Soil carbon and nitrogen dynamics along a latitudinal transect in Western Siberia, Russia

An 1800-km South to North transect (N 53°43′ to 69°43′) through Western Siberia was established to study the interaction of nitrogen and carbon cycles. The transect comprised all major vegetation zones from steppe, through taiga to tundra and corresponded to a natural temperature gradient of 9.5°C mean annual temperature (MAT). In order to elucidate changes in the control of C and N cycling along this transect, we analyzed physical and chemical properties of soils and microbial structure and activity in the organic and in the mineral horizons, respectively. The impact of vegetation and climate exerted major controls on soil C and N pools (e.g., soil organic matter, total C and dissolved inorganic nitrogen) and process rates (gross N mineralization and heterotrophic respiration) in the organic horizons. In the mineral horizons, however, the impact of climate and vegetation was less pronounced. Gross N mineralization rates decreased in the organic horizons from south to north, while remaining nearly constant in the mineral horizons. Especially, in the northern taiga and southern tundra gross nitrogen mineralization rates were higher in the mineral compared to organic horizons, pointing to strong N limitation in these biomes. Heterotrophic respiration rates did not exhibit a clear trend along the transect, but were generally higher in the organic horizon compared to mineral horizons. Therefore, C and N mineralization were spatially decoupled at the northern taiga and tundra. The climate change implications of these findings (specifically for the Arctic) are discussed.     

Carbon mineralization in the southern Sonoran Desert

We measured carbon mineralization in four different desert habitats (Arroyos, Hillsides, Canopies-Plains and Open-Plains) and the separate effect of litter addition from annual and perennial plants on soil microbial respiration using two laboratory soil incubation experiments. The differences in total aboveground phytomass among habitats correlates with soil nutrient content, soil particulate organic matter (POM) and consequently, C mineralization. The Arroyos habitat with the highest perennial plant phytomass and litter production, had the highest soil nutrient content, soil POM and C mineralization. Litter from annual plants had twice the P concentration than litter from the perennials, but only half the N concentration. Soil microbial respiration was higher with annual plant litter than with perennial plant litter in the Hillsides and Canopies-Plains, suggesting that microbial activity in both habitats was improved by litter with a higher C quality. In contrast, in the poorest habitat, the Open-Plains, the better response to the addition of perennial plant litter suggests that microbial activity may have been constrained by N input.     

Microbial activity and soil respiration under nitrogen addition in Alaskan boreal forest

Climate warming could increase rates of soil organic matter turnover and nutrient mineralization, particularly in northern high‐latitude ecosystems. However, the effects of increasing nutrient availability on microbial processes in these ecosystems are poorly understood. To determine how soil microbes respond to nutrient enrichment, we measured microbial biomass, extracellular enzyme activities, soil respiration, and the community composition of active fungi in nitrogen (N) fertilized soils of a boreal forest in central Alaska. We predicted that N addition would suppress fungal activity relative to bacteria, but stimulate carbon (C)‐degrading enzyme activities and soil respiration. Instead, we found no evidence for a suppression of fungal activity, although fungal sporocarp production declined significantly, and the relative abundance of two fungal taxa changed dramatically with N fertilization. Microbial biomass as measured by chloroform fumigation did not respond to fertilization, nor did the ratio of fungi : bacteria as measured by quantitative polymerase chain reaction. However, microbial biomass C : N ratios narrowed significantly from 16.0 ± 1.4 to 5.2 ± 0.3 with fertilization. N fertilization significantly increased the activity of a cellulose‐degrading enzyme and suppressed the activities of protein‐ and chitin‐degrading enzymes but had no effect on soil respiration rates or ¹⁴C signatures. These results indicate that N fertilization alters microbial community composition and allocation to extracellular enzyme production without affecting soil respiration. Thus, our results do not provide evidence for strong microbial feedbacks to the boreal C cycle under climate warming or N addition. However, organic N cycling may decline due to a reduction in the activity of enzymes that target nitrogenous compounds.     

