Stand-scale spatial patterns of soil microbial biomass in natural cold-temperate beech forests along an elevation gradient

This study focuses on spatial heterogeneity in the soil microbial biomass (SMB) of typical climax beech (Fagus crenata) at the stand scale in forest ecosystems of the cold-temperate mountain zones of Japan. Three beech-dominated sites were selected along an altitudinal gradient and grid sampling was used to collect soil samples at each site. The highest average SMB density was observed at the site 1500 m a.s.l. (44.9 gC m⁻²), the lowest was recorded at the site 700 m a.s.l. (18.9 gC m⁻²); the average SMB density at the 550 m site (36.5 gC m⁻²) was close to the overall median of all three sites. Geostatistics, which is specifically designed to take spatial autocorrelation into account, was then used to analyze the data collected. All sites generally exhibited stand-scale spatial autocorrelation at a lag distance of 10-18 m in addition to the small-scale spatial dependence noted at <3.5 m at the 550 m site. Correlation analysis with an emphasis on spatial dependency showed SMB to be significantly correlated with bulk density at the 550 and 1500 m sites, dissolved organic carbon (DOC) at the 700 and 1500 m sites, and nitrogen (N) at the 550 and 700 m sites. However, no soil parameter showed a significant correlation with SMB at every site, and some variables were also differently correlated (negative or positive) with SMB at different sites. This suggests that the factors controlling the spatial distribution of SMB are very complex and responsive to local in situ conditions. SMB regression models were generated from both the ordinary least-squares (OLS) and generalized least-squares (GLS) models. GLS performance was only superior to OLS when cross-variograms were accurately fitted. Geostatistics is preferable, however, since these techniques take the spatial non-stationarity of samples into account. In addition, the sampling numbers for given minimum detectable differences (MDD_s) are provided for each site for future SMB monitoring.     

华北平原典型农田土壤微生物生物量碳氮磷库的县域分布特征 ——以河北省曲周县为例

土壤微生物生物量是土壤中的活性养分库,直接参与土壤碳氮磷硫等元素的形态转化与生物地球化学循环过程,是反映土壤肥力与质量的重要生物指标.基于网格法采样,运用地统计学方法分析华北平原典型农田土壤微生物生物量碳氮磷库的空间分布特征及影响因子.结果表明:河北省曲周县域农田耕层(0～30 cm)土壤微生物生物量库在空间上呈斑块状...     

Rapid estimation of microbial biomass in grassland soils by ultra-violet absorbance

A reliable and simple technique for estimating soil microbial biomass (SMB) is essential if the role of microbes in many soil processes is to be quantified. Conventional techniques are notoriously time-consuming and unreproducible. A technique was investigated that uses the UV absorbance at 280 nm of 0.5 M K₂SO₄ extracts of fumigated and unfumigated soils to estimate the concentrations of carbon, nitrogen and phosphorus in the SMB. The procedure is based on the fact that compounds released after chloroform fumigation from lysed microbial cells absorb in the near UV region. Using 29 UK permanent grassland soils, with a wide range of organic matter (2.9–8.0%) and clay contents (22–68%), it was demonstrated that the increase in UV absorbance at 280 nm after soil fumigation was strongly correlated with the SMB C (r=0.92), SMB N (r=0.90) and SMB P (r=0.89), as determined by conventional methods. The soils contained a wide range of SMB C (412–3412 μg g⁻¹ dry soil), N (57–346 μg g⁻¹ dry soil) and P (31–239 μg g⁻¹ dry soil) concentrations. It was thus confirmed that the UV absorbance technique described was a rapid, simple, precise and relatively inexpensive method of estimating soil microbial biomass.     

