Free amino sugar reactions in soil in relation to soil carbon and nitrogen cycling

Amino sugars represent a major constituent of microbial cell walls and hydrolyzed soil organic matter. Despite their potential importance in soil nitrogen cycling, comparatively little is known about their dynamics in soil. The aim of this study was therefore to quantify the behaviour of glucosamine in two contrasting grassland soil profiles. Our results show that both free amino sugars and amino acids represented only a small proportion of dissolved organic N and C pool in soil. Based upon our findings we hypothesize that the low concentrations of free amino sugars found in soils is due to rapid removal from the soil solution rather than slow rates of production. Further, we showed that glucosamine removal from solution was a predominantly biotic process and that its half-life in soil solution ranged from 1 to 3 h. The rates of turnover were similar to those of glucose at low substrate concentrations, however, at higher glucosamine concentrations its microbial use was much less than for glucose. We hypothesized that this was due to the lack of expression of a low affinity transport systems in the microbial community. Glucosamine was only weakly sorbed to the soil's solid phase (K_d=6.4±1.0) and our results suggest that this did not limit its bioavailability in soil. Here we showed that glucosamine addition to soil resulted in rapid N mineralization and subsequent NO₃⁻ production. In contrast to some previous reports, our results suggest that free amino sugars turn over rapidly in soil and provide a suitable substrate for both microbial respiration and new biomass formation.     

Plant capture of free amino acids is maximized under high soil amino acid concentrations

Free amino acids (AA's) represent a significant source of available N for some plants and soil microorganisms. It can be expected, however, that significant competition will exist between plants and microorganisms for this organic N resource. Our study indicated that microbial capture and utilization of glycine was very rapid at a range of soil solution concentrations (0.1 μM to 10 mM) indicating that significant competition will exist between roots and soil microorganisms. Plant capture of free AA's was maximal at high soil solution concentrations where microbial utilization was slowest. Our results suggest that plant capture of soil dissolved organic N may primarily occur in organic rich patches in soil where concentrations of free AA's are high.     

Soil nitrogen pools and turnover in native woodland and managed pasture soils

Dissolved organic nitrogen (DON) is a significant nitrogen (N) pool in most soils and is considered to be important for N cycling. The present study focused on paired sites of native remnant woodland and managed pasture at three locations in south-eastern Australia. Improved understanding of N cycling is important for assessing the impact of agriculture on soil processes and can guide conservation and restoration soil management strategies to maintain remnant native woodland systems, which currently exist as small pockets of woodland within extensive managed pasture landscapes. Organic and inorganic N pools were quantified, as well as the rates of amino acid and peptide mineralisation in the paired native woodland and managed pasture systems. Soil DON dominated the soil N pool in both land uses, and the proportion of DON to other N pools was greatest at the most N-limited site (up to ∼70% of extractable N). In both land uses soil ammonium and free amino acid concentrations were similar (∼20% of extractable N), and soil nitrate formed the smallest N pool (<∼5% of extractable N). Mineralisation of ¹⁴C-labelled amino acid and peptide substrates was rapid (<3 h), and more amino acid was respired than peptide in both the native woodland and managed pasture soils. Soil C:N ratio was important in separating site and land use differences, and contrasting relationships between soil physico-chemical properties and organic N uptake rates were identified across sites and land uses.     

Differential acquisition of amino acid and peptide enantiomers within the soil microbial community and its implications for carbon and nitrogen cycling in soil

l-isomeric amino acids and oligopeptides are thought to represent a key nitrogen (N) source for plants and soil microorganisms, bypassing the need to take up inorganic N, whilst self-cycling of d-enantiomers within peptidoglycan-containing bacteria may provide a further short circuit within the N cycle. Here we use stable isotope profiling (SIP) to identify the fate of organic N within soil microbial communities. We followed the incorporation of ¹³C-labelled d- or l-labelled amino acids/peptides into phospholipid fatty acids (PLFAs). l-alanine and its peptides were taken up more rapidly than d-enantiomers by Gram-positive bacteria with ¹³C incorporation being predominantly into anteiso- and iso-fatty acids typically associated with Gram-positive bacteria. d-enantiomer uptake was found not to differ significantly between the microbial groups, providing little support for the view that soil bacteria may self-cycle d-forms of amino acids and peptides. There was no consistent association between peptide chain length and incorporation. The concentrations of l- and d-isomeric amino acids in soil solution were 866 nM and 72 nM, respectively. We conclude that Gram-positive bacteria appear to be the primary competitors for l-enantiomeric forms of amino acids and their peptides, but that both d- and l-enantiomers are available N and C sources for bacteria and fungi.     

