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.     

Surface and subsurface organic carbon, microbial biomass and activity in a forest soil sequence

The distribution of organic carbon, microbial biomass and activity, from the surface down to 70 cm, was investigated through three semiarid Mediterranean soils: (1) a Typic Calcixeroll covered with a native pinewood (NP), (2) a Typic Calcixerept under a mature pine plantation (PP) on abandoned agricultural terraces and (3) a Typic Haploxerept under a grassland (GS). NP and GS had the highest and lowest soil organic carbon (SOC) pools, respectively. Both of them had decreasing SOC contents with depth. PP, which held intermediate SOC levels, showed an increase in total organic C and humic substances C with depth due to their mineralization in the anciently ploughed topsoil layer. The soils were similarly ranked as regards their microbial biomass and activity: NP>PP>GS. In general, the microbial communities were less dense and active towards the deeper horizons. More specifically, PP and GS had a very populated and active top 20-cm layer, which was attributed to the dense root system of their grass cover. NP maintained high microbial biomass and activity levels from 0 to 70 cm, progressively diminishing along with shrub root density (e.g. microbial biomass C dropped from 2342 to 394 mg kg⁻¹ soil). The latter soil presented the sharpest drop of its microbial properties with depth, what was considered an indicator of its quality. Generally decreasing patterns of microbial biomass and activity were not always coincident with previously published gradients of microbial metabolic abilities and genetic structure. This reinforces the need of combining biomass, activity and biodiversity measurements if the ecosystem's functioning is to be fully understood and a real monitoring of degradation processes and restoration strategies is to be achieved.     

Soil microbial communities under different soybean cropping systems: Characterization of microbial population dynamics, soil microbial activity, microbial biomass, and fatty acid profiles

This work analyzes the direct effect of soil management practices on soil microbial communities, which may affect soil productivity and sustainability. The experimental design consisted of two tillage treatments: reduced tillage (RT) and zero tillage (ZT), and three crop rotation treatments: continuous soybean (SS), corn–soybean (CS), and soybean–corn (SC). Soil samples were taken at soybean planting and harvest. The following quantifications were performed: soil microbial populations by soil dilution plate technique on selective and semi-selective culture media; microbial respiration and microbial biomass by chloroform fumigation-extraction; microbial activity by fluorescein diacetate hydrolysis; and fatty acid methyl ester (FAME) profiles. Soil chemical parameters were also quantified. Soil organic matter content was significantly lower in RT and SS sequence crops, whereas soil pH and total N were significantly higher in CS and SC sequence crops. Trichoderma and Gliocladium populations were lower under RTSS and ZTSS treatments. Except in a few cases, soil microbial respiration, biomass and activity were higher under zero tillage than under reduced tillage, both at planting and harvest sampling times. Multivariate analyses of FAMEs clearly separated both RT and ZT management practices at each sampling time; however, separation of sequence crops was less evident. In our experiments ZT treatment had highest proportion of 10Me 16:0, an actinomycetes biomarker, and 16:1ω9 and 18:1ω7, two fatty acids associated with organic matter content and substrate availability. In contrast, RT treatment had highest content of branched biomarkers (i15:0 and i16:0) and of cy19:0, fatty acids associated with cell stasis and/or stress. As cultural practices can influence soil microbial populations, it is important to analyze the effect that they produce on biological parameters, with the aim of conserving soil richness over time. Thus, in a soybean-based cropping system, appropriate crop management is necessary for a sustainable productivity without reducing soil quality.     

