Properties of Microbial Biomass in Acid Soils and Their Turnover

The soil microbial biomass is important such as pool of plant nutrients and is also driving force of the cycling of C, N, P and S in soil. However, the microbial biomass in acid soil has not been fully investigated due to the limitation of methods, i.e. chloroform-fumigation incubation or substrate-induced respiration because of decreased basal mineralization in chloroform-fumigated soil under acid conditions. This paper reviews improvement and application of these methods and vertical distribution of microbial biomass in two kinds of acid soils; namely, Andisols as dominant upland soils in Japan and tropical peat soils as potentially important lowland soils for agriculture, and also discuss on C and N turnover of microbial biomass in Andisols. Microbial succession in acid soil has also not been investigated so much, but, some studies in another important acid soil, i.e. acid sulfate soil, were also reviewed briefly.     

Tillage effects on microbial biomass and nutrient distribution in soils under rain-fed corn production in central-western Mexico

Quantifying how tillage systems affect soil microbial biomass and nutrient cycling by manipulating crop residue placement is important for understanding how production systems can be managed to sustain long-term soil productivity. Our objective was to characterize soil microbial biomass, potential N mineralization and nutrient distribution in soils (Vertisols, Andisols, and Alfisols) under rain-fed corn (Zea mays L.) production from four mid-term (6 years) tillage experiments located in central-western, Mexico. Treatments were three tillage systems: conventional tillage (CT), minimum tillage (MT) and no tillage (NT). Soil was collected at four locations (Casas Blancas, Morelia, Apatzingán and Tepatitlán) before corn planting, at depths of 0–50, 50–100 and 100–150 mm. Conservation tillage treatments (MT and NT) significantly increased crop residue accumulation on the soil surface. Soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were highest in the surface layer of NT and decreased with depth. Soil organic C, microbial biomass C and N, total N and extractable P of plowed soil were generally more evenly distributed throughout the 0–150 mm depth. Potential N mineralization was closely associated with organic C and microbial biomass. Higher levels of soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were directly related to surface accumulation of crop residues promoted by conservation tillage management. Quality and productivity of soils could be maintained or improved with the use of conservation tillage.     

Nitrogen mineralization and microbial activity in permanent pastures amended with nitrogen fertilizer or dung

 Gross rates of soil processes and microbial activity were measured in two grazed permanent pasture soils which had recently been amended with N fertilizer or dung. ¹⁵N studies of rates of soil organic matter turnover showed gross N mineralization was higher, and gross N immobilization was lower, in a long-term fertilized soil than in a soil which had never received fertilizer N. Net mineralization was also found to be higher in the fertilized soil: a consequence of the difference between the opposing N turnover processes of N mineralization and immobilization. In both soils without amendments the soil microbial biomass contents were similar, but biomass activity (specific respiration) was higher in the fertilized soil. Short-term manipulation of fertilizer N input, i.e. adding N to unfertilized soil, or witholding N from previously fertilized soil, for one growing season, did not affect gross mineralization, immobilization or biomass size and activity. Amendments of dung had little effect on gross mineralization, but there was an increase in immobilization in both soils. Total biomass also increased under dung in the unfertilized soil, but specific respiration was reduced, suggesting changes in the composition of the biomass. Dung had a direct effect on the microbial biomass by temporarily increasing available soil C. Prolonged input of fertilizer N increases soil C indirectly as a result of enhanced plant growth, the effect of which may not become evident within one seasonal cycle. Received: 18 December 1998     

Effect of arsenic contamination on bacterial and fungal biomass and enzyme activities in tropical arsenic-contaminated soils

