Soil Microbial Biomass Activity Research Articles

Multiple Functions of Microbes to Improve Food and Soil Nutrition in Support of Environment Management to Maintain Global Agricultural Practices

Plant-microbe interactions are interesting phenomena Soil microbial biomass is considered the living part of the soil organic matter, which acts as an agent of transformation of organic matter and is a source of available nutrients. Activities of soil microbes can change the soil environment rapidly and influence the rate of nutrient cycling. Soil microbes also play a significant role in the formation of macro aggregates. The potential of soil microorganisms has been recognized widely for the improvement of soil quality, soil formation, aggregation, and revegetation. Soil microbes are involved in various ecosystem processes, such as litter decomposition, phosphate solubilization, nitrogen fixation, oxidation of various inorganic components of soil or mineralization of inorganic components, and mycorrhizal symbiosis. Implications of plants and their symbionts like mycorrhizal fungi, nitrogen-fixing bacteria, and free-living rhizosphere populations of bacteria promote plant establishment and growth. Therefore, active soil microbial biomass is an essential component for long-term soil fertility. The application of microbial technology which we can refer to as green technology has become an integral element of forestry as well as agriculture and has substantially contributed to forest and agricultural productivity and ensuring the ecosystem’s sustainability. This article reviews some of the important aspects of microbial applications in agriculture and forestry and discusses different facets of microbial functions, their interaction with plants and soil, and their role in the sustainability of natural resources, forests, and agroecosystems.

Phosphate fertilization and bioactivator influences on fractions of organic matter and soil microbial biomass

Soil organic matter is considered a potential source of phosphorus (P) for plants because of biological cycling. Bioactivators are characterized as tools that can help soil microbial activity and the availability of P demanded by plants. The objective of this study was to evaluate the influence of the use of bioactivators and phosphate fertilization on the activity of soil microbial biomass and humified fractions of organic matter. The experimental design was completely randomized in a factorial scheme (6 × 2), with the first factor consisting of six doses of phosphorus applied to the soil (0, 30, 60, 90, 120, and 150 kg ha-1 of P2O5), using triple superphosphate as the source, and the second factor being the presence or absence of soil and plant bioactivator application, with four replications. The evaluated characteristics were soil microbial biomass carbon (SMBC), soil basal respiration (SBR), microbial biomass nitrogen (MBN), metabolic quotient (qCO2), microbial quotient (qmic), total organic carbon (TOC), fulvic acid fraction carbon (FAC), humic acid fraction carbon (HAC), and humin fraction carbon (HUMC). The use of soil and plant bioactivators and mineral phosphate fertilizer promoted changes in soil microbial activity. Increasing doses of phosphate fertilization promoted a reduction in the carbon characteristics of microbial biomass and basal soil respiration in the presence or absence of soil and plant bioactivators. Moreover, the use of bioactivator promoted higher averages for soil microbial biomass carbon, soil basal respiration, metabolic quotient, humic acid and humin and reduced the levels of microbial quotient, total organic carbon and fulvic acid.

Does grass-legume intercropping change soil quality and grain yield in integrated crop-livestock systems?