Soil organic carbon and microbial activity: east Antarctica

Schirmacher oasis (SO) (70°46′04′′S to 70°44′24′′S; 11°49′54′′[± 48]E to 11°26′03′′[± 02]E), one of the smallest oases in Antarctica, is situated on Dronning Maud Land, which is about 70 km south of the Princess Astrid Coast of east Antarctica. This oasis is a small rocky moraine, surrounded by the vast polar ice cap and Antarctic ice shelf. It represents cold desert conditions, devoid of any higher plant and animal life, except for a few patches of microbiotic crusts in some suitable niches. Melted water from glaciers, ice and snow produces about 30 fresh water lakes and many streams, which supply essential nutrients to the autotrophs (algae, mosses and lichens). Between 30 December 1999 and 29 January 2000, soil samples were collected from 14 sparsely vegetated sites to study the soils and ecology over this oasis. Bio‐physicochemical parameters, organic carbon and microbial activities (dehydrogenase) of the soils were determined. The average surface air temperature and wind velocities during the sampling period were −0.87°C and 4.3 m s⁻¹. The average pH, plant biomass (standing crops) and moisture content were 7.5, 22.5 g m⁻² and 375.4%. Total organic carbon (TOC) contents of the soil samples ranged from 1.16 to 2.58% and the mean value was 1.58%. Dehydrogenase activity (DHA) was low and the average was 0.008 mg Triphenyl formazan (TPF) g soil⁻¹ day⁻¹. The low DHA in SO suggests that anaerobic oxidation of organic C is poor. TOC and DHA are negatively correlated with plant biomass (r=−0.14, P= 0.62 and r=−0.21, P= 0.48). The organic C and microbial activities are dependent on the amount of the autotrophic productivity and abundances. TOC correlated significantly with DHA (r= 0.85, P < 0.001), which indicates that organic C is an important factor in controlling the development of DHA in the SO. The averaged bio‐physicochemical data in the oasis do not deviate much from the respective mean values and the TOC is expected to remain in the range of 1–2%; however, a small change in human activity is likely to cause long‐term impacts in this pristine ecosystem.     

章古台沙地松树人工林土壤无机氮、微生物生物量碳及微生物生物量氮

The dynamics of soil inorganic nitrogen （NH4^＋ -N and NO3^- -N） and microbial biomass carbon （Cmic） and nitrogen （Nmic） under 30-year-old fenced Pinus sylvestris L. var. mongolica Litvin （SF）, unfenced P. sylvestris L. var. mongolica Litvin （SUF）, and unfenced Pinus densiflora Siebold et Zucc. （DUF） plantations in the Zhanggutai sandy soil of China were studied during Apr. to Oct. 2004 by the in situ closed-top core incubation method. All mentioned C and N indices in each stand type fluctuated over time. The ranges of inorganic N, Cmic, and Nmic contents in the three stand types were 0.7-2.6, 40.0-128.9, and 5.4-15.2 μg g^-1, respectively. The average contents of soil NH4^＋ -N and Cmic under the three 30-year-old pine plantations were not different. However, soil NO3^ -N and total inorganic N contents decreased in the order of SUF ≥ SF ≥ DUF, the Nmic content was in the order of SF = SUF 〉 DUF, and the Cmic：Nmic ratio was in the order of SUF = DUF 〉 SF. Seasonal variations were observed in soil inorganic N, microbial biomass, and plant growth. These seasonal variations had certain correlations with microbe and plant N use in the soil, and their competition for NH4^＋ -N was mostly regulated by soil N availability. The influence of tree species on inorganic N and Nmic were mainly because of differences in litter quality. Lack of gazing decreased the Cmic：Nmic ratio owing to decreased carbon output and increased the ability of soil to supply N. The soil N supply under the P. sylvestris var. mongolica plantation was lower than under the P. densiflora plantation.     

Soil microbial biomass and organic matter composition in soils under cultivation

To determine whether there is a relationship between the composition of soil organic matter and the activity of the soil microbial biomass, the composition of the organic matter in 12 typical arable soils in Northwest Germany was investigated by wet chemical analysis and CPMAS cross polarization magic angle spinning ¹³C-NMR spectroscopy. The data were correlated with the microbial biomass as estimated by substrate-induced respiration. A strong correlation between the microbial biomass and alkylic C compounds was observed (r=-0.960^***). Recalcitrant substances were enriched in this fraction, which were classified as humic acids according to the wet chemical procedure. The microbial decomposition of these humic acids is probably retarded, due to their chemical structure and/or physical bonding, when the soil microbial biomass activity is limited.     

