The effects of fumigation on organic C extractable by 0.5 M K2SO4 were examined in a contrasting range of soils. EC (the difference between organic C extracted by 0.5 M K2SO4 from fumigated and non-fumigated soil) was about 70% of FC (the flush of CO2-C caused by fumigation during a 10 day incubation), meaned for ten soils. There was a close relationship between microbial biomass C, measured by fumigation-incubation (from the relationship Biomass C = FC/0.45) and EC given by the equation: Biomass C = (2.64 ± 0.060) EC that accounted for 99.2% of the variance in the data. This relationship held over a wide range of soil pH (3.9–8.0).

ATP and microbial biomass N concentrations were measured in four of the soils. The (ATP)(EC) ratios were very similar in the four soils, suggesting that both ATP and the organic C rendered decomposable by CHCl3 came from the soil microbial biomass. The C:N ratio of the biomass in a strongly acid (pH 4.2) soil was greater (9.4) than in the three less-acid soils (mean C:N ratio 5.1).

We propose that the organic C rendered extractable to 0.5 m K2SO4 after a 24 h CHCl3-fumigation (EC) comes from the cells of the microbial biomass and can be used to estimate soil microbial biomass C in both neutral and acid soils.

An extraction method for measuring soil microbial biomass c

A new “direct extraction” method for measuring soil microbial biomass nitrogen (biomass N) is described. The new method (fumigation-extraction) is based on CHC13 fumigation, followed by immediate extraction with 0.5 M K2SO4 and measurement of total N released by CHC13 in the soil extracts. The amounts of NH4-N and total N extracted by K2SO4 immediately after fumigation increased with fumigation time up to 5 days. Total N released by CHC13 after 1 day fumigation (1 day CHC13-N) and after 5 days fumigation (5 day CHC13-N) were positively correlated with the flush of mineral N (FN) in 37 soils that had been fumigated, the fumigant removed and the soils incubated for 10 days (fumigation-incubation). The regression equations were 1 day CHC13-N = (0.79 ± 0.022) FN and 5 day CHC13-N = (1.01 ± 0.027) FN, both regressions accounting for 92% of the variance in the data. In field soils previously treated with 15N-labelled fertilizer, the amounts of labelled N, measured after fumigation-extraction, were very similar to the amounts of labelled N mineralized during fumigation-incubation; both were about 4 times as heavily labelled as the soil N as a whole. These results suggest that fumigation-extraction and fumigation-incubation both measure the same fraction of the soil organic N (probably the cytoplasmic component of the soil microbial biomass) and that measurement of the total N released by CHC13 fumigation for 24 h provides a rapid method for measuring biomass N.

Chloroform fumigation and the release of soil nitrogen: A rapid direct extraction method to measure microbial biomass nitrogen in soil

Priming effects are strong short-term changes in the turnover of soil organic matter caused by comparatively moderate treatments of the soil. In the course of priming effects large amounts of C, N and other nutrients can be released or immobilized in soil in a very short time. These effects have been measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of such phenomena. The aim of this overview is to reveal possible causes and processes leading to priming actions using the references on agricultural ecosystems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative real and apparent priming effects induced after the addition of different organic and mineral substances to the soil. Soil microbial biomass plays the key role in the processes leading to the real priming effects. The most important mechanisms for the real priming effects are the acceleration or retardation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and different uncontrolled losses of mineralized N from the soil are responsible for the apparent N priming effects. Other multiple mechanisms (predation, competition for nutrients between roots and microorganisms, preferred uptake, inhibition, etc.) in response to addition of different substances are also discussed. These mechanisms can be distinguished from each other by the simultaneous monitoring of C and N release dynamics; its comparison with the course of microbial activity; and by the labelling of different pools with 14 C or 13 C and 15 N. Quantitative methods for describing priming effects and their dynamics using 14 C and 15 N isotopes, as well as for non-isotopic studies are proposed.

Review of mechanisms and quantification of priming effects.