In sustainable tropical agriculture, grass-legume intercropping can increase productivity, enhance the nutritional quality of the crop and affect soil microbiological activity, leading to higher yields in succession crops. This study evaluated the impact of grass-legume intercropping, during the pasture phase of integrated crop-livestock systems under no-tillage (ICLS–NT), on soil microbiota, pasture quality, and soybean productivity in the Brazilian Cerrado. We evaluated the effect of intercropping on total soil contents of organic carbon and nitrogen, soil microbial activity, animal and soybean productivity (2017 and 2018 crop seasons), and pasture production and quality. The experiment consisted of twelve treatments under grass-legume intercropping in the pasture phase of ICLS–NT. The grasses Urochloa ruziziensis ‘Kennedy’, U. brizantha ‘Paiaguás’, Panicum maximum ‘Tamani’ and U. brizantha ‘Piatã’ were grown in monoculture and intercropped with cowpea (Vigna unguiculata) ‘Tumucumaque’ or pigeon pea (Cajanus cajan) ‘Mandarin’. A randomized block design, with three repetitions, was used in a split-plot arrangement, considering the grasses as plots and the legumes as subplots. The results showed that ICLS–NT improved the activity of soil microbial biomass when compared to monoculture systems. Grass-legume intercropping during the pasture phase, mainly with cowpea (Vigna unguiculata), increased soybean productivity, and grass dry matter. Total organic carbon (TOC), carbon management index (CMI%), microbial biomass carbon (MBC), soil basal respiration (BR), and metabolic quotient (Qmic) are the most sensitive markers when it comes to the effects of intercropping on soil microbiological activity and soybean productivity in ICLS–NT. Identifying the best grass-legume combinations allows the establishment of more productive crop-livestock integration systems under no-tillage better adapted to the bio-edaphoclimatic conditions of the Brazilian Cerrado.

The Effect of Mineral and Organic Fertilization on Common Osier (Salix viminalis L.) Productivity and Qualitative Parameters of Naturally Acidic Retisol

One of the potential options for sewage sludge as an alternative organic material is the fertilization of energy crops. To evaluate the effect of granulated sewage sludge and mineral fertilization N60P60K60 on common osier’s (Salix viminalis L.) biomass productivity and soil parameters, field trials were held in Western Lithuania’s naturally acidic Retisol (WB 2014; pHKCl 4.35–4.58). After four years of cultivation and dependent on fertilization type, common osier dry matter (DM) yield varied from 49.60 to 77.92 t ha−1. Higher DM yield was related to an increased number of stems/plants. The application of a 90 t ha−1 sewage sludge rate had a significant and positive impact on common osier productivity, as well as on the increment of soil organic carbon, total N, and mobile P2O5 content in the upper 0–30 cm soil layer. The use of both sewage sludge rates (45 and 90 t ha−1) had a similar impact on soil bulk density, water-stable aggregates, and the active soil microbial biomass. Annual mineral fertilization had little effect on the parameters studied. When growing common osier in Retisol, 45 t ha−1 of a single sewage sludge rate was enough to maintain both plant and soil productivity.

Altitude and Vegetation Affect Soil Organic Carbon, Basal Respiration and Microbial Biomass in Apennine Forest Soils

Both altitude and vegetation are known to affect the amount and quality of soil organic matter (SOM) and the size and activity of soil microbial biomass. However, when altitude and vegetation changes are combined, it is still unclear which one has a greater effect on soil chemical and biochemical properties. With the aim of clarifying this, we tested the effect of altitude (and hence temperature) and vegetation (broadleaf vs pine forests) on soil organic carbon (SOC) and soil microbial biomass and its activity. Soil sampling was carried out in two adjacent toposequences ranging from 500 to 1000 m a.s.l. on a calcareous massif in central Italy: one covered only by Pinus nigra J.F. Arnold forests, while the other covered by Quercus pubescens Willd., Ostrya carpinifolia Scop. and Fagus sylvatica L. forests, at 500, 700 and 1000 m a.s.l., respectively. The content of SOC and water-extractable organic carbon (WEOC) increased with altitude for the pine forests, while for the broadleaf forests no trend along the slope occurred, and the highest SOC and WEOC contents were observed in the soil at 700 m under the Ostrya carpinifolia forest. With regard to the soil microbial community, although the size of the soil microbial biomass (Cmic) generally followed the SOC contents along the slope, both broadleaf and pine forest soils showed similar diminishing trends with altitude of soil respiration (ΣCO2-C), and ΣCO2-C:WEOC and ΣCO2-C:Cmic ratios. The results pointed out that, although under the pine forests’ altitude was effective in affecting WEOC and SOC contents, in the soils along the broadleaf forest toposequence this effect was absent, indicating a greater impact of vegetation than temperature on SOC amount and pool distribution. Conversely, the similar trend with altitude of the microbial activity indexes would indicate temperature to be crucial for the activity of the soil microbial community.

