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.     

Effect of<Emphasis Type="Italic"> Eisenia foetida</Emphasis> earthworms on mineralization kinetics,microbial biomass,enzyme activities,respiration and labile C fractions of three soils treated with a composted organic residue

The aim of this work was to assess and compare the influence of Eisenia foetida Savigny earthworms on C mineralization rate, labile C fractions (water-soluble C and water-soluble carbohydrates), microbial biomass C, and enzyme activities (dehydrogenase, urease, phosphatase and ß-glucosidase) in three soils of varying texture treated with a composted organic residue and cropped with Avena sativa L. Mineralization decreased with the addition of earthworms to the sandy and clay-loam soils, especially in sandy soil (by about 4 µg CO ₂-C g ^-1 day ^-1). There were no significant effects on the amount of CO ₂ evolved from clay soil due to the addition of E. foetida. The addition of E. foetida to sandy soil significantly decreased microbial biomass C and increased microbial metabolic quotient the qCO ₂ (CO ₂-C to biomass C ratio). The addition of E. foetida did not affect the microbial biomass or the qCO ₂ of the clay-loam and clay soils.     

Decomposition of wheat straw differing in nitrogen content in soils under conventional and organic farming management

An incubation experiment was carried out to investigate the interactions of two straw qualities differing in N content and two soils differently accustomed to straw additions. One soil under conventional farming management (CFM) regularly received straw, the other soil under organic farming management (OFM) only farmyard manure. The soils of the two sites were similar in texture, pH, cation‐exchange capacity, and glucosamine content. The soil from the OFM site had higher contents of organic C, total N, muramic acid, microbial biomass C and N (C_mic and N_mic), but a lower ergosterol content and lower ratios ergosterol to C_mic and fungal C to bacterial C. The straw from the CFM had threefold higher contents of total N, twofold higher contents of ergosterol and glucosamine, a 50% higher content of muramic acid, and a 30% higher fungal C–to–bacterial C ratio. The straw amendments led to significant net increases in C_mic, N_mic, and ergosterol. Microbial biomass C showed on average a 50% higher net increase in the organic than in the CFM soil. In contrast, the net increases in N_mic and ergosterol differed only slightly between the two soils after straw amendment. The CO₂ evolution from the CFM soil always exceeded that from the OFM, by 50% or 200 µg (g soil)^–1 in the nonamended control soil and by 55% or additional 600 µg (g soil)^–1 in the two straw treatments. In both soils, 180 µg g^–1 less was evolved as CO₂‐C from the OFM straw. The metabolic quotient qCO₂ was nearly twice as high in the control and in the straw treatments of the CFM soil compared with that of the OFM. In contrast, the difference in qCO₂ was insignificant between the two straw qualities. Differences in the fungal‐community structure may explain to a large extent the difference in the microbial use of straw in the two soils under different managements.     

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.     

Modelling the transformations and sequestration of soil organic matter in two contrasting ecosystems of the Andes

The mechanisms linking soil respiration to climate and soil physical properties are important for modelling transformation and sequestration of C and N in the soil. We investigated them by incubating ¹⁴C and ¹⁵N labelled straw in soils of the dry puna (Bolivian altiplano, semi‐arid shrubland at 3789 m above sea level) and the humid paramo (Venezuelan tropical alpine vegetation at 3400 m). These two ecosystems of the high Andes are comparable in terms of altitude, mean temperature and land use, but are very different regarding organic matter content, rainfall patterns and soil physical properties. Total ¹⁴C and ¹⁵N, microbial‐biomass ¹⁴C and ¹⁵N, soil moisture and meteorological data were recorded over 2 years. Daily soil moisture was predicted from a water balance model. The data from the paramo site were used to calibrate MOMOS‐6, a model of organic matter decomposition based on microbial activity and requiring only kinetic constant parameters to describe: (i) inputs to microbial biomass from plant debris and microbial metabolites, and (ii) losses from the biomass by mortality and respiration (respiration coefficient and microbial metabolic quotient qCO₂). The simulated qCO₂–¹⁴C agrees well with qCO₂–¹⁴C and qCO₂ measured at the calibration site and with published data. To apply MOMOS‐6 to the puna site, only the respiration coefficient of the biomass was re‐estimated. The dynamics of ¹⁴C and ¹⁵N were very different in the two systems. In the puna, the transformation processes stop during the long dry periods, though total annual mineralization is greater than in the paramo. The change in the value of the respiration coefficient enables us to predict that the amount of C and N sequestered in the stable humus is greater in the paramo than in the puna. The data in this paper can be used to estimate values of the respiration coefficient so that MOMOS‐6 can be applied to other systems.     