Using the continuous-quality theory to predict microbial biomass and soil organic carbon following organic amendments

Soil microbial biomass and microbial quotient (the ratio of soil microbial biomass to soil organic carbon) are considered to be useful as rapidly responding indicators of perturbations of soil properties. In this paper we will use a well‐tested model (the continuous‐quality theory) to analyse these variables in a Swedish 35‐year‐old field experiment with a black fallow, crop with no N addition, crop with calcium nitrate addition, and six treatments with organic amendments: straw, green manure, peat, farmyard manure, sawdust and sewage sludge. The model predicts correctly that the amount of microbial biomass increases for all the treatments with organic amendments compared with the black fallow treatment. The microbial biomass quotient increases also for all the amended treatments, except peat and sewage sludge, and decreases for the other treatments. The microbial biomass and microbial quotient increase with both the amounts of organic matter added (crop residues and amendments) and the quality of the added matter. However, to fully explain the observations it is also necessary to have an increasing microbial mortality with substrate quality. Moreover, short‐term observations can be misleading with respect to both the magnitude and direction of long‐term changes in biomass and related variables. Special attention must be paid to such amendments as sewage sludge, where contaminants such as heavy metals may determine process rates. We find no relation between microbial biomass or microbial quotient and yields.     

Soil microbial biomass and activity in soil from different grassland management treatments stored under controlled conditions

Soil taken during summer from permanent pasture on a clay loam in S.W. England which had been under four different N management treatments for at least 3 yr was stored under aerobic conditions at 18°C. Total soil microbial biomass (SMB), measured by fumigation-extraction, was higher in the unfertilized grass and grass–clover treatments than in the fertilized grass and clover treatments and these differences persisted for 30 weeks. Over this period total SMB decreased by 25% in all the treatments, soil respiration also decreased, but more rapidly and to a greater extent (50%). The respiration response of the biomass to glucose, estimating “active” biomass, was measured by a substrate induced respiration (SIR) procedure; it showed no differences between the four treatments but was reduced by 66% over 30 weeks. Concentrations of NO₃⁻–N increased progressively in each treatment by more than 15-fold. At the end of the 30 weeks addition of available C in the form of glucose resulted in a small increase in total SMB and respiratory activity and a large decrease in NO₃⁻–N in all treatments: the SIR response of the biomass was completely restored. It is suggested that the differences in total SMB in the management treatments throughout the incubation were attributable to the “inactive” fraction of the biomass.     

Soil and microbial biomass stoichiometry regulate soil organic carbon and nitrogen mineralization in rice-wheat rotation subjected to long-term fertilization

Journal of Soils and Sediments - Soil microbial biomass (SMB), as the source and sink of soil nutrients, and its stoichiometry play a key role in soil organic carbon (SOC) and nitrogen (N)...     

Relationships of soil microbial biomass carbon and organic carbon with environmental parameters in mountainous soils of southwest China

The relationships between microbial biomass C, organic C, and environmental parameters were studied in soils under corn (Zea mays. L) in the mountainous areas of southwest China. Three yellowish-red (Ultisols), yellow (Ultisols) and yellowish-brown (Alfisols) soils were relatively weathered, leached and impoverished, with most having a low input of aboveground corn residues. Seasonal changes in soil microbial C at 0-10 cm depth were significant at each sampling site, with the highest value (120 g C m^-2) in winter, and lowest value in summer (21 g C m^-2). Microbial biomass C was significantly and negatively correlated with site elevation and positively correlated with mean annual temperature. The seasonal change in microbial biomass C was significantly correlated with total soil organic C. The decline in microbial biomass C estimated as a percentage of the total soil organic C was negatively correlated with the elevation above sea level, ranging from 3.9ǂ.9% below 600 m to 1.4ǂ.5% above 1,500 m, suggesting higher turnover rates of soil microbial biomass C at warmer air temperatures. Temperature influenced the decomposition of organic C in soils mainly through its effects on microbial biomass C, and the microbial biomass C/organic C ratio appears to be a sensitive index of the change in organic matter content of soil.     

Soil organic matter,microbial properties,and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K₂SO₄-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K₂SO₄-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.     

水力侵蚀影响下土壤有机碳和微生物数量动态变化特征

土壤侵蚀是土壤有机碳(Soil organic carbon,SOC)动态过程的重要驱动因素,明确土壤侵蚀如何影响土壤微生物进而作用于SOC,有助于准确把握土壤侵蚀在全球碳循环中的作用。通过野外径流小区模拟降雨试验,结合定量聚合酶链式反应(quantitative Polymerase Chain Reaction,q PCR)技术,研究了水力侵蚀后短期内(10 d)坡耕地表层土壤微生物数量和SOC含量动态变化特征,并在此基础上探讨了微生物与SOC间的关系。结果表明:与雨前相比,降雨侵蚀后表层土壤SOC含量没有显著差异,而表层土壤细菌数量显著降低,为雨前细菌数量的58.76%(坡上)、55.22%(坡中)、55.82%(坡下);降雨侵蚀同样显著改变了表层土壤真菌数量,雨后真菌数量为雨前真菌数量的105.51%(坡上)、2.29%(坡中),12.20%(坡下);降雨侵蚀后,SOC、细菌和真菌数量均在短时间内显著增加,达到峰值后下降;相关性分析表明,细菌和真菌数量与SOC之间的关系均未表现出显著正相关关系,仅有坡下细菌,坡中、坡下以及整个坡面真菌与SOC含量表现出显著正相关关系。     