宁南山区植被恢复对土壤团聚体养分特征及微生物特性的影响

测定了宁夏黄土丘陵区植被恢复近30年的天然草地和农地不同粒径团聚体的土壤养分含量、微生物生物量、呼吸特性和生态化学计量比等指标,探索黄土丘陵区植被恢复对不同粒径土壤团聚体的养分特性和微生物学性质的影响.结果表明: 微团聚体(粒径<0.25 mm)质量百分比、各粒径土壤团聚体养分(有机碳、全氮、速效钾)含量、C/N均表现为天然草地大于农地,其中1～2 mm粒径团聚体有机碳、全氮含量在天然草地和农地中均最高,C/N也较高,说明植被恢复能有效促进土壤团粒的形成,适宜养分积累和有机碳的汇集,且在1～2 mm粒径团聚体上表现最为突出;天然草地各粒径土壤团聚体微生物生物量(碳、氮)、基础呼吸强度均高于农地,而呼吸熵低于农地,可见植被恢复措施可有效提高各粒径土壤微生物生物量与活性,并使土壤生境趋于稳定;但由于养分特性的差异,不同粒径团聚体微生物特性对植被修复的响应存在差异,其中天然草地土壤1～2 mm粒径团聚体微生物生物量碳,<0.25、0.25～1、1～2 mm粒径团聚体微生物生物量氮,以及1～2、>5 mm粒径团聚体基础呼吸强度显著高于其他粒径,即上述粒径团聚体的微生物生物量和微生物活性在植被恢复过程中逐渐被改善.表明宁南山区植被恢复有效改善了土壤团聚体的肥力状况与结构特征,且1～2 mm粒径团聚体的改良效果最为突出.     

Carbon sequestration potential of soils in southeast Germany derived from stable soil organic carbon saturation

Sequestration of atmospheric carbon (C) in soils through improved management of forest and agricultural land is considered to have high potential for global CO₂ mitigation. However, the potential of soils to sequester soil organic carbon (SOC) in a stable form, which is limited by the stabilization of SOC against microbial mineralization, is largely unknown. In this study, we estimated the C sequestration potential of soils in southeast Germany by calculating the potential SOC saturation of silt and clay particles according to Hassink [Plant and Soil 191 (1997) 77] on the basis of 516 soil profiles. The determination of the current SOC content of silt and clay fractions for major soil units and land uses allowed an estimation of the C saturation deficit corresponding to the long‐term C sequestration potential. The results showed that cropland soils have a low level of C saturation of around 50% and could store considerable amounts of additional SOC. A relatively high C sequestration potential was also determined for grassland soils. In contrast, forest soils had a low C sequestration potential as they were almost C saturated. A high proportion of sites with a high degree of apparent oversaturation revealed that in acidic, coarse‐textured soils the relation to silt and clay is not suitable to estimate the stable C saturation. A strong correlation of the C saturation deficit with temperature and precipitation allowed a spatial estimation of the C sequestration potential for Bavaria. In total, about 395 Mt CO₂‐equivalents could theoretically be stored in A horizons of cultivated soils – four times the annual emission of greenhouse gases in Bavaria. Although achieving the entire estimated C storage capacity is unrealistic, improved management of cultivated land could contribute significantly to CO₂ mitigation. Moreover, increasing SOC stocks have additional benefits with respect to enhanced soil fertility and agricultural productivity.     

