Extractability of labelled microbial biomass N by electro-ultrafiltration and CaCl2 extraction

A laboratory soil incubation and a pot experiment with ryegrass were carried out in order to examine the extractability of microbial biomass N by using either 10-mM CaCl₂ extraction or the electro-ultrafiltration (EUF) method. The aim of the experiment was to test the hypothesis whether the organic N (Norg) extracted by EUF or CaCl₂ from dried soil samples represents a part of the microbial biomass. For the laboratory incubation a ¹⁵N-labelled Escherichia coli suspension was mixed with the soil. For the pot experiment a suspension of ¹⁵N-labelled bacteria was applied which had previously been isolated from the soil used. Soil samples of both treatments, with and without applied bacterial suspension, were extracted by EUF and CaCl₂. The extractability of applied microbial biomass was estimated from the difference in extractable Norg between the two treatments. In addition, the N isotopic composition in the upper plant matter, in the soil, and in organic and inorganic N fractions of EUF and CaCl₂ extracts was analysed. Both experiments showed that the applied microbial biomass was highly accessible to mineralization and thus represented potentially mineralizable N. However, this mineralizable N was not extractable by CaCl₂ or by the EUF method. It was, therefore, concluded that the organic N released on soil drying and which was thus extractable was derived from the non-biomass soil organic matter. The result suggests that both extraction methods may provide a suitable index for mineralizable N only in cases where the decomposable organic substrates are derived mainly from sources other than the living soil biota.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

The proportional mineralisation of microbial biomass and organic matter caused by air-drying and rewetting of a grassland soil

During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO₂-C flushes that resulted. To do this, a UK grassland soil was amended with ¹⁴C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO₂-C evolved (varying from 83 to 240 μg g⁻¹ soil), compared to the control with, overall, less CO₂-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and ¹⁴C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g⁻¹ biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO₂-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO₂-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.     

Warming and elevated CO2 combine to increase microbial mineralisation of soil organic matter

The net annual exchange of carbon between the atmosphere and terrestrial ecosystems is of prime importance in determining the concentration of CO₂ ([CO₂]) in the atmosphere and consequently future climate. Carbon loss occurs primarily through soil respiration; it is known that respiration is sensitive to the global changes in [CO₂] and temperature, suggesting that the net carbon balance may change in the future. However, field manipulations of temperature and [CO₂] alter many important environmental factors so it is unclear how much of the observed alterations in soil respiration is due to changes of microbial function itself instead of changes to the physical and chemical environment. Here we focus on resolving the importance of changes in the microbial community in response to warming and elevated [CO₂] on carbon mineralisation, something not possible in field measurements. We took plant material and soil inocula from a long running experiment where native grassland had been exposed to both warming and elevated CO₂ and constructed a reciprocal transplant experiment. We found that the rate of decomposition (heterotrophic respiration) was strongly determined by the origin of the microbial community. The combined warming + elevated CO₂ treatment produced a soil community that gave respiration rates 30% higher when provided with shoot litter and 70% for root litter than elevated CO₂ treatment alone, with the treatment source of the litter being unimportant. Warming, especially in the presence of elevated CO₂, increased the size of the apparent labile carbon pool when either C₃ or C₄ litter was added. Thus, the metabolic activity of the soil community was affected by the combination of warming and elevated CO₂ such that it had an increased ability to mineralise added organic matter, regardless of its source. Therefore, soil C efflux may be substantially increased in a warmer, high CO₂ world. Current ecosystem models mostly drive heterotrophic respiration from plant litter quality, soil moisture and temperature but our findings suggest equal attention will need to be paid to capturing microbial processes if we are to accurately project the future C balance of terrestrial ecosystems and quantify the feedback effect on atmospheric concentrations of CO₂.     

