Impact of reduced tillage on carbon and nitrogen storage of two Haplic Luvisols after 40 years

It is broadly accepted that reduced tillage increases soil organic carbon (C_org) and total nitrogen (N) concentrations in arable soils. However, the underlying processes of sequestration are not completely understood. Thus, our objectives were to investigate the impact of a minimum tillage (MT) system (to 5–8 cm depth) on aggregates, on particulate organic matter (POM), and on storage of C_org and N in two loamy Haplic Luvisols in contrast to conventional tillage (CT) (to 25 cm). Surface soils (0–5 cm) and subsoils (10–20 cm) of two experimental fields near Göttingen, Germany, were investigated. Each site (Garte-Süd and Hohes Feld) received both tillage treatments for 37 and 40 years, respectively. In the bulk soil of both sites C_org, N, microbial carbon (C_mic), and microbial N (N_mic) concentrations were elevated under MT in both depths. Likewise, water-stable macroaggregates (>0.25 mm) were on average 2.6 times more abundant under MT than under CT but differences in the subsoils were generally not significant. For surface soils under MT, all aggregate size classes <1 mm showed approx. 35% and 50% increased C_org concentrations at Garte-Süd and Hohes Feld, respectively. For greater macroaggregates (1–2, 2–10 mm), however, differences were inconsistent. Elevations of N concentrations were regular over all size classes reaching 61% and 52%, respectively. Density fractionation of the surface soils revealed that tillage system affected neither the yields of free POM nor occluded POM nor their C_org and N concentrations. Moreover, more C_org and N (15–238%) was associated within the mineral fractions investigated under MT in contrast to CT. Overall, similar to no-tillage, a long-term MT treatment of soil enhanced the stability of macroaggregates and thus was able to physically protect and to store more organic matter (OM) in the surface soil. The increased storage of C_org and N did not occur as POM, as reported for no-tillage, but as mineral-associated OM.     

Carbon flow into microbial and fungal biomass as a basis for the belowground food web of agroecosystems

The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat + maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat + maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (C_org), extractable organic C (EOC), microbial biomass (C_mic) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to C_org, EOC, C_mic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass.     

Microbial respiration activities of soils from different climatic regions of European Russia

The aim of this study was to survey and evaluate the microbial respiration of main soil types (gleyic Cryosols, umbric Albeluvisols, albic Luvisols, luvic Chernozems, Kastanozems) across European Russia, from semiarid to polar climatic zones. Soil was sampled from 0–5 and 5–10 cm layers at natural (forest, grassland, fallow) and corresponding sites under agricultural land use. Soil microbial biomass carbon (C_mic) determined by the substrate-induced respiration method and basal respiration (BR) were measured under standardized laboratory conditions (22 °C, 60% WHC). The ratios of BR/C_mic and C_mic/C_org were also calculated. C_mic and BR were highest in polar (gleyic Cryosols) and temperate (albic Luvisols, luvic Chernozems) climatic zones, the lowest were in boreal (umbric Luvisols) and semiarid (Kastanozems). C_mic, BR and C_mic/C_org ratios were higher in 0–5 cm layers compared to the corresponding 5–10 cm and in natural sites versus in arable. Principal component analysis yielded a clear separation of the vegetation zones with respect to the several principal components (PC). PC 1 was composed of C_mic, BR, soil chemical (C_org, N_tot) and texture parameters. PC 2 was composed of climatic (MAT, MAP) and soil pH variables. Three-way ANOVA indicated that “soil type”, “ecosystem” and “layer” factors, and their interactions accounted for almost 98 and 99% of the total variance in C_mic and BR, respectively.     