Regulation of amino acid biodegradation in soil as affected by depth

Dissolved organic nitrogen (DON) and in particular free amino acids represent a key pool in the terrestrial soil C and N cycle. The factors controlling the rate of turnover of this pool in soil, however, remain poorly understood. We investigated the factors regulating the rate of amino acid turnover at different depths (up to 1.2 m) in five low-input, acid soil profiles. Within the root zone (0–60 cm), amino acids constituted 8% of the DON and represented only a small fraction of plant available N. In all the soil profiles, the rate of amino acid mineralisation decreased progressively with depth. The average half-life of the exogenously added amino acids in the soil was 5.8 h in topsoils (0–10 cm), falling to 20 h at a depth of 50 cm and to 33 h at 100 cm. Generally, the rate of amino acid mineralisation correlated positively with total soil C and N, soil microbial activity (basal soil respiration rate) and soil content. The relatively rapid rates of microbial amino acid assimilation in subsoils below the root zone (>60 cm) indicate that long-term transport of amino acids (e.g. from soil to freshwaters) will be low. Based upon the C-to-N ratio of the amino acid substrate and the microbial C assimilation efficiency, we estimate that approximately 40–60% of the amino acid-N will be excreted as . In conclusion, the rapid rate of free amino acid turnover and their low concentration in soil solution indicate that the formation of inorganic N ( and ) in soil is limited primarily by the rate of free amino acid appearance in soil and not by the rate of amino acid mineralisation.     

Dissolved soil organic nitrogen and carbon in a Norway spruce stand and an adjacent clear-cut

 The aims of this study were to characterize dissolved soil organic N (DON) and C (DOC) in a coniferous stand and an adjacent clear-cut, and to evaluate the importance of DON in N leaching. The study was carried out in a Norway spruce stand and a clear-cutting treatment in the same forest stand. Concentrations of DON in soil solution were monitored for 5 years after clear-cutting with gravity lysimeters. In the Norway spruce stand DON comprised 62–83% of the total N in soil solution over the 5-year period. The concentrations of DON in the clear-cut were higher than in the forest stand, but the proportion of total N was lower. To characterize dissolved organic matter, soil samples were aerobically incubated for 6 weeks in the laboratory, and the quantity, molecular size distribution and chemical nature of both DON and DOC were determined from water extracts made before and after the incubation. In the soil samples from the Norway spruce stand, C-rich compounds with a high C/N ratio and large molecular size were formed. In contrast, after the incubation the major carriers of DON in soil samples from the clear-cut were N-rich organic compounds with a low C/N ratio and a small molecular size. The distribution of different chemical fractions of DOC in soil did not differ much whether recovered from the Norway spruce stand or the clear-cut. It was (from highest to lowest concentration): hydrophobic acids>hydrophilic acids>phenols>hydrophilic neutrals. A major part of DON was also carried by these fractions. During incubation the concentration of N-containing hydrophilic acids increased, especially in the soil from the clearcut. In soil samples from the Norway spruce stand, the rate of net N mineralization was low and no NO₃ was formed, whilst the rate of net N mineralization was high and net nitrification was intensive in soil from the clear-cut. Received: 12 June 2000     

Carbon-to-nitrogen ratio is a poor predictor of low molecular weight organic nitrogen mineralization in soil

Carbon-to-nitrogen ratio (C:N) has frequently been shown to be a good predictor of the speed of organic residue decomposition and N mineralization in soil. While this relationship appears to work well for complex organic materials (e.g. plant litter), its applicability to smaller organic substrates containing N remains unknown. Here we evaluated whether the intrinsic properties of amino acids and peptides could be used to predict their rate of microbial uptake and subsequent N mineralization. In an agricultural grassland soil we found that C:N, molecular weight, aromaticity and sulphur content provided poor indicators of amino acid bioavailabilityand subsequent NH₄⁺ release into soil. We therefore hypothesize that the position of amino acids along microbial biosynthetic pathways together with internal demand for individual amino acids rather than their C or N content is the primary determinant of N mineralization.     

