The relationships between soil texture and the proportions of soil organic C and N present in the microbial biomass, the amounts of C and N mineralized per unit of microbial biomass and the C:N ratio of the microbial biomass in Dutch grassland soils were investigated. The proportions of both soil C and N in the microbial biomass were higher in fine-textured soils than in coarse-textured soils. The ratios between C mineralization and microbial biomass C (activity of the microbial biomass) and between N mineralization and microbial biomass C were both negatively correlated with the percentage of soil organic C in the microbial biomass. The activity of the biomass was twice as large in an average sandy or loam soil than in an average clay. While the activity of the microbial biomass was the same in an average sandy and loam soil, the amount of N mineralized per amount of microbial biomass was larger in the sandy soils. This was associated with a higher C:N ratio of the microbial biomass in the sandy soils (average 8) than in the loams (average 5). The amount of N mineralized per amount of microbial biomass was lowest in the clays. This was associated with the lower activity of the microbial biomass and its relatively low C:N ratio (average 6). The observed differences in N mineralization between soil types could be calculated well with a simple food web model using the observed C:N ratios of the microbial biomass.

The microbial immobilization and subsequent transformation of SO4(2-)-S were monitored in six soils following amendment with 35S-labelled SO4(2-)-S (20/micrograms S g-1 soil) with or without glucose (2000 micrograms C g-1 soil). In the soils receiving glucose, 21-34% of the added SO4(2-)-35S was immobilized within 3 days whereas without addition of glucose, only 0.5-13% was immobilized. Similar proportions of the added SO4(2-)-35S were converted into microbial biomass (biomass-35S) over the same time. Over 127 days, both amendments (with or without glucose) had little effect on soil SO4(2-)-S and extractable-S. There was no measurable effect of the glucose-free amendment on soil biomass-S. However, in soils receiving glucose, total biomass-S increased markedly over the first 3 days (11-24 micrograms S g-1 soil), whereas in these soils, the amounts of labelled biomass-S formed over the same period (3.3-8.7 micrograms S g-1 soil) were much lower. The additional increase in total biomass-S was due to a priming effect of the glucose amendment on the decomposition of soil organic-S. Between 3 and 10 days, total biomass-S in soils receiving glucose declined to concentrations similar to those in soils receiving the glucose-free amendment. The S (both labelled and unlabelled) lost from this decrease in total biomass-S was converted into soil organic-S since it was undetectable in the SO4(2-)-S pool. The data suggest that the turnover of microbial biomass is the primary process by which S is transformed into soil organic matter.

Values for microbial biomass C and N, as assayed by a fumigation-incubation and a fumigation-extraction method were compared for a range of acid soils. Values for microbial biomass C obtained from CO2-evolution data using the fumigation-incubation method, were far less than those calculated from ninhydrin-reactive N obtained by the fumigation-extraction method, and in some cases were negative. For a given soil, discrepancies between the two methods of assay increased with soil acidity (pH range, 4-6), and were attributed to an over-correction in the fumigation-incubation assay for CO2 values of the unfumigated control soil. The extent of overcorrection seemed to be related to the failure of the fumigated, re-inoculated soils to convert organic C to CO2 at the same rates as in the unfumigated control soils, but not to the amounts per se of readily-decomposable substrate in the soils. By contrast, the amounts of NH4+ N released during oxidative decomposition of killed microbial cells in the fumigation-incubation method, compared closely with the amounts of NH4+ N plus amino acid N derived by autolysis and deamination in the fumigation-extraction method. The amounts of nitrogenous products released by the two methods, and hence calculated values for biomass N, were not significantly different. Biomass N (fumigation-incubation) = 1.05 X Biomass N (fumigation-extraction). The performance of the fumigation-extraction assay for biomass C was further evaluated by measuring biomass C in each of the acid soils, as soil pastes in water, and in buffers with pHs ranging between 4-6. The magnitude of the ninhydrin-reactive N flush from chloroform-fumigated soils was increased by soil water content and was affected by soil pH. For soil pastes at 100% water holding capacity, there was a linear relationship between estimated values of biomass C and assay pH. This relationship determined the extent of adjustment of assayed biomass C values to correct for the effects of different soil pHs arising from specific agronomic practices. Biomass C Adjustment Factor = e0.301(delta soil pH) Thus, a one unit increase in pH, over the pH range 4-6, gave a 35% increase in the estimated biomass C value. The effects of soil pH and water content on biomass values demonstrates the need for closer standardization of assay indicators, preferably with a wider pH range of soils extending to those of neutral to alkaline pH.