Impact of plant species and atmospheric CO2 concentration on rhizodeposition and soil microbial activity and community composition

Both plant species and CO₂ concentration can potentially affect rhizodeposition and consequently soil microbial activity and community composition. However, the effect differs based on plant developmental stage. We focused on the effect of three plant species (forbs, grasses, and N₂‐fixers) at an early stage of development on root C deposition and fate, soil organic matter (SOM) mineralization and soil microbial community composition at ambient (aCO₂) and elevated (eCO₂) CO₂ levels. Plants were grown from seed, under continuous ¹³C‐labelling atmospheres (400 and 800 µmol mol^?1 CO₂), in grassland soil for three weeks. At the end of the growth period, soil respiration, dissolved organic C (DOC) and phospholipid fatty acid (PLFA) profiles were quantified and isotopically partitioned into root‐ and soil‐derived components. Root‐derived DOC (0.53 ± 0.34 and 0.26 ± 0.29 µg mL soil solution^?1) and soil‐derived CO₂ (6.14 ± 0.55 and 5.04 ± 0.44 µg CO₂‐C h^?1) were on average two times and 22% higher at eCO₂ than at aCO₂, respectively. Plant species differed in exudate production at aCO₂ (0.11 ± 0.11, 0.10 ± 0.18, and 0.58 ± 0.58 µg mL soil solution^?1 for Plantago, Festuca, and Lotus, respectively) but not at eCO₂ (0.20 ± 0.28, 0.66 ± 0.32, and 0.75 ± 0.15 µg mL soil solution^?1 for Plantago, Festuca, and Lotus, respectively). However, no differences among plant species or CO₂ levels were apparent when DOC was expressed per gram of roots. Relative abundance of PLFAs did not differ between the two CO₂ levels. A higher abundance of actinobacteria and G‐positive bacteria occurred in unplanted (8.07 ± 0.48 and 24.36 ± 1.18 mol%) and Festuca‐affected (7.63 ± 0.31 and 23.62 ± 0.69 mol%) soil than in Plantago‐ (7.04 ± 0.36 and 23.41 ± 1.13 mol%) and Lotus‐affected (7.24 ± 0.17 and 23.13 ± 0.52 mol%) soil. In conclusion, the differences in root exudate production and soil respiration are mainly caused by differences in root biomass at an early stage of development. However, plant species evidently produce root exudates of varying quality affecting associated microbial community composition.     

Effect of temperature on soil microbial biomass and its metabolic quotient in situ under different tillage systems

Variations in the microbial biomass and the in situ metabolic quotient (qCO₂) due to climatic conditions were determined in a typical soil from the Argentine Rolling Pampa. Microbial C was evaluated by fumigation-incubation and qCO₂ was calculated using soil respiration in the field. An inverse relationship between microbial C and soil temperature was fitted to a model (r ²=0.90, P=0.01). No significant association with the soil water content was detected because the soil was generally near field capacity and thus water availability did not limited microbial growth and activity. Values of qCO₂ increased (r ²=0.89, P=0.01) as the result of metabolic activatìon, likely induced by a higher maintenance energy requirement at high temperatures. The highest values of qCO₂ were obtained when microbial C was the lowest, which was attributed to self consumption of microbial C in the presence of high temperatures. Consequently, microbial C was generally higher (P=0.05) in winter than in summer. Therefore, when microbial C is used as an index of soil biological activity, the influence of temperature should be taken into account.     