城市不同功能区绿地土壤微生物量碳氮及溶解性碳氮分布特征及影响因素

为研究城市功能分区对绿地土壤活性碳氮的影响,选择合肥市不同功能区(老商业区、老工业区、居民服务区、近郊森林公园)绿地土壤为研究对象,分析了土壤微生物量碳氮(MBC、MBN)、溶解性碳氮(DOC、DON)、有机碳(SOC)、全氮(TN)以及土壤基本理化性质的差异及其相互关系。结果表明,功能区类型对土壤微生物量碳氮及溶解性碳氮影响显著(P0.05),且0~30 cm土层含量均为近郊森林公园高于城区内各功能区。土层厚度对MBC、DOC、DON含量影响显著,MBC、DON在各功能区中均随土层深度增加而递减,森林公园MBN、DOC均为10~20 cm含量最高。不同功能区MBC/MBN在6~14间波动,以此预测本研究范围内土壤微生物可能以真菌和放线菌为主,细菌较少。本研究未证实DOC与MBC、MBC与DON、DON与MBN间的相关性,MBC+DOC、MBN+DON比MBC、DOC、DON、MBN更能反应土壤活性碳氮库与土壤微生物量。总体来说,相比城区内各功能区,近郊森林公园更利于土壤生物量碳氮及溶解性有机碳氮的积累。     

Minimal direct contribution of arbuscular mycorrhizal fungi to DOC leaching in grassland through losses of glomalin-related soil protein

Arbuscular mycorrhizal fungi (AMF) have multiple influences on ecosystem C cycling, but most research has focused on ecosystem C gains. We explore here the possibility of direct contributions of AMF to ecosystem C losses, namely via leaching of glomalin-related soil protein (GRSP). We tested the hypothesis that GRSP, an operationally defined SOM pool to which AMF contribute (especially as evidenced with monoclonal antibody MAb32B11-based detection), is mobile in soils and can be lost in leachate. For two New Zealand soils, we showed that only insignificant amounts of GRSP were lost: a maximum of 0.03% of MAb32B11-immunoreactive GRSP present in soils was lost during the week-long experiment, representing a minute fraction of total leachate dissolved organic carbon (0.06%). Our data showed that this pathway of C loss may be relatively unimportant in many soils. However, other indirect contributions of AMF to soil C losses remain yet to be explored.     

Fast turnover of low molecular weight components of the dissolved organic carbon pool of temperate grassland field soils

Large amounts of low molecular weight (LMW;<250 Da) carbon (C) are lost from roots into the rhizosphere as a consequence of root turnover and exudation. Their rates of turnover after release into the soil remain poorly understood. We extracted soil solution from a temperate grassland Eutric Cambisol, isotopically labeled the glucose and amino acid components, and then re-injected the solution back into the soil. We followed the subsequent evolution of ¹⁴CO₂ and incorporation of the LMW C into the soil microbial biomass or grasses for 48 h. The experiments were performed both on grazed and un-grazed swards in the field, and in the laboratory. In the field, we showed that glucose and amino acids had short half-lives (t_1/2) in soil solution (t_1/2=20-40 min), but that they persisted in soil microbes for much longer. A first-order double exponential model fitted the experimental data well and gave rate constant (k) values of 1.21-2.14 h⁻¹ for k₁ and 0.0025-0.0048 h⁻¹ for k₂. Only small amounts of the added ¹⁴C were recovered in plant biomass (<5% of total added to soil) indicating that plant roots are poor competitors for LMW dissolved organic C (DOC) in comparison to soil microorganisms. The first phase of glucose and amino acid mineralization in the laboratory was slower (t_1/2=40-60 min) than measured in the field reinforcing the importance of making flux measurements in situ. Whilst grazing stimulated below-ground respiration, it exerted only a small influence on the turnover of LMW DOC suggesting that the increase in respiration was due to increased root respiration and not turnover of soil organic matter (SOM). Our results suggest that some components of the LMW DOC pool are turned over extremely rapidly (ca. 4000 times annually).     

