Arylsulfatase activity of microbial biomass in soils as affected by cropping systems

 The impacts of crop rotations and N fertilization on different pools of arylsulfatase activity (total, intracellular, and extracellular) were studied in soils of two long-term field experiments in Iowa to assess the contibution of the microbial biomass to the activity of this enzyme. Surface-soil samples were taken in 1996 and 1997 in corn, soybeans, oats, or meadow (alfalfa) plots that received 0 or 180 kg N ha^–1 before corn, and an annual application of 20 kg P ha^–1 and 56 kg K ha^–1. The arylsulfatase activity in the soils was assayed at optimal pH (acetate buffer, pH 5.8) before and after chloroform fumigation; microbial biomass C (C_mic) and N (N_mic) were determined by chloroform-fumigation methods. All pools of arylsulfatase activity in soils were significantly affected by crop rotation and plant cover at sampling time, but not by N fertilization. Generally, the highest total, intracellular, and extracellular arylsulfatase activities were obtained in soils under cereal-meadow rotations, taken under oats or meadow, and the lowest under continuous cropping systems.Total, intracellular, and extracellular arylsulfatase activities were significantly correlated with C_mic (r>0.41, P<0.01) and N_mic (r>0.38, P<0.01) in soils. The averages of specific activity values, i.e., of arylsulfatase activity of the microbial biomass, expressed per milligram C_mic, ranged from 315 to 407 μg p-nitrophenol h^–1. The total arylsulfatase activity was significantly correlated with the intracellular activity, with r values >0.79 (P<0.001). In general, about 45% of the total arylsulfatase activity was extracellular, and 55% was associated with the microbial biomass in soils, indicating the importance of the microflora as an enzyme source in soils. Received: 23 April 1998     

Soil microbial biomass carbon and nitrogen as affected by cropping systems

 The impacts of crop rotations and N fertilization on microbial biomass C (C_mic) and N (N_mic) were studied in soils of two long-term field experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. The C_mic and N_mic values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The C_mic and N_mic values were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest C_mic and N_mic contents were found in the multicropping systems (4-year rotations) taken in oats or meadow plots, and the lowest values were found in continuous corn and soybean systems. On average, C_mic made up about 1.0% of the organic C (C_org), and N_mic contributed about 2.4% of the total N (N_tot) in soils at both sites and years of sampling. The C_mic values were significantly correlated with C_org contents (r≥0.41**), whereas the relationship between C_mic and N_tot was significant (r≤0.53***) only for the samples taken in 1996 at the NERC site. The C_mic : N_mic ratios were, on average, 4.3 and 6.4 in 1996, and 7.6 and 11.4 in 1997 at the NERC and CWRC sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fertilization showed no effect at either site. In general, multicropping systems resulted in greater C_mic : C_org (1.1%) and N_mic : N_tot (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively). Received: 9 February 1999     

Soil microbial biomass C and N changes in relation to forest conversion in the Northwestern Turkey

Tree species differ in their effect on soil development and nutrient cycling. Conversion of beech coppice to pine plantations can alter soil physical and chemical properties, which in turn may have significant impacts on soil microbial biomass C and N (C_mic, N_mic). The major objective of this study was to evaluate soil quality changes associated with the forest conversion in humid NW Turkey. Results from this study showed that levels of soil organic carbon (C_org), total nitrogen (N_t), moisture, C_mic and N_mic under beech coppice were consistently higher but levels of pH, CaCO₃ and EC were lower compared to pine plantation. Differences between the forest stands in C_mic and N_mic were mainly related to the size of the C_org stores in soil and to tree species. In addition, high level of CaCO₃ is likely to reduce pools of soil organic C and possibly even microbial biomass C and N in pine forests. The average C_mic:N_mic ratios were higher in soils under beech coppice than pine plantation, while C_mic:C_org and N_mic:N_t percentages were similar in both forest types. These results revealed the differences in microbial community structure associated with different tree species and the complex interrelationships between microbial biomass, soil characteristics, litter quantity and quality. Copyright © 2008 John Wiley & Sons, Ltd.     

Nitrogen pools and turnover in arable soils under different durations of organic farming: II: Source‐and‐sink function of the soil microbial biomass or competition with growing plants?