The importance of assessing the impacts of soil arsenic (As) contamination on microbial properties lay on the fact that microbes are instrumental in nutrient cycling and are therefore indicators of soil quality. In this study, soil chemical extraction methods were used to extract labile and freely exchangeable As (water-soluble As and sodium bicarbonate-extractable As), amorphous/crystalline Fe and Mn oxide-bound As (acid ammonium oxalate-extractable As and hydroxylamine hydrochloride-extractable As), and their impacts on microbial biomass (microbial biomass C, total bacterial and fungal biomass, active bacterial and fungal biomass), enzyme activities representing four major soil biogeochemical cycles, i.e., C (β-glucosidase activity), N (urease activity), P (acid phosphomonoesterase activity), S (acryl-sulfatase activity), and microbial activity (fluorescein diacetate hydrolysis and dehydrogenase activity) were investigated in As-contaminated soils of Ambagarh Chauki block, Chhattisgarh, Central India. The results revealed that the majority of the As in soils resided in the Fe/Mn oxide-bound fraction. The microbial biomass C, total and active fungal biomass, and enzyme activities were significantly inhibited by all the forms of As. However, water-soluble As, even though occupying only a small portion of the total As (0.9–2.9 %), exerted the greatest impact. Interestingly, total and active bacterial biomass was not significantly affected by As toxicity, suggesting their resistance to As. Urease activity was not affected by As pollution.     

Soil microorganisms are less susceptible than crop plants to potassium deficiency

To evaluate the relationship between the potassium (K) status in the microbial community and the exchangeable K concentration in soils, the effects of K addition on microbial activity were assessed in cultivated Andisols not having received K fertilizer. Potassium limitation was not observed in the microbial community, even in a soil amended with only nitrogen (N) and phosphorus (P) but not K since 1938, though crop plants in this soil showed severe K deficiency symptoms. Furthermore, in a soil amended with NP + compost, microbial activity was limited by K only after limitation of carbon (C) and N. These results suggest that soil microorganisms demand more C and N than K, even in soils with low K availability, and also that the soil microbial community is less susceptible to K deficiency than are crop plants.     

Effect of liming on some chemical,biochemical, and microbiological properties of acid soils under spruce (Picea abies L.)

Summary Soil pH, total organic C, total N, exchangeable Al, available P, CO₂ evolution, microbial biomass C and N, phosphatase and dehydrogenase activities were determined in acid soils sampled under spruce subjected to acid deposition, before and after liming. A slight decrease in pH values was observed from the edge of a tree canopy to the base of the trunk in acid soils. Liming drastically reduced exchangeable Al and increased CO₂ evolution, microbial biomass, and the metabolic quotient. The microbial biomass C to total organic C ratio increased after liming but did not reach 2%, the average value considered valid in soils where the C content is in equilibrium, that is when C inputs are equal to C outputs. The microbial biomass C:N ratio decreased after liming, thus indicating that bacteria became predominant over fungi when soil acidity decreased. Dehydrogenase activity but not phosphatase activity was increased by liming. The decrease in phosphatase activity was not completely related to the increase in available P, but was also dependent on microbial growth and the decrease in acid phosphatase, the predominant component of acid soils.     

FORMATION OF MICROBIAL BIOMASS DURING THE DECOMPOSITION OF 14C LABELLED RYEGRASS IN SOIL

Ryegrass uniformly labelled with ^I4C was incubated aerobically at 25°C for 62 days in two contrasting soils, a near-neutral (pH 6.8) palcudalf from England and a strongly acid (pH 3.6) haplorthox from Brazil. Decomposition of the labelled plant material was faster in the near-neutral soil throughout the whole of the incubation period. In neither soil did the addition of fresh plant material significantly accelerate the evolution of CO₂ from organic matter already in the soil, i.e. there was no priming action. In the near-neutral soil there was a rapid build up of labelled microbial biomass in the first 6 days, followed by a much slower increase that continued throughout the whole incubation period. After 62 days 22.5% of the labelled C remaining in the near-neutral soil was in the biomass. The yield coefficient (the fraction of the incoming plant C converted to microbial C) of this stabilized or ‘resting’ biomass was 0.15. Much less labelled microbial biomass was formed in the acid soil than in the near-neutral soil. By the end of 62 days only 6.2% of the labelled carbon remaining in the acid soil was in the biomass. Biomass C measurements in strongly acid soils must however be treated with caution as the technique used has not yet been adequately validated for such soils.     