Mobile and readily available C and N fractions and their relationship to microbial biomass and selected enzyme activities in a sandy soil under different management systems

Within different land‐use systems such as agriculture, forestry, and fallow, the different morphology and physiology of the plants, together with their specific management, lead to a system‐typical set of ecological conditions in the soil. The response of total, mobile, and easily available C and N fractions, microbial biomass, and enzyme activities involved in C and N cycling to different soil management was investigated in a sandy soil at a field study at Riesa, Northeastern Germany. The management systems included agricultural management (AM), succession fallow (SF), and forest management (FM). Samples of the mineral soil (0—5, 5—10, and 10—30 cm) were taken in spring 1999 and analyzed for their contents on organic C, total N, NH₄⁺‐N and NO₃^—‐N, KCl‐extractable organic C and N fractions (Corg_(KCl) and Norg_(KCl)), microbial biomass C and N, and activities of β‐glucosidase and L‐asparaginase. With the exception of Norg_(KCl), all investigated C and N pools showed a clear relationship to the land‐use system that was most pronounced in the 0—5 cm profile increment. SF resulted in greater contents of readily available C (Corg_(KCl)), NH₄⁺‐N, microbial biomass C and N, and enzyme activities in the uppermost 5 cm of the soil compared to all other systems studied. These differences were significant at P ≤ 0.05 to P ≤ 0.001. Comparably high C_mic:C_org ratios of 2.4 to 3.9 % in the SF plot imply a faster C and N turnover than in AM and FM plots. Forest management led to 1.5‐ to 2‐fold larger organic C contents compared to SF and AM plots, respectively. High organic C contents were coupled with low microbial biomass C (78 μg g^—1) and N contents (10.7 μg g^—1), extremely low C_mic : C_org ratios (0.2—0.6 %) and low β‐glucosidase (81 μg PN g^—1 h^—1) and L‐asparaginase (7.3 μg NH₄‐N g^—1 2 h^—1) activities. These results indicate a severe inhibition of mineralization processes in soils under locust stands. Under agricultural management, chemical and biological parameters expressed medium values with exception for NO₃^—‐N contents which were significantly higher than in SF and FM plots (P ≤ 0.005) and increased with increasing soil depth. Nevertheless, the depth gradient found for all studied parameters was most pronounced in soils under SF. Microbial biomass C and N were correlated to β‐glucosidase and L‐asparaginase activity (r ≥ 0.63; P ≤ 0.001). Furthermore, microbial biomass and enzyme activities were related to the amounts of readily mineralizable organic C (i.e. Corg_(KCl)) with r ≥ 0.41 (P ≤ 0.01), suggesting that (1) KCl‐extractable organic C compounds from field‐fresh prepared soils represent an important C source for soil microbial populations, and (2) that microbial biomass is an important source for enzymes in soil. The Norg_(KCl) pool is not necessarily related to the size of microbial biomass C and N and enzyme activities in soil.<?show $6#>     

Coffee mucilage impact on young coffee seedlings and soil microorganisms

A greenhouse pot experiment was carried out to assess the effects of fermented coffee mucilage applied as mulch together with maize leaves on the growth of young coffee plants of two different varieties and on soil microbial biomass indices. The coffee variety Catuai required 32% more water per g plant biomass than the variety Yellow Caturra, but had a 49% lower leaf area, 34% less shoot and 46% less root biomass. Maize and mucilage amendments did not affect leaf area, shoot and root yield, or the N concentration in shoot and root dry matter. The amendments always reduced the water use efficiency values, but this reduction was only significant in the maize+mucilage‐14 (= 14 g mucilage pot^?1) treatment. Soil pH significantly increased from 4.30 in the control to 4.63 in the maize+mucilage‐14 treatment. Microbial biomass C increased by 18.5 µg g^?1 soil, microbial biomass N by 3.1 µg g^?1 soil, and ergosterol by 0.21 µg g^?1 soil per g mucilage added pot^?1. The presence of mucilage significantly reduced the microbial biomass‐C/N ratio from a mean of 13.4 in the control and maize treatments to 9.3, without addition rate and coffee variety effects. The application of non‐composted mucilage is recommended in areas where drought leads to economic losses and in coffee plantations on low fertility soils like Oxisols, where Al toxicity is a major constraint.     