Heterogeneous distribution may substantially decrease initial decomposition, long-term microbial growth and N-immobilization from high C-to-N ratio resources

The effect of heterogeneous resource distribution in soil has seldom been studied under controlled laboratory conditions, even though this type of distribution is the rule rather than the exception in both agricultural and undisturbed soil systems. We use distribution trials to test the effect of stratified distribution of chopped maize, sheep faeces derived from maize, and chopped rape stems compared with even distribution. In all treatments, CO₂ mineralization decreased initially in the stratified experiments and in the rape treatment less (39 versus 43%) of the added carbon was mineralized even after 202 days of incubation at 15°C. In both maize and rape residue treatments, we observed much less immobilization of N_min and less microbial growth in the stratified experiments. The data set for rape was sufficiently detailed to allow a model interpretation using a spreadsheet version of the soil organic matter module from DAISY, a soil‐plant‐atmosphere system model. This indicated that the observed differences between the stratified and evenly distributed experiments could be largely understood by assuming diffusion limitation of nitrate from the bulk soil to the residuesphere. The residuesphere is the part of soil that is immediately affected by the decomposition of residue, and was assumed to be included in the first 10 mm of soil surrounding the residues in the stratified experiments at all times. Only a very small part of the variation in CO₂ respiration, N_min and soil microbial biomass N (SMB‐N) could be explained by additionally assuming less substrate utilization efficiency of the less decomposable fraction of rape in the stratified experiment. We observed greater N mineralization in the evenly distributed experiment with faeces, and smaller concentrations of soil microbial biomass nitrogen extracted by chloroform fumigation. We assume that most of the additional SMB‐N determined in the faeces treatments was from microorganisms present in the faeces at the time they were added to soil, and that these were less susceptible to microbial predation in the stratified than in the distributed experiment. We conclude that the spatial distribution of decomposing litters in soil significantly affects the C and N dynamics. Diffusion limitation of available N in the active zone of decomposition was the main causal agent, which induced a decrease in microbial growth and substrate utilisation.     

Changes in chemical and biochemical soil properties induced by 11-yr repeated additions of different organic materials in maize-based forage systems

The repeated addition of organic materials to the soil greatly affects the physical, chemical and biological characteristics. In the present work, we analyzed changes in soil quality properties of the tilled layer caused by different agronomic managements of maize which supply different amounts of carbon (C) and nitrogen (N) through the addition of slurry, farmyard manure or plant residues. The agronomic history of the analyzed soils, which derived from a medium-term (11 yr) field experiment located in NW Italy, represents typical managements of maize for this region. The area is characterized by highly intensive agriculture, with consequent risks to soil degradation that could be limited by the efficient utilization of organic inputs and by recycling within cropping systems, the large amounts of manure that are produced from the many animal breeding farms in this region. We used a combination of both different chemical (soil organic C and total N) and biochemical indicators (potential soil respiration, potentially mineralizable N (PMN) and potential soil microbial biomass (SMB)). We considered the suitability of the selected biochemical indicators to describe the changes in soil characteristics resulting from the past management.The results showed that the application of the different organic materials, in addition to urea-N fertilizer, increased SOM contents and altered the selected soil biochemical properties compared with the unfertilized treatment, especially in the upper 15 cm of the 0?30 cm tilled soil layer. Farmyard manure applications caused the greatest increase in SOM content, PMN and potential SMB, whilst return of maize straw produced the largest increase in potential soil respiration, but had less effect on total soil organic C and SMB. The use of slurry only caused a moderate increase in SOM and showed intermediate changes in biochemical properties. Also, the rate of C accumulation in the soil per unit of C applied was higher for farmyard manure application than for slurry and straw incorporation in the soil. Fertilization with only mineral N did not induce an increase in C_org and N_tot and even reduces soil N mineralization potential.Because of the high variability in the data, potential SMB carbon could be considered as a less successful indicator for differentiating between past agronomic histories and effects on soil quality, whilst microbial activity (measured by potential soil respiration) and PMN, gave a more reliable and useful indication of the amount of easily decomposable organic carbon.     

Poplar plantations in coastal China: towards the identification of the best rotation age for optimal soil carbon sequestration