Topographic variation of soil nitrogen mineralization and microbial biomass in Japanese cedar (Cryptomeria japonica D. Don) stands at Myougodani watershed,Toyama, Japan

The spatial variability of nitrogen (N) mineralization, nitrification, and microbial biomass was investigated using surface soils from various topographic positions at a relatively small watershed with Japanese cedar (Crgptomeria japonica D. Don) plantations. The watershed topography was characterized using a topographic index derived from GIS analysis. The topographic index reasonably reflected the spatial variability of the soil water conditions, total soil carbon (C) and N contents, and exchangeable base concentrations. However, this index was not significantly correlated with the spatial variability of net N mineralization and microbial biomass. Topographic index and soil properties (total soil C and N contents, C / N ratio, exchangeable base concentrations, and clay content) were subjected to principal component analysis to eliminate multiple-collinearity among the variables, and express the variables as new orthogonal variables. Principal component analysis showed that the soil properties could be divided into two groups: PC1 (soil nutrient pools) and PC2 (soil clay content). The topographic index was closely correlated with PC1 and not significantly correlated with PC2. Regression of PC scores on net N mineralization and microbial biomass indicated the relatively high contribution of PC2 to the variability in N mineralization and microbial biomass. This result suggested that not only topographic factors but also the clay content exerted an important influence on the spatial pattern of N mineralization and microbial biomass within a watershed with single species forests.     

Drying and rewetting effects on C and N mineralization and microbial activity in surface and subsurface California grassland soils

Rewetting a dry soil has long been known to cause a burst of respiration (the “Birch Effect”). Hypothesized mechanisms for this involve: (1) release of cellular materials as a result of the rapid increase in water potential stress and (2) stimulating C-supply to microbes via physical processes. The balance of these factors is still not well understood, particularly in the contexts of multiple dry/wet cycles and of how resource and stress patterns vary through the soil profile. We evaluated the effects of multiple dry/wet cycles on surface and subsurface soils from a California annual grassland. Treatments included 4, 6, and 12 cycles that varied the length of the drying period between rewetting events. Respiration was monitored after each wetting event while extractable C and N, microbial biomass, and microbial activity were assayed initially, after the first rewetting event, and at the end of the experiment. Initially, microbial biomass and activity (respiration, dehydrogenase, and N mineralization) in subsurface soils were ca. 10% and 20% of surface soil levels. After multiple cycles, however, subsurface soil microbial biomass and activity were enhanced by up to 8-fold, even in comparison to the constantly moist treatment. By comparison, surface soil microbial biomass and activity were either moderately (i.e. 1.5 times increase) or not affected by wetting and drying. Drying and rewetting led to a cascade of responses (soluble C release, biomass growth, and enhanced activity) that mobilized and metabolized otherwise unavailable soil carbon, particularly in subsurface soils.     

Microbial use of organic amendments in saline soils monitored by changes in the C/C ratio

An incubation experiment was carried out to investigate whether salinity at high pH has negative effects on microbial substrate use, i.e. the mineralization of the amendment to CO₂ and inorganic N and the incorporation of amendment C into microbial biomass C. In order to exploit natural differences in the ¹³C/¹²C ratio, substrate from two C₄ plants, i.e. highly decomposed and N-rich sugarcane filter cake and less decomposed N-poor maize leaf straw, were added to two alkaline Pakistani soils differing in salinity, which had previously been cultivated with C₃ plants. In soil 1, the additional CO₂ evolution was equivalent to 65% of the added amount in the maize straw treatment and to 35% in the filter cake treatment. In the more saline soil 2, the respective figures were 56% and 32%. The maize straw amendment led to an identical immobilization of approximately 48 μg N g⁻¹ soil over the 56-day incubation in both soils compared with the control soils. In the filter cake treatment, the amount of inorganic N immobilized was 8.5 μg N g⁻¹ higher in soil 1 than in soil 2 compared with the control soils. In the control treatment, the content of microbial biomass C₃-C in soil 1 was twice that in soil 2 throughout the incubation. This fraction declined by about 30% during the incubation in both soils. The two amendments replaced initially similar absolute amounts of the autochthonous microbial biomass C, i.e. 50% of the original microbial biomass C in soil 1 and almost 90% in soil 2. The highest contents of microbial biomass C₄-C were equivalent to 7% (filter cake) and 11% (maize straw) of the added C. In soil 2, the corresponding values were 14% lower. Increasing salinity had no direct negative effects on microbial substrate use in the present two soils. Consequently, the differences in soil microbial biomass contents are most likely caused indirectly by salinity-induced reduction in plant growth rather than directly by negative effects of salinity on soil microorganisms.     