Long-term monitoring of microbial biomass, N mineralisation and enzyme activities of a Chernozem under different tillage management

 We investigated the influence of tillage (conventional, minimum and reduced) on selected soil microbial properties of a fine-sandy loamy Haplic Chernozem over a period of 8 years. The microbial biomass and soil microbial processes were affected mostly by type of tillage and to a lesser extent by the date of soil sampling. Whereas xylanase activity was significantly higher in the 0 to 10-cm soil layer of the reduced and minimum tillage systems within the first year of the experiment (protease and phosphatase activities were significantly higher in the second year), significant treatment effects on microbial biomass, N mineralisation and potential nitrification were observed after a 4-year period. The slow response of substrate-induced respiration to the change in type of tillage may have been due to the differences in the biomass C turnover rates. After a 4-year period, the stratification of the soil microbial biomass within the profile of reduced and minimum tillage systems was probably responsible for the more intensive soil microbial processes near the soil surface compared with conventional tillage. In the 20 to 30-cm layer, N mineralisation, potential nitrification and xylanase activity in the conventional treatment were significantly higher than in the minimum and reduced tillage plots due to buried organic materials. Discriminant analysis underlined the similarity of the enzyme activity patterns in the top layer of the reduced and minimum tillage treatments, and in both layers of the conventional tillage system. The trend towards a significant increase in functional diversity caused by reduced tillage became obvious within the first year of the experiment, and this effect was still manifest after 8 years. All relationships suggested that there were differences in available resources (e.g. organic matter) along the sequence of different tillage systems; this was reflected in part by enhanced enzymatic and microbial activities in the soil layers. In conclusion, this study showed that soils affected by tillage may be classified on the basis of their functional diversity. Therefore, the soil microbial properties chosen for microbiological soil monitoring (microbial biomass, N mineralisation and enzyme activities involved in C, N and P cycling) provide a reliable tool with which to estimate early changes in the dynamics and distribution of soil microbial processes within soil profiles. Received: 3 February 1998     

Impact of biochar on mineralisation of C and N from soil and willow litter and its relationship with microbial community biomass and structure

Using a laboratory experiment, we investigated the effect of applying willow biochar to short rotation coppice soil on C and N dynamics and microbial biomass and community composition, in the presence and absence of willow litter. Application of biochar at a rate of 0.5 % had no effect on net CO₂ mineralisation in the presence or absence of litter. However at a rate of 2 %, net CO₂ mineralisation was reduced by 10 and 20 % over a 90-day period in the absence and presence of litter respectively. Biochar reduced N mineralisation when applied at both 0.5 and 2 % concentrations. pH was increased by application of 2 % biochar to soil. Phospholipid fatty acid analysis demonstrated that both concentrations of biochar affected microbial community composition, although the effect of biochar was not as great as the effect of time or litter application in shaping community structure. In particular, the amount of bacterial biomass was increased by biochar application to soil, and there was evidence for increased abundance of Gram-negative bacteria and actinobacteria following biochar application. The data is discussed in the context of microbial mechanisms underlying impacts of biochar on C cycling in soil, and the coupling of C and N cycles following amendment of soil with biochar.     

Applicability of 0.01 M CaCl2 as a single extraction solution for the assessment of the nutrient status of soils and other diagnostic purposes

Abstract

The applicability of 0.01 M CaCl₂ solution as a single extraction agent for soils as a basis for fertilizer recommendation was tested on a variety of soils both from the Netherlands and from some tropical countries. Air‐dry soil samples were subjected to extraction with 0.01 M CaCl₂ and to several conventional extraction procedures, and the results were compared. In the soil suspensions pH was measured, whereas in the extracts Na, K, Mg, P, different extractable N‐forms and Zn were measured. The values found in CaCl₂ extracts are discussed in relation to results of other extraction procedures and as to their potential value in soil quality assessment. It is concluded that a single extraction procedure with 0.01 M CaCl₂ can be applied for fertilizer recommendation purposes. The possibility of determining different extractable N‐forms (NH₄, NO₃, soluble organic N) significantly enhances the value of the method in predicting the N‐fertilizer needs. Furthermore it was found that the concentration of Zn in 0.01 M CaCl₂ extracts was a good indicator of phytotoxicity in a polluted area. Additional advantages of this extraction are low costs, simplicity and repro‐ducibility.     