A calorimetric assessment of the effects of sodium chloride and sodium bromide on the microbiological and thermokinetic characteristics of soil

Excess salinity may lead to degradation of arable land and exclusion from agricultural production by worsening of their properties. The evaluation of the characteristics of saline soils is very important from the point of view of the use of agricultural soil. The aim of this study was to determine changes in the physicochemical properties of soil, the content and activity of soil microbial biomass (SMB) and the thermokinetics of glucose biodegradation in soil samples containing different doses of sodium chloride and sodium bromide (0.0263; 0.0526; 0.1052; 0.1578; 0.2104; 0.2630mmolg−1 DM soil), and to compare the effects of both salts. The pH values of soil samples decreased (from 6.1 to 5.4 for NaCl and NaBr) and electrical conductivity increased (from 0.20dSm−1 to 2.61dSm−1for NaCl and 2.30dSm−1 for NaBr) with increasing doses of both salts. SMB content, determined based on substrate-induced respiration (SIR), decreased with increasing doses of NaCl and NaBr (from 737mg C to 348mg C for NaCl and 379mgCkg−1 DM soil for NaBr). Both salts inhibited glucose biodegradation processes in soil proportionally to their increasing doses (NaBr exerted a greater inhibitory effect). This was confirmed by lower values of the maximum rate of heat production (RHPmax) and the apparent growth rate constant (k), and by higher values of peak time (PT), generation time (tG) and the inhibitory ratio (I). Changes in pH, SIR-SMB and Qtmax (total heat production) were modeled based on the applied doses of NaCl and NaBr. The empirical data were well approximated by the proposed mathematical models representing relationships between soil pH, SIR-SMB and Qtmax vs. the applied doses of NaCl and NaBr. They can be used to reliably predict the effects of the analyzed salts on the tested soil parameters.

Activity of soil microbial biomass altered by land use in the southwestern Amazon

ABSTRACT The increasing demand for food creates environmental problems, mainly due to the removal of native vegetation cover for agriculture expansion in Brazil. These changes in land use lead to changes in the soil organic matter dynamics. Microorganisms represent the most biological and physiological diversity in soil, as well as are responsible for more than 95% of the decomposition and nutrient cycling processes. The objective in this research was to check if there is difference of patterns in activity of soil microbial biomass under varied natural vegetation, pastures in use and agricultural systems recently established. The area covered by this study corresponds to the states of Rondônia and Mato Grosso. Canonical variate analysis was used in physical, chemical and microbiological factors in each ecoregion and land use, looking for patterns and variables that can differentiate them. The native areas showed distinct patterns in the dynamics of microbiological attributes mainly related to the amount of litter in each biome studied. For the disturbed areas, there were similar results between pastures and native areas, significantly different from the results obtained in agricultural areas, which, due to differences in management and kind of cultures analyzed, showed a great variability in the final result. The results support the recommendation for use of microbiological attributes as indicators of land use change, combined with chemical and physical factors of the soil.

Glucose promotes the reduction of hexavalent chromium in soil

In this study we evaluated the dynamics of Cr(VI) reduction in contaminated soil amended or not with glucose together with investigation on its effects on soil microbial biomass size and activity. The influence of native or glucose-induced soil microbial biomass on chromate reduction was assessed. During the incubation period, the concentration of Cr(VI) in soil decreased over time whether glucose was added to soil or not, but with different reduction rates. Soil displayed a natural capacity towards chromate reduction, but the addition of glucose increased the reduction rate by both abiotic and biotic mechanisms. Cr(VI) negatively affected both the size and activity of soil microbial biomass. However, our data suggest that glucose is likely to have exerted an indirect role in the increased rate of Cr(VI) reduction, by promoting growth of indigenous microbial biomass.