祁连山3种典型生态系统土壤微生物活性和生物量碳氮含量

 测定分析了祁连山高寒草甸、山地森林和干草原土壤中微生物活性、生物量碳氮含量。结果显示：就土壤微生物生物量碳含量,森林比干草原和高寒草甸中分别高60%和120%以上,干草原比高寒草甸中高40%以上(p<0.05)。就土壤微生物生物量氮含量,0～5 cm土层,森林比高寒草甸和干草原中分别高64%和111%以上,高寒草甸比干草原中高29%;5～15 cm土层,森林比干草原和高寒草甸中分别高7%和191%以上,干草原比高寒草甸中高171% 以上(p<0.05)。森林和干草原中土壤微生物生物量碳比例比高寒草甸中高32%以上,0～5和5～15 cm土层,森林和干草原中土壤微生物生物量氮比例比高寒草甸中高150%以上（p<0.05）。就土壤微生物活性,0～5和5～15 cm土层,森林和高寒草甸比干草原中高26%以上;15～35 cm土层,森林比干草原和高寒草甸中高28%以上 (p<0.05)。土壤微生物生物量碳氮含量与有机碳含量及微生物生物量氮含量和比例与微生物生物量碳含量和比例呈现正相关（r²>0.30,p<0.000 1）。土壤微生物生物量氮含量、微生物生物量碳氮含量比例、微生物活性与土壤pH值呈显著负相关,土壤微生物生物量碳氮含量及其比例、微生物活性与土壤湿度呈正相关。说明祁连山3种生态系统土壤中微生物生物量和活性受气候要素、植被、有机碳、pH值和湿度等因素 的共同影响。     

不同退化沙地土壤碳的矿化潜力

通过实验室土壤培养试验 ,研究了科尔沁退化沙质草地不同生境 (流动沙地 ,半固定沙地 ,固定沙地和丘间低地 )下土壤碳的矿化潜力及不同凋落物在沙地土壤中的分解。经 33d的室内培养 ,不同生境土壤 CO2 - C的释放有极显著的差异 ,与生境植被盖度 ,凋落物积累 ,土壤沙化程度 ,土壤有机碳和全氮含量的分布有显著相关。流动沙地土壤有极低的土壤有机碳和氮的含量及其微弱的土壤微生物呼吸 ,表明土地沙漠化不仅导致土壤有机碳库衰竭 ,也使土壤微生物活性丧失。在有机质含量很低的流动沙地和半固定沙地土壤中 ,含氮量高的小叶锦鸡儿 (Caragana microphylla)凋落物比含氮量低、C/N比高的差巴嘎蒿(Artemisia halodendron)和 1年生植物凋落物有较快的分解。在沙漠化的演变中 ,土壤的粗粒化 ,有机物质和养分及微生物活性的丧失制约着凋落物在土壤中的矿化潜力。灌木的存在使更多的有机物质和养分积聚在灌丛下 ,形成灌丛肥岛 ,因而显著贡献于碳的固存。     

Does long-term soil warming affect microbial element limitation? A test by short-term assays of microbial growth responses to labile C,N and P additions

Increasing global temperatures have been reported to accelerate soil carbon (C) cycling, but also to promote nitrogen (N) and phosphorus (P) dynamics in terrestrial ecosystems. However, warming can differentially affect ecosystem C, N and P dynamics, potentially intensifying elemental imbalances between soil resources, plants and soil microorganisms. Here, we investigated the effect of long-term soil warming on microbial resource limitation, based on measurements of microbial growth (¹⁸O incorporation into DNA) and respiration after C, N and P amendments. Soil samples were taken from two soil depths (0–10, 10–20 cm) in control and warmed (>14 years warming, +4°C) plots in the Achenkirch soil warming experiment. Soils were amended with combinations of glucose-C, inorganic/organic N and inorganic/organic P in a full factorial design, followed by incubation at their respective mean field temperatures for 24 h. Soil microbes were generally C-limited, exhibiting 1.8-fold to 8.8-fold increases in microbial growth upon C addition. Warming consistently caused soil microorganisms to shift from being predominately C limited to become C-P co-limited. This P limitation possibly was due to increased abiotic P immobilization in warmed soils. Microbes further showed stronger growth stimulation under combined glucose and inorganic nutrient amendments compared to organic nutrient additions. This may be related to a prolonged lag phase in organic N (glucosamine) mineralization and utilization compared to glucose. Soil respiration strongly positively responded to all kinds of glucose-C amendments, while responses of microbial growth were less pronounced in many of these treatments. This highlights that respiration–though easy and cheap to measure—is not a good substitute of growth when assessing microbial element limitation. Overall, we demonstrate a significant shift in microbial element limitation in warmed soils, from C to C-P co-limitation, with strong repercussions on the linkage between soil C, N and P cycles under long-term warming.     