Grassland plants affect dissolved organic carbon and nitrogen dynamics in soil

Dissolved organic carbon (DOC) and nitrogen (DON) are central in many nutrient cycles within soil and they play an important role in many pedogenic processes. Plants provide a primary input of DOC and DON into soil via root turnover and exudation. Under controlled conditions we investigated the influence of 11 grass species alongside an unplanted control on the amount and nature of DOC and DON in soil. Our results showed that while the presence of plants significantly increases the size of a number of dissolved nutrient pools in comparison to the unplanted soil (e.g. DOC, total phenolics in solution) it has little affect on other pools (e.g. free amino acids). Grass species, however, had little effect on the composition of the DOC, DON or inorganic N pools. While the concentration of free amino acids was the same in the planted and unplanted soil, the flux through this pool was significantly faster in the presence of plants. The presence of plants also affected the biodegradability of the DOC pool. We conclude that while the presence of plants significantly affects the quantity and cycling of DOC and DON in soil, comparatively, individual grass species exerts less influence.     

Amounts of carbon mineralised and leached as DOC during decomposition of Norway spruce needles and fine roots

Changes in climate or forest management practices leading to increased litter production will most likely cause increased leaching rates of dissolved organic carbon (DOC) from the O horizon. The rhizosphere is often assumed to have a large carbon flux associated with root turnover and exudation. However, little has been done to quantify the amount of DOC originating from root litter. We studied decomposition of fine root and needle litter of Norway spruce (Picea abies) through a combined incubation and leaching experiment in the laboratory using five different litter types: fresh needle litter, aged needles from the litter layer, fresh and dead roots from mineral soil samples, and seven-year-old roots from a previous litterbag study. After respiration measurements, the samples were percolated with artificial throughfall water and DOC and UV absorbance were measured in the leachate. Mineralisation of dissolved organic matter in the leachate and sorption of DOC to ferrihydrite were determined as a measure of DOC ability to be stabilised by iron (hydr)oxide surfaces.The mineralisation rate and DOC production rate of root samples were always lower than that of needle samples. However, root and needle derived dissolved organic matter (DOM) were similar in terms of aromaticity, as indicated by their specific UV absorbance, and ability to be sorbed by ferrihydrite. For seven-year-old roots, a significantly higher fraction of carbon was lost as DOC (30%) than for younger roots (20%). Furthermore, DOM from old roots bound more strongly to ferrihydrite and is mineralised at a lower rate than DOC from younger roots, suggesting that roots at late stages of decomposition, although a small fraction of total litter, significantly contribute to carbon build-up in mineral soils. The slower decomposition rate of roots compared with needles must be taken into account when modelling litter decomposition.     

Modelling low molecular weight organic acid dynamics in forest soils

Low molecular weight organic acids such as citrate and oxalate have been hypothesized to play a key role in rhizosphere ecology and pedogenesis. A mathematical site-specific model, DYNLOW, was constructed to describe the temporal and spatial dynamics of these organic acids in coniferous forest soils using the modelling software STELLA^®. Experimentally derived values for biodegradation, adsorption, and daily values of soil temperature, moisture and hydrological flow were used to parameterize the model. The model describes the dynamics and downward movement of oxalate and citrate through the horizons (O, AE, E, Bhs, Bs) of three podzolic soil profiles in Sweden. After calibration, the model predicted average soil solution organic acid concentrations ranging from <1 to 90 μM, which was in agreement with experimental measurements (<1 to 116 μM). The model results indicated that microbial degradation of organic acids was in quantitative terms the biggest process regulating soil solution concentrations. Primary production rates of organic acid in the soil were predicted to be high (<1 to 1250 nmol g⁻¹ soil d⁻¹) in comparison to the amount present at steady state in the soil solution pool (<0.1 to 240 nmol g⁻¹ soil). The downward transfer of organic acids between soil horizons due to mass flow was predicted to be a small flux (<0.1 to 3% of the total loss) compared to that lost by microbial biodegradation. The model predicted that the amount of basal soil respiration that could be attributable to the microbial turnover of organic acids was on average 19±22% of the basal CO₂ production across all sites and horizons for citrate and 7±7% for oxalate. The model results are discussed in the context of pedogenesis, forest soil respiration and organic matter production.     