Carbon and nitrogen enhancement in Cambisols and Vertisols by Acacia spp. in eastern Burkina Faso: Relation to soil respiration and microbial biomass

The current study tested the contribution of native Acacia species of the Sudano-Sahelian zone to improving organic carbon and nitrogen level in Cambisols and Vertisols with specific focus on variation in microbial biomass (C_mic), soil basal respiration (C_resp) and metabolic quotient (qCO₂). The results show enrichment in total organic carbon (C_total), in total nitrogen (N_total) and higher clay content under Acacia canopies as compared to adjacent open grasslands. The relative nutrient concentration in Acacia cover showed an increase in C_mic ranging from 203 to 572 μg g⁻¹ whereas in adjacent open grassland it varied from 100 to 254 CO₂–C μg g⁻¹. As a function of C_mic (r = 0.60), C_total (r = 0.70) and N_total (r = 0.70), C_resp was higher under Acacia canopies than open grassland and this difference was more pronounced when measured over lengthier incubation periods (10–21 days). A lower qCO₂ under Acacia cover (except for one site) demonstrated a change in microorganisms communities structure and higher substrate use efficiency as compared to open grassland. The results also show that soil texture, as well as vegetation cover, influenced microbial processes. The negative correlation between clay content and carbon mineralization (C_resp/C_total, qCO₂), and positive linear relation between clay and C_mic supported the hypothesis that finer soil texture protects soil microbial biomass against degradation and limits organic matter mineralization. The specific effects of soil typology and vegetation cover on C_mic and qCO₂ variability were significant, but the greater effects were attributed to vegetation cover.     

Soil microbiological and biochemical properties affected by plant growth and different long-term fertilisation

The purpose of this study was to measure the effects of plant growth on soil microbial biomass C (C_mic) and soil enzyme activities. In a pot experiment using spring barley and sugar beet, we investigated the response of C_mic, hot water extractable C and N fractions (C_hwe, N_hwe), and enzyme activities involved in C, N and P cycling in a loess-derived Chernozem from Bad Lauchstädt (Central Germany). The study site has been receiving the same fertilisation treatments for 100 years. The soil originated from plots fertilised with 15 t ha⁻¹ farmyard manure (FYM) year⁻¹ + mineral fertiliser (NPK), or 15 t ha⁻¹ FYM year⁻¹, or NPK or from an unfertilised control. Pots were sampled monthly, and alkaline phosphatase- (AP), β-glucosidase- and protease-activities were analysed. At the beginning and the end of study, levels of C_mic, C_hwe and N_hwe were also measured. All three enzyme activities and C_mic were significantly and positively correlated with C_org and C_hwe. Results suggest that the enzyme activities measured originated mostly from microorganisms and that C_hwe is an important C source for soil microorganisms. β-Glucosidase and AP activities were higher in summer months than at other times. In contrast, protease activity changed only slightly during the growing period.     

Influence of reduced tillage on earthworm and microbial communities under organic arable farming

Although reduced tillage is an agricultural practice reported to decrease soil erosion and external inputs while enhancing soil fertility, it has still rarely been adopted by European organic farmers. The objective of this study was to assess the long-term interactive effects of tillage (conventional (CT) vs. reduced (RT)) and fertilization (slurry (S) vs. composted manure/slurry (MCS)) on earthworms and microbial communities in a clay soil under spelt in an organic 6-year crop rotation. Earthworm populations (species, density and biomass, cocoons) were investigated by handsorting the soil nine years after initial implementation of the treatments. Soil microbial carbon (C_mic) and nitrogen (N_mic) were measured by chloroform-fumigation extraction and a simplified phospholipid fatty acid (PLFA) analysis was used to separate for populations of bacteria, fungi and protozoa. Significantly increased total earthworm density in RT plots was mainly attributed to increased numbers of juveniles. Moreover, we found five times more cocoons with RT. Species richness was not affected by the treatments, but tillage treatments had differentially affected populations at the species-level. In addition, cluster analysis at the community level revealed two distinct groups of plots in relation to tillage treatments. In RT plots C_mic increased in the 0–10 cm and 10–20 cm soil layers, while PLFA concentrations indicative of Gram-negative bacteria, fungi and protozoa only increased in the topsoil. Lower bacteria-to-fungi ratios in the upper soil layer of RT plots indicated a shift to fungal-based decomposition of organic matter whereas a higher C_mic-to-C_org ratio pointed towards enhanced substrate availability. Slurry application decreased microbial biomass and enhanced density of juvenile anecic earthworms but overall fertilization effect was weak and no interactions with tillage were found. In conclusion, tillage is a major driver in altering communities of earthworms and microorganisms in arable soils. The use of reduced tillage provides an approach for eco-intensification by enhancing inherent soil biota functions under organic arable farming.     