The measurement of labile N as soil exchangeable, soil solution and soil microbial biomass pools over time in the 0-15 cm puddled layer in intensively cultivated irrigated rice indicated that inorganic N was immobilized within the soil microbial biomass until flowering. This biomass pool size was constant after flowering, indicating that any subsequent crop N uptake was derived from fertilizer top dressing or mineralization of soil organic matter. Such measurements provide great insight into the mechanisms of N supply in the system. Given that the impact of fish during the crop cycle may be to charge both the quantities and characteristics of N supply, continuous monitoring of the N content in the crop for each management practice is needed. Non-destructive plant sampling to measure nitrogen concentration of leaves should be used. This can be done using a chlorophyll meter, correcting values for the specific leaf weight. Specific experiments to establish a mechanistic understanding of the seasonal N supply should be linked with studies of long impact of rice-fish management. It is important to evaluate the performance of the system through measurements such as total factor productivity as well as specific components such as soil fertility. A key component determining the long-term N fertility of soils is soil organic matter. Changes in the quantity and nature of soil organic matter will inevitably affect soil fertility. A large proportion of total soil carbon is relatively inert and thus its measurement is a poor indicator of soil fertility. Much research has identified the soil microbial biomass as a labile organic matter pool that is sensitive to impacts due to management practice. When the soil system is at a steady state, soil microbial biomass will form a constant proportion of total carbon (C). Where soil organic matter is accumulating or declining, this relationship with total C breaks down, however the relationship between biomass and labile C will remain constant

The influence of heavy metal contamination on the efficiency of conversion of fresh substrates into new microbial biomass in a pasture soil was examined. Three soils covering a range of chromium, copper and arsenic concentrations, and an uncontaminated control soil, were amended with [U-14C]glucose and incubated for 28 days. During incubation, microbial biomass C was determined using the fumigation-extraction technique. The amounts of 14CO2 evolved during incubation were monitored, and residual 14C concentrations were determined. Throughout the incubation, the microbial biomass-14C formed following addition of glucose was consistently lower in the metal-contaminated soils than in the uncontaminated control soil. Soils differed significantly in their rates of 14CO2 evolution. More glucose-derived 14CO2 was evolved from contaminated soil than from the uncontaminated control. The ratio of both (total respired C):(total biomass-C) and (respired 14CO2):(biomass-14C) was greater in the contaminated soils than in the uncontaminated soil. The results suggest that the microbial biomass in soils contaminated with heavy metals are less efficient in the utilization of substrates for biomass synthesis and need to expend more energy for maintenance requirements.

Soil temperature, water potential, and substrate (C) availability are the primary constraints on microbial activity within soil and display substantial seasonal variation within northern deciduous forests. Following autumn litterfall, soil C availability is relatively high, and N should be assimilated by soil microorganisms to maintain or form new biomass. Conversely, N should be mineralized from microbial biomass during midsummer when C availability is relatively low and soil temperatures are high. Because N availability is directly controlled by microbial activity, we hypothesized that microbial biomass and net N mineralization are inversely related on a seasonal basis. To test this hypothesis, we studied the temporal relationship between microbial biomass (C and N) and rates of net N mineralization in two different northern hardwood ecosystems, one dominated by sugar maple (Acer saccharum Marsh.) and basswood (Tilia americana L.) and the other by sugar maple and red oak (Quercus rubra L.). In situ buried bags were used to estimate net N mineralization and nitrification at monthly intervals for 1 yr. Microbial C and N contents of the incubated soil were determined using the chloroform fumigation-incubation method. Net N mineralization displayed marked seasonal variability, ranging from 35 to 115 mg N m⁻² d⁻¹ during the growing season. In contrast, microbial biomass C and N were relatively constant throughout the year, averaging 112 g C m⁻² and 17 g N m⁻². Neither microbial biomass (C or N) nor the change in microbial biomass between sampling dates were significantly inversely correlated with mean daily rates of net N mineralization. As such, our data do not support the idea that N availability is controlled by large seasonal fluctuations in soil microbial biomass. Rather, our results suggest that N availability is primarily controlled by changes in the turnover rate of microbial biomass such that a relatively constant pool is maintained through time. In addition, mean annual rates of net N mineralization and nitrification did not differ significantly from those previously measured by others in the same stands, suggesting that annual rates may be relatively consistent in climatically similar years.

Soil temperature, water potential, and substrate (C) availability are the primary constraints on microbial activity within soil and display substantial seasonal variation within northern deciduous forests. Following autumn litterfall, soil C availability is relatively high, and N should be assimilated by soil microorganisms to maintain or form new biomass. Conversely, N should be mineralized from microbial biomass during midsummer when C availability is relatively low and soil temperatures are high. Because N availability is directly controlled by microbial activity, we hypothesized that microbial biomass and net N mineralization are inversely related on a seasonal basis. To test this hypothesis, we studied the temporal relationship between microbial biomass (C and N) and rates of net N mineralization in two different northern hardwood ecosystems, one dominated by sugar maple (Acer saccharum Marsh.) and basswood (Tilia americana L.) and the other by sugar maple and red oak (Quercus rubra L.). In situ buried bags were used to estimate net N mineralization and nitrification at monthly intervals for 1 yr. Microbial C and N contents of the incubated soil were determined using the chloroform fumigation-incubation method. Net N mineralization displayed marked seasonal variability, ranging from 35 to 115 mg N m(-2) d(-2) during the growing season. In contrast, microbial biomass C and N were relatively constant throughout the year, averaging 112 g C m(-2) and 17 g N m(-2). Neither microbial biomass (C or N) nor the change in microbial biomass between sampling dates were significantly inversely correlated with mean daily rates of net N mineralization. As such, our data do not support the idea that N availability is controlled by large seasonal fluctuations in soil microbial biomass. Rather, our results suggest that N availability is primarily controlled by changes in the turnover rate of microbial biomass such that a relatively constant pool is maintained through time.