The following parameters were measured on seven field plots at 3 sites which had been under organic farming for different periods of time: mineral nitrogen (N _min) contents, in situ net nitrogen mineralization (N _net), soil microbial biomass carbon (C _mic), and nitrogen (N _mic) contents, and extractable organic N contents. The measurements were conducted every three weeks from spring 1995/1996 to autumn 1997. The objective was to test whether, under organic farming: 1) temporal fluctuations of N_mic contents over the course of the year are indicative for a source‐and‐sink function for plant‐available N of the soil microbial biomass, and 2) temporal variations in N_mic content can be related with in situ N_net or plant N uptake. N_min contents gradually increased after ploughing in autumn until late winter. During intensive plant growth in spring, values rapidly declined. In situ N_net fluctuated only moderately and reached high values during intensive plant growth (May—July) as well as after soil cultivation in autumn. The C_mic and N_mic contents generally were low in winter, increased in spring and reached maxima in late spring or summer. In spring, the increase in C_mic contents preceded the increase in N_mic contents, resulting in elevated C_mic:N_mic ratios until shooting of winter wheat. This corresponds to an uptake of available soil nitrogen by the plants at the expense of soil micro‐organisms. The subsequent increase in N_mic contents, coinciding with high plant N uptake rates, indicates an enhanced, plant‐induced N mobilization at that time. Possible mobilization mechanisms are discussed. Soil microbial biomass exerted a source‐and‐sink function for extractable organic N on some of the field plots. Estimates of in situ N_net measurements were neither correlated significantly with soil microbial biomass N, N_mic flux, N_mic turnover, nor with plant N uptake. Lower N_mic turnover rates on 41 years versus 3 years organically managed fields indicate a stabilizing effect of organic farming on soil microflora.     

Urease activity of microbial biomass in soils as affected by cropping systems

 The impacts of crop rotations and N fertilization on different pools of urease activity were studied in soils of two long-term field experiments in Iowa; at the Northeast Research Center (NERC) and the Clarion-Webster Research Center (CWRC). Surface soil samples (0–15 cm) were taken in 1996 and 1997 in corn, soybeans, oats, or meadow (alfalfa) plots that received 0 or 180 kg N ha^–1, applied as urea before corn and an annual application of 20 kg P and 56 kg K ha^–1. The urease activity in the soils was assayed at optimal pH (THAM buffer, pH 9.0), with and without toluene treatment, in a chloroform-fumigated sample and its nonfumigated counterpart. The microbial biomass C (C_mic) and N (N_mic) were determined by chloroform fumigation methods. The total, intracellular, extracellular and specific urease activities in the soils of the NERC site were significantly affected by crop rotation, but not by N fertilization. Generally, the highest total urease activities were obtained in soils under 4-year oats–meadow rotations and the lowest under continuous corn. The higher total activities under multicropping systems were caused by a higher activity of both the intracellular and extracellular urease fractions. In contrast, the highest values for the specific urease activity, i.e. of urease activity of the microbial biomass, were found in soils under continuous soybean and the least under the 4-year rotations. Total and extracellular urease activities were significantly correlated with C_mic (r>0.30* and >0.40**) and N_mic (r>0.39** and >0.44**) in soils of the NERC and CWRC sites, respectively. Total urease activity was significantly correlated with the intracellular activity (r>0.73***). About 46% of the total urease activity of the soils was associated with the microbial biomass, and 54% was extracellular in nature. Received: 25 May 1999     

Medium-term effects of a single application of mustard residues on soil microbiota and C content of vertisols

 A study of the effects of different qualities (fresh and composted) and rates (equivalent to 120, 240, and 360 kg N ha^–1) of mustard meal application on wheat yields on humid tropical vertisol was started in 1990 at Ginchi Research Station in Ethiopia. After continuous wheat cropping for 7 years and without any further fertilisation, soil microbial parameters (basal respiration, microbial biomass-C and N, organic-C, and ecophysiological quotients) were studied during one growth period. After 7 years of application, mustard meal still exerted a significant positive effect on microbial biomass, basal respiration, organic-C, C_mic : N_mic ratio, and metabolic quotient (qCO₂). Organic-C, qCO₂ and C_mic : N_mic ratios were higher for the compost-amended plots than plots amended with fresh mustard meal. Basal respiration, C_mic, and C_mic : N_mic ratio showed a clear seasonality, but only in manured plots. The data indicate shifts in microbial community structure (from bacteria to fungi and from r to K strategists) and suggest positive medium-term effects of mustard meal on humid tropical vertisol biological qualities. Received: 25 May 1999     

