Maintaining long-term soil productivity requires development of cropping systems that provide maintenance or improvement in soil structure and an understanding of associated rhizosphere microbial populations. The objectives of this study were to determine the effects of several crops on soil biomass C and biomass N contents, their within-season variability, and the relationships between changes in soil biomass C, biomass N, and soil structure on a Brookston clay loam soil (fine-loamy, mixed, mesic Typic Argiaquoll). Soil microbial biomass C, biomass N, and soil-structure parameters (wet aggregate stability [WAS], organic carbon [OC], dry aggregate mean weight diameter [MWD], bulk density, and total and air-filled porosity) were measured during the third year of corn (Zea mays L.), soybean (Glycine max [L.], Merr.) alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), reed canarygrass (Phalaris arundinacea L.), orchardgrass (Dactylis glomerata L.), and no-crop (bare, covered, and shaded) plots at monthly intervals (June, July, August, and September). Reed canarygrass resulted in greater biomass-C contents than both the corn and soybean at all four sampling dates. Soil biomass C under alfalfa was significantly greater than under corn and soybean for both the first and last sampling dates. Forage species did not affect the soil biomass-N content. No consistent effects of the no-crop treatment on biomass C or biomass N were observed between sampling dates. Biomass C was significantly correlated with WAS, OC, and MWD for the cropping treatments. Wet aggregate stability was negatively correlated with soil water content for both the no-crop and cropping treatments, indicating that improvements in structure were not solely the result of the cropping treatments and associated rhizosphere populations. Reed canarygrass resulted in greater soil biomass C/N than the alfalfa, corn, and orchard grass plots, suggesting that fungal activity, and therefore soil structure, may be preferentially enhanced in the presence of reed canarygrass. This study demonstrated the influence of forage species and seasonal variability on concurrent changes in microbial biomass and soil structural properties.

Maintaining long-term soil productivity requires development of cropping systems that provide maintenance or improvement in soil structure an understanding of associated rhizosphere microbial populations. The objectives of this study were to determine the effects of several crops on soil biomass C and biomass N contents, their within-season variability, and the relationships between changes in soil biomass C, biomass N, and soils structure on a Brookston clay loam soil (fine-loamy, mixed, mesic Typic Argiaquoll). Soil microbial biomass C, biomass N, and soil-structure parameters (wet aggregate stability [WAS], organic carbon [OC], dry aggregate mean weight diameter [MWD], bulk density, and total and air-filled porosity) were measured during the third year of corn (Zea mays L.), soybean (Glycine max [L.], Merr.) alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), reed canarygrass (Phalaris arundinacea L.), orchardgrass (Dactylis glomerata L.), and no-crop (bare, covered, and shaded) plots at monthly intervals (June, July, August, and September). Reed canarygrass resulted in greater biomass-C contents than both the corn and soybean at all four sampling dates. Soil biomass C under alfalfa was significantly greater than under corn and soybean for both the first and last sampling dates. Forage species did not affect the soil biomass-N content. No consistent effects of the no-crop treatment on biomass C or biomass N were observed between sampling dates. Biomass C was significantly correlated with WAS, OC, and MWD for the cropping treatments. Wet aggregate stability was negatively correlated with soil water content for both the no-crop and cropping treatments, indicating that improvements in structure were not solely the result of the cropping treatments and associated rhizosphere populations. Reed canarygrass resulted in greater soil biomass C/N than the alfalfa, corn, and orchard grass plots, suggesting that fungal activity, and therefore soil structure, may be preferentially enhanced in the presence of reed canarygrass. This study demonstrated the influence of forage species and seasonal variability on concurrent changes in microbial biomass and soil structural properties.

An experiment was done to see if changes in total soil microbial biomass N and 15N-labelled biomass N could be measured by fumigation-extraction during the 5-20 day period following addition of 15N-labelled wheat straw to soil. Nitrogen-15 labelled wheat straw was added to a clay-loam soil at a rate equivalent to 76 micrograms N g(-1) soil, with or without an addition of 50 micrograms N g(-1) soil as unlabelled NH4NO3. Measurements were made 5, 12 and 20 days later. Non-amended soil was incubated similarly. Total soil microbial biomass N (biomass N) remained constant throughout the 20 day incubation in the unamended soil. Addition of straw alone increased biomass N from about 46 (in the unamended soil) to about 80 micrograms N g(-1) soil by day 5, and it remained at this level thereafter. Inorganic N addition caused a further increase in biomass N of about 20 micrograms N g(-1) soil by day 5, it then slowly declined until day 20. About 30% of the N originally in the straw was held in the biomass in the soil incubated with straw alone by day 5 and about 25% when N was added with the straw. These percentages hardly changed upon further incubations, and were equivalent to 23 and 19 micrograms 15N-labelled biomass N respectively, meaned over the three sampling times. Thus a relatively large addition of inorganic N with the straw (50 micrograms N g(-1) soil) only slightly decreased the consumption of straw-N by the biomass and did not prolong the complete consumption of the straw-N. It was concluded that fumigation-extraction could reliably measure changes in biomass N and 15N-labelled biomass N shortly (i.e. at 5 days) after addition of 15N-labelled straw to soil. Most of the inorganic N initially added with the straw could not be measured 20 days later. While this loss could have been due to denitrification, it seems unlikely under the aerobic incubation conditions used. Other factors may have operated.