Phosphorus limitation in microorganisms leads to high phosphomonoesterase activity in acid forest soils

We addressed the mechanistic basis for a negative correlation between soil pH and phosphomonoesterase activity, often found in various soils. Also in the present study, a significant negative correlation was observed between soil pH and phosphomonoesterase activity measured at pH 6.5 in Japanese acidic forest soils (3 Inceptisols, 3 non-allophanic Andisols, and 2 allophanic Andisols). A hypothesis that higher activity of phosphomonoesterase in acid forest soils results from increased synthesis of phosphomonoesterase by microorganisms in response to P limitation was tested. Soils with lower pH showed a lower optimum pH for phosphomonoesterase activity and greater activity at the optimum pH than other soils. To assess nutrient limitations of the soil microbial community, the effects of addition of C, N, or P on phosphomonoesterase and dehydrogenase activities, which is an intracellular enzyme and the activity of which reflects overall microbial activity, were examined in the soil samples. Addition of P increased dehydrogenase activity in some forest soils. Also, microorganisms in some soils were co-limited by C, N, and P. Response ratios (RR) of phosphomonoesterase and dehydrogenase activities in P-amended soil to their activity in non-amended soil were used to evaluate the response of soil microorganisms to P limitation. The ratio of RR-dehydrogenase to RR-phosphomonoesterase was strongly correlated with phosphomonoesterase activity at the optimum pH (P < 0.01). The results indicate that P limitation accounts for higher phosphomonoesterase activity in the more acid forest soils.     

Nitrogen pools and turnover in arable soils under different durations of organic farming: I: Pool sizes of total soil nitrogen,microbial biomass nitrogen,and potentially mineralizable nitrogen

Soil organic matter contents, soil microbial biomass, potentially mineralizable nitrogen (N) and soil pH values were investigated in the Ap horizons of 14 field plots at 3 sites which had been under organic farming over various periods. The objective was to test how these soil properties change with the duration of organic farming. Site effects were significant for pH values, microbial biomass C and N, and for potentially mineralizable N at 0—10 cm depth. The contents of total organic C, total soil N, and potentially mineralizable N tended to be higher in soils after 41 versus 3 years of organic farming, but the differences were not significant. Microbial biomass C and N contents were higher after 41 years than after 3 years of organic farming at 0—10 cm depth, and the pH values were increased at 10—27 cm depth. Nine years of organic farming were insufficient to affect soil microbial biomass significantly. Increased biomass N contents help improve N storage by soil micro‐organisms in soils under long‐term organic farming.     

Impact of activated charcoal and tannin amendments on microbial biomass and residues in an irrigated sandy soil under arid subtropical conditions

Effects of goat manure application combined with charcoal and tannins, added as feed additives or mixed directly, on microbial biomass, microbial residues and soil organic matter were tested in a 2-year field trial on a sandy soil under Omani irrigated subtropical conditions. Soil microbial biomass C revealed the fastest response to manure application, followed by microbial residue C, estimated on the basis of fungal glucosamine and bacterial muramic acid, and finally soil organic C (SOC), showing the slowest, but still significant response. At the end of the trial, microbial biomass C reached 220 μg g^?1 soil, i.e. contents similar to sandy soils in temperate humid climate, and showed a relatively high contribution of saprotrophic fungi, as indicated by an average ergosterol to microbial biomass C ratio of 0.35 % in the manure treatments. The mean fungal C to bacterial C ratio was 0.55, indicating bacterial dominance of microbial residues. This fraction contributed relatively low concentrations of between 20 and 35 % to SOC. Charcoal added to manure increased the SOC content and the soil C/N ratio, but did not affect any of the soil microbial properties analysed. Tannins added to manure reduce the 0.5 M K₂SO₄-extractable N to N total ratio compared to manure control. These effects occurred regardless of whether charcoal or tannins were supplied as feed additive or directly mixed to the manure.     