Soil microbial and extractable C and N after wildfire

 The effect of wildfire on soil microbes and extractable C (C_ext) and N (N_ext) changed with respect to the time from burning and soil depth. Initially, microbial biomass C (C_mic) and N (N_mic) were drastically reduced in the soil surface layer (0–5 cm) and reduced by 50% in the subsurface (5–10 cm), whereas C_ext increased by 62% in the surface layer and did not significantly change in the subsurface. These parameters were affected for the following 4 years, during which the average reductions in the soil surface and subsurface layers were, respectively, 60% and 50% for C_mic, 70% and 45% for N_mic, 60% and 40% for the ratio C_mic: organic C (C_org) and 70% and 30% for the ratio N_mic: total N (N_tot), while for C_ext the surface layer was the only zone consistently affected and C_ext decreased by up to 59%. Immediately after a fire, the C_ext : C_org ratio increased by 3.5-fold and 2-fold in the surface and subsurface layers, respectively; thereafter for 2 years, it decreased in the surface layer (by up to 45%) while the effect on the subsurface layer was not consistent. The effect of burning on N_ext lasted 1 year, in which N_ext increased by up to 7- and 3-fold in the surface and subsurface layers, respectively, while the average N_ext : N_tot ratio doubled in the surface layer and increased by 34% in the subsurface. During the time in which each parameter was affected by burning, the soil factor explained a high percentage of variance in the fluctuations of C_mic, N_mic, C_mic : C_org and N_mic : N_tot, while those of N_ext and N_ext : N_tot, but not those of C_ext and C_ext : C_org depended on both the soil and its depth. In the burned soils similar patterns of response were found between the following parameters listed in pairs: C_mic and N_mic; C_mic : C_org and N_mic : N_tot; C_ext and N_ext; and C_ext : C_org and N_ext : N_tot. However, after the fire relationships found previously between the parameters studied and many other soils properties were either no longer evident, or were inverted. Although the addition of cellulose to the burned soil favoured fungal mycelium development and increased C_mic and C_ext contents, the negative effect of burning on the microbial biomass and the C_ext was not counteracted even under incubation conditions suitable for both microbial growth and C mineralization. Received: 28 May 1997     

蒙古高原草原土壤微生物量碳氮特征

沿着水分梯度采集了蒙古高原不同草原类型表层土壤样品 1 44个 ,分析了土壤微生物量C、N含量及其与年平均温度和降雨量的关系。结果表明 :蒙古高原草原土壤微生物量C、N与土壤有机C、全N、降雨量、温度均表现出了很好的相关性。微生物量C变化在5 1 7～ 797mgkg- 1之间 ,微生物量N变化在 1 1 0～ 1 1 8 6mgkg- 1之间。微生物量C∶N比变化在 5～ 9之间。土壤微生物量碳 (Cmic)占土壤有机碳 (Corg)的比例 (Cmic Corg)变化在 1 1 5 %～ 4 1 %之间 ,Cmic Corg与土壤有机C、全N、降雨量均成显著的负相关。土壤呼吸表现为草甸草原土壤 >典型草原 >荒漠草原 ,土壤呼吸与降雨量显著正相关 ,与温度显著负相关。呼吸熵 (QCO2 )与降雨量成二次抛物线关系。放牧对微生物量的影响与不同草原类型和放牧率有关。     

Near-infrared spectroscopy for analysis of chemical and microbiological properties of forest soil organic horizons in a heavy-metal-polluted area

