was determined to be 95 ± 2%. Samples were incubated in the laboratory for periods up to 96 h at a temperature of 18 ± 2°C with periodic changes of the NaOH trap. After 24 or 96 h, the amount of 14C label remaining in solution and held on exchange sites was determined by extracting the soils with 4 mL of 1 M KCl (15 min) followed by centrifugation (15000 g, 5 min) and counting the 14C label remaining in the supernatant solution (Joergensen, 1996). No difference in extractable 14C label was observed between extractions performed with 1 M KCl or 0.5 M K2SO4. All samples were performed in duplicate. All radioactivity was determined by liquid scintillation counting using a Wallac 1414 counter and a NaOH compatible scintillation fluid (Wallac Optiphase 3, EG&G Ltd., Milton Keynes, UK).

The amount of 14CO2 produced by each soil with time was determined on a cumulative basis. The standard errors shown in Fig. 1 to 5 are therefore additive. The amount of 14C label present in the microbial biomass at the end of the incubation period was determined by difference as follows:

(1)
while microbial biomass yield was calculated as follows:

(2)



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  		Fig. 1 Mineralization of amino acids to CO2 as influenced by soil type. Soils were incubated with of a mixture of 15 14C-labeled amino acids (5 mM) for 96 h, during which cumulative 14CO2 production was measured. Horizon designations for each soil sample are provided in the legend. Values represent means ± SE, n = 2

 


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  		Fig. 2 Impact of microbial disrupting agents on the mineralization of 14C-labeled amino acids to 14CO2. Inhibitor treatments included, autoclaving (121°C, 0.5 h), CHCl3 fumigation (12 h), freeze–thaw (-5°C, 12 h), and HgCl2 (10 mM). Values represent means ± SE, n = 2

 


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  		Fig. 3 Effect of soil sieving (2 mm), storage temperature (4 and 25°C), and storage time (0–40 d) on the mineralization of 14C-labeled amino acids to 14CO2. The legend is the same for all panels. Values represent means ± SE, n = 2

 


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  		Fig. 4 Effect of soil sieving (2 mm), storage temperature (4 and 25°C), and storage time (0–40 d) on (a) amino acid half-life in soil and (b) microbial biomass yield. The legend is the same for both panels; dotted lines represent linear regression fits. Values represent means ± SE, n = 2

 


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  		Fig. 5 Mineralization of 14C-labeled amino acids as measured by 14CO2 production, as influenced by increasing soil additions of inorganic N in six contrasting soils. NH4NO3 was added at Day 0 at different rates corresponding with 0 (•), 30 ({circ}), 60 ({blacktriangledown}), and 120 ({triangledown}) kg N ha-1. Soils were then incubated under aerobic conditions for up to 40 d, at which time the mineralization of a mixture of 15 14C-labeled amino acids (5 mM) was followed for 24 h. Horizon designation is provided after the soil number in the Day 1 panels. Values represent means ± SE, n = 2

 
Amino acid half-life was estimated by determining the point at which half the maximal 14CO2 had been produced (i.e., that observed after 96 h). Half-life values were obtained from interpolation of the experimental data points using the curve fitting package Sigmaplot 4.01 (SPSS, Chicago, IL) with a time interval of 0.05 h.

Incubations with Microbial Activity Disrupters
Biochemical inhibitors and thermal treatments were applied to Soil 1 to demonstrate the biological nature of the mineralization process and its ability to respond to changes in microbial activity or biomass. Amino acid assays identical to those described above were carried out except in the presence of HgCl2 (10 mM) or carried out on soil which had either been autoclaved (121°C, 0.5 h), subjected to one freeze–thaw cycle (-5°C, 12 h), or CHCl3 fumigated (12 h). Freeze–thawing was achieved by placement of soil in a laboratory freezer overnight. In the case of the autoclaved and freeze–thawed sample, soil was allowed to equilibrate to room temperature for 20 min prior to the addition of substrate.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Amino Acid Mineralization Rates
The amino acid mineralization kinetics were soil-type dependent with enhanced amino acid mineralization to CO2 observed in the topsoils (Soils 1, 2, 3, and 5) compared with the subsoils (Soils 4 and 6; Fig. 1). This is presumably due to the higher microbial activity within these surface horizons as reflected by the presence of higher amounts of organic C and soil respiration (Table 1). Extraction of the soils with 1 M KCl indicated that very little 14C label remained either free in the soil solution or held on the exchange complex after an incubation of 96 h (mean ± SE = 2.6 ± 0.6%; Table 2) . This exhaustion of the 14C-amino acid pool by the soil's microbial biomass is also supported by all the respiration profiles which show saturation after approximately 24 h, indicating a lack of available substrate. Assuming the maximum amount of 14CO2 that the microbes could produce is that observed at the 96 h time point, an estimate of amino acid half-life in the soil can be determined by interpolation of the respiration profiles. The mean half-life (± SE) of the amino acids was estimated to be 4.3 ± 1.0 h for the topsoils and 19.6 ± 5.2 h for the subsoils (Table 1). These rapid rates of utilization are similar to mineralization rates and half-lives reported for other low molecular weight solutes released from plant cells into the rhizosphere (e.g., organic acids, sugars; Coody et al., 1986; Jones et al., 1996). For the four topsoils, it was calculated that the mean biomass yields (± SE) after 96 h were 0.65 ± 0.01 µmol biomass C µmol substrate-C-1, while for subsoils it was slightly lower at 0.59 ± 0.02 µmol biomass C µmol substrate C-1. These yields are higher than previously reported for sugars (0.56) and organic acids (0.25), and probably reflect the ease by which amino acids can be directly fed into protein synthesis cycles (Coody et al., 1986; Jones et al., 1996). As the respiration profiles indicated that amino acid mineralization was maximal within the first 24 h, all the later studies were performed within this period.