SOIL BIOLOGICAL AND BIOCHEMICAL QUALITY OF WHEAT-MAIZE CROPPING SYSTEM IN LONG-TERM FERTILIZER EXPERIMENTS

SUMMARYTwo long-term field experiments, each consisting of three treatments (organic compost treatment, chemical fertilizer treatment and an untreated control) were established in 1993 and 1997, respectively. Soil samples were collected from each plot in June 2004 and 2005 after crop harvest and were used to determine soil physical-chemical properties, biological and biochemical activity, and the nematode community. Soil physicochemical parameters, microbial biomass, biological activities and nematode communities were significantly influenced by long-term application of organic compost. In general, soil total organic carbon, dissolved organic carbon, total nitrogen, alkaline-hydrolysable nitrogen, available phosphorus, and available potassium, microbial biomass, basal respiration, urease activities, total number of nematodes and bacterial-feeding nematodes were significantly higher in the compost plots than in the chemical fertilizer and control plots at two experimental sites and two sampling dates. Soil bulk density and pH values were significantly lower in the compost plots. We conclude that soil physical-chemical properties, size and activity of soil microbial biomass, metabolic quotient (qCO2), urease activity, total number of nematodes and bacteria-feeding nematodes could be used as indicators of soil quality.

The Influence of Selected Meteorological Factors on Microbial Biomass and Mineralization of Two Organic Fertilizers

A mesocosm study was conducted in order to evaluate the effects of short-term rainfall and temperature variation on soil microbial biomass and bacteria to fungi ratio. In addition, the relation between the decomposition process of two organic fertilizers, cattle manure and barley straw, and the activity of soil microbial biomass was also studied. In order to assess the effect of biological activity on soil fertility the dynamics of soil pH, N-NO3-, N-NH4+, Corg and Nt during plant growing season was measured. The results suggest that short-term variation of climate had a significant effect on microbial biomass with dry periods distinguished by a reduced microbial biomass compared to wet periods. The ratio bacteria to fungi seems also to be sensitive to variations in rainfall and temperature regime, however further studies are required to draw a definitive conclusion. Regarding the type of fertilizer used, the straw treatments showed higher microbial biomass than the manure treatments, but higher decomposition rate was observed in manure fertilized soil. The effect of soil biological activity on soil pH was limited for both manure and straw treatments while the changes of the soil nitrate amounts are related to the microbial biomass. The study indicates that nitrate immobilization and mineralization processes are influenced by meteorological conditions and microbial biomass dynamics. In contrast, soil organic carbon and total nitrogen did not seem to be affected by variations in temperature, rainfall and microbial activity.

Soil microbial biomass in organic farming system

Agricultural production systems have to combine management practices in order to sustain soil's profitability and quality. Organic farming is gaining worldwide acceptance and has been expanding at an annual rate of 20% in the last decade, accounting for over 24 million hectares worldwide. Organic practices avoid applications of synthetic fertilizers and pesticides, rely on organic inputs and recycling for nutrient supply, and emphasize cropping system design and biological processes for pest management, as defined by organic farming regulation in the world. In comparison with conventional farming, organic farming has potential benefits in improving food quality and safety. Plant production in organic farming mainly depends on nutrient release as a function of mineralization processes in soils. The build-up of a large and active soil microbial biomass is important pool of accessible nutrients, therefore, is an important priority in organic farming. In organic farming, there is positive effect of quantity and quality of inputs of organic residues on soil microbial biomass. In this way, the organic systems are extremely important for the increase of the soil fertility and the maintenance of the environmental sustainability.

Soil microbial biomass in organic farming system

Agricultural production systems have to combine management practices in order to sustain soil's profitability and quality. Organic farming is gaining worldwide acceptance and has been expanding at an annual rate of 20% in the last decade, accounting for over 24 million hectares worldwide. Organic practices avoid applications of synthetic fertilizers and pesticides, rely on organic inputs and recycling for nutrient supply, and emphasize cropping system design and biological processes for pest management, as defined by organic farming regulation in the world. In comparison with conventional farming, organic farming has potential benefits in improving food quality and safety. Plant production in organic farming mainly depends on nutrient release as a function of mineralization processes in soils. The build-up of a large and active soil microbial biomass is important pool of accessible nutrients, therefore, is an important priority in organic farming. In organic farming, there is positive effect of quantity and quality of inputs of organic residues on soil microbial biomass. In this way, the organic systems are extremely important for the increase of the soil fertility and the maintenance of the environmental sustainability.