Poplar plantations are an important resource in China, which possess significant potential to offset carbon (C) emissions through the sequestration of atmospheric carbon dioxide (CO₂) within biomass and soil. The traditional rotation age of poplar plantations is determined by maximizing the economic return from timber production. However, the optimal rotation age that results in the highest level of carbon sequestration within the soil remains unclear. In this study, we examined the total C, nitrogen (N) and microbial biomass (SMB) content of soils, as well as other properties in 0–10, 10–25 and 25–40 cm soil profiles along a 0‐ to 20‐yr chronosequence in a coastal region of Eastern China. Soil C stocks were determined for 1 m soil profiles, and the stand biomass in poplar plantations of different ages was investigated. We found that C concentrations within soils increased with plantation age, primarily in the topsoil layers. The periodic annual increment of C in soils peaked between stand ages of from 6 to 10 yr (0.71 t/ha/yr) and then decreased considerably at 17.5 yr, while the mean annual increment of C in soils was the highest at 15 yr (0.573 t/ha/yr). Soil C accumulation (i.e. soil C sequestration) was positively correlated with poplar biomass, soil N and SMB, and negatively correlated with soil potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na), but not with sulphur (S) or phosphorus (P). Our results suggest that a rotation age of 15 yr is optimal for the sequestration of atmospheric CO₂ in poplar plantations in the coastal region of Eastern China. The C sequestration capacity of soil was primarily controlled by poplar biomass, soil N and SMB.     

Spatial patterns of soil biological and physical properties in a ridge tilled and a ploughed Luvisol

The present study was conducted to determine the spatial heterogeneity of bulk density, soil moisture, inorganic N, microbial biomass C, and microbial biomass N in the ridge tillage system of Turiel compared to conventional mouldboard ploughing on three sampling dates in May, July, and August. The soil sampling was carried out under vegetation representing the ridge in a high spatial resolution down the soil profile. Bulk density increased with depth and ranged from 1.3 g cm⁻³ at 10 cm depth to 1.6 g cm⁻³ at 35 cm in ploughed plots and from 1.0 g m⁻³ at 5 cm to 1.4 g m⁻³ at 35 cm in the ridges. In the ploughed plots, the contents of microbial biomass C and microbial biomass N remained roughly constant at 215 and 33 μg g⁻¹ soil, respectively, throughout the experimental period. The microbial biomass C/N ratio varied in a small range around 6.4. In the ridged plots, the contents of microbial biomass C and microbial biomass N were 5% and 6% higher compared to the ploughed plots. Highest microbial biomass C contents of roughly 300 μg g⁻¹ soil were always measured in the crowns in July. The lowest contents of microbial biomass C of 85–137 μg g⁻¹ soil were measured in the furrows. The ridges showed strong spatial heterogeneity in bulk density, soil water content, inorganic nitrogen and microbial biomass.     

Microbial immobilisation and turnover of N labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass N dynamics were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas in an agricultural field. The objective of the study was to determine the size and activity of soil microbial biomass in the soils of the different yield areas and to compare these data obtained under field and laboratory conditions. Soils were amended with ¹⁵N labelled mustard (Sinapis alba) residues (both experiments) and labelled nitrate (laboratory only) at 30 μg N g⁻¹ dry soil. Soil microbial biomass (SMB) N, mineral N (N_min) and total N content was monitored both in the field and in the laboratory. N₂O efflux was additionally measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, for all other parameters only in the laboratory treatments. In the laboratory less amounts of added substrate N were immobilised by the SMB in HY soils compared to LY soils, whereas in the field immobilisation of added N by SMB was higher in HY soils initially and slightly lower after 40 days of incubation. Calculated turnover times in the laboratory nitrate, laboratory mustard and field mustard amendments were 0.18, 0.27 and 0.74 years (HY) and 0.22, 0.61 and 1.01 years (LY), respectively. The turnover times of added substrate N always showed the trend to be faster in HY soils compared to LY soils. A faster turnover of nutrients in the HY soils may involve a better nutrient supply of the plants, which coincides with the higher agricultural yield observed in these areas.     

The effect of warming on the vulnerability of subducted organic carbon in arctic soils