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  		Table 2 Fate and persistence of 14C-labeled amino acids added to soil after 96 h. Values represent means ± SE (n = 2)

 
Impact of Microbial Activity Disrupters on Amino Acid Mineralization
The impact of various heat and chemical treatments on amino acid mineralization by the soil's microbial biomass is shown in Fig. 2. Of the disrupting agents chosen, autoclaving was found to almost completely inhibit mineralization, in agreement with Skipper and Westermann (1973). Similarly, the simultaneous addition of the cytotoxin HgCl2, or prior fumigation with CHCl3 also caused a 90% inhibition of 14CO2 production. In contrast, a single freeze–thaw treatment had only a small, albeit notable, effect on mineralization rate. However, this small freeze–thaw effect could simply be due to substrate competition from other non-14C-labeled organic compounds whose release into the soil solution is known to be stimulated by soil freezing (Vaz et al., 1994). These results indicate that the amino acid mineralization assay can respond to changes in the soil biomass activity.

Impact of Soil Sieving and Storage Temperature
The impact of sieving soil to pass 2 mm and subsequent storage temperature (4 or 18°C) on the mineralization of amino acids to CO2 in shown in Fig. 3 and 4. In general, soil sieving appeared to reduce amino acid mineralization rate within the first 24 h; however, subsequent incubation for up to 40 d revealed no consistent differences in amino acid mineralization rate between sieved and nonsieved samples (Fig. 3 and 4). Even when small differences were observed, none were of the same magnitude as measured with the microbial disrupting agents shown in Fig. 2. The effect of soil storage at either 4 or 18°C also produced no consistent differences between amino acid mineralization rate during the 40-d incubation period (Fig. 3). The effect of sample preparation, storage time, and temperature on microbial C use efficiency is shown in Fig. 4. Again, little difference was observed in microbial biomass yield at any of the time points studied.

Inorganic Nitrogen Effect on Amino Acid Mineralization
The total inorganic N content of the six test soils varied considerably from 0.23 to 9.76 mmol N kg-1. The levels of added N, relative to the background levels already present, consequently varied greatly between samples. The addition of high levels of inorganic N (equivalent to 120 kg N ha-1) resulted in an increase in soil NO-3 ranging from 50-fold in Soil 4 to 14-fold in Soil 3. Similarly, N additions also resulted in an increase in soil NH+4 ranging from 36-fold in Soils 4 and 6, to 5-fold in Soil 3. On average for the six soils, inorganic-N levels were instantaneously raised 17-fold by the addition of 120 kg N ha-1 NH4NO3.

The effect of increased loadings of inorganic N (NH4NO3) on amino acid mineralization by the soil microbial biomass is shown in Fig. 5. In all of the soils tested, the addition of varying amounts of inorganic N had no major effect on the utilization or C use efficiency of the amino acid C by the soil's microbial biomass. This lack of response was consistent for all the incubation times tested (1, 5, 10, 20, and 40 d after N addition) and all N levels. During the 40-d incubation, some slight changes in the kinetics of the 14CO2 respiration profiles were evident (e.g., compare Day 5 and Day 10 in Soil 4); however, generally the mineralization rate of the soil microorganisms remained similar at all sampling times, in agreement with Fig. 3. Although soil N levels at the end of 40 d were not measured, considering the aerobic nature of the samples, it is unlikely that significant losses due to denitrification occurred.


   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
It has been proposed that amino acid mineralization to CO2 and NH+4 can occur in soil due to chemical oxidation by Mn oxides (Wang and Lin, 1993; Sherigara et al., 1995). However, the results obtained here for a mixture of amino acids indicate that at least for the soils in this study, amino acid mineralization is largely biological in nature. This is also in agreement with results presented by Jones et al. (1996) for the breakdown of low molecular weight organic acids in soil.

