Roots from beech (Fagus sylvatica L.) and ash (Fraxinus excelsior L.) differentially affect soil microorganisms and carbon dynamics

Knowledge about the influence of living roots on decomposition processes in soil is scarce but is needed to understand carbon dynamics in soil. We investigated the effect of dominant deciduous tree species of the Central European forest vegetation, European beech (Fagus sylvatica L.) and European ash (Fraxinus excelsior L.), on soil biota and carbon dynamics differentiating between root- and leaf litter-mediated effects. The influence of beech and ash seedlings on carbon and nitrogen flow was investigated using leaf litter enriched in ¹³C and ¹⁵N in double split-root rhizotrons planted with beech and ash seedlings as well as a mixture of both tree species and a control without plants. Stable isotope and compound-specific fatty acid analysis (¹³C-PLFA) were used to follow the incorporation of stable isotopes into microorganisms, soil animals and plants. Further, the bacterial community composition was analyzed using pyrosequencing of 16S rRNA gene amplicons. Although beech root biomass was significantly lower than that of ash only beech significantly decreased soil carbon and nitrogen concentrations after 475 days of incubation. In addition, beech significantly decreased microbial carbon use efficiency as indicated by higher specific respiration. Low soil pH probably increased specific respiration of bacteria suggesting that rhizodeposits of beech roots induced increased microbial respiration and therefore carbon loss from soil. Compared to beech δ¹³C and δ¹⁵N signatures of gamasid mites in ash rhizotrons were significantly higher indicating higher amounts of litter-derived carbon and nitrogen to reach higher trophic levels. Similar δ¹³C signatures of bacteria and fine roots indicate that mainly bacteria incorporated root-derived carbon in beech rhizotrons. The results suggest that beech and ash differentially impact soil processes with beech more strongly affecting the belowground system via root exudates and associated changes in rhizosphere microorganisms and carbon dynamics than ash.     

农田土壤紧实度和容重空间变异性研究

为了揭示长期人为管理与利用过程对于土壤质量的作用与影响,本试验研究了陕西关中地区旱地农田土壤紧实度和容重空间变异特征。研究结果表明:在水平方向上土壤紧实度和容重具有中等变异强度,土壤紧实度和容重以村庄为中心向外逐渐增大;在垂直方向上土壤紧实度和容重同样存在明显的变异性。进一步分析表明,土壤紧实化的水平空间变异特征是人为长期采用就近原则进行有机培肥,对土壤质量产生的作用与影响;而垂直剖面上的差异性则是多年旋耕产生的结果。所以,加强土壤培肥,改善现行耕作制度显得极为重要。     

关中农田土壤有机质和碳酸钙空间变异特征及其机理分析

为了研究人为活动对于农田土壤有机质含量和碳酸钙空间变异性的作用与影响,在陕西杨凌杜寨村研究了农田土壤有机质和碳酸钙空间变异性,并对其变异机理进行了分析.试验表明:表层0～10 cm和底层20～40 cm土壤有机质平均值分别20.64 g/kg和10.81 g/kg,变异系数为0.087和0.013,均属弱变异程度;亚表层10～20 cm土壤有机质平均值为16.96 g/kg,变异系数为0.130,属中等变异程度;碳酸钙均属弱变异程度.经过对土壤特性空间分布等值线图的分析得出:在水平方向上,土壤有机质和碳酸钙均具有相同的变异规律,距离村庄越近土壤有机质和碳酸钙含量越高,以村庄为圆心土壤有机质和碳酸钙含量呈同心圆式的空间变异特征.