不同耕作方式对土壤有机碳、微生物量及酶活性的影响

【目的】依托8年长期(2005~2012)固定道定位试验,研究不同耕作方式对土壤有机碳、土壤微生物量、土壤酶活性在0—90 cm土层的分布特征,为优化中国西北干旱区的耕作方式提供理论依据。【方法】试验包括固定道垄作(PRB)、固定道平作(PFT)与传统耕作(CT)三种耕作模式下的土壤有机碳土壤总有机碳(TOC)、颗粒有机碳(POC)、土壤微生物量碳(MBC)、土壤微生物量氮(MBN)、土壤微生物量磷(MBP)、蔗糖酶、过氧化氢酶、脲酶及小麦产量进行了测定和分析。【结果】在0—90 cm土层,不同耕作方式下的TOC、POC、MBC、MBN、MBP、蔗糖酶活性、脲酶活性均随着土层的增加呈下降趋势,过氧化氢酶活性呈先下降后增大的分布特征;在0—60 cm,固定道保护性耕作能够显著增加心土层作物生长带土壤有机碳储量,有机碳储量大小为PRBPFTCT;PRB、PFT较CT可以显著增加0—10 cm作物生长带TOC、POC、MBC、MBN、MBP含量、蔗糖酶、脲酶活性,其大小为PRBPFTCT;耕作方式对过氧化氢酶活性影响不显著;TOC、POC、MBC、MBN、MBP、蔗糖酶活性、脲酶活性、过氧化氢酶活性之间均达到了显著或极显著相关。【结论】PRB较PFT、CT能够提高耕作层(0—10 cm)土壤有机碳含量、土壤微生物量、土壤酶活性, 增加作物产量, 增大0—60 cm土层有机碳储量,耕作方式(PRB、PFT及CT)对10 cm以下土层土壤环境改善作用不明显。     

Soil microbial biomass and activity under a potato crop fertilised with N with and without C

Summary A range of soil microbiological parameters were measured at intervals throughout the growing season of a potato crop. Treatments applied to the soil at sowing were zero N fertilisation of N fertilisation at 120 kg N ha^–1, either alone or supplemented with straw or sucrose at 1200 kg C ha^–1. C and N flushes determined by fumigation-incubation and fumigation-extraction, and substrate-induced respiration, were measured as indicators of microbial biomass. Microbial activity was measured as respiration (CO₂ production) and dehydrogenase activity (formazan production). The greatest effects were obtained from the addition of N plus sucrose. Both biomass size and activity were significantly stimulated for up to 25 days after incorporation, with the magnitude of the effects consistently diminishing over time. By 125 days after planting, there was no detectable legacy from any of the treatmentson any of the biomass parameters that were measured, and all values had reverted to those prevalent at planting. There was no consistent effect from adding N, either alone or supplemented with straw, on any of the biomass parameters. There was no evidence for crop-induced stimulation of the biomass. The experiment demonstrates that biomass is only influenced where the quantity, quality, and rate of incorporation of C into the soil is appropriate, in this case, only by adding C as a pulse of sucrose.     

Tebuconazole application decreases soil microbial biomass and activity

A short-term mesocosm experiment was conducted to ascertain the impact of tebuconazole on soil microbial communities. Tebuconazole was applied to soil samples with no previous pesticide history at three rates: 5, 50 and 500 mg kg⁻¹ DW soil. Soil sampling was carried out after 0, 7, 30, 60 and 90 days of incubation to determine tebuconazole concentration and microbial properties with potential as bioindicators of soil health [i.e., basal respiration, substrate-induced respiration, microbial biomass C, enzyme activities (urease, arylsulfatase, β-glucosidase, alkaline phosphatase, dehydrogenase), nitrification rate, and functional community profiling]. Tebuconazole degradation was accurately described by a bi-exponential model (degradation half-lives varied from 9 to 263 days depending on the concentration tested). Basal respiration, substrate-induced respiration, microbial biomass C and enzyme activities were inhibited by tebuconazole. Nitrification rate was also inhibited but only during the first 30 days. Different functional community profiles were observed depending on the tebuconazole concentration used. It was concluded that tebuconazole application decreases soil microbial biomass and activity.     