It has been demonstrated many times that preparation of soil for use in laboratory studies can significantly alter the soil's chemical and biological status, subsequently biasing results (e.g., air drying; Comford et al., 1991). Further, it is known that microbial activity is significant even at low temperatures, while sieving to pass 2 mm breaks fungal hyphae and causes a significant flush of nutrients capable of supporting microbial activity (Chapman et al., 1997). However, the results presented here indicate that sieving, incubation temperature, and incubation time had little overall effect on the amino acid mineralization rate and C use efficiency of the microbial biomass. It must be noted that our findings represent the sum of the mineralization kinetics of fifteen amino acids. However, as the individual amino acids are frequently taken into microbial cells by different transport systems, we also speculate that similar results would have been obtained for singly added amino acids. Our results are also in agreement with other studies which indicate that storage temperature and sieving has little impact on microbial community structure and bacterial survival (Peterson and Klug, 1994; Biederbeck and Geissler, 1993). In contrast, microbial activity in soils that contain a significant amount of toxins do appear to be significantly affected by storage conditions either because of a gradual loss of organic pollutants or an increase heavy metal toxicity with storage time (Ehrlichmann et al., 1997).

Many factors are known to affect the functional diversity of the soil biota (Jarvis et al., 1996; McCarty et al., 1995). The results presented here indicate that the use of amino acids for either microbial respiration or biomass production is not directly affected by the application of inorganic N. This is in contrast to previous findings by Smith et al. (1989), who, using a similar technique, found that inorganic N stimulated the decomposition of some amino acids. Our results are in agreement with laboratory incubations performed by Recous et al. (1995), who also reported that NH4NO3 amendments (0–7 mmol N kg-1; {approx}0–120 kg N ha-1) had little effect on basal soil respiration in the absence of C substrates. Similarly, Lovell and Jarvis (1998) also demonstrated that while long-term inorganic additions caused a decrease in microbial biomass, it did not significantly affect microbial activity as assayed by substrate (glucose)–induced respiration. Field experiments have also shown that the long-term addition of inorganic fertilizer at rates up to 200 kg N ha-1 yr-1 has little effect on microbial biomass and activity (Rochette and Gregorich, 1998; Carter, 1986; Fauci and Dick, 1994; McCarty et al., 1995).

In other studies, N additions have been shown to reduce both the levels of those enzymes required to process organic N–containing substrates (e.g., amidase, urease) and the size of the population itself (Dick, 1992; McAndrew and Malhi, 1992; Smolander et al., 1994). Where decreases in microbial activity have been observed, it has been attributed in part to NH4NO3-induced drops in soil pH of up to 1.5 units (McAndrew and Malhi, 1992). This pH drop will have a direct effect on the microbial population, selecting for more acid-tolerant species, but will also act indirectly, reducing plant growth, which in turn determines substrate quantity and quality. However, in other studies where pH drops have not been observed but N-induced changes in biomass have occurred, the reasons behind these shifts and the nature of these changes still remains unknown (Smolander et al., 1994).

It has been demonstrated many times that cultured microbes directly take up intact amino acids into the cells by using specific transport proteins (Anraku, 1980), but until recently it was thought that microbes living in soil deaminated the amino acids outside the cell prior to the assimilation of the remaining C skeleton (Stevenson, 1982). While this may be a significant route for the removal of NH+4 from polymeric proteins and soil organic matter that cannot be transported into the cell, in the case of simple and generally weakly sorbed amino acids the direct route appears to predominate (Barraclough, 1997). Therefore, the use of amino acid C and amino acid N by the soil microbial biomass must be inextricably linked. It has been shown that due to the low C/N nature of amino acids , excess microbial NH+4 is frequently excreted into the soil (Barraclough, 1997). Although the excretion of amino acid–derived NH+4 into the soil was not measured in this study, it can be hypothesized that inorganic-N additions had little impact on the rate of microbial NH+4 release. Assuming that deamination must occur prior to the use of any amino acid–derived C in microbial respiration, it can be expected that large changes in intracellular amino acid deamination would also produce a concomitant rise in C skeletons and therefore an increased use of 14C label in 14CO2 production. As no changes in microbial C use efficiency were seen in any of the treatments investigated here, it is unlikely that inorganic-N additions increased microbial NH+4 excretion into the soil.

In conclusion, our results indicate that short term (<40 d) additions of inorganic N to soils that either do or do not receive regular additions of inorganic fertilizers, has no detectable direct or indirect effect on the rate of amino acid mineralization by the soil's microbial biomass. In addition, soil sieving and storage temperature also appear to have little impact on the capacity of the soil microbial biomass to biodegrade amino acids.Jansson Persson 1982; Petersen Klug 1994


   ACKNOWLEDGMENTS
 
We greatly appreciate funding for this project provided in part by the Nuffield Foundation and The Royal Society. Thanks are also due to Ian Kelso and Hilton Trow for advice and help on the soil characterization.

Received for publication August 19, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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