The role of “effective microorganisms” in the composting of banana (Musa ssp.) residues

“Effective microorganisms” (EM) are a poorly defined mixture of supposedly beneficial microorganisms that are claimed to enhance microbial turnover in compost and soil. In Costa Rica, EM are used to produce organic compost (bokashi) from banana residues (Musa ssp.). Given the scarcity of scientific data about the effects of EM on the mineralization of plant residues, this study aimed at investigating the effects of EM addition on the decomposition of banana residues during Bokashi production. To this end, the following non‐EM treatments were compared to EM Bokashi: Bokashi produced with water (W), with molasses (M) as an EM additive, and with sterilized EM (EMst). Subsequently, the effects of the resulting Bokashi treatments on the growth of young banana plants were evaluated. Compared with non‐EM controls, the effect of EM on the mineralization of banana material was negligible. Dry‐matter losses of the composts with different EM treatments were similar, with about 78% over 5 weeks. Ergosterol concentration was highest in EM Bokashi (77 µg (g dry soil)^–1) and lowest in EMst Bokashi (29 µg (g dry soil)^–1). Microbial biomass carbon (C_mic) and microbial biomass nitrogen (N_mic) were both lowest in EM (C_mic = 3121 µg g^–1; N_mic = 449 µg g^–1), while C_mic was highest in Bokashi produced with molasses (3892 µg g^–1) and N_mic was highest in EMst (615 µg g^–1). Treatment effects on adenylate concentrations and adenylate energy charge were negligible. Application of all Bokashi variants to young banana plants significantly increased shoot growth under greenhouse conditions compared to plants grown in a control soil without amendments. However, these effects were similar for all Bokashi treatments, even if EM Bokashi increased the K concentrations in banana leaves significantly compared to Bokashi produced with EMst and the control. Bokashi produced with only molasses and EM Bokashi decreased the number of root nematodes under greenhouse conditions compared to the control. Overall, the results confirmed the expected influence of composting on the degradation of organic material and the effect of compost application on plant growth. Hower, under the conditions of this study, EM showed no special effects in this, except for increasing the K concentrations in the leaves of young banana plants.     

Effect of organic and inorganic sources of nutrients on soil microbial activity and soil organic carbon build-up under rice in west coast of India

The effect of medium-term (5 years) application of organic and inorganic sources of nutrients (as mineral or inorganic fertilizers) on soil organic carbon (SOC), SOC stock, carbon (C) build-up rate, microbial and enzyme activities in flooded rice soils was tested in west coast of India. Compared to the application of vermicompost, glyricidia (Glyricidia maculate) (fresh) and eupatorium (Chromolaena adenophorum) (fresh) and dhaincha (Sesbania rostrata) (fresh), the application of farmyard manure (FYM) and combined application of paddy straw (dry) and water hyacinth (PsWh) (fresh) improved the SOC content significantly (p < 0.05). The lowest (p < 0.05) SOC content (0.81%) was observed in untreated control. The highest (p < 0.05) SOC stock (23.7 Mg C ha^?1) was observed in FYM-treated plots followed by recommended dose of mineral fertilizer (RDF) (23.2 Mg C ha^?1) and it was lowest (16.5 Mg C ha^?1) in untreated control. Soil microbial biomass carbon (C_mb) (246 µg g^?1 soil) and C_mb/SOC (1.92%) were highest (p < 0.05) in FYM-treated plot. The highest (p < 0.05) value of metabolic quotient (qCO₂) was recorded under RDF (19.7 µg CO₂-C g^?1 C_mb h^?1) and untreated control (19.6 µg CO₂-C g^?1 C_mb h^?1). Application of organic and inorganic sources of nutrients impacted soil enzyme activities significantly (p < 0.05) with FYM causing highest dehydrogenase (20.5 µg TPF g^?1 day^?1), phosphatase (659 µg PNP g^?1 h^?1) and urease (0.29 µg urea g^?1 h^?1) activities. Application of organic source of nutrients especially FYM improved the microbial and enzyme activities in flooded and transplanted rice soils. Although the grain yield was higher with the application of RDF, but the use of FYM as an organic agricultural practice is more useful when efforts are intended to conserve more SOC and improved microbial activity.     