Measurement of soil microbial biomass C by fumigation-extraction—an automated procedure

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.

https://www.cfc.umt.edu/research/biogeochemistry/files/publications/BGC_85_232%20copy.pdf

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

Immobilization of N was measured in a fumigated and in an unfumigated soil by adding (15NH4)2SO4 and following the disappearance of inorganic label from the soil solution and its simultaneous conversion to soil organic N. Calculations based on the measurement of organically-bound 15N gave more consistent values for immobilization than did calculations based on the measurement of the disappearance of label from solution. The fumigated soil immobilized 6.6 μg N g−1 N g−1 soil in 10 days at 25°C, the unfumigated control 4.8 μg. The corresponding gross mineralization rates were 34.9 and 5.6 μg N g−1 soil in 10 days.

Addition of 58 μg N as (15NH4)2SO4 to the fumigated soil increased the quantity of the ynlabelled NH4-N extracted at the end of 10 days from 33.8 to 37.8 μg Ng−1 soil, i.e. there was a positive Added Nitrogen Interaction (ANI). The added labelled N produced this ANI, not by increasing the rate of mineralization of organic N, but by standing proxy for unlabelled N that otherwise would have been immobilized.

A procedure for calculating biomass N from the size of the flush of mineral N caused by fumigation is proposed. Biomass N (BN) is calculated from the relationship BN = F'N/0.68 where F'N is [(N in fumigated soil incubated for 10 days — (N in unfumigated soil incubated for 10 days)].

Mineralization and immobilization of nitrogen in fumigated soil and the measurement of microbial biomass nitrogen

There was a small decrease in grain yield (from 10.8 to 10.5 t ha−1) when wheat straw (3 t dry matter ha−1) was incorporated into a silty clay loam soil sown to winter wheat. In the absence of straw, 60% of autumn-applied 15N-labelled nitrate was lost from the crop:soil system. Straw incorporation decreased this loss to 47%. There was little overall effect on the uptake of N by the crop, presumably because straw immobilised inorganic N that would otherwise have been leached from the soil during winter. Only 12% of the N in 15N-labelled straw was recovered by the crop; 78% still remained in the soil one year after incorporation.

The effect of straw incorporation on the uptake of nitrogen by winter wheat.

A complete procedure for analysing soil and plant samples for total N and atom % excess15N is described. The salicylic acid version of the Kjeldahl method for measuring total N was modified for use in a digestion block, giving quantitative reduction of nitrate in both soil and plant material. Procedures for minimising cross-contamination between samples are specified, including a double-distillation procedure that eliminates ‘memory effects’ when distilling NH3 from Kjeldahl digests. A simple and robust apparatus for converting (NH4)2SO4 to N2 gas for mass spectrometric determination of atom % excess15N is described. The coefficient of variation for replicate measurements of total N in soil and plant material over the range 0.1–2.2% N was 1.0%. The coefficient of variation for measurements of15N in plant material over the range 0.4–2.9 atom % excess15N was 0.2%.

The measurement of15N in soil and plant material

A new method is described for reducing nitrate prior to Kjeldahl digestion of soil and crop samples. Zinc is used to reduce CrIII to CrII, which in turn reduces NO3—N. Zinc powder is mixed with the sample, a solution of CrK(SO4)2 in H2SO4 added and the mixture allowed to stand at room temperature for 2h. The usual Kjeldahl reagents (K2SO4, CuSO4 and H2SO4) are then added and the digestion completed in a digestion block at 360°C. Nitrate is quantitatively converted to NH4—N, whether on its own, in plant material or in soil.

G. Pruden

Papers

Chloroform fumigation and the release of soil nitrogen: A rapid direct extraction method to measure microbial biomass nitrogen in soil

Mineralization and immobilization of nitrogen in fumigated soil and the measurement of microbial biomass nitrogen

The effect of straw incorporation on the uptake of nitrogen by winter wheat.

The measurement of15N in soil and plant material

Reduction of nitrate prior to kjeldahl digestion