Soil organic carbon stocks, storage profile and microbial biomass under different crop management systems in a tropical agricultural ecosystem

 We investigated the soil organic C and N stocks, storage profiles and microbial biomass as influenced by different crop management systems in a tropical agricultural ecosystem. The different crop management systems significantly affected the C and N stocks and microbial biomass C and N at different soil depths. Amongst the systems evaluated, the rice-wheat system maintained a higher soil organic C content. Inclusion of legumes in the system improved the soil organic matter level and also soil microbial biomass activity, vital for the nutrient turnover and long-term productivity of the soil. Irrespective of the cropping system, approximately 58.4%, 25.7% and 15.9% of the C was distributed in 0–15, 15–30 and 30–60 cm depths, respectively. Received: 10 October 1999     

Size and activity of the soil microbial biomass under grass and arable management

 The effects of 5 years of continuous grass/clover (Cont grass/clover) or grass (Cont grass) pasture or 5 years of annual grass under conventional (Ann grass CT) or zero tillage (Ann grass ZT) were compared with that of 5 years of continuous barley (LT arable) on a site which had previously been under arable crops for 11 years. For added comparison, a long-term grass/clover pasture site (LT past) nearby was also sampled. Soil organic C (C_org) content followed the order LT arable=Ann grass CT<Ann grass ZT<Cont grass=Cont grass/clover<LTpast. Trends with treatment for microbial biomass C (C_mic), basal respiration, flourescein diacetate (FDA) hydrolytic activity, arginine ammonification rate and the activities of dehydrogenase, protease, histidase, acid phosphatase and arylsulphatase enzymes were broadly similar to those for C_org. For C_mic, FDA hydrolysis, arginine ammonification and the activities of histidase, acid phosphatase and arylsulphatase, the percentage increase caused by 5 years of continuous pasture (in comparison with LT arable) was 100–180%, which was considerably greater than that for organic C (i.e. 60%). The microbial metabolic quotient (qCO₂) was higher for the two treatments which were mouldboard ploughed annually (LT arable and Ann grass CT) than for the undisturbed sites. At the undisturbed sites, C_org declined markedly with depth (0–15 cm) and there was a similar stratification in the size and activity of C_mic and enzyme activity. The microbial quotient (C_mic/C_org) declined with depth whilst qCO₂ tended to increase, reflecting a decrease in the proportion of readily available substrate with depth. Received: 7 July 1998     

Grasses, litter, and their interaction affect microbial biomass and soil enzyme activity

Plant effects on ecosystem processes are mediated through plant-microbial interactions belowground and soil enzyme assays are commonly used to directly relate microbial activity to ecosystem processes. Live plants influence microbial biomass and activity via differences in rhizosphere processes and detrital inputs. I utilized six grass species of varying litter chemistry in a factorial greenhouse experiment to evaluate the relative effect of live plants and detrital inputs on substrate-induced respiration (SIR, a measure of active microbial biomass), basal respiration, dissolved organic carbon (DOC), and the activities of β-glucosidase, β-glucosaminidase, and acid phosphatase. To minimize confounding variables, I used organic-free potting media, held soil moisture constant, and fertilized weekly. SIR and enzyme activities were 2-15 times greater in litter-addition than plant-addition treatments. Combining live plants with litter did not stimulate microbial biomass or activity above that in litter-only treatments, and β-glucosidase activity was significantly lower. Species-specific differences in litter N (%) and plant biomass were related to differences in β-glucosaminidase and acid phosphatase activity, respectively, but had no apparent effect on β-glucosidase, SIR, or basal respiration. DOC was negatively related to litter C:N, and positively related to plant biomass. Species identity and living plants were not as important as litter additions in stimulating microbial activity, suggesting that plant effects on soil enzymatic activity were driven primarily by detrital inputs, although the strength of litter effects may be moderated by the effect of growing plants.