Accounting for variability in soil microbial communities of temperate upland grassland ecosystems

This study aimed to determine the factors which regulate soil microbial community organisation and function in temperate upland grassland ecosystems. Soil microbial biomass (C_mic), activity (respiration and potential carbon utilisation) and community structure (phospholipid fatty acid (PLFA) analysis, culturing and community level physiological profiles (CLPP) (Biolog^®)) were measured across a gradient of three upland grassland types; Festuca–Agrostis–Galium grassland (unimproved grassland, National Vegetation Classification (NVC) — U4a); Festuca–Agrostis–Galium grassland, Holcus–Trifolium sub-community (semi-improved grassland, NVC — U4b); Lolium–Cynosurus grassland (improved grassland, NVC — MG6) at three sites in different biogeographic areas of the UK over a period of 1 year. Variation in C_mic was mainly due to grassland type and site (accounting for 55% variance, v, in the data). C_mic was significantly (P<0.001) high in the unimproved grassland at Torridon (237.4 g C m⁻² cf. 81.2 g C m⁻² in semi- and 63.8 g C m⁻² in improved grasslands) and Sourhope (114.6 g C m⁻² cf. in 44.8 g C m⁻² semi- and 68.3 g C m⁻² in improved grasslands) and semi-improved grassland at Abergwyngregyn (76.0 g C m⁻² cf. 41.7 g C m⁻² in un- and 58.3 g C m⁻² in improved grasslands). C_mic showed little temporal variation (v=3.7%). Soil microbial activity, measured as basal respiration was also mainly affected by grassland type and site (n=32%). In contrast to C_mic, respiration was significantly (P<0.001) high in the improved grassland at Sourhope (263.4 l h⁻¹m⁻² cf. 79.6 l h⁻¹m⁻² in semi- and 203.9 l h⁻¹m⁻² unimproved grasslands) and Abergwyngregyn (198.8 l h⁻¹m⁻² cf. 173.7 l h⁻¹m⁻² in semi- and 88.2 l h⁻¹m⁻² unimproved grasslands). Microbial activity, measured as potential carbon utilisation, agreed with the respiration measurements and was significantly (P<0.001) high in the improved grassland at all three sites (A₅₉₀ 0.14 cf. 0.09 in semi- and 0.07 in unimproved grassland). However, date of sampling also had a significant (P<0.001) impact on C utilisation potential (v=24.7%) with samples from April 1997 having highest activity at all three sites. Variation in microbial community structure was due, predominantly, to grassland type (average v=23.6% for bacterial and fungal numbers and PLFA) and date of sampling (average v=39.7% for bacterial and fungal numbers and PLFA). Numbers of culturable bacteria and bacterial PLFA were significantly (P<0.001) high in the improved grassland at all three sites. Fungal populations were significantly (P<0.01) high in the unimproved grassland at Sourhope and Abergwyngregyn. The results demonstrate a shift in soil microbial community structure from one favouring fungi to one favouring bacteria as grassland improvement increased. Numbers of bacteria and fungi were also significantly (P<0.001) higher in August than any other sampling date. Canonical variate analysis (CVA) of the carbon utilisation data significantly (P<0.05) differentiated microbial communities from the three grassland types, mainly due to greater utilisation of sugars and citric acid in the improved grasslands compared to greater utilisation of carboxylic acids, phenolics and neutral amino acids in the unimproved grasslands, possibly reflecting substrate availability in these grasslands. Differences in C_mic, activity and community structure between grassland types were robust over time. In addition, broad scale measures of microbial growth and activity (C_mic and respiration) showed little temporal variation compared to measures of soil microbial community structure, which varied quantitatively with respect to environmental variables (temperature, moisture) and plant productivity, hence substrate supply.     

Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piauí state,Brazil

The aim of this work was to investigate the response of soil microbial biomass and activity to practices in organic and conventional farming systems. The study was carried out at the Irrigation District of Piauí, Brazil. Five different plots planted with “acerola” orchard (Malpighia glaba) and established at the following management were evaluated: (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24). Soil microbial biomass C (C_mic), basal respiration, organic carbon (C_org), C_mic-to-C_org ratio and metabolic quotient (qCO₂) were evaluated in soil samples collected at 0–10 cm depth. The highest C_org and C_mic levels occurred in organic system plots ORG18 and ORG24 compared to the conventional system. Soil respiration and C_mic-to-C_org ratio were significantly enhanced by the organic system plots. The qCO₂ was greater in conventional than in organic system. These results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C.     

The relationships among microbial parameters and the rate of organic matter mineralization in forest soils,as influenced by forest type

Vegetation type influences the rate of accumulation and mineralization of organic matter in forest soil, mainly through its effect on soil microorganisms. We investigated the relationships among forest types and microbial biomass C (MBC), basal respiration (R_B), substrate-induced respiration (R_S), N mineralization (N_min), specific growth rate μ, microbial eco-physiology and activities of seven hydrolytic enzymes, in samples taken from 25 stands on acidic soils and one stand on limestone, covering typical types of coniferous and deciduous forests in Central Europe. Soils under deciduous trees were less acidic than soils of coniferous forests, which led to increased mineralizing activities R_B and N_min, and a higher proportion of active microbial biomass (R_S/MBC) in the Of horizon. This resulted in more extractable organic C (0.5 M K₂SO₄) in soils of deciduous forests and a higher accumulation of soil organic matter (SOM) in coniferous forest soil. No effect of forest type on the microbial properties was detected in the Oh horizon and in the 0–10 cm layer. The microbial quotient (MBC/C_org), reflecting the quality of organic matter used for microbial growth, was higher in deciduous forests in all three layers. The metabolic quotient qCO₂ (R_B/MBC) and the specific growth rate μ, estimated using respiration growth curves, did not differ in soils of both forest types. Our results showed that the quality of SOM in coniferous forests supported microorganisms with higher activities of β-glucosidase, cellobiosidase and β-xylosidase, which suggested the key importance of fungi in these soils. Processes mediated by bacteria were probably more important in deciduous forest soils with higher activities of arylsulphatase and urease. The results from the stand on limestone showed that pH had a positive effect on microbial biomass and SOM mineralization.     

Effects of chemical conditions in re-wetted peats on temporal variation in microbial biomass and acid phosphatase activity within the growing season

Possible effects of chemical alterations in peat following re-wetting on their microbial characteristics are insufficiently known. Microbial biomass carbon (C_mic), nitrogen (N_mic), phosphorus (P_mic) and acid phosphatase activity were investigated in re-wetted virtually undisturbed and differently degraded peatlands (Histosols) in northeast Germany to assess re-wetting effects on microbial biomass production and phosphorus (P) cycling in one growing season. The virtually undisturbed Eutri-Ombric Histosol had the largest content of microbial biomass (C_mic: 2132 mg/kg, N_mic: 309 mg/kg and P_mic: 48 mg/kg; means of six sampling dates, upper 10 cm). Increasingly lower contents of microbial biomass were observed in the more strongly degraded peats of two Ombri-Sapric Histosols. Furthermore, the proportions of P_mic as a percent of total P (P_t) were smallest in the strongly degraded Ombric-Sapric Histosol (1.6% of P_t) and gradually larger with better peat conservation (2.6% of P_t in the moderately degraded Ombri-Sapric Histosol and 3.0% of P_t in the virtually undisturbed Eutri-Ombric Histosol). The acid phosphatase activity was always greatest in May, irrespective of peat degradation. This maximum was lower for the Eutri-Ombric Histosol (2633 μg nitrophenol/(g h)) than for the two Ombri-Sapric Histosols (3963 and 3212 μg nitrophenol/(g h)). In the two degraded peats, the temporal variation in phosphatase activity was also more pronounced. Our results, in particular the higher peak phosphatase activity combined with an incorporation of P into microbial biomass, indicate that peat degradation may enhance the phosphate input to soil solution. Thus, it is concluded that modified biological P cycling could contribute to increased risks of P losses to adjacent surface water after re-wetting of degraded peats.     