In industrial areas, heavy metals may accumulate in forest soil organic horizons, affecting soil microorganisms and causing changes in the chemical composition of the accumulated organic matter. The objectives of this study were to test the ability of near-infrared spectroscopy (NIRS) to detect heavy metal effects on the chemical composition of forest soil O horizons and to test whether NIRS may be used to quantitatively determine total and exchangeable concentrations of Zn and Pb (Zn_t, Pb_t, Zn_ex, Pb_ex) and other chemical and microbial properties in forest soil O horizons polluted with heavy metals. The samples of O horizons (n = 79) were analyzed for organic C (C_org), total N and S (N_t, S_t), Zn_t, Pb_t, Zn_ex, Pb_ex, basal respiration (BR), microbial biomass (C_mic) and C_mic-to-C_org ratio. Spectra of the samples were recorded in the Vis-NIR range (400–2,500 nm). To detect heavy-metal-induced changes in the chemical composition of O horizons principal components (PC1–PC7) based on the spectral data were regressed against Zn_t + Pb_t values. A modified partial least squares method was used to develop calibration models for prediction of various chemical and microbial properties of the samples from their spectra. Regression analysis revealed a significant relationship between PC3 and PC5 (r = −0.27 and −0.34, respectively) and Zn_t + Pb_t values, indicating an effect of heavy metal pollution on the spectral properties of the O horizons and thus on their chemical composition. For quantitative estimations, the best calibration model was obtained for C_org-to-N_t ratio (r = 0.98). The models for C_org, N_t, and microbial properties were satisfactory but less accurate. NIRS failed to accurately predict S_t, C_org-to-S_t, Zn_t, Pb_t, Zn_ex, and Pb_ex.     

Soil organic carbon pools and productivity in relation to nutrient management in a 20-year-old rice–berseem agroecosystem

Labile fractions of soil organic C (SOC) can respond rapidly to changes in C supply and are considered to be important indicators of soil quality. An attempt is made in this paper to investigate into the dynamics of total organic C (C _tot), oxidisable organic C (C _oc), very labile C (C _{frac 1}), labile C (C _{frac 2}), less labile C (C _{frac 3}), non-labile C (C _{frac 4}), microbial biomass C (C _mic), mineralizable C (C _min) and particulate organic C (C _p) in relation to the system productivity of a 20-year-old rice (Oryza sativa L)–berseem (Trifolium alexandrium L) cropping system with different management strategies [no fertilization, only NPK and NPK + FYM (farmyard manure) applied in different seasons] in the hot humid, subtropics of India. Cultivation over the years caused a net decrease, while balanced fertilization with NPK maintained the SOC. About 62% of the C applied as FYM was stabilized into SOC. The passive pool (C _{frac 3} + C _{frac 4}) constituted about 55% of the C _tot. A larger proportion (63%) of applied C was stabilized in the passive pool of SOC. Of the analysed pools, C _{frac 1}, C _mic, C _p and C _min were influenced most by the treatments imposed and explained higher per cent variability in the yield of the crops.     

Effect of microbial nitrogen immobilization during the growth period on the availability of nitrogen fertilizer for winter cereals

 Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca(¹⁵NO₃)₂ was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot^–1, 200 mg N pot^–1 or 300 mg N pot^–1 were applied at each N application date. At crop maturity, ¹⁵N-labelled fertilizer N immobilization was highest at the highest N rate (3×300 mg N pot^–1). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total ¹⁵N immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley (one site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51–255 kg N ha^–1) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54–97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (N_mic) was determined for cropped plots at GS 31. Although N_mic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in N_mic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low. Received: 6 July 1999     

Soil microbial biomass C and N along a climatic transect in the Mongolian steppe

The relationships of soil microbial biomass C (C_mic) or N (N_mic) with mean annual precipitation and temperature were studied along a climatic transect in the Mongolian steppe. Soil organic C (C_org) and total N (N_t), respiration rate, C_mic and N_mic at depths of 0–5 and 5–10 cm decreased with increasing aridity. The contents of C_org and N_t in the 0- to 5-cm soil layers decreased linearly with precipitation reduction along the transect. C_mic and N_mic changes with precipitation were not linear, with higher changes between 330 and 128 mm mean annual precipitation. C_mic/C_org and N_mic/N_t increased with increasing aridity. The metabolic quotient qCO₂ of 0- to 5-cm soil layers was low between 330 and 273 mm precipitation. The relationship between the qCO₂ of the 0- to 5-cm soil layers and the mean annual precipitation was well fitted with a quadratic function y =0.0006x² –0.40x +86.0, where y is the qCO₂ (µmol CO₂-C mmol^–1 C_mic) and x is the mean annual precipitation (mm). C_org, N_t, C_mic, N_mic and respiration rate decreased exponentially with increasing mean annual temperature in both the 0- to 5- and 5- to 10-cm soil layers, and change rate was lower when the mean annual temperature was higher than 2.6°C. The close relationships of the mean annual precipitation or temperature with soil C_org, N_t, C_mic, N_mic, C_mic/C_org and qCO₂ indicate that each parameter can be calculated by determining the other parameters in this specific climatic range.     