Organic matter, microbial biomass and enzyme activity of soils under different crop rotations in the tropics

Soil organic matter level, soil microbial biomass C, ninhydrin-N, C mineralization, and dehydrogenase and alkaline phosphatase activity were studied in soils under different crop rotations for 6 years. Inclusion of a green manure crop of Sesbania aculeata in the rotation improved soil organic matter status and led to an increase in soil microbial biomass, soil enzyme activity and soil respiratory activity. Microbial biomass C increased from 192 mg kg^–1 soil in a pearl millet-wheat-fallow rotation to 256 mg kg^–1 soil in a pearl millet-wheat-green manure rotation. Inclusion of an oilseed crop such as sunflower or mustard led to a decrease in soil microbial biomass, C mineralization and soil enzyme activity. There was a good correlation between microbial biomass C, ninhydrin-N and dehydrogenase activity. The alkaline phosphatase activity of the soil under different crop rotations was little affected. The results indicate the green manuring improved the organic matter status of the soil and soil microbial activity vital for the nutrient turnover and long-term productivity of the soil. Received: 7 January 1996     

The relationship between N mineralization or microbial biomass N with micromorphological properties in beech forest soils with different texture and pH

To test relationships between net N-mineralization, organic matter and soil organisms, we combined micromorphology with laboratory incubation experiments over a soil gradient. Microbial biomass N generally increased with pH, and from sandy to loamy soil, but net N-mineralization showed the opposite, and was highest in acid, sandy soil. Twenty-two micromorphological characteristics were analyzed with principal component analysis. PC1 had high eigenvalue (0.70), and clearly separated fungi from earthworms, microarthropods and bacteria. PC2 was less important (0.15). Organic layer and sand content clearly correlated with the fungi-end of PC1, but pH and C-content of the Ah with the opposite. Microbial N also correlated with the earthworm–bacteria end, but net N-mineralization did not. Efficiency of N-mineralization per unit microbe even correlated with the fungi end of PC1, in both organic layer and mineral topsoil. The results support the hypothesis that high (or low) litter turnover and biological activity can be counteracted by high (or low) microbial N-demand.     

Organic matter and microbial biomass in a soil incubated in the field for 20 years with 14C-labelled barley straw

¹⁴C-labelled barley straw was incubated under field conditions in a sandy soil. After 8 yr, 16% of the labelled C added remained in the soil; after 20 yr 9.3%. During the 8 to 20 yr period the labelled organic matter in the soil decayed at a rate corresponding to a half life of 15 yr. The percentage of residual labelled C in amino acids remained almost constant during the period, being on average 21%. The soil contained 1.9% native C in organic matter; during the 8 to 20 yr period this decayed at a rate corresponding to a half life of 91 yr. The percentage of native C in amino acids increased significantly, from about 14% to about 16%.During the 8 to 20 yr period, microbial biomass was determined yearly by the chloroform fumigation technique. The proportion of total labelled C in biomass remained nearly constant during this period, on average 2.7%. Labelled C in biomass decreased at a rate corresponding to a half life of 8 yr. The native biomass increased during the same period from 0.7 to 1.4% of the total native soil C.As measured by CO₂ produced during periods of 3 months, laboratory incubation increased the rate of decay of the labelled organic matter by a factor of 1.2 and that of the native organic matter by a factor of 4.3.     

Dynamics of microbial biomass nitrogen as influenced by organic matter application in paddy fields

The effects of annual application of rice straw or cow manure compost for 17–20 y on the dynamics of fertilizer N and soil organic N in Gley paddy fields were investigated by using the ¹⁵N tracer technique during the rice cropping season. The chloroform fumigation-extraction method was evaluated to determine the properties of soil microbial biomass under submerged field conditions at the tillering stage before mid-summer drainage, with special reference to the fate of applied NH₄ ⁺-¹⁵N.