A field study of gross rates of N mineralization and nitrification and their relationships to microbial biomass and enzyme activities in soils treated with dairy effluent and ammonium fertilizer

Abstract. Gross N mineralization and nitrification rates were measured in soils treated with dairy shed effluent (DSE) (i.e. effluent from the dairy milking shed, comprising dung, urine and water) or ammonium fertilizer (NH₄Cl) under field conditions, by injecting ¹⁵N-solution into intact soil cores. The relationships between gross mineralization rate, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) as affected by the application of DSE and NH₄Cl were also determined. During the first 16 days, gross mineralization rate in the DSE treated soil (4.3–6.1 μg N g^?1 soil day^?1) were significantly (P 14;< 14;0.05) higher than those in the NH₄Cl treated soil (2.6–3.4 μg N g^?1 soil day^?1). The higher mineralization rate was probably due to the presence of readily mineralizable organic substrates in the DSE, accompanied by stimulated microbial and extracellular enzyme activities. The stable organic N compounds in the DSE were slow to mineralize and contributed little to the mineral N pool during the period of the experiment. Nitrification rates during the first 16 days were higher in the NH₄Cl treated soil (1.7–1.2 μg N g^?1 soil day^?1) compared to the DSE treated soil (0.97–1.5 μg N g^?1 soil day^?1). Soil microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) increased after the application of the DSE due to the organic substrates and nutrients applied, but declined with time, probably because of the exhaustion of the readily available substrates. The NH₄Cl application did not result in any significant increases in microbial biomass C, protease or urease activities due to the lack of carbonaceous materials in the ammonium fertilizer. However, it did increase microbial biomass N and deaminase activity. Significant positive correlations were found between gross N mineralization rate and soil microbial biomass, protease, deaminase and urease activities. Nitrification rate was significantly correlated to biomass N but not to the microbial biomass C or the enzyme activities. Stepwise regression analysis showed that the variations of gross N mineralization rate was best described by the microbial biomass C and N.     

Responses of carbon,nitrogen and phosphorus to two consecutive drying–rewetting cycles in soils

Drying and rewetting cycles are known to be important for the dynamics of carbon (C), phosphorus (P), and nitrogen (N) in soils. This study reports the short‐term responses of these nutrients to consecutive drying and rewetting cycles and how varying soil moisture content affects microbial biomass C and P (MBC and MBP), as well as associated carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions. The soil was incubated for 14 d during which two successive drying–rewetting episodes were imposed on the soils. Soils subjected to drying (DRW) were rewetted on the seventh day of each drying period to return them to 60% water holding capacity, whilst continually moist samples (M), with soil maintained at 60% water holding capacity, were used as control samples. During the first seven days, the DRW samples showed significant increases in extractable ammonium, total oxidized nitrogen, and bicarbonate extractable P concentrations. Rewetting after the first drying event produced significant increases only in CO₂ flux (55.4 µg C g^?1 d^?1). The MBC and MBP concentrations fluctuated throughout the incubation in both treatments and only the second drying–rewetting event resulted in a significantly MBC decrease (416.2 and 366.8 mg kg^?1 in M and DRW soils, respectively). The two drying–rewetting events impacted the microbial biomass, but distinguishing the different impacts of microbial versus physical impacts of the perturbation is difficult. However, this study, having a combined approach (C, N, and P), indicates the importance of understanding how soils will react to changing patterns of drying–rewetting under future climate change.     