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.     

Applying an oxygen-based respiratory assay to assess soil microbial responses to substrate and N availability

Documented approaches for measuring soil microbial activities and their controlling factors under field conditions are needed to advance understanding of soil microbial processes for numerous applications. We manipulated field plots with carbon (C) and nitrogen (N) additions to test the capability of a respiratory assay to: (1) measure respiration of endogenous soil C in comparison to field-measured CO₂ fluxes; (2) determine substrate-induced respiratory (SIR) activities that are consistent with substrate availability in the field; and, (3) report N availability in the field based on assay responses with and without added N. The respiratory assay utilizes a microplate containing an oxygen-sensitive fluorescent ruthenium dye. Respiratory activities measured with this approach have previously been shown to occur within short (6–8 h) incubation periods using low substrate concentrations that minimize enrichment during the assay. Field treatments were conducted in a randomized full-factorial design with C substrate (casamino acids, glucose, or none) and inorganic N (±) as the treatment factors. With one exception, we found that respiration of endogenous soil C in the assay responded to the field treatments in a similar manner to CO₂ fluxes measured in the field. Patterns of SIR with low concentrations of added amino acid or carbohydrate substrate (200 μg C g⁻¹ soil) were consistent with field treatments. The ratio (N_ratio) of carbohydrate respiration with added N (25 μg N g⁻¹ soil) to the same without N in the assay was significantly (P < 0.05) decreased by field N amendment. The carbohydrate N_ratio exhibited a logarithmic relationship (r = 0.64, P < 0.05) with extractable inorganic soil nitrate and ammonium concentrations. These data significantly extend and support the capability of this oxygen-based respiratory assay to evaluate in situ soil activities and examine factors that limit these activities.     

Formation and use of microbial residues after adding sugarcane sucrose to a heated soil devoid of soil organic matter

A 67-day incubation experiment was carried out with a soil initially devoid of any organic matter due to heating, which was amended with sugarcane sucrose (C₄-sucrose with a δ¹³C value of ?10.5‰), inorganic N and an inoculum for recolonisation and subsequently at day 33 with C₃-cellulose (δ¹³C value of ?23.4‰). In this soil, all organic matter is in the microbial biomass or in freshly formed residues, which makes it possible to analyse more clearly the role of microbial residues for decomposition of N-poor substrates. The average δ¹³C value over the whole incubation period was ?10.7‰ in soil total C in the treatments without C₃-cellulose addition. In the CO₂ evolved, the δ¹³C values decreased from ?13.4‰ to ?15.4‰ during incubation. In the microbial biomass, the δ¹³C values increased from ?11.5‰ to ?10.1‰ at days 33 and 38. At day 67, 36% of the C₄-sucrose was left in the treatment without a second amendment. The addition of C₃-cellulose resulted in a further 7% decrease, but 4% of the C₃-cellulose was lost during the second incubation period. Total microbial biomass C declined from 200 μg g^?1 soil at day 5 to 70 μg g^?1 soil at day 67. Fungal ergosterol increased to 1.5 μg g^?1 soil at day 12 and declined more or less linearly to 0.4 μg g^?1 soil at day 67. Bacterial muramic acid declined from a maximum of 35 μg g^?1 soil at day 5 to a constant level of around 16 μg g^?1 soil. Glucosamine showed a peak value at day 12. Galactosamine remained constant throughout the incubation. The fungal C/bacterial C ratio increased more or less linearly from 0.38 at day 5 to 1.1 at day 67 indicating a shift in the microbial community from bacteria to fungi during the incubation. The addition of C₃-cellulose led to a small increase in C₃-derived microbial biomass C, but to a strong increase in C₄-derived microbial biomass C. At days 45 and 67, the addition of N-free C₃-cellulose significantly decreased the C/N ratio of the microbial residues, suggesting that this fraction did not serve as an N-source, but as an energy source.     