Priming effects in different soil types induced by fructose, alanine, oxalic acid and catechol additions

It is well established that certain substrate additions to soils may accelerate or retard the mineralisation of soil organic matter. But up to now, research on these so called ‘priming effects’ was almost exclusively conducted with arable soils and with plant residues or glucose as additives. In this study, the effects of the uniformly ¹⁴C-labelled substrates fructose, alanine, oxalic acid and catechol on the mineralisation of soil organic carbon (SOC) from different horizons of two forest soils (Haplic Podzol and Dystric Cambisol) and one arable soil (Haplic Phaeozem) under maize and rye cultivation were investigated in incubation experiments for 26 days. Apart from the controls, all samples received substrate additions of 13.3 μg substrate-C mg⁻¹ C_org. During the incubation, CO₂-evolution was measured hourly and the amount of ¹⁴CO₂ was determined at various time intervals. In almost all soils, priming effects were induced by one or several of the added substrates. The strongest positive priming effects were induced by fructose and alanine and occurred in the Bs horizon of the Haplic Podzol, where SOC mineralisation was nearly doubled. In the other soil samples, these substrates enhanced SOC mineralisation by +10 to +63%. Catechol additions generally reduced SOC mineralisation by −12 to −43% except in the EA horizon of the Haplic Podzol where SOC-borne CO₂-evolution increased by +46%. Oxalic acid also induced negative as well as positive priming effects ranging from −24 to +82%. The data indicate that priming effects are ubiquitously occurring in surface and subsoil horizons of forest soils as well as in arable soils. Although a broad variety of soils was used within this study, relationships between soil properties and priming effects could not be ascertained. Therefore, a prediction on occurrence and magnitude of priming effects based on relatively easily measurable chemical and physical soil properties was not possible. Nevertheless, the data suggest that positive priming effects are most pronounced in forest soils that contain SOC of low biodegradability, where the added substrates may act as an important energy source for microbial metabolism.     

Microbial immobilization and mineralization of dissolved organic nitrogen from forest floors

Dissolved organic nitrogen (DON) plays a key role in the N cycle of many ecosystems, as DON availability and biodegradation are important for plant growth, microbial metabolism and N transport in soils. However, biodegradation of DON (defined as the sum of mineralization and microbial immobilization) is only poorly understood. In laboratory incubations, biodegradation of DON and dissolved organic carbon (DOC) from Oi and Oa horizons of spruce, beech and cypress forests ranged from 6 to 72%. Biodegradation of DON and DOC was similar in most samples, and mineralization of DON was more important than microbial immobilization. Nitrate additions (0-10 mg N L⁻¹) never influenced either DON immobilization by microorganisms or mineralization. We conclude that soil microorganisms do not necessarily prefer mineral N over DON for meeting their N demand, and that biodegradation of DON seems to be driven by the microbial demand for C rather than N. Quantifying the dynamics of DON in soils should include consideration of both C and N demands by microbes.     

Soil- and enantiomer-specific metabolism of amino acids and their peptides by Antarctic soil microorganisms

Most nitrogen (N) enters many Arctic and Antarctic soil ecosystems as protein. Soils in these polar environments frequently contain large stocks of proteinaceous organic matter, which has decomposed slowly due to low temperatures. In addition to proteins, considerable quantities of d-amino acids and their peptides enter soil from bacteria and lengthy residence times can lead to racemisation of l-amino acids in stored proteins. It has been predicted that climate warming in polar environments will lead to increased rates of soil organic N turnover (i.e. amino acids and peptides of both enantiomers). However, our understanding of organic N breakdown in these soils is very limited. To address this, we tested the influence of chain length and enantiomeric composition on the rate of breakdown of amino acids and peptides in three contrasting tundra soils formed under the grass, moss or lichen-dominated primary producer communities of Signy Island in the South Orkney Islands. Both d- and l-enantiomers of the amino acid monomer were rapidly mineralized to CO₂ at rates in line with those found for l-amino acids in many other terrestrial ecosystems. In all three soils, l-peptides were decomposed faster than their amino acid monomer, suggesting a different route of microbial assimilation and catabolism. d-peptides followed the same mineralization pattern as l-peptides in the two contrasting soils under grass and lichens, but underwent relatively slow decomposition in the soil underneath moss, which was similar to the soil under the grass. We conclude that the decomposition of peptides of l-amino acids may be widely conserved amongst soil microorganisms, whereas the decomposition of peptides of d-amino acids may be altered by subtle differences between soils. We further conclude that intense competition exists in soil microbial communities for the capture of both peptides and amino acids produced from protein breakdown.     