Effects of microbial inoculations on soil chemical,biochemical and microbial biomass carbon of cassava (Manihot esculenta Crantz) growing Vertisols

Cassava is an important subsidiary food in the tropics. In Tamil Nadu, India, microbial cultures were used to eradicate the tuberous root rot of cassava. Hence, an experiment was conducted for two consecutive years to test the effects of coinoculation of microbes on soil properties. The surface soil from the experimental site was analysed for soil available nutrients, soil enzyme activities and microbial biomass carbon. The treatment of Azospirillum with Trichoderma at the 50% recommended N:P₂O₅:K₂O (NPK) rate (50:25:50 kg ha^?1) significantly increased soil available nitrogen (142.81 kg ha^?1) by 72.66% over uninoculated control. There was a significant increase in available phosphorus in soil by the inoculation of AM (arbuscular mycorrhizal) fungi with Trichoderma at the 50% recommended NPK rate (41.04 kg ha^?1) compared to other treatments. The application of Pseudomonas fluorescens with Trichoderma at the 50% recommended NPK rate significantly increased available iron (19.34 µg g^?1) in soil. The treatment of Azospirillum with Trichoderma increased urease enzyme activity at the recommended NPK rate (816.32 μg urea hydrolyzed g^?1 soil h^?1). Soil application of all cultures at the 50% recommended NPK rate significantly increased dehydrogenase activity (88.63 μg TPF g^?1 soil) and β-glucosidase activity (48.82 μg PNP g^?1 soil) in soil. Inoculation of Trichoderma alone at the 50% recommended NPK rate significantly increased microbial biomass carbon (3748.85 μg g^?1 soil). Thus, the microbial inoculations significantly increased soil available nutrient contents, enzyme activities such as urease, dehydrogenase and β-glucosidase activity and microbial biomass carbon by reducing the amount of the required fertilizer.     

Microbial respiration and biomass (substrate-induced respiration) in soils of old-growth and regenerating forests on northern Vancouver Island,British Columbia

In studying the basal respiration, microbial biomass (substrate-induced respiration, SIR), and metabolic quotient (qCO₂) in western red cedar (Thuja plicata Donn ex D. Don)-western hemlock [(Tsuga heterophylla Raf.) Sarg.] ecosystems (old-growth forests, 3- and 10-year-old plantations) on northern Vancouver Island, British Columbia, Canada, we predicted that (1) soil basal respiration would be reduced by harvesting and burning, reflecting the reduction in microbial biomass and activities; (2) the microbial biomass would be reduced by harvesting and slash-burning, due to the excessive heat of the burning or due to reduced substrate availability; (3) microbial biomass in the plantations would tend to recover to the preharvesting levels with growth of the trees and increased substrate availability; and (4) microbial biomass measured by the SIR method would compare well with that measured by the fumigation-extraction (FE) method. Decaying litter layer (F), woody F (Fw) and humus layer (H) materials were sampled four times in the summer of 1992. The results obtained supported the four predictions. Microbial biomass was reduced in the harvested and slash-burned plots. Both SIR and FE methods provided equally good estimates of microbial biomass in the samples [SIR microbial C (mg g^-1)=0.227+0.458 FE microbial C (mg g^-1), r=0.63, P=0.0001] and proved suitable for microbial biomass measurements in this strongly acidic soil. Basal respiration was significantly greater in the old-growth forests than in the young plantations (P<0.05) in both F and H layers, but not in the Fw layer. For the 3- and 10-year-old plantations, there was no difference in basal respiration in F, Fw, and H layers. Basal respiration was related to changes in air temperature, precipitation, and the soil moisture contant at the time of sampling. The qCO₂ values were higher in the old-growth stands than in the plantations. Clear-cutting followed by prescribed burning did not increase soil microbial respiration, but CO₂ released from slash-burning and that contributed from other sources may be of concern to increasing atmospheric CO₂ concentrations.     