Soil characteristics determine soil carbon and nitrogen availability during leaf litter decomposition regardless of litter quality

Climate and litter quality have been identified as major drivers of litter decomposition, but our knowledge of how soil characteristics (e.g. microbial community and chemical properties) determine carbon (C) and nitrogen (N) availability derived from the decomposition of litter of different qualities is still scarce. We conducted a microcosm experiment to evaluate how soils with contrasting microbial communities and soil properties (denoted Soils A and B hereafter, where Soil B has higher bacterial and fungal abundance, fungal:bacterial ratio, and organic C than Soil A) determine the availability of soil C (carbohydrates, proteins, amino acids and phenols) and N (dissolved organic and inorganic N, microbial biomass N and available N) during the decomposition of litter of contrasting quality (C:N ratios ranging from 20 to 102). We also evaluated the relative importance of soil characteristics and litter quality as drivers of C and N inputs to the soil during this process. Overall, higher soil C and N availability after litter decomposition was found in Soil B than in Soil A. Soil characteristics had a higher positive effect on soil C and N contents than litter quality during litter decomposition. We also found that changes in N availability and organic matter quality registered after litter decomposition, linked to different soil characteristics, were able to promote dissimilarities in the potential mineralization rates. In conclusion, our study provides evidence that soil characteristics (e.g. microbial communities and chemical properties) can be more important than litter quality in determining soil C and equally important for N availability during the decomposition of leaf litter.     

Soil fungi influence the distribution of microbial functional groups that mediate forest greenhouse gas emissions

The distribution of microbial functional groups in soil may be governed by the interaction between the soil environment and the presence of other microbial competitors or facilitators. In forest soils, one of the most important groups of organisms are fungi, which are vital to many ecosystem processes such as nutrient cycling and decomposition, and can form direct connections to primary producers. Nevertheless, the overall effect of soil fungi on the structure and distribution of the other soil microbial functional groups has not been thoroughly investigated. We hypothesized that by altering the soil environment, fungi create favorable conditions for Archaea, methane oxidizing bacteria (MOB) and denitrifying bacteria (DNB), thereby potentially influencing the ability of forest soils to produce or consume greenhouse gases. To test these hypotheses, we studied the distribution of microbial functional groups and fungi in forest soil using molecular methods and related that distribution to soil environment and extracellular enzyme activity as a measure of microbial activity and metabolic effort. Non-metric multidimensional scaling of terminal restriction fragment length (TRFLP) profiles found that DNB and MOB largely separated within ordination space, suggesting little overlap of these bacteria in soil cores. In addition, DNB were significantly positively correlated with fungal biomass and with chitinase activity while MOB were negatively correlated with both. Most archaeal TRFs were also negatively correlated with fungal biomass, suggesting that forest Archaea and MOB have similar relationships to fungal biomass. Soil chemistry including soil carbon (C), nitrogen (N) and bicarbonate extractable phosphorus (P) were not significantly correlated with DNB, MOB or Archaea. Our results suggest that soil fungi might influence the spatial distribution of important prokaryotic groups in forests, including some groups that mediate the production and consumption of important greenhouse gases.     

Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation

Cultivation is known to influence the organic matter status and structural stability of soil. We investigated the effects of 69 yr of cultivation on the nature, distribution and activity of microbial biomass (MB) in different aggregate size classes of an Orthic Brown Chernozemic soil. Cultivation decreased MB content, its activity and enzyme activity in soil. Microaggregate (<0.25mm) size classes in both native and cultivated soils contained lower organic-C, MB-C, fungal biomass, arylsulfatase, acid phosphatase and respiratory activities as compared to macroaggregates. However, the negative effects of cultivation were more pronounced on macroaggregate size classes. Nutrient ratios of both whole aggregates and microbial biomass were narrower in aggregates from cultivated soil as compared to native soil. In both native and cultivated soils, mineralization of C. N and S was greater in macroaggregates as compared to that in microaggregates. The greatest effect of cultivation on nutrient and microbial characteristics was observed in the 0.25 to 1.00 mm dia size classes. These results suggest that microbial biomass, especially fungal biomass, plays an important role in the formation of macroaggregates and is the labile organic matter that serves as the primary source of C and nutrients released following cultivation.     