Long-term effects of leguminous cover crops on biochemical and biological properties in the organic and mineral layers of soils of a coconut plantation

The primary aim of the study was to determine the long-term (12 years) effects of leguminous cover crops like Atylosia scarabaeoides, Centrosema pubescens, Calopogonium mucunoides and Pueraria phaseoloides on important soil biochemical and biological properties and their interrelationships in the organic (fresh litter layer, F and fermented + humus layer, F + H) and mineral (0–10 and 10–20 cm) layers of soils of a 19-year-old coconut plantation.The total biomass production (above-ground) for the 12-year period varied significantly between the cover crops and ranged from 34.86 (calopo) to 90.43 (pueraria) Mg ha^–1. Total N and C additions at the cover cropped (CC) site for the 12-year period were 0.97–3.07 Mg ha^–1 and 16.90–43.34 Mg ha^–1, respectively. Irrespective of layers, the levels of organic C, total N, organic substrates viz., dissolved organic C and N, labile organic N, water soluble carbohydrates, and light fraction organic matter-C and were markedly higher in the CC site compared to the control. Consequently, the levels of microbial biomass-C (C_MIC), -N (N_MIC) and -P (P_MIC), net N mineralization rates, CO₂ evolution, metabolic quotient (qCO₂) and the activities of l-asparaginase, l-glutaminase and β-glucosaminidase were significantly higher in the CC site compared to the corresponding levels in the control site. Between layers, the levels of various chemical, biochemical and microbial parameters were consistently higher in the organic layers compared to the mineral layers at all the sites including control. Among the ratios of various microbial indices, the ratios of C_MIC: organic C and C_MIC: P_MIC did not differ significantly between the layers and sites. However, the ratio of C_MIC: N_MIC was relatively higher in the mineral layers and control site. The variation in individual soil properties between layers and sites reflected the concomitant changes occurring in soil organic matter content. Apparently, microbial activity was limited by the supply of biologically available substrates in the mineral layers and the control site. Contrarily, the more direct supply of nutrients from decomposing plant litter and the indirect supply of nutrients from the mineralization of organic matter led to significantly higher levels of microbial biomass in the organic layers.     

Response of microbial characteristics to heavy metal pollution of mining soils in central Tibet,China

Soil microbial activity plays a crucial role in soil microbiological processes, which can be used as a useful indicator to determine the ecological effects of heavy metal pollution on soils. The objective was to determine the effects of heavy metal pollution on mining soils at the Lawu mine of central Tibet, China on soil enzyme activities (sucrase, urease and acid phosphatase), microbial biomass C, N and P (MBC, MBN, and MBP), basal respiration, metabolic quotients, and N mineralization. Sixteen soil samples around the mine were sampled, and one soil sample, 2 km from the mine center, was taken as the control. Compared to the control, mining soils were polluted by heavy metals, Cu, Zn, Pb and Cd, resulting in decreases of sucrase activities, urease activities, acid phosphatase activities, MBC, MBN, MBP, and N mineralization, and increases of basal respiration and qCO₂. Multivariate analysis (cluster analysis [CA], principle component analysis [PCA] and canonical correlation analysis [CCA]) indicated nine microbial variables were only reduced to one principal component explaining 72% of the original variances, and MBC (R² = 0.93) had the highest positive loadings on the principal component. Mining soils polluted by heavy metals were perfectly clustered into four groups, which were highly distinguished by MBC. There were significant canonical correlations between soil heavy metals and microbial indexes on two canonical variates (R1 = 0.99, p < 0.001; R2 = 0.97, p < 0.01), which further demonstrated significant correlations between soil heavy metal contents and microbial characteristics. Hence, our results suggested that MBC may be used a sensitive indicator for assessing changes in soil environmental quality in metal mine of central Tibet.     