Response of soil carbon dioxide fluxes,soil organic carbon and microbial biomass carbon to biochar amendment: a meta‐analysis

Biochar as a carbon‐rich coproduct of pyrolyzing biomass, its amendment has been advocated as a potential strategy to soil carbon (C) sequestration. Updated data derived from 50 papers with 395 paired observations were reviewed using meta‐analysis procedures to examine responses of soil carbon dioxide (CO₂) fluxes, soil organic C (SOC), and soil microbial biomass C (MBC) contents to biochar amendment. When averaged across all studies, biochar amendment had no significant effect on soil CO₂ fluxes, but it significantly enhanced SOC content by 40% and MBC content by 18%. A positive response of soil CO₂ fluxes to biochar amendment was found in rice paddies, laboratory incubation studies, soils without vegetation, and unfertilized soils. Biochar amendment significantly increased soil MBC content in field studies, N‐fertilized soils, and soils with vegetation. Enhancement of SOC content following biochar amendment was the greatest in rice paddies among different land‐use types. Responses of soil CO₂ fluxes and MBC to biochar amendment varied with soil texture and pH. The use of biochar in combination with synthetic N fertilizer and waste compost fertilizer led to the greatest increases in soil CO₂ fluxes and MBC content, respectively. Both soil CO₂ fluxes and MBC responses to biochar amendment decreased with biochar application rate, pyrolysis temperature, or C/N ratio of biochar, while each increased SOC content enhancement. Among different biochar feedstock sources, positive responses of soil CO₂ fluxes and MBC were the highest for manure and crop residue feedstock sources, respectively. Soil CO₂ flux responses to biochar amendment decreased with pH of biochar, while biochars with pH of 8.1–9.0 had the greatest enhancement of SOC and MBC contents. Therefore, soil properties, land‐use type, agricultural practice, and biochar characteristics should be taken into account to assess the practical potential of biochar for mitigating climate change.     

Increased microbial growth,biomass, and turnover drive soil organic carbon accumulation at higher plant diversity

Species‐rich plant communities have been shown to be more productive and to exhibit increased long‐term soil organic carbon (SOC) storage. Soil microorganisms are central to the conversion of plant organic matter into SOC, yet the relationship between plant diversity, soil microbial growth, turnover as well as carbon use efficiency (CUE) and SOC accumulation is unknown. As heterotrophic soil microbes are primarily carbon limited, it is important to understand how they respond to increased plant‐derived carbon inputs at higher plant species richness (PSR). We used the long‐term grassland biodiversity experiment in Jena, Germany, to examine how microbial physiology responds to changes in plant diversity and how this affects SOC content. The Jena Experiment considers different numbers of species (1–60), functional groups (1–4) as well as functional identity (small herbs, tall herbs, grasses, and legumes). We found that PSR accelerated microbial growth and turnover and increased microbial biomass and necromass. PSR also accelerated microbial respiration, but this effect was less strong than for microbial growth. In contrast, PSR did not affect microbial CUE or biomass‐specific respiration. Structural equation models revealed that PSR had direct positive effects on root biomass, and thereby on microbial growth and microbial biomass carbon. Finally, PSR increased SOC content via its positive influence on microbial biomass carbon. We suggest that PSR favors faster rates of microbial growth and turnover, likely due to greater plant productivity, resulting in higher amounts of microbial biomass and necromass that translate into the observed increase in SOC. We thus identify the microbial mechanism linking species‐rich plant communities to a carbon cycle process of importance to Earth's climate system.     