The transfer ratios from applied NH₄ ⁺-¹⁵N to immobilized N in soil and to uptake N by rice during given periods varied with the rice growth stages and were affected by organic matter application. The accumulated amounts of netmineralized soil organic N (net-M_j ), immobilized N (I_j ), and denitrified N (D_j ) during the cropping season were estimated to be 14.0–22.5, 6.3–11.2, and 3.4–5.3 g N m^-2, respectively. Values of net-M_j and I_j were larger in the following order: cow manure compost plot > rice straw plot > plot without organic matter application, and their larger increase by the application of cow manure compost contributed to a decrease of the D_j values, as compared with rice straw application.

Values of E _N extra extractable soil total N after fumigation, increased following organic matter application, ranging from 2.1 to 5.4 g N m^-2. Small residual ratios of applied ¹⁵N in the fraction E _N at the end of the given period indicated that re-mineralization of newly-assimilated ¹⁵N through the easily decomposable fraction of microbial biomass had almost ended. Thus, the applicability to paddy field soils of the chloroform fumigation-extraction method was confirmed.     

Extraction with 0.01 m CaCl2 underestimates the concentration of phosphorus in the soil solution

The aim of this paper was to compare the concentration of P in soil extracts prepared with water and a ‘soil solution proxy’ (‘SSP’, that is, a salt solution similar in ionic composition and strength to the actual soil solution) with that in 0.01 m CaCl₂ extracts, which is usually taken as a measure of soil P intensity. Seventy widely ranging agricultural soils from the Mediterranean part of Spain were used. Soil/solution ratio was 1:10 and extraction time 3 days. For 0.01 m CaCl₂, a short extraction time of 30 min was also used as the reference method. CaCl₂‐P(3 days) and CaCl₂‐P(30 min) were not significantly different for the 40 noncalcareous soils group, but CaCl₂‐P(3 days) was significantly larger than CaCl₂‐P(30 min) for the 30 calcareous soils group. The Water‐P/CaCl₂‐P(30 min) ratio was not significantly related to any soil property, its mean being 6.3 for the noncalcareous and 5.8 for the calcareous soils group. The mean SSP‐P/CaCl₂‐P(30 min) ratio was 2.6 for the noncalcareous and 3.1 for the calcareous soils group, and decreased slightly with increasing ionic strength of the soil solution in the noncalcareous soils group. These results are consistent with the promoting influence of the Ca ion and ionic strength on P adsorption by permanent‐charge soils. The fact that extraction with 0.01 m CaCl₂ generally results in underestimation of the actual concentration of P in the soil solution should be considered when CaCl₂‐P is used as a soil P test.     

C and N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is ¹³C, and especially ¹⁵N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was ¹⁵N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and ¹³C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also ¹³C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but ¹³C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.     

Influence of the rhizosphere on microbial biomass and recently formed organic matter

We have aimed to quantify the effect of roots on the size of the soil microbial biomass, and their influence on the turnover of soil organic matter and on the extent of the rhizosphere. We sampled a sandy clay loam topsoil from two subplots with different treatment histories. One had a normal arable fertilization record, the other had received only inorganic nitrogen fertilizer but no phosphorus and potassium for 30 years. Glucose labelled with ¹⁴C was added to both samples which were then incubated for 4 weeks before the soil was packed in cylinders and planted with ryegrass. In both soils, microbial biomass at the root surface doubled during the first 8 days. At day 15, the microbial biomass had further increased in the fertile soil, and the rhizosphere effect had extended 2.5 mm into the fertile soil, but to only 1 mm in the infertile soil. The microbial ¹⁴C increased threefold near the roots in the fertile soil as a result of assimilation of previously formed microbial residues, but in the infertile soil there was no increase. There was a close relation between the increase in microbial ¹⁴C and a decrease in ¹⁴C soluble in 2 m KCl, indicating that the microbial residues were more weakly adsorbed in the fertile soil. We conclude that the increased microbial population living near the root surfaces re‐assimilated part of the ¹⁴C‐labelled microbial residues in the fertile soil. In the infertile soil, microbial residues resisted decomposition because they were more strongly sorbed on to soil surfaces.     