Arctic permafrost soils contain large stocks of organic carbon (OC). Extensive cryogenic processes in these soils cause subduction of a significant part of OC-rich topsoil down into mineral soil through the process of cryoturbation. Currently, one-fourth of total permafrost OC is stored in subducted organic horizons. Predicted climate change is believed to reduce the amount of OC in permafrost soils as rising temperatures will increase decomposition of OC by soil microorganisms. To estimate the sensitivity of OC decomposition to soil temperature and oxygen levels we performed a 4-month incubation experiment in which we manipulated temperature (4–20 °C) and oxygen level of topsoil organic, subducted organic and mineral soil horizons. Carbon loss (C_LOSS) was monitored and its potential biotic and abiotic drivers, including concentrations of available nutrients, microbial activity, biomass and stoichiometry, and extracellular oxidative and hydrolytic enzyme pools, were measured. We found that independently of the incubation temperature, C_LOSS from subducted organic and mineral soil horizons was one to two orders of magnitude lower than in the organic topsoil horizon, both under aerobic and anaerobic conditions. This corresponds to the microbial biomass being lower by one to two orders of magnitude. We argue that enzymatic degradation of autochthonous subducted OC does not provide sufficient amounts of carbon and nutrients to sustain greater microbial biomass. The resident microbial biomass relies on allochthonous fluxes of nutrients, enzymes and carbon from the OC-rich topsoil. This results in a “negative priming effect”, which protects autochthonous subducted OC from decomposition at present. The vulnerability of subducted organic carbon in cryoturbated arctic soils under future climate conditions will largely depend on the amount of allochthonous carbon and nutrient fluxes from the topsoil.     

C and N mineralization and microbial biomass in heavy-metal contaminated soil

Heavy metals such as arsenic (As), lead (Pb), copper (Cu) and zinc (Zn) can be found in large concentrations in mine spills in Mexico. Interest in contamination by these heavy metals has increased recently as they can change the functioning of soil ecosystems qualitatively and quantitatively. They disturb the activities of soil fauna and contaminate drinking water in large parts of the world, which severely affects human health. Little, however, is known how heavy metals might affect the biological functioning of a soil. Soil was sampled from eight locations along a gradient of heavy-metal contamination with distance from a mine in San Luis Potosí (Mexico) active since about 1800 AD. Microbial biomass was determined with the original chloroform fumigation incubation (CFI) as well as extraction (CFE) techniques and the substrate induced respiration (SIR) technique while C and N mineralization were measured. Total concentrations of As in the top 0–10 cm soil layer ranged from 8 to 22992 mg kg^–1, from 31 to 1845 mg kg^–1 for Pb, from 27 to 1620 mg kg^–1 for Cu and from 81 to 4218 mg kg^–1 for Zn. There was a significant negative correlation (P < 0.0001) between microbial biomass, soil organic carbon, total N and C mineralization and the heavy metal content of the soil. The microbial biomass C to organic C ratio, which varied from 0.4 to 1.9%, specific respiratory activity (qCO₂), and oxidation of NO₂^– were not affected by heavy metals. It was found that long-term contamination of soil with heavy metals had an adverse effect on the amount of soil microorganisms as evidenced by a marked decrease in microbial biomass C, but not some of their characteristics. According to principal components analysis (PCA), the correlation matrix showed three distinct factors explaining 71% of the variance. A first factor including heavy metals (As, Pb, Cu and Zn) with a negative loading and total N, organic C, soil microbial biomass with a positive loading characterized the soil organic matter and contamination status. Loam and sand combined for the second factor characterizing the textural classification while the third factor was loaded by CEC and clay content.     

Dynamics of soil microbial biomass and nitrogen availability in a flooded rice soil amended with different C and N sources

 A greenhouse experiment was conducted to compare effects of different C and N sources applied to a flooded soil on soil microbial biomass (SMB) C and N, extractable soil organic N (N_ORG), and NH₄ ⁺-N in relation to plant N accumulation of rice (Oryza sativa L.). In addition to a control without inputs (CON), four treatments were imposed receiving: prilled urea (PU), rice straw (RS), RS and PU (RS+PU), or Sesbania rostrata as green manure (SES). Treatments were arranged according to a completely randomized design with four replicates and further consisted of pots with and without transplanted rice. While plant effects on the SMB were relatively small, the application of organic N sources resulted in a rapid increase in SMB until 10 days after transplanting (DAT) followed by a gradual decline until 73 DAT. Plant N accumulation data in these treatments clearly indicated that the SMB underwent a transition from a sink to a source of plant-available soil N during the period of crop growth. Seasonal variation of the SMB was small in treatments without amendment of organic material (CON, PU) presumably due to a lack of available C as energy source. Extractable N_ORG was significantly affected by soil planting status and organic N source amendment, but represented only a small N pool with little temporal variation despite an assumed rapid turnover. Among the three treatments receiving the same amount of N from different sources, the recovery efficiency of applied N was 58% for PU and 28% for both RS+PU and SES treatments at 73 DAT. The N uptake of rice, however, was not driven by N availability alone, as most evident in the RS+PU treatment. We assume that root physiological functions were impeded after application of organic N sources. Received: 1 June 1999     