Seasonal dynamics of soil microbial biomass C shows close correlation with environmental factors in natural Fagus crenata forests

Abstract

Soil microbial biomass (C_mic) is an important factor regulating a number of ecosystem processes. In this study, we investigated seasonal variations in soil microbial biomass in natural climax beech (Fagus crenata) forests in a typical cold-temperate mountain region of Japan. Four permanent tower sites along an altitudinal gradient were selected and soil samples were collected once every month during the growing season of 2007. Soil microbial biomass (by fumigation-extraction method) and soil properties were later measured in the laboratory, while environmental factors (soil temperature, soil moisture) were continuously recorded in the field. Our results indicated large seasonal variations (130.4 ~ 5558.0 µg g^?1) in soil microbial biomass in beech forests – a range that is much larger than previously reported. Statistically significant correlations are noted between soil properties with C_mic, but largely due to spatial linkages. On the other hand, the environmental factors of soil temperature and especially soil moisture largely control seasonal variations in C_mic. Furthermore, pH could be an important factor influencing seasonal change in C_mic at the 20–30 cm deep soil layer. The study suggests no direct correlation between plant eco-physiology and soil microbial biomass in seasonal courses of the forests.     

Initial results from a long-term, multi-site field study of the effects on soil fertility and microbial activity of sludge cakes containing heavy metals

In a long‐term study of the effects on soil fertility and microbial activity of heavy metals contained in sewage sludges, metal‐rich sludge cakes each with high Zn, Cu or Cd concentrations were applied annually for 4 years (1994–1997) to nine sites throughout Britain. These sites were selected to represent agricultural soils with a range of physical and chemical properties, typical of those likely to be amended with sewage sludge. The aim was to establish individual total Zn (approx. 60–450 mg kg^?1), total Cu (approx. 15–200 mg kg^?1) and total Cd (approx. 0.2–4 mg kg^?1) metal dose–response treatments at each site. Sludges with low metal concentrations were added to all treatments to achieve as constant an addition of organic matter as possible. Across the nine sites, soil pH was the single most important factor controlling Zn (P < 0.001; r² = 92%) and Cd extracted with 1 m NH₄NO₃ (P < 0.001; r² = 72%), and total iron content the most important factor controlling Cu extracted with 1 m NH₄NO₃ (P < 0.001; r² = 64%). There were also positive relationships (P < 0.001) between soil organic carbon (C) concentrations and soil biomass C and respiration rates across the nine sites. Oxidation of sludge C following land application resulted in approximately 45% of the digested sludge cake C and approximately 64% of the ‘raw’ sludge cake C being lost by the end of the 4‐year application period. The sludge cake applications generally increased soil microbial biomass C and soil respiration rates, whilst most probable numbers of clover Rhizobium were generally unchanged. Overall, there was no evidence that the metal applications were damaging soil microbial activity in the short term after the cessation of sludge cake addition.     