Analysis of soil microbial biomass nitrogen and 15N with a high background of labeled mineral N

Abstract

Microbial biomass is determined from the excess of extractable organic N released from fumigated soil samples. In the presence of relatively high contents of labeled mineral N, small differences in organic N and N may not be detectable. Two approaches were tested to determine organic N content and its N enrichment in the presence of considerably greater concentrations of labeled mineral N: (i) Removal of mineral N from mixed solutions of alanine and NH₄ or NO₃ by reduction and boiling under alkaline conditions, prior to Kjeldahl digestion. (ii) Including mineral N in Kjeldahl N analysis, by reduction under acidic conditions prior to digestion and calculating organic N and N content by subtracting mineral N and N. The removal of mineral N was either incomplete‐ particularly regarding labeled mineral N, or partly destroyed organic N as well. When mineral N was included in the digest, the recovery of N and ¹⁵N was sufficiently accurate to obtain good results of organic N and ¹⁵N by subtracting the known quantities of mineral N, even when organic N was only 20% of the total Kjeldahl N. This procedure was used to determine the flush of Kjeldahl N in fumigated soils that were incubated with labeled mineral N and cellulose.     

Legacies of organic matter removal: decreased microbial biomass nitrogen and net N mineralization in New Zealand Pinus radiata plantations

The effect of variations in organic matter removal during harvesting on microbial biomass nitrogen was determined in four Pinus radiata plantations between 9 and 17 years after harvesting. Variation in microbial biomass nitrogen with season and the response of net nitrogen mineralization to organic matter removal after 9 and 17 years were also determined at two of the sites. The microbial biomass nitrogen in the fermentation-humus (FH) layer was correlated with litter fall characteristics and did not vary with organic matter removal, but the total mass of microbial biomass nitrogen in the FH layer was significantly reduced by increased organic matter removal. The microbial biomass nitrogen in the mineral soil was decreased by increased organic matter removal and was strongly correlated with moisture content and total nitrogen concentration. The FH layer microbial biomass nitrogen did not vary with season, but mineral soil microbial biomass nitrogen varied with season at one site, probably due to increased moisture availability. Net nitrogen mineralization in the mineral soil, determined by an anaerobic incubation, was decreased by increased organic matter removal and was strongly correlated with microbial biomass nitrogen. The persistence of the significant differences in microbial biomass nitrogen and net nitrogen mineralization indicated that variations in organic matter removal during harvesting have long-term effects on soil microbial properties and activity during the life of the subsequent rotation. This has implications for the selection of harvesting techniques to promote the maintenance of site productivity and to minimise disruption to the soil biota.     

Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N

Summary A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of ¹⁵N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of ¹⁵N-labelled microbial biomass. At a stage when all of the applied ¹⁵N was in organic forms, as biomass and products, the soil samples were analysed for biomass N by the chloroform (CHCl₃) fumigation-extraction method, which involves exposure of the soil to CHCl₃ vapour for 24 h followed by extraction with 500 mM K₂SO₄. A correction is made for inorganic and organic N in 500 mM K₂SO₄ extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized ¹⁵N extracted by 500 mM K₂SO₄ containing different amounts of CHCl₃. The extraction time varied from 0.5 to 4 h.The amount of N extracted ranged from 27 to 270 g g^–1, the minimum occurring at the lowest (67 g g^–1) and the maximum at the highest (333 g g^–1) N-addition rate. Extractability of biomass ¹⁵N ranged from 25% at the lowest N-addition rate to 65%a for the highest rate and increased consistently with an increase in the amount of ¹⁵N and glucose added. The amounts of both soil N and immobilized ¹⁵N extracted with 500 mM K₂SO₄ containing CHCl₃ increased with an increase in extraction time and in concentration of CHCl₃. The chloroform fumigation-extraction method gives low estimates for biomass N because some of the organic N in K₂SO₄ extracts of unfumigated soil is derived from biomass.     