The microbial biomass and activity in soil with shrub (Caragana korshinskii K.) plantation in the semi-arid loess plateau in China

Caragana korshinskii K. is a shrub species which is adapted to arid and semi-arid environments and plays an important role in soil protection. The objective of this study was to determine the influence of this shrub plantation on the soil ecosystem functions driven by microorganisms in the long-term. The changes in the size and activity of soil microbial biomass and the relationship between soil microbial biomass and chemical properties were investigated under shrub plantations aged 6, 18 and 26 years. The results showed that the pH value in the soil decreased gradually, while soil organic carbon (OC) and total nitrogen (TN) significantly increased with the age of C. korshinskii. Although microbial biomass carbon (MBC) and MBC/OC ratio gradually increased, the ratio of basal respiration to MBC (qCO₂) decreased with the age of C. korshinskii. The microbial biomass nitrogen (MBN) and MBC had a positive relationship with soil TN and OC, respectively. The flux of CO₂ decreased with the age of C. korshinskii which had a significant negative relationship with soil OC, TN, MBN and MBC. The results indicate that C. korshinskii plantations may help to improve microbially driven ecosystem functioning through long-term creation of resource-island.     

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.     

Change in soil microbial biomass and regulating factors in an alpine meadow site on the Qinghai-Tibetan Plateau

ABSTRACT

The change in soil microbial biomass (SMB) content and its direct links to soil organic matter (SOM) and environmental factors are not well understood for high-elevation regions. Therefore, this research investigated the temporal variation of SMB and its relationship with SOM and environmental factors in an alpine meadow site on the Qinghai-Tibetan Plateau. The soil organic carbon (SOC) and total nitrogen (TN) contents in alpine meadows showed monthly and seasonal variations and were higher in colder months, and the soil C/N ratio was higher in winter and in autumn than it was in the other seasons (P < 0.05). In addition, the changes in the SMB C and SMB N contents were notable at monthly and seasonal scales, whereas the SMB C and SMB N contents were higher in the winter and spring than they were in the other seasons (P < 0.05); the ratio of SMB C/SOC was higher in spring than it was in the other seasons; the ratio of the SMB N/TN content was higher in cold or cool months than it was in the other months; and the ratio of the SMB C/SMB N was highest in August (P < 0.05). Moreover, the SMB C and SMB N contents were significantly positively correlated to the SOC and TN contents (P < 0.01), and the SMB content was also significantly positively correlated to soil water content and air temperature (P < 0.05), or to soil bulk density and soil pH (P < 0.05). The results suggested that the change in the SMB in the alpine meadows was highly regulated by the SOC and TN and by monthly and seasonal changes in the soil bulk density, the soil pH, soil moisture, and air temperatures.     

A microcosm experiment to evaluate the influence of location and quality of plant residues on residue decomposition and genetic structure of soil microbial communities

The effects of location (soil surface vs. incorporated in soil) and nature of plant residues on degradation processes and indigenous microbial communities were studied by means of soil microcosms incubation in which the different soil zones influenced by decomposition i.e. residues, soil adjacent to residues (detritusphere) and distant soil unaffected by decomposition (bulk soil) were considered. Plant material decomposition, organic carbon assimilation by the soil microbial biomass and soil inorganic N dynamics were studied with ¹³C labelled wheat straw and young rye. The genetic structure of the community in each soil zone were compared between residue locations and type by applying B- and F-ARISA (for bacterial- and fungal-automated ribosomal intergenic spacer analysis) directly to DNA extracts from these different zones at 50% decomposition of each residue. Both location and biochemical quality affected residue decomposition in soil: 21% of incorporated ¹³C wheat straw and 23% left at the soil surface remained undecomposed at the end of incubation, the corresponding values for ¹³C rye being 1% and 8%. Residue decomposition induced a gradient of microbial activity with more labelled C incorporated into the microbial biomass of the detritusphere. The sphere of influence of the decomposing residues on the dynamics of soluble organic C and inorganic N in the different soil zones showed particular patterns which were influenced by both residue location and quality. Residue degradation stimulated particular genetic structure of microbial community with a gradient from residue to bulk soil, and more pronounced spatial heterogeneity for fungal than for bacterial communities. The initial residue quality strongly affected the resulting spatial heterogeneity of bacteria, with a significance between-zone discrimination for rye but weak discrimination between the detritusphere and bulk soil, for wheat straw. Comparison of the different detrituspheres and residue zones (corresponding to different residue type and location), indicated that the genetic structure of the bacterial and fungal communities were specific to a residue type for detritusphere and to its location for residue, leading to conclude that the detritusphere and residue corresponded to distinct trophic and functional niches for microorganisms.     