Annual C inputs from plant production in terrestrial ecosystems only meet the maintenance energy requirements of soil microorganisms, allowing for little or no net annual increase in their biomass. Because microbial growth within soil is limited by C availability, we reasoned that plant production should, in part, control the biomass of soil microorganisms. We also reasoned that soil texture should further modify the influence of plant production on soil C availability because fine—textured soils typically support more microbial biomass than coarse—textured soils. To test these ideas, we quantified the relationship between aboveground net primary production (ANPP) and soil microbial biomass in late—successional ecosystems distributed along a continent—wide gradient in North America. We also measured labile pools of C and N within the soil because they represent potential substrate for microbial activity. Ecosystems ranged from a Douglas—fir forest in the western United States to the grasslands of the mid—continent to the hardwood forest in the eastern U.S. Estimates of ANPP obtained from the literature ranged from 82 to 1460 g°m— ²°yr— ¹. Microbial biomass C and N were estimated by the fumigation—incubation technique. Labile soil pools of C and N and first—order rate constants for microbial respiration and net N mineralization were estimated using a long—term (32 wk) laboratory incubation. Regression analyses were used to relate ANPP and soil texture with microbial biomass and labile soil C and N pools. Microbial biomass carbon ranged from 2 g/m² in the desert grassland to 134 g/m² in the tallgrass prairie; microbial N displayed a similar trend among ecosystems. Labile C pools, derived from a first—order rate equation, ranged from 115 g/m² in the desert grassland to 491 g/m² in the southern hardwood forest. First—order rate constants for microbial respiration (k) fell within a narrow range of values (0.180 to 0.357 wk— ¹), suggesting that labile C pools were chemically similar among this diverse set of ecosystems. Potential net N mineralization rates over the 32—wk incubation were linear in most ecosystems with first—order responses only in the alpine tundra, tallgrass prairie, and forests. Microbial biomass C displayed a positive, linear relationship with ANPP (r² = 0.51), but was not significantly related to soil texture. Labile C also was linearly related to ANPP (r² = 0.32) and to soil texture (r² = 0.33). Results indicate that microbial biomass and labile organic matter pools change predictably across broad gradients of ANPP, supporting the idea that microbial growth in soil is constrained by C availability.

The fumigation-extraction (FE) method, applied after extraction of inorganic N and sieving was used to measure soil microbial biomass C, N and carbohydrate C contents at a depth of 0-10 cm under wheat every week in a field study lasting from October 1990 to November 1991. A minimum of 180 micrograms microbial biomass C g-1 soil and a maximum of 363 micrograms g-1 were recorded, the corresponding values for biomass N being 35 micrograms g-1 soil (= 53 kg ha-1) and 66 micrograms g-1 soil (= 99 kg ha-1). An increase was observed from December 1990 to November 1992, with minor fluctuations during periods with strong variations in weather conditions: cold and very wet in winter, extremely dry in summer. All three biomass components measured in the 0.5 M K2SO4 extracts of the FE procedure were significantly correlated. The intercepts of the linear relationships between microbial biomass C, N and carbohydrate C were non-significant, indicating constant component ratios. The amount of carbon additionally made extractable by fumigation consisted of 17% carbohydrate C. The biomass C:N ratio fluctuated closely around a mean value of 5.5.

Im Feldversuch in Timmerlah mit unterschiedlichen mechanischen Bodenbelastungsvarianten konnte gezeigt werden, dass Bodenverdichtungen auf Mikroorganismen und mikrobiologische Prozesse wirken. Im einzelnen wurde mit zunehmender Bodendichte, verursacht durch mechanische Bodenbelastungen, festgestellt: Zunahme der N2 und N2O-Verluste, Zunahme des Erhaltungsbedarfes der mikrobiellen Biomasse, Abnahme der Gehalte an mikrobieller Biomasse, Abnahme der Gehalte an organischer Substanz. Aber die Auspraegung dieser Effekte im Feldversuch waren nur relativ gering aufgrund: hoher Trockenheit (wenige O2-Mangelsituationen), gleichzeitigem verdichtungsbedingten Rueckgang der Fressfeinde (Collembolen) und erhoehter Besiedlungsdichte (Substratverfuegbarkeit).