中国东部森林土壤有机碳组分的纬度格局及其影响因子

土壤有机碳是森林碳库的重要组成部分,其活性有机碳组分不仅是土壤碳周转过程的重要环节,还是气候变化最敏感的指标。以中国东部南北森林样带(NSTEC,North-South Transect of Eastern China)为对象,选择了9个典型森林生态系统(尖峰岭、鼎湖山、九连山、神农架、太岳山、东灵山、长白山、凉水和呼中),涵盖了我国热带森林、亚热带森林和温带森林的主要类型,测定其0—10 cm土壤有机碳(SOC)、易氧化有机碳(EOC)、微生物碳(MBC)和可溶性有机碳(DOC)含量,结合气候、土壤质地、土壤微生物和植被生物量等因素,探讨了森林土壤有机碳组分的纬度格局及其主要影响因素。实验结果表明:SOC、EOC、MBC和DOC含量分别为23.12—77.00 g/kg、4.62—17.24 g/kg、41.92—329.39 mg/kg和212.63—453.43 mg/kg。SOC、EOC和MBC随纬度增加呈指数增长(P0.05),而DOC则随纬度增加呈指数降低(P0.05)。在不同气候带上,SOC和EOC含量表现为热带森林亚热带森林温带森林(P0.05),DOC含量表现为热带森林亚热带森林温带森林(P0.001)。气候、植被生物量、土壤质地和土壤微生物可解释土壤有机碳组分纬度格局的大部分空间变异(SOC 74%;EOC 65%;MBC 51%和DOC 76%)。其中,气候是土壤有机碳组分呈现纬度格局的主要影响因素,土壤质地是SOC和EOC的次要影响因素,而土壤微生物和植被生物量是MBC和DOC的次要影响因素。     

宁南山区典型植物根际与非根际土壤微生物功能多样性

选择宁南山区9种典型植物的根际与非根际土壤为研究对象,采用Biolog方法对土壤微生物功能多样性进行了研究。结果表明:9种不同植物根际土壤与非根际土壤的微生物活性(AWCD)、微生物多样性指数和微生物均匀度指数均存在明显差异;除冰草外,其他各种植物的根际土壤的微生物活性AWCD、微生物多样性指数和微生物均匀度指数均比非根际土壤的高;9种典型植物根际土壤微生物主要碳源利用类型是羧酸类和氨基酸类,非根际土壤微生物主要碳源利用类型是羧酸类、胺类、氨基酸类;微生物活性、微生物多样性指数和微生物均匀度指数两两之间均达到了极显著相关,与土壤化学性质各指标之间均未达到显著相关水平。     

Mechanisms of soil carbon accrual and storage in bioenergy cropping systems

Annual row cropping systems converted to perennial bioenergy crops tend to accrue soil C, likely a function of increased root production and decreased frequency of tillage; however, very little is known about the mechanisms governing the accrual and stability of this additional soil C. To address this uncertainty, we assessed the formation and stability of aggregates and soil organic C (SOC) pools under switchgrass, giant miscanthus, a native perennial grass mix and continuous corn treatments in Michigan and Wisconsin soils differing in both texture and mineralogy. We isolated different aggregate size fractions, >2 mm, 0.5–2 mm, and <0.5 mm, using a procedure intended to minimize alterations to aggregate biological and chemical properties. We determined SOC, permanganate oxidizable C (POXC), and microbial activities (i.e. enzyme activities and soil respiration rates) associated with these aggregates. Soil type strongly influenced the trajectory of aggregate formation and stabilization with differences between sites in mean aggregate size, stability, SOC and microbial activity under perennial vs. corn cropping systems. At the Michigan site, soil microbial activities were highest in the >2 mm aggregates, and higher under the perennial grasses compared to corn. Contrastingly, in Wisconsin soils, microbial activities were highest in the <0.5 mm aggregates and evidence for soil C accrual under perennial grasses was observed only in a fast turnover pool in the <0.5 mm aggregate class. Our results help explain cross‐site variability in soil C accrual under perennial bioenergy crops by demonstrating how interactions between belowground productivity, soil type, aggregation processes and microbial communities influence the rates and extent of SOC stabilization. Bioenergy cropping systems have the potential to be low‐C energy sources but first we must understand the complex interactions controlling the formation and stabilization of SOC if we are to maximize soil C accrual.     