Seasonal and long-term resource-related variations in soil microbial communities in wheat-based rotations of the Canadian prairie

Like all other living organisms, microorganisms depend on nutrients, carbon and energy. Since microorganisms are central to most soil processes, the sustainable management of agricultural soils may need to consider the impact of soil fertility management on the soil microbial community. We tested the hypothesis that different rates of N and P fertilizers, and cropping frequency (modifying C input to soil) influence the size, structure and physiological condition of soil microbial populations residing in the plough layer (top 7.5 cm). For this study, we used a 37-yr old long-term wheat-based rotation experiment located in the semiarid Brown soil zone of Saskatchewan. The experiment included (1) four input treatments: (i) no N or (ii) no P fertilizer application to wheat (Triticum aestivum L.) grown in fallow-wheat-wheat (F-W-W) rotations, and (iii) recommended rates of both N and P fertilizer applied to fallow-wheat (F-W) and (iv) to F-W-W; (2) two rotation phases: fallow and wheat-after-fallow; and (3) four sampling times: 8 June, 4 July, 5 August and 16 September 2003. Increased partitioning into storage lipids of the arbuscular mycorrhizal fungi (AMF) fatty acid methyl ester (FAME) biomarker 16:1ω5 (P=0.04), suggested the accumulation of storage material under low soil N availability. Discriminant analysis detected modifications in soil microbial community structure due to cropping frequency (P=0.001) and sampling time, the effect of which was different in the fallow (P<0.0001) and wheat-after-fallow (P<0.0001) phases of the rotations. Correlation analysis of soil variables conducted in plots growing wheat revealed a dual effect of plants, which stimulated active soil microbial biomass (SMB), possibly through the release of soluble extractable C (C_sol−ext) in soil and, at the same time, SMB competed with wheat for soil water and N. The 37 y of different nutrient input treatments had no effect upon the active soil microbial biomass according to PLFA measurements, despite changes in soil resource-related variables (soil water potential, soil PO₄-P and NO₃-N fluxes, and C_sol−ext concentrations) (P?0.003). The biomass of each of three microbial populations monitored was lowest on 4 July, when the amounts of the soil resources monitored were average, and greatest on 5 August, when N, P and soil moisture availability was lowest. The temporal effect on the biomass of microbial populations seemed unrelated to variation in nutrient or water availability. We conclude that the soil microbial community is adaptable to a wide range of soil conditions. We propose therefore that the occurrence of sudden and dramatic events, such as a heavy rainfall on a dry soil, is the most important determinant of seasonal variation in active soil microbial biomass.     

Microbial immobilisation and turnover of C labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass C immobilisation and turnover were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas within an agricultural field. We compared the size and activity of soil microbial biomass (SMB) in the soils of the different yield areas under field and laboratory conditions. Soils were amended with ¹³C labelled mustard (Sinapis alba) residues (both experiments) and labelled glucose (laboratory only) at 500 μg C g⁻¹ dry soil. SMB-C, dissolved organic carbon (DOC) and total C content were monitored in the field and the laboratory. CO₂-efflux was also measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, but other variables only in the laboratory treatments. A positive priming effect was measured in three of four laboratory treatments. Priming was induced after a significant increase of soil derived C in the microbial biomass. Thereafter, the total C loss through priming was always smaller than or equal to the decline in microbial biomass C. In field and laboratory experiments SMB in the HY soil immobilised less of the added substrate C than LY soil SMB. Calculated turnover times in the laboratory glucose amendment were 0.24 (HY) and 0.31 y (LY), in the laboratory mustard treatment 0.58 (HY) and 0.44 y (LY) and in the field mustard amendments 1.09 (HY) and 1.25 y (LY). In both the field mustard and laboratory glucose treatments turnover in the HY soil tended to exceed that in the LY soil. These turnover times as well as the reaction of SMB-C to drying-rewetting and substrate addition, indicated that the HY soil possessed a more active microbial community with a more rapid C turnover than the LY soil. As C turnover is considered to be closely linked to nutrient cycles, faster turnover in the HY soil may involve a better nutrient supply for crops resulting in higher agricultural yield.     