Response of feather moss associated N2 fixation and litter decomposition to variations in simulated rainfall intensity and frequency

Feather mosses in boreal forests form a dense ground‐cover that is an important driver of both nutrient and carbon cycling. While moss growth is highly sensitive to moisture availability, little is known about how moss effects on nutrient and carbon cycling are affected by the dynamics of moisture input to the ecosystem. We experimentally investigated how rainfall regimes affected ecosystem processes driven by the dominant boreal feather moss Pleurozium schreberi by manipulating total moisture amount, frequency of moisture addition and moss presence/absence. Moisture treatments represented the range of rainfall conditions that occur in Swedish boreal forests as well as shifts in rainfall expected through climate change. We found that nitrogen (N) fixation by cyanobacteria in feather mosses (the main biological N input to boreal forests) was strongly influenced by both moisture amount and frequency, and their interaction; increased frequency had greater effects when amounts were higher. Within a given moisture amount, N fixation varied up to seven‐fold depending on how that amount was distributed temporally. We also found that mosses promoted vascular litter decomposition rates, concentrations of litter nutrients, and active soil microbial biomass, and reduced N release into soil solution. These effects were usually strongest under low moisture amount and/or frequency, and revealed a buffering effect of mosses on the decomposer subsystem under moisture limitation. These results highlight that both the amount and temporal distribution of rainfall, determine the effect of feather mosses on ecosystem N input and the decomposer subsystem. They also emphasize the role of feather mosses in mediating moisture effects on decomposer processes. Finally, our results suggest that projected shifts in precipitation in the Swedish boreal forest through climate change will result in increased moss growth and N2 fixation but a reduced dependency of the decomposer subsystem on feather moss cover for moisture retention.

The microbial biomass and activity in soil with shrub ( Caragana korshinskii K.) plantation in the semi-arid loess plateau in China

Caragana korshinskii K. is a shrub species which is adapted to arid and semi-arid environments and plays an important role in soil protection. The objective of this study was to determine the influence of this shrub plantation on the soil ecosystem functions driven by microorganisms in the long-term. The changes in the size and activity of soil microbial biomass and the relationship between soil microbial biomass and chemical properties were investigated under shrub plantations aged 6, 18 and 26 years. The results showed that the pH value in the soil decreased gradually, while soil organic carbon (OC) and total nitrogen (TN) significantly increased with the age of C. korshinskii. Although microbial biomass carbon (MBC) and MBC/OC ratio gradually increased, the ratio of basal respiration to MBC (qCO 2) decreased with the age of C. korshinskii. The microbial biomass nitrogen (MBN) and MBC had a positive relationship with soil TN and OC, respectively. The flux of CO 2 decreased with the age of C. korshinskii which had a significant negative relationship with soil OC, TN, MBN and MBC. The results indicate that C. korshinskii plantations may help to improve microbially driven ecosystem functioning through long-term creation of resource-island.

Autoclaving kills soil microbes yet soil enzymes remain active

Biocidal treatment of soil is used to remove or inhibit soil microbial activity, and thus provides insight into the relationship between soil biology and soil processes. Chemical (soil pH, phosphodiesterase, protease) and biological (substrate induced respiration) characteristics of three contrasting soils from tropical savanna ecosystems in north Queensland, Australia were measured in field fresh samples and following autoclaving (121 °C/103 kPa for 30 min on two consecutive days). Autoclaving treatment killed the active soil microbial biomass and significantly decreased protease activity (∼90%) in all three soils. Phosphodiesterase activity in kaolinitic soils also significantly decreased by 78% and 92%. However, autoclave treatment of smectitic soil only decreased phosphodiesterase activity by 4% only. This study demonstrates phosphodiesterase can remain stable in extreme conditions. This might be a characteristic vital to the cycling of phosphorus in shrink–swell clays in Australian tropical savanna ecosystems.