The fumigation-extraction method was used to measure changes in soil microbial biomass in a field experiment following incorporation of 10 t ha-1 wheat straw, with and without 100 kg N ha-1 (as NH4NO3), into a silty clay loam soil in autumn 1987. The first measurements were 7 days after straw incorporation and then periodically for ca 1 yr. The amount of biomass (measured as biomass C, N and ninhydrin-reactive N) roughly doubled (from initial levels of ca 340 kg C, 76 kg N and 17 kg ninhydrin-N ha-1) within 7 days of straw incorporation, remained constant for the next 27 days and then slowly declined. The increase in biomass was similar when N was incorporated with the straw, although maximal amounts were a little greater (by ca 30 kg N ha-1) and attained a little later (14 days after incorporation). At the last sampling (363 days after incorporation) the biomass had declined to about half of its size shortly after straw addition; this was still ca 20% more than in the unamended soil. Initial soil inorganic N contents were small (< 10 kg N ha-1) so that the rapid increase in biomass in the soil amended with straw alone could not be explained by immobilization of inorganic N. Similarly, in the soil receiving straw and 100 kg ha-1 inorganic N, nearly all of this N was still present in inorganic form 7 days after straw incorporation, yet by this time the biomass had increased by ca 50 kg N ha-1. In both treatments, most of the newly synthesized biomass probably came from N already present in the straw. Although straw incorporation increased the size of the biomass, there were no significant differences in biomass C to N ratios, or in biomass C to ninhydrin-N ratios between treatments and at different times. The mean ratios were biomass C/N = 4.7; biomass C/ninhydrin-N = 21.4; biomass N/ninhydrin-N = 4.6. A feature of this work is the extraction of relatively large samples of soil for biomass measurements with minimal sample preparation; a procedure that gave reproducible results even with the very heterogeneous mixtures of soil and straw encountered.

The amounts of soil microbial biomass in metal-contaminated (high-metal) soils of the Woburn Market Garden Experiment in the U.K. are now about half those in similar uncontaminated (low-metal) soils from the same experiment. The metal-contamination was caused by applications of metal-rich sewage-sludge which ceased about 30 yr ago. Soil metal concentrations in the high-metal soils are now at, or a little above, current European Community limits. This work was designed to see if heavy metals decreased soil microbial biomass by decreasing the input of plant material to the soil or if the synthesis of microbial biomass is less efficient in the presence of heavy metals. Either or both of these mechanisms could explain the effects of heavy metals on microbial biomass in the Woburn experiment. Sunflower (Helianthus annus L., cultivar Sunbred 246) seedlings were grown for 31 days under controlled conditions (12 h day at 20 degrees C, 12 h night at 17 degrees C) in a low-metal or a high-metal soil from the Woburn experiment. From days 21 to 31 of growth the plants were supplied with 14C-labelled CO2 on alternate days. The final dry matter yield (shoots plus roots) of the plants grown on the low-metal soil was about 30% greater than that of the plants grown on the high-metal soil. The distribution of total C and 14C-labelled C between the various plant and soil compartments viz. plant shoots, roots, soil microbial biomass and bulk soil, were measured 2 days after the final 14(CO)2 labelling. The percentage distribution of 14C within shoots, roots, soil microbial biomass and bulk soil was quite similar in both soils. Plant-derived 14C-labelled organic C inputs into the high-metal soil were about 20% less than in the low-metal soil. About 35% less of this 14C-labelled C was in the microbial biomass in the high-metal soil than the low-metal soil at harvest. The plants caused an increase in total biomass C of about 22 and 42 micrograms C g-1 soil respectively in the high-metal and low-metal soil at harvest, about half of which was 14C-labelled in both cases. These increases in biomass were thus in the same ratio as those of the biomass in high-metal and low-metal soils taken directly from the field. These results suggest that both mechanisms (i.e. decreased inputs of C from plants to the soil and decreased efficiency of conversion of this C into new biomass C) operate in causing smaller biomasses in metal-contaminated soils of the Woburn experiment. The latter mechanism would appear more important than the former.