Soil microbial responses to experimental warming and clipping in a tallgrass prairie

Global surface temperature is predicted to increase by 1.4–5.8°C by the end of this century. However, the impacts of this projected warming on soil C balance and the C budget of terrestrial ecosystems are not clear. One major source of uncertainty stems from warming effects on soil microbes, which exert a dominant influence on the net C balance of terrestrial ecosystems by controlling organic matter decomposition and plant nutrient availability. We, therefore, conducted an experiment in a tallgrass prairie ecosystem at the Great Plain Apiaries (near Norman, OK) to study soil microbial responses to temperature elevation of about 2°C through artificial heating in clipped and unclipped field plots. While warming did not induce significant changes in net N mineralization, soil microbial biomass and respiration rate, it tended to reduce extractable inorganic N during the second and third warming years, likely through increasing plant uptake. In addition, microbial substrate utilization patterns and the profiles of microbial phospholipid fatty acids (PLFAs) showed that warming caused a shift in the soil microbial community structure in unclipped subplots, leading to the relative dominance of fungi as evidenced by the increased ratio of fungal to bacterial PLFAs. However, no warming effect on soil microbial community structure was found in clipped subplots where a similar scale of temperature increase occurred. Clipping also significantly reduced soil microbial biomass and respiration rate in both warmed and unwarmed plots. These results indicated that warming‐led enhancement of plant growth rather than the temperature increase itself may primarily regulate soil microbial response. Our observations show that warming may increase the relative contribution of fungi to the soil microbial community, suggesting that shifts in the microbial community structure may constitute a major mechanism underlying warming acclimatization of soil respiration.     

黄土丘陵区草地土壤微生物C、N及呼吸熵对植被恢复的响应

以野外样地调查和室内分析法研究了黄土丘陵区不同植被恢复年限下草地土壤微生物C、N及土壤呼吸熵的变化.结果表明,土壤微生物量碳明显地随着植被恢复年限的增加而增加.在恢复前23a, 土壤微生物量碳在0～20 cm土层年增加率为24.1%;20～40 cm为104.4%.植被恢复23a后,0～20 cm土层增长率为0.83%,20～40 cm为0.19%.土壤微生物量N表现为在植被恢复的初期略有下降,3a后,开始出现明显增加.0～20 cm土层年增长率为20.14%,20～40 cm为15.11%.在植被恢复23a后,0～20 cm土层的年增长率为0.14%,20～40 cm变化不大.土壤微生物呼吸强度随着恢复年限的增加逐渐加强;土壤呼吸熵随植被封育时间的增加而呈对数降低趋势.土壤呼吸熵(qCO2)在反映土壤的生物质量变化时,显得更加稳定,受植物生长状况影响较小.相关分析表明,土壤微生物量和土壤微生物活性与土壤有机质、碱解氮和粘粒含量显著正相关;与土壤粉粒含量明显负相关;表层土壤pH值对其也有明显影响.草地植被自然恢复过程可增加土壤微生物活性,有利于土壤质量的提高.     

Elevated temperature increases the accumulation of microbial necromass nitrogen in soil via increasing microbial turnover

Microbial‐derived nitrogen (N) is now recognized as an important source of soil organic N. However, the mechanisms that govern the production of microbial necromass N, its turnover, and stabilization in soil remain poorly understood. To assess the effects of elevated temperature on bacterial and fungal necromass N production, turnover, and stabilization, we incubated ¹⁵N‐labeled bacterial and fungal necromass under optimum moisture conditions at 10°C, 15°C, and 25°C. We developed a new ¹⁵N tracing model to calculate the production and mineralization rates of necromass N. Our results showed that bacterial and fungal necromass N had similar mineralization rates, despite their contrasting chemistry. Most bacterial and fungal necromass ¹⁵N was recovered in the mineral‐associated organic matter fraction through microbial anabolism, suggesting that mineral association plays an important role in stabilizing necromass N in soil, independently of necromass chemistry. Elevated temperature significantly increased the accumulation of necromass N in soil, due to the relatively higher microbial turnover and production of necromass N with increasing temperature than the increases in microbial necromass N mineralization. In conclusion, we found elevated temperature may increase the contribution of microbial necromass N to mineral‐stabilized soil organic N.     