Seasonal changes in the microbial biomass of Brazilian soils with different organic matter contents

Abstract

Carbon and nitrogen levels of microbial biomass were studied in four plots located in Rio de Janeiro State, Brazil. Two samplings were carried out, the first one, on November 1992 when rainfall was high which led the soil to high levels of moisture, and the second one on March 1993 when there was a decrease in rainfall coupled with high temperatures. Microbial carbon (MBC) and microbial nitrogen (MBN) assessments were done by the fumigation‐extraction method. The results showed significant differences for MBC and MBN between the sampling times and between different plots. Moreover, MBN showed differences as a function of sampling depth. In the 1993 sampling, developed under moisture conditions of soil which promoted the mineralization of organic matter, lesser values of MBC and MBN were found, whereas there was no difference in the organic carbon content. This fact shows a major sensitivity of biomass measurements to reflect changes which occur in the soil organic matter content. Significant correlations were obtained between MBC and organic carbon (r = 0.35, P < 0.01, n = 68), MBN and total N (r = 0.62; P < 0.07, n = 47), and MBC with NBM (r 0.74, P < 0.01, n = 54).     

Nitrogen compounds extracted by electroultrafiltration (EUF) or CaCl2 solution and their relationships to nitrogen mineralization in soils

The objective of the investigation was to identify the most important organic N-containing fractions extracted from soils by electroultrafiltration (EUF) or a CaCl₂ solution, respectively, and their importance for nitrogen mineralization. The investigation comprised 19 agricultural and one forest top soil. Net N mineralization was tested in Mitscherlich pot experiments with three treatments: (1) fallow soil without N fertilizer, (2) soil cultivated with rye grass without N fertilizer, (3) soil cultivated with rye grass with N fertilizer. The highest proportion of N in the extracts was the amino N fraction (amino acids + peptides) amounting to approximately 60% of the total N extracted by CaCl₂ and to about 40% of the total N extracted by EUF. The proportion of amino sugars from total N extracted was in average 10% for the CaCl₂ and 5.2% for the EUF extracts. The proportion of heterocyclic N bases derived from nucleic acids amounted in average to 4.8% and 3.6% for the CaCl₂ and EUF extract, respectively. Amino N (amino acids + peptides) were correlated best with net N mineralization (EUF, r = 0.81***, CaCl₂, r = 0.86***). The correlation between amino sugars and net N mineralization was r = 0.55* for the EUF extract and r = 0.49* for the CaCl₂ extract. The heterocyclic N bases did not correlate with net N mineralization. Correlations between Norg extracted by CaCl₂ versus net N mineralization were higher than those obtained by the EUF extract. Net N mineralization was about four times higher in the fallow soils than in the treatment with grass and no N fertilizer. In the treatment with grass + N fertilizer on average no net N mineralization occurred, moreover there was a tendency of N immobilization. It is assumend that in the treatments with grass cultivation, organic C released by roots stimulated the assimilation of mineral N and amino acids by soil microorganisms resulting in a low net N mineralization. Net N mineralization led to a highly significant depletion in the Norg pools and particularly in the amino N and amino sugar pools in the treatment with grass and without N fertilizer. This depletion was particularly evident in the CaCl₂ extracts. The results justify the conclusion that the Norg obtained with both extraction methods originates from a dynamic N pool into which N flows in and out. The amino N extractable with EUF or CaCl₂ is a reliable indicator for the net N mineralization potential of soils.     

Ecophysiology of the soil microbial biomass and its relation to the soil microbial N pool

Abstract. Microbial osmoregulation as a factor regulating the nitrogen and carbon contents of soil microbial biomass was studied in two experiments. In the first the percentages of the carbon and nitrogen occurring in the cytoplasm of Aspergillus flavus and Pseudomonas sp. were shown to be strongly influenced by osmotic stress. In the second, biomass carbon and nitrogen initially increased with increasing water stress (osmotic and matric) up to −1.0 and −1.5 MPa, respectively, but declined under greater osmotic stress. As the soil microbial carbon and nitrogen pools are affected by these stresses, allowance must be made for them when interpreting biomass measurements in water-stressed soils.