Soil microbial activity and biomass is stimulated by leguminous cover crops

The leguminous cover crops Atylosia scarabaeoides (L.) Benth., Centrosema pubescens Benth., and Pueraria phaseoloides (Roxb.) Benth., were grown in the interspaces of a 19 y–old coconut plantation and incorporated into the soil at the end of the monsoon season every year. At the end of the 12th year, soils from different depths were collected and analyzed for various microbial indices and their interrelationships. The objectives were to assess the effects of long‐term cover cropping on microbial biomass and microbial‐community structure successively down the soil profile. In general, total N (TN), organic C (OC), inorganic N, extractable P, and the levels of biological substrates viz., dissolved organic C (DOC) and N (DON), labile organic N (LON), and light‐fraction organic matter (LFOM) C and N decreased with depth at all the sites. Among sites, the cover‐cropped (CC) sites possessed significantly greater levels of TN, OC, DOC, DON, and LON compared to the control. Consequently, microbial biomass C (MBC), N (MBN), and P (MBP), CO₂ evolution, and ATP levels, in general, decreased with depth at all sites and were also significantly higher in the CC sites. Among the ratios of various microbial indices, the ratio of MBC to OC and metabolic quotient (qCO₂) declined with depth. Higher MBC‐to‐OC ratios and large qCO₂ levels in the surface soils could be ascribed to greater levels of readily degradable C content and indicated short turnover times of the microbial biomass. In contrast, the ratios of MBC to MBN and MBC to MBP increased with depth due to low N/P availability and relatively higher C availability in the subsoils. Cover cropping tended to enhance the ratios of MBC to OC, MBC to MBN, MBC to MBP, and ergosterol to MBC and decreased the ATP‐to‐MBC ratio at all depths. The relatively lower ATP‐to‐MBC ratios in the CC site, especially in the subsoil indicated microbial‐community structure possibly dominated by fungi. By converting the ergosterol content to fungal biomass, it was observed that fungi constituted 52%–63% of total biomass C at the CC site, but only 33%–40% of total biomass C at the control site. Overall, the study indicated that leguminous cover crops like P. phaseoloides or A. scarabaeoides significantly enhanced the levels of OC, N and microbial activity in the soils, even down to 50 cm soil depth.     

Application of eco-physiological quotients (qCO2 and qD) on microbial biomasses from soils of different cropping histories

Metabolic quotients for CO₂C (qCO₂C) and microbial-C-loss (qD) were studied on soil microbial communities under long-term monoculture (M) or continuous crop rotations (CR). Under defined laboratory conditions the mean qCO₂C (unit CO₂C unit⁻¹ C_mic h⁻¹) of different microbial biomasses from 17 M systems amounted to 1.097 μg CO₂qCO₂CC as compared to 0.645 μg CO₂C of microbial biomasses from 19 CR systems. The 1.7 times higher CO₂C release per unit biomass and time of microbial biomasses from M systems was significantly different at the P =0.001 level.In addition, microbial C-loss in samples from M or CR plots was followed for 5 weeks. Again, mean qD per unit microbial biomass and time was 1.6 times higher (P = 0.01) for microbial biomasses from M systems (0.301 μg C, 14 soils) when compared with CR systems (0.188μg C, 14 soils).These differences were not related to soil texture, C_org or pH of these soils. The effects of environmental influences (soil management) on the microbial pool in terms of a changing energy demand are discussed.     

Microbial biomass and activity in salt affected soils under arid conditions

The effects of salinity on the size, activity and community structure of soil microorganisms in salt affected arid soils were investigated in Shuangta region of west central Anxi County, Gansu Province, China. Eleven soils were selected which had an electrical conductivity (EC) gradient of 0.32–23.05 mS cm⁻¹. There was a significant negative exponential relationship between EC and microbial biomass C, the percentage of soil organic C present as microbial biomass C, microbial biomass N, microbial biomass N to total N ratio, basal soil respiration, fluorescein diacetate (FDA) hydrolysis rate, arginine ammonification rate and potentially mineralizable N. The exponential relationships with EC demonstrate the highly detrimental effect that soil salinity had on the microbial community. In contrast, the metabolic quotient (qCO₂) was positively correlated with EC, and a quadratic relationship between qCO₂ and EC was observed. There was an inverse relationship between qCO₂ and microbial biomass C. These results indicate that higher salinity resulted in a smaller, more stressed microbial community which was less metabolically efficient. The biomass C to biomass N ratio tended to be lower in soils with higher salinity, reflecting the bacterial dominance in microbial biomass in saline soils. Consequently, our data suggest that salinity is a stressful environment for soil microorganisms.