Soil biochemical and microbial indices in wet tropical forests: Effects of deforestation and cultivation

Little information is available about the long‐term effects of deforestation and cultivation on biochemical and microbial properties in wet tropical forest soils. In this study, we evaluated the general and specific biochemical properties of soils under evergreen, semi‐evergreen, and moist deciduous forests and adjacent plantations of coconut, arecanut, and rubber, established by clear felling portions of these forests. We also examined the effects of change in land use on microbial indices and their interrelationships in soils. Significant differences between the sites occurred for the biochemical properties reflecting soil microbial activity. Microbial biomass C, biomass N, soil respiration, N mineralization capacity, ergosterol, levels of adenylates (ATP, AMP, ADP), and activities of dehydrogenase and catalase were, in general, significantly higher under the forests than under the plantations. Likewise, the activities of various hydrolytic enzymes such as acid phosphomonoesterase, phosphodiesterase, casein‐protease, BAA‐protease, β‐glucosidase, CM‐cellulase, invertase, urease, and arylsulfatase were significantly higher in the forest soils which suggested that deforestation and cultivation markedly reduced microbial activity, enzyme synthesis and accumulation due to decreased C turnover and nutrient availability. While the ratios of microbial biomass C : N and microbial biomass C : organic C did not vary significantly between the sites, the ratios of ergosterol : biomass C and ATP : biomass C, qCO2 and AEC (Adenylate Energy Charge) levels were significantly higher in the forest sites indicating high energy requirements of soil microbes at these sites.     

中国北方温带草原土壤微生物对气温变暖与添加氮素的响应特征

The responses of soil microbes to global warming and nitrogen enrichment can profoundly affect terrestrial ecosystem functions and the ecosystem feedbacks to climate change. However, the interactive effect of warming and nitrogen enrichment on soil microbial community is unclear. In this study, individual and interactive effects of experimental warming and nitrogen addition on the soil microbial community were investigated in a long-term field experiment in a temperate steppe of northern China. The field experiment started in 2006 and soils were sampled in 2010 and analyzed for phospholipid fatty acids to characterize the soil microbial communities. Some soil chemical properties were also determined. Five-year experimental warming significantly increased soil total microbial biomass and the proportion of Gram-negative bacteria in the soils. Long-term nitrogen addition decreased soil microbial biomass at the 0-10 cm soil depth and the relative abundance of arbuscular mycorrhizal fungi in the soils. Little interactive effect on soil microbes was detected when experimental warming and nitrogen addition were combined. Soil microbial biomass positively correlated with soil total C and N, but basically did not relate to the soil C/N ratio and pH. Our results suggest that future global warming or nitrogen enrichment may significantly change the soil microbial communities in the temperate steppes in northern China.     

Carbon and nitrogen turnover in two acid forest soils of southeast Australia as affected by phosphorus addition and drying and rewetting cycles

The effects of adding P and of drying and rewetting were studied in two acid forest soils from southeast Australia. The soils were a yellow podzolic with a low soil organic matter content (3.75% C) and a red earth with a high organic matter content (13.5% C). C and N mineralization and microbial C and N contents were investigated in a laboratory incubation for 151 days. Microbial C and N were estimated by a hexanol fumigation-extraction technique. Microbial C was also determined by substrate-induced respiration combined with a selective inhibition technique to separate the fungal and the bacterial biomass. The results obtained by the selective inhibition technique were not conclusive. Adding P to the soil and drying and rewetting the soil reduced microbial N. This effect was more pronounced in rapidly and frequently dried soils. Microbial C was generally less affected by these treatments. Compared with the control, the addition of P caused a reduction in respiration in the red earth (-13%) but an increase in the yellow podzolic soil (+12%). In the red earth net N mineralization was highest following the addition of P. In the yellow podzolic soil highest N mineralization rates were obtained when the soil was subjected to drying and rewetting cycles. In both soils increased N mineralization was associated with a decrease in microbial N, indicating that the mineralized N was of microbial origin. Nitrification decreased with rapid drying and rewetting. The addition of P promoted heterotrophic nitrification in both soils.     

Bestimmung mikrobieller Biomasse im Boden: Eine kritische Betrachtung

Estimation of Microbial Biomass in Soil: A Critical View Unrepresentative factors have been applied to calculate microbial C in a wide range of soils without consideration of soil properties, physiological state of microbial populations, and experimental conditions. In this paper principal and technical difficulties of different methods to estimate microbial biomass in soils as well as interpretation of results are discussed. The author concludes, that a standard method to measure microbial C in a wide range of soils remains an elusive aim. Neither is there a need to convert microbial data into biomass, especially for many questions related to the role of microorganisms in soils (soil fertility, ecotoxicology, the influence of climatic factors, soil treatments and cultivation, etc.).