Secondary salinity effects on soil microbial biomass

Secondary soil salinilization is a big problem in irrigated agriculture. We have studied the effects of irrigation-induced salinity on microbial biomass of soil under traditional cotton (Gossypium hirsutum L.) monoculture in Sayhunobod district of the Syr-Darya province of northwest Uzbekistan. Composite samples were randomly collected at 0–30 cm depth from weakly saline (2.3 ± 0.3 dS m⁻¹), moderately saline (5.6 ± 0.6 dS m⁻¹), and strongly saline (7.1 ± 0.6 dS m⁻¹) replicated fields, 2-mm sieved, and analyzed for pH, electrical conductivity, total C, organic C (C_Org), and extractable C, total N and P, and exchangeable ions (Ca²⁺, Mg²⁺, K⁺, Na⁺, Cl⁻, and CO₃²⁻), microbial biomass (C_mic). The Na⁺ and Cl⁻ concentrations were 36-80% higher in strongly saline compared to weakly saline soil. The C_Org concentration was decreased by 10% and C_Ext by 40% by increasing soil salinity, whereas decrease in C_mic ranged from 18-42% and the percentage of C_Org present as C_mic from 8% to 26%. We conclude that irrigation-induced secondary salinity significantly affects soil chemical properties and the size of soil microflora.     

Utilization of Distillation Waste–Based Vermicompost and Other Organic and Inorganic Fertilizers on Improving Production Potential in Geranium and Soil Health

Vermicompost (VC) produced from distillation waste of geranium (Pelargonium graveolens), farmyard manure (FYM) produced from animal excreta mixed with pine needle (Pinus sp.), and biofertilzer (Azotobacter) were utilized for this experiment. The plant growth attributes, biomass, and oil yield of geranium were significantly increased with integrated nutrient supply, and maximum increase was found in T₈ treatments (N₁₀₀P₆₀ K₆₀ + 5t VC). Soil organic carbon (C_org) significantly increased by 4.2% to 81.8% in T₄ and T₈ treatments, respectively, over the control. Data obtained on total nitrogen (N_t) and available N, phosphorus (P), and potassium (K) clearly showed that the integrated nutrient supply considerably improved the soil health and sustainability. The soil respiration and microbial biomass C (C_mic) and N (N_mic) were increased by the manures according to the application rate. The C_mic accounted for 1.8 to 2.7% of the soil C_org content and microbial N accounted for 3.9 to 5.8 % of N_t under different treatment combinations.     

Effects of increasing periods under intensive arable vegetable production on biological, chemical and physical indices of soil quality

 The effects on soil condition of increasing periods under intensive cultivation for vegetable production on a Typic Haplohumult were compared with those of pastoral management using soil biological, physical and chemical indices of soil quality. The majority of the soils studied had reasonably high pH, exchangeable cation and extractable P levels reflecting the high fertilizer rates applied to dairy pasture and more particularly vegetable-producing soils. Soil organic C (C_org) content under long-term pasture (>60 years) was in the range of 55 g C kg^–1 to 65 g C kg^–1. With increasing periods under vegetable production soil organic matter declined until a new equilibrium level was attained at about 15–20 g C kg^–1 after 60–80 years. The loss of soil organic matter resulted in a linear decline in microbial biomass C (C_mic) and basal respiratory rate. The microbial quotient (C_mic/C_org) decreased from 2.3% to 1.1% as soil organic matter content declined from 65 g C kg^–1 to 15 g C kg^–1 but the microbial metabolic quotient (basal respiration/C_mic ratio) remained unaffected. With decreasing soil organic matter content, the decline in arginine ammonification rate, fluorescein diacetate hydrolytic activity, earthworm numbers, soil aggregate stability and total clod porosity was curvilinear and little affected until soil organic C content fell below about 45 g C kg^–1. Soils with an organic C content above 45 g C kg^–1 had been under pasture for at least 30 years. At the same C_org content, soil biological activity and soil physical conditions were markedly improved when soils were under grass rather than vegetables. It was concluded that for soils under continuous vegetable production, practices that add organic residues to the soil should be promoted and that extending routine soil testing procedures to include key physical and biological properties will be an important future step in promoting sustainable management practices in the area. Received: 18 November 1997     