Microbiological characteristics of a sandy loam soil exposed to tebuconazole and λ-cyhalothrin under laboratory conditions

Changes in microbiological properties of a sandy loam soil in response to the addition of different concentrations of fungicide tebuconazole and pyrethroid insecticide lambda-cyhalothrin were assessed under laboratory conditions. To ascertain these changes, the potentially active soil microbial biomass, concentrations of ammonium and nitrate ions, numbers of total culturable bacteria, fungi, nitrogen-fixing bacteria, nitrifying and denitrifying bacteria were determined. Substrate-induced respiration (SIR) increased with time in both control (ranged from 13.7 to 23.7 mg/O(2)/kg(-1)/dry soil/h(-1)) and pesticide treated soil portions. For both pesticides, SIR values ranged from 12-13 to 23-25 mg/O(2)/kg(-1)/dry soil/h(-1) on days 1 and 28, respectively. Also, concentrations of nitrate and ammonium ions, numbers of total culturable bacteria, denitrifying bacteria, nitrogen-fixing bacteria (for the insecticide) and fungi (for the insecticide) were either unaffected or even stimulated by the pesticide treatments. The adverse impacts of the pesticides were observed for nitrate concentrations (on days 1 or 7), numbers of nitrifying bacteria (on day 1), denitrifying bacteria (for the insecticide on days 1 and 14), nitrogen-fixing bacteria (for tebuconazole on day 1) as well as numbers of fungi in tebuconazole-treated soil (on days 1 and 14).

Effects of long-term heavy metal contamination on soil microbial characteristics

In this study, total heavy metal content and its effects on soil microbiological characteristics were investigated in soil samples from an area with known long-term pollution problems. The total heavy metal concentrations of contaminated soil samples were 109 and 1,558 mg/kg for Hg and As, respectively. Key microbiological parameters measured included dehydrogenase activity, ATP content and number of culturable aerobic bacteria, actinomycetes, fungi and asymbiotic nitrogen-fixers. Quantitative analysis of soil microbial populations shows a marked decrease in total culturable numbers of the different microbial groups of the contaminated soil samples. Certain groups of soil microbes were particularly sensitive to long-term contamination (asymbiotic nitrogen-fixers and heterotrophic bacteria). Dehydrogenase activity was found to be a sensitive assay for determining the effect of heavy metals on physiologically active soil microbial biomass and sustains the high applicability of this parameter for soil ecotoxicological testing as reported by other authors.

Seasonal and long-term resource-related variations in soil microbial communities in wheat-based rotations of the Canadian prairie

Like all other living organisms, microorganisms depend on nutrients, carbon and energy. Since microorganisms are central to most soil processes, the sustainable management of agricultural soils may need to consider the impact of soil fertility management on the soil microbial community. We tested the hypothesis that different rates of N and P fertilizers, and cropping frequency (modifying C input to soil) influence the size, structure and physiological condition of soil microbial populations residing in the plough layer (top 7.5 cm). For this study, we used a 37-yr old long-term wheat-based rotation experiment located in the semiarid Brown soil zone of Saskatchewan. The experiment included (1) four input treatments: (i) no N or (ii) no P fertilizer application to wheat ( Triticum aestivum L.) grown in fallow-wheat-wheat (F-W-W) rotations, and (iii) recommended rates of both N and P fertilizer applied to fallow-wheat (F-W) and (iv) to F-W-W; (2) two rotation phases: fallow and wheat-after-fallow; and (3) four sampling times: 8 June, 4 July, 5 August and 16 September 2003. Increased partitioning into storage lipids of the arbuscular mycorrhizal fungi (AMF) fatty acid methyl ester (FAME) biomarker 16:1ω5 ( P=0.04), suggested the accumulation of storage material under low soil N availability. Discriminant analysis detected modifications in soil microbial community structure due to cropping frequency ( P=0.001) and sampling time, the effect of which was different in the fallow ( P<0.0001) and wheat-after-fallow ( P<0.0001) phases of the rotations. Correlation analysis of soil variables conducted in plots growing wheat revealed a dual effect of plants, which stimulated active soil microbial biomass (SMB), possibly through the release of soluble extractable C (C sol−ext) in soil and, at the same time, SMB competed with wheat for soil water and N. The 37 y of different nutrient input treatments had no effect upon the active soil microbial biomass according to PLFA measurements, despite changes in soil resource-related variables (soil water potential, soil PO 4-P and NO 3-N fluxes, and C sol−ext concentrations) ( P⩽0.003). The biomass of each of three microbial populations monitored was lowest on 4 July, when the amounts of the soil resources monitored were average, and greatest on 5 August, when N, P and soil moisture availability was lowest. The temporal effect on the biomass of microbial populations seemed unrelated to variation in nutrient or water availability. We conclude that the soil microbial community is adaptable to a wide range of soil conditions. We propose therefore that the occurrence of sudden and dramatic events, such as a heavy rainfall on a dry soil, is the most important determinant of seasonal variation in active soil microbial biomass.