Long-term application of compost influences microbial biomass and enzyme activities in a tropical Aeric Endoaquept planted to rice under flooded condition

In a field study, long-term application of compost to a tropical Aeric Endoaquept under continuous rice growing in a rice-rice-fallow sequence resulted in the stimulation of microbial biomass and select soil enzyme activities. Mean seasonal soil microbial biomass-C (C_mic) increased by 42%, 39% and 89% in inorganic fertilizer, compost and compost+inorganic fertilizer treatments, respectively, over the unamended control. C_mic content was also influenced by the rice crop growth stage and was highest at maximum tillering stage irrespective of treatments and declined thereafter. Soil organic C (C_org) content showed highly significant positive correlation with dehydrogenase, urease, cellulase, β-glucosidase and fluorescein di-acetate (FDA) hydrolysis activity, and a positive but not significant correlation with invertase and amidase activity. C/N ratio which was lowest in unamended control plots showed a significant positive relationship with only the enzymes involved in C cycle. Stepwise regression analysis revealed that for prediction of both total organic C and total N, FDA hydrolysis activity contributed significantly for the variance and explained up to 85-96% variability. Results demonstrated that microbial biomass and soil enzyme activity is sensitive in discriminating between long-term organic residue amendment practices.     

Adaptive responses of soil microbial communities under experimental acid stress in controlled laboratory studies

Soil samples from nine different beech forest sites (A_h horizon) with a mean initial soil pH close to neutral (6.4, SD 0.6) were treated with different amounts of H⁺-ions using acid water of pH 0.5 (H₂SO₄). The H⁺-input needed to lower the pH by one or more than three units was in the range between 0.006 and 0.6 mg H⁺ kg⁻¹, designated as mild, strong or extreme acid stress. The soil samples were incubated for a maximum of 200 days at 20°C and their microbial biomass-C (C_mic), qCO₂ and pH was measured at intervals. In addition, the ratio of fungal:bacterial contributions to total respiration was determined at the beginning and end of the experimental period. The extent of microbial biomass-C loss (32–87%) and the increase in the qCO₂ (1.8–>7 times) in comparison to acid-untreated samples followed the amount of initial H⁺-ion input. Differences between treatments based on one-way ANOVA were significant for C_mic depression at day 8 (p<0.01) and day 80 (p<0.05), and for qCO₂ at the beginning (p<0.001) and at day 80 only between mild and extreme acid stress (p<0.027). Over time some recovery of the microbial biomass was observed with a concomitant decrease in the qCO₂, an indication of adaptation to acidic conditions by the surviving and newly formed biomass. After 80 days of incubation microbial biomass values expressed as percent microbial-C in total soil carbon (C_mic:C_org) resembled those recorded for natural sites at comparable soil pH. There was a strong reduction in bacterial respiration following mild, strong or extreme acid treatment. A recovery here over time was only noted for mild or strong acid treatments. The results confirm that soil pH is a significant controlling parameter for microbial biomass build-up and the fungal:bacterial ratio as found previously with natural site studies.     

Characterization of the forest humus microbial community in a heavy metal polluted area

Various parameters of the soil microbial community may be used in soil quality evaluation and environmental risk assessment. The objectives of this study were to assess the effects of different environmental factors on the characteristics of forest humus microbial communities, and to test which environmental factors most affect the gross microbial indices and physiological profiles of these communities. Samples were taken at 71 plots located in a heavily polluted area of the Krakowsko-Cze¸stochowska upland in southern Poland. The samples were analyzed for pH in KCl (pH_KCl), organic C (C_org), total N (N_t) and S (S_t), and for total and soluble Zn, Pb and Cd concentrations. The considered microbial parameters included basal respiration (BAS), microbial biomass (C_mic), C_mic-to-C_org ratio, and community-level physiological profiles (CLPPs) studied using BIOLOG^® Ecoplates. Multiple regression analysis was used to estimate the effects of humus properties on the microbial parameters. It indicated that S_t and C_org-to-N_t ratio were the most important factors positively affecting C_mic (β=0.15 and 0.11, respectively) and BAS (β=0.13 and 0.08, respectively). The C_mic-to-C_org ratio was related positively to S_t (β=0.12) but negatively to N_t (β=−0.08). The effects of pH_KCl and heavy metals on the gross microbial indices were significant but less important. The most important effect on microbial activity on BIOLOG^® plates and CLPPs was from pH_KCl. The other significant variables included S_t, C_org-to-N_t and interactions of heavy metals with pH_KCl. It was concluded that C_mic, C_mic-to-C_org and BAS might be good indicators of the general status of soil microbial communities, but their use in studying heavy metal effects may entail difficulties in separating the effects of other factors. The sensitivity of the BIOLOG^® test to pH_KCl suggests that it may be useful for studying the effects of acidification or liming on soil microbial communities. The significant effect of the interactions between heavy metals and other variables on physiological profiles indicated that high heavy metal content affects the metabolic functions of soil microbial populations.     