Effects of fire on soil respiration,ATP content and enzyme activities in Mediterranean maquis

Question: Do low or high intensity fires affect micro‐organism activity in the upper soil layer of Mediterranean maquis? Location: 600 m from the sea in the Nature Reserve of Castel Volturno (Campania, southern Italy, 40°57’N; 13°55’E). Methods: Soil respiration was measured in situ on intact soil; enzyme activity (cellulase, xylanase, invertase, trehalase and protease) and ATP content were measured on soil samples collected under three species of maquis vegetation: Phillyrea angustifolia L., Myrtus communis L. and Cistus incanus L. Results: Soil microbial respiration showed no significant differences in CO₂ flux in treated and untreated plots, but the ATP content in the soil under C. incanus and M. communis was lower in the treated plots for most of the study period. In the soil under Ph. angustifolia, ATP content was low only for one week after fire. The reduction was more marked in the samples from ‘high fire intensity’ than from ‘low fire intensity’ plots. Soil respiration and ATP content exhibited seasonal variations linked to soil water content. Among the enzyme activity measured in the soil under the three plant covers, only invertase declined in burned plots throughout the study period, particularly in the ‘high fire intensity’ plots. Activity of the enzymes cellulase, xylanase, trehalase and protease had a different sensitivity depending on the respective shrub cover. Conclusions: Impact of fire on soil microbial activity is largely dependent on vegetation mosaic and species identity.     

酸性矿山废水污染的水稻田土壤中重金属的微生物学效应

采样调查了广东大宝山地区受酸性采矿废水长期污染的亚热带水稻田的土壤理化性质 ,重金属 Cu、Pb、Zn、Cd的全量及其 DTPA浸提量 ,以及微生物生物量及其呼吸活性等指标。利用主成分和逐步回归分析了影响土壤重金属的有效性及其微生物学效应的因素。结果表明 :土壤高含硫 ,强酸性 ,有机碳、全氮较低 ,4种金属的全量普遍超标。DTPA可提取态金属含量较高 ,不仅与其全量呈显著正相关 ,而且与土壤酸度和粘粒含量正相关 ,和 Mn含量负相关。过量的金属显著降低了土壤微生物生物量 C、N、微生物商、生物量 N/全 N比 ,并抑制了微生物呼吸强度和对有机碳的矿化率 ,导致了土壤 C/N比的升高。同时 ,金属对微生物群落及生理代谢指标 ,如微生物生物量 C/N比和代谢商的影响不显著。 DTPA可提取态金属 ,特别是 DTPA- Cu是导致微生物生物量和活性指标变化的主要因素。以有机碳 (或全氮 )为基数的复合微生物指标降低了土壤性质差异造成的干扰 ,较单一指标更能准确指示微生物对金属胁迫的反应。土壤硫没有对金属有效性和微生物指标产生明显影响 ,但其氧化过程可能引起酸化和金属离子的释放     

根系分泌物C∶N对刺槐林地土壤理化特征和土壤呼吸的影响

为探究根系分泌物C∶N对土壤养分循环及微生物活性的影响,本研究以黄土高原人工刺槐林为对象,在生境条件基本一致的15、25、35、45 a刺槐林地取原位土壤,通过模拟不同C∶N的根系分泌物(只添加N、C∶N=10、C∶N=50、C∶N=100和只添加C)添加至土壤,以去离子水作为对照,分析根系分泌物C∶N对土壤碳、氮、磷、pH值等理化特征和土壤呼吸的影响。结果表明: 1)有机碳含量与根系分泌物C∶N呈正相关,根系分泌物C∶N=10时土壤有机碳(SOC)分解较快,高根系分泌物C∶N(C∶N=100)能延缓SOC分解,而只添加C处理对SOC无显著影响。2)不同C∶N根系分泌物处理对全氮的影响不明显,碳添加能促进微生物对铵态氮的吸收,氮添加能促进铵态氮的硝化,随着根系分泌物C∶N增加,土壤中铵态氮含量下降。3)氮添加会导致土壤pH值下降,增加土壤全磷含量。4)刺槐林地土壤呼吸值与根系分泌物C∶N呈正相关,随着C∶N增加,根系分泌物对25和35 a人工刺槐林土壤呼吸的促进作用更显著。综上,根系分泌物C∶N值越高,对人工刺槐林土壤呼吸的促进作用越显著。研究结果进一步加深了对森林根系-土壤-微生物互作过程的认识。