Microbial C,N, and P relationships in moisture‐stressed soils of Potohar,Pakistan

In 11 rain‐fed arable soils of the Potohar plateau, Pakistan, the amounts of microbial‐biomass C (C_mic), biomass N (N_mic), and biomass P (P_mic) were analyzed in relation to the element‐specific total storage compartment, i.e., soil C_org, N_t, and P_t. The effects of climatic conditions and soil physico‐chemical properties on these relationships were highlighted with special respect to crop yield levels. Average contents of soil C_org, N_t, and P_t were 3.9, 0.32, and 0.61 mg (g soil)^–1, respectively. Less than 1% of P_t was extractable with 0.5 M NaHCO₃. Mean contents of C_mic, N_mic, and P_mic were 118.4, 12.0, and 3.9 µg (g soil)^–1. Values of C_mic, N_mic, P_mic, soil C_org, and N_t were all highly significantly interrelated. The mean crop yield level was closely connected with all soil organic matter– and microbial biomass–related properties, but showed also some influence by the amount of precipitation from September to June. Also the fraction of NaHCO₃‐extractable P was closely related to soil organic matter, soil microbial biomass, and crop yield level. This reveals the overwhelming importance of biological processes for P turnover in alkaline soils.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

Microbial biomass in arable soils of Germany during the growth period of annual crops

Results from several field studies involving numerous measurements were used to describe the change of soil microbial biomass C (C_mic) and N (N_mic) during the growth period of annual crops (years 1988–1992, 1994, 1995) under the temperate climatic conditions of central Europe. The data were taken from our own investigations as well as from the literature. Only studies with at least eight measurements on one plot during the growth period were used. The total number of farms (cash crop–production farms) was 7, that of experimental plots was 15. The evaluation of these results through regression analysis demonstrated that C_mic and N_mic from the beginning of a year increased only slightly until summer and subsequently decreased until autumn to their initial levels. This increase on an average corresponded to a C assimilation of approx. 100 kg ha^–1 and an N immobilization of approx. 20 kg ha^–1 (30 cm)^–1. The increase in N_mic alone could not explain N immobilization rates frequently observed in different studies using ¹⁵N‐labeled fertilizers. Most of the labeled N that was immobilized (>50 kg N ha^–1) might have accumulated in the matrix of soil organic matter (SOM). Therefore, the changes in microbial biomass may be of less importance for changes in soil N storage as frequently assumed.     

Spatial structure of microbial biomass and activity in prairie soil ecosystems

Management of soil ecosystems requires assessment of key soil physicochemical and microbial properties and the spatial scale over which they operate. The objectives were to determine the spatial structure of microbial biomass and activity and related soil properties, and to identify spatial relationships of these properties in prairie soils under different management histories. Soil were sampled along a transect at 0.2 m intervals in each of five long-term treatments, namely, undisturbed, cattle grazed at two intensities, and cultivated with either wheat (Triticum aestivum L.) or cotton (Gossypium hirsutum L.). Contents of organic carbon (C_org), dissolved organic C (DOC), soluble nitrogen (N_sol), and microbial biomass C (C_mic) and N (N_mic) as well as dehydrogenase activity (DH) in 70 samples were evaluated. Results showed that long-term soil management altered the spatial structure and dependence of C_org and microbial biomass and activity. Cultivation has contributed to high nugget variance for C_org, C_mic, N_mic and DH which interfered with detection of spatial structure at the sampling scale used. Contents of C_org were spatially connected to microbial biomass and activity and to DOC in the uncultivated but not in the cultivated soils, indicating that various factors affected by management may operate at different spatial scales.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.