Microbial immobilisation and turnover of 15N labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass N dynamics were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas in an agricultural field. The objective of the study was to determine the size and activity of soil microbial biomass in the soils of the different yield areas and to compare these data obtained under field and laboratory conditions. Soils were amended with 15N labelled mustard ( Sinapis alba) residues (both experiments) and labelled nitrate (laboratory only) at 30 μg N g −1 dry soil. Soil microbial biomass (SMB) N, mineral N (N min) and total N content was monitored both in the field and in the laboratory. N 2O efflux was additionally measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, for all other parameters only in the laboratory treatments. In the laboratory less amounts of added substrate N were immobilised by the SMB in HY soils compared to LY soils, whereas in the field immobilisation of added N by SMB was higher in HY soils initially and slightly lower after 40 days of incubation. Calculated turnover times in the laboratory nitrate, laboratory mustard and field mustard amendments were 0.18, 0.27 and 0.74 years (HY) and 0.22, 0.61 and 1.01 years (LY), respectively. The turnover times of added substrate N always showed the trend to be faster in HY soils compared to LY soils. A faster turnover of nutrients in the HY soils may involve a better nutrient supply of the plants, which coincides with the higher agricultural yield observed in these areas.

Microbial immobilisation and turnover of 13C labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass C immobilisation and turnover were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas within an agricultural field. We compared the size and activity of soil microbial biomass (SMB) in the soils of the different yield areas under field and laboratory conditions. Soils were amended with 13C labelled mustard ( Sinapis alba) residues (both experiments) and labelled glucose (laboratory only) at 500 μg C g −1 dry soil. SMB–C, dissolved organic carbon (DOC) and total C content were monitored in the field and the laboratory. CO 2-efflux was also measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, but other variables only in the laboratory treatments. A positive priming effect was measured in three of four laboratory treatments. Priming was induced after a significant increase of soil derived C in the microbial biomass. Thereafter, the total C loss through priming was always smaller than or equal to the decline in microbial biomass C. In field and laboratory experiments SMB in the HY soil immobilised less of the added substrate C than LY soil SMB. Calculated turnover times in the laboratory glucose amendment were 0.24 (HY) and 0.31 y (LY), in the laboratory mustard treatment 0.58 (HY) and 0.44 y (LY) and in the field mustard amendments 1.09 (HY) and 1.25 y (LY). In both the field mustard and laboratory glucose treatments turnover in the HY soil tended to exceed that in the LY soil. These turnover times as well as the reaction of SMB–C to drying-rewetting and substrate addition, indicated that the HY soil possessed a more active microbial community with a more rapid C turnover than the LY soil. As C turnover is considered to be closely linked to nutrient cycles, faster turnover in the HY soil may involve a better nutrient supply for crops resulting in higher agricultural yield.