Metal-contaminated soils (produced by past long-term applications of contaminated sewage-sludge) from the Woburn Market Garden Field Experiment were previously shown to contain only about half the amounts of microbial biomass as other soils from the experiment which received farmyard manure during the same period. In some cases, the amounts of biomass in the metal-contaminated soils were even smaller than in other soils from the experiment which received inorganic fertilizer throughout. It is possible that the metals were causing decreased efficiency of substrate utilization by the microbial biomass, leading, in turn, to a smaller microbial population. This was investigated in a laboratory experiment by adding 14C-labelled glucose and 14C-labelled maize shoots (maize) separately to a metal-contaminated and a non-contaminated soil from the field experiment. Microbial biomass C, ninhydrin-N, soil ATP content and CO2 evolution were measured during the next 50 days following glucose addition and 100 days following maize addition in both soils. The biomass formed following addition of glucose or maize was consistently smaller in the metal-contaminated soil throughout the incubations. Overall, about 15-32% less glucose-derived and 25-60% less maize-derived biomass was formed in the metal-contaminated soil. In contrast, more CO2-C was evolved from the metal-contaminated soil than from the non-contaminated soil. This suggests that the biomass in the metal-contaminated soil was less efficient in the utilization of substrates for biomass synthesis. It is suggested that this may be a major reason for the smaller biomass in the metal-contaminated Woburn soils.

Amounts of microbial biomass were measured in soils from two different U.K. field experiments, one on a sandy loam (15% clay) at Luddington (Wick series) and the other on a silty loam soil (21% clay) at Lee Valley (Hamble series), where sewage sludges, mainly enriched with single metals, were applied 22 yr ago. No single metal (Zn, Cu, Ni and Cd) at or below current EC permitted total soil metal roncentrations, or limits, decreased the amounts of soil microbial biomass. However, Cu at about two and a half times permitted metal limits decreased the amounts of biomass by about 40% at both sites and caused an increased accumulation of organic C and total N of about 30% in the sandy loam and about 13% in the silty loam soil. Zinc, at about the same concentration, decreased the biomass by about 40% in the sandy loam and 30% in the silty loam soil while soil organic matter accumulation increased by only 9-14%. Cadmium, at about twice current EC limits did not affect the amount of biomass or soil organic matter in the silty loam-soil. Similarly, neither were affected by Ni at 2-3 times current metal limits. The amount of microbial biomass C as a percentage of total soil organic C was much lower (< 1.0%) in soils contaminated with Zn and Cu at about two-and-a-half times current permitted limits than in soils containing less metal. This also suggested that the metals were causing decreased microbial biomass at these metal concentrations.

The relationships between denitrification, soil microbial biomass and selected soil properties were investigated in 13 soils which varied in physical and chemical properties. It was hypothesized that estimates of biomass C may be directly related to the background (unamended) denitrification rates. Denitrification potential (with C and NO3(-) amendments) was also compared to biomass C and the indigenous soil properties. The background and potential denitrification rates were measured in these soils during 75 h of anaerobiosis. Background denitrification was highly correlated with biomass C, organic C content and moisture content at field capacity (-33 kPa). Soil organic C was also highly correlated with microbial biomass C. Nitrate was not limiting for most soils as in all but two soils the residual NO3(-) content following incubation exceeded 10 mg N kg-1. Hence, NO3(-) concentration was not strongly correlated with the background denitrification rate. The denitrification potential was 5 times greater than background denitrification. In our study, C was the factor limiting denitrification. Further, microbial biomass appeared to be a sensitive indicator of both soil C content and background denitrification. There was no significant relationship between any of the other soil chemical and physical properties measured and the denitrification potential.

Estimations of soil microbial biomass are now frequently made, because of the importance of soil organisms in nutrient cycling and their role as a source and sink of plant nutrients (Jenkinson, 1988: Smith and Paul, 1990). Various estimation procedures have been developed, but all appear to have their deficiencies under some conditions. Usually, microbial biomass is measured as soon as possible after soil sampling, in either "fresh" samples or those that have been "pre-incubated" for a few days (Jenkinson and Powlson. 1976); in some situations, however, preliminary storage of the soil is required. The effects of prolonged storage on the microbial biomass of a soil (a Fluvaquentic Eutrochrept) sampled under crops and pasture is here determined, and a comparison made of several procedures for estimating microbial C in the soil under pasture.