Assessment of soil structure repair due to wetting and drying cycles through 2D tomographic image analysis

Soil structure is very important in agriculture since it affects soil and plant root attributes, such as root system distribution, soil water and nutrient transport, and heat transfer. Degraded soil structures may be repaired by wetting and drying cycles due to changes in the soil pore system. Gamma-ray computed tomography (CT) was used as a tool to evaluate the effect of wetting/drying cycles on soil structure repair, using samples collected in aluminum cylinders. A first-generation tomograph with an ²⁴¹Am source and a 7.62 cm × 7.62 cm NaI(Tl) scintillation crystal detector coupled to a photomultiplier tube were employed. Image analysis and tomographic unit profiles showed that CT can provide an insight into sample structure in order to evaluate repairs and so improves the use of this tool in relation to the judgement of the quality of measured soil physical properties.     

Priming depletes soil carbon and releases nitrogen in a scrub-oak ecosystem exposed to elevated CO2

Elevated atmospheric CO₂ tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO₂ concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO₂. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO₂ plots at 0-10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO₂ stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO₂ increased net N mineralization in the 10-30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO₂. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO₂ studies. Increased mineralization of stable soil organic carbon by a CO₂-induced priming effect may diminish the terrestrial carbon sink globally.     

Carbon availability regulates soil respiration response to nitrogen and temperature

The response of soil CO₂ fluxes (R_soil) to interactions between carbon (C) and nitrogen (N) availability or C and temperature conditions is not well understood, but may increasingly affect future C storage under the combined anthropogenic impacts of N deposition and climate change. Here we addressed this uncertainty through a series of laboratory incubation experiments using soils from three contrasting ecosystems to investigate how changes in C, N, and temperature regulate R_soil through changes to Michaelis–Menten parameters (i.e. V_max and K_m). Results of this study demonstrate that R_soil response to N enrichment and changes in temperature are dependent on the C availability of soil substrates. N addition influenced R_soil through both the maximum rate (V_max) and the half saturation constant (K_m). The increase in K_m corresponded to a decrease in R_soil when C was limited. Alternatively, when C was abundant, N enrichment increased R_soil, which corresponded to an increase in V_max. Regulation of temperature sensitivity through V_max and K_m was also dependent on C availability. Both V_max and K_m demonstrated positive temperature responses, supporting the hypothesis of a canceling effect at low C concentrations. While temperature sensitivity was influenced by both C quantity and C complexity, our results suggested that C quantity is a stronger predictor. Despite strong differences in climate, vegetation, and management of our soils, C–N and C-temperature interactions were markedly similar between sites, highlighting the importance of C availability in the regulation of R_soil and justifying the use of Michaelis–Menten kinetics in biogeochemical modeling.     

Protein breakdown represents a major bottleneck in nitrogen cycling in grassland soils

Proteins represent the dominant input of organic N into most ecosystems and they also constitute the largest store of N in soil organic matter. The extracellular protease mediated breakdown of proteins to amino acids therefore represents a key step regulating N cycling in soil. In this study we investigated the influence of a range of environmental factors on the rate of protein mineralization in a grazed grassland and fallow agricultural soil. The protein turnover rates were directly compared to the rates of amino acid mineralization under the same conditions. Uniformly ¹⁴C-labelled soluble protein and amino acids were added to soil and the rate of ¹⁴CO₂ evolution determined over 30 d. Our results indicate that the primary phase of protein mineralization was approximately 20 ± 3 fold slower that the rate of amino acid mineralization. The addition of large amounts of inorganic NO₃⁻ and NH₄⁺ to the soil did not repress the rate of protein mineralization suggesting that available N does not directly affect protease activity in the short term. Whilst protein mineralization was strongly temperature sensitive, the presence of plants and the addition of humic and tannic acids had relatively little influence on the rate of soluble protein degradation in this fertile grassland soil. Our results suggests that the extracellular protease mediated cleavage of proteins to amino acids rather than breakdown of amino acids to NH₄⁺ represents the limiting step in soil N cycling.