Response of soil chemical and biological variables to small and large scale changes in climatic factors

This paper studies the effect of large- and small-scale changes of soil temperature and humidity on soil microbial biomass C and N, ergosterol, carbon utilization potential, organic and inorganic N and rate of C and N mineralization at 25°C. Large-scale variations are identified with seasonal changes in temperature and humidity. To simulate small-scale changes, soil temperature and humidity were manipulated in the field. The treatment resulted in damping of temperature fluctuations and a decrease of soil humidity.The majority of the studied variables exhibit pronounced seasonality, showing a clear-cut distinction between summer (July–August) and winter (December). In summer, C mineralization rate and carbon utilization potential was high but microbial and fungal biomass (ergosterol) was low.C and N mineralization rate and microbial and fungal biomass were only affected by sampling date, demonstrating that gross parameters of biomass and activity of microorganisms are not affected by small-scale changes in temperature and humidity. In contrast, variables relating to N availability (organic N, NH₄⁺ and NO₃⁻, microbial biomass N) and carbon utilization potential of the microbial community were highly affected by small-scale changes in soil abiotic conditions. The results suggest that changes in N dynamics induced by small-scale changes of temperature and humidity are caused by shifts in the structure of the microbial community rather than by variations in microbial biomass.     

Soluble organic C and N and their relationships with soil organic C and N and microbial characteristics in moso bamboo (Phyllostachys edulis) plantations along an elevation gradient in Central Taiwan

Purpose

Moso bamboo (Phyllostachys edulis), an important economic crop, is distributed from low- to medium-elevation mountains in Taiwan. Bamboo is a fast-growing herbaceous species with an extensive rhizome structure. With the hypothesis that the characteristics of soil organic matter and microbes might change after long-term bamboo plantation, we investigated different fractions of organic C and N as well as soil microbial biomass and activities in five moso bamboo plantations along an elevation gradient in Central Taiwan.

Materials and methods

Five soil samples (top 10 cm of soil) were collected from each bamboo plantation (600, 800, 1,000, 1,200, and 1,400 m above sea level (asl)) in January 2011. Soil was processed and analyzed for soil total C and N contents, biologically available C, potentially mineralizable N, soil microbial biomass and soil respiration (CO₂). Two extraction methods (2 M KCl and hot-water extraction) were used to estimate soil soluble organic C and N (S_bOC and S_bON) and soil inorganic N (NH₄ ⁺ and NO₃ ^?) concentrations to evaluate the relationship with soil organic matter and microbe characteristics in bamboo plantations.

Results and discussion

Soil total C and N contents as well as soil microbial biomass and soil respiration (CO₂) of the bamboo plantations increased along the elevation gradient. Temperature changes along elevation contributed to such variations observed among the selected bamboo plantations. The S_bON in hot-water extracts was highest in the 1,200-m plantation, then in the 1,400-m plantation, and lowest in the low-elevation plantations (600, 800, and 1,000 m). However, S_bON in 2 M KCl extracts did not differ by elevation. The S_bON was strongly correlated with soil total N in both 2 M KCl and hot-water extracts, but only S_bON in hot-water extracts was strongly correlated with microbial biomass N and potentially mineralizable N. S_bOC was strongly correlated with soil total C content, microbial biomass C, and biologically available C in both 2 M KCl and hot-water extracts.

Conclusions

Soil total C and N, S_bOC and S_bON, and microbial biomass characteristics increased in the moso bamboo plantations with increasing elevation. No altitudinal difference in specific soil respiration (CO₂) rate suggested that the enhanced potentially mineralizable N and soil respiration (CO₂) in the high-elevation plantations were associated with increased microbial biomass rather than microbial activities.