Changes in the fungal-to-bacterial respiratory ratio and microbial biomass in agriculturally managed soils under free-air CO2 enrichment (FACE) - A six-year survey of a field study

In soil ecology, microbial parameters have been identified as sensitive indicators of changes in the soil environment. The Braunschweig FACE project provided the opportunity to study the effects of elevated CO₂ (550 μmol mol⁻¹) as compared to ambient CO₂ (370 μmol mol⁻¹) on total microbial biomass (C_mic), C_mic-to-C_org ratio and the fungal-to-bacterial respiratory ratio together with total C_org, N_t, C:N ratio and pH over a six-year period. Field management followed a typical crop rotation system of this region with either a crop-related full nitrogen supply (N100) or 50% reduced N supply (N50). The soil microbial parameters responded to the elevated CO₂ treatment in varying intensities and time spans. The fungal-to-bacterial respiratory ratio was the most sensitive parameter in responding to an elevated CO₂ treatment with highly significant differences to ambient CO₂-treated control plots in the third year of CO₂ fumigation. After six years bacterial respiratory activity had increased in ascending order to 34% in FACE-treated plots (N50 and N100) as compared to control plots. Soil microbial biomass (C_mic) responded more slowly to the FACE treatment with highly significant increases of >12% after the fourth year of CO₂ fumigation. The C_mic-to-C_org ratio responded very late in the last two years of the CO₂ treatment with a significant increase of >7.0% only in the N100 variant. Total C_org and N_t were slightly but significantly increased under FACE around 10.0% with ascending tendency over time starting with the second year of CO₂ treatment. No significant FACE effects could be recorded for the C:N ratio or pH.These results suggest that under FACE treatment changes in the soil microbial community will occur. In our study the fungal-to-bacterial respiratory ratio was superior to total C_mic as microbial bioindicators in reflecting changes in the soil organic matter composition.     

Effect of lithological substrate on microbial biomass and enzyme activity in brown soil profiles in the northern Apennines (Italy)

The aim of the present study was to investigate the microbial activity along forest brown soil profiles sequence developed on different lithological substrates (carbonate or non-carbonated cement in sandstone formations) at different altitudes. The main question posed was: does carbonate affect the biochemical activity of brown soil profiles at different altitudes? For the purpose of this study, four soil profiles with different amounts and compositions of SOM developed on different lithological substrates were selected: two with carbonate (MB and MZ) and the other two with non-carbonated cement in the sandstone formations (MF1 and MF2). Chemical and biochemical properties of soil were analysed along soil profiles in order to assess the SOM quantity and quality, namely total organic C (C_org), water extractable organic C (WEOC) and humification indices (HI, DH, HR). Microbial biomass (C_mic and N_mic) content, as well as the specific activities of acid phosphatase, β-glucosidase and chitinase enzymes were chosen as indicators of biochemical activity. The soil biochemical properties provided evidence of better conditions for microorganisms in MB than in MF1, MF2 and MZ soil profiles, since patterns of microbial biomass content and activity might be expected in response to the amount and quality of organic substances. The different lithological substrates did not show any clear effect on soil microbial biomass content, since similar values were obtained in MF1, MF2 (with non-carbonated cement) and MZ (with carbonate). However, the specific activities of acid phosphatase (per unit of C_org and per unit of C_mic) were higher in soils with no carbonate (MF1 and MF2) than in soils with carbonate (MB and MZ). In conclusion, the biochemical activity along brown soil profiles was mainly regulated by different soil organic matter content and quality, while the two different lithological substrates (with carbonate or non-carbonated cement in the sandstone formations) did not show any direct effect on microbial biomass and its activity. However, the activity of acid phosphatase per unit of C was particularly enhanced in soil with non-carbonate cement in the sandstone formations.