Soil nitrogen turnover in proximal and distal stem areas of European beech trees
Soil nitrogen turnover in proximal and distal stem areas of European beech trees
复制标题
欧洲山毛榉树近端和远端茎区的土壤氮周转
作者：
Shih;E. Matzner
在欧洲山毛榉(Fagus sylvatica L.)在森林中，输入到土壤中的水和离子的很大一部分是由树干径流造成的，这在树干附近形成了一个高元素通量的土壤微区。树干附近的土壤具有不同的氮素周转速率，这可能会影响林分尺度上氮素周转速率的估计。在之前的一项研究中，我们报告了德国施泰格瓦尔德一片以欧洲山毛榉为主的森林中，树干附近存在高硝酸盐通量和渗漏。在这里，我们研究了近端(定义为山毛榉茎周围1平方米)和远端茎干区域表层15厘米土壤的氮素周转。通过实验室培养和原位连续取芯培养，确定了氨化、硝化和根系吸收矿质氮的净速率。在实验室培养条件下，与树干远端相比，树干近端林地的净氮矿化和硝化速率较高。在矿物土壤样本中，没有观察到与茎相关的差异。相反，原位培养显示在茎近端的矿质土壤中硝化速率较高，而近端和远端的净氮矿化作用相同。在原位培养中，近端的硝化/氨化比平均为0.85，远端的平均硝化/氨化比为0.34。两个地区的氮素净矿化量均为4.4gNm-2.90d-1。矿化氮几乎全部被树根吸收，氮素形态以氨氮为主。硝态氮/氨氮吸收比近端为0.69，远端为0.20。近端茎段土壤含水率较高被认为是硝化速率增加的主要原因。在林分尺度上，不同氮素周转率对土壤氮素周转率没有影响。因此，观测到的高硝酸盐通量和茎附近的渗漏归因于树干径流输入的高氮素，而不是土壤氮素的周转。
In European beech (Fagus sylvatica L.) forests, a large proportion of the water and ion input to the soil results from stemflow which creates a soil microsite of high element fluxes proximal to the tree trunk. The soil proximal to the stem is considered to have different rates of nitrogen turnover which might influence the estimation of N-turnover rates at the stand scale. In a previous study we reported high nitrate fluxes with seepage proximal to the stems in a forest dominated by European beech in Steigerwald, Germany. Here, we investigated the soil nitrogen turnover in the top 15 cm soil in proximal (defined as 1 m2 around beech stems) and distal stem areas. Laboratory incubations and in situ sequential coring incubations were used to determine the net rates of ammonification, nitrification, and root uptake of mineral nitrogen. In the laboratory incubations higher rates of net nitrogen mineralization and nitrification were found in the forest floor proximal to the stem as compared to distal stem areas. No stem related differences were observed in case of mineral soil samples. In contrast, the in situ incubations revealed higher rates of nitrification in the mineral soil in proximal stem areas, while net nitrogen mineralization was equal in proximal and distal areas. In the in situ incubations the average ratio of nitrification/ammonification was 0.85 in proximal and 0.34 in distal stem areas. The net nitrogen mineralization was 4.4 g N m-2 90 day-1 in both areas. Mineralized nitrogen was almost completely taken up by tree roots with ammonium as the dominant nitrogen species. The average ratio of nitrate/ammonium uptake was 0.69 in proximal and 0.20 in distal areas. The higher water content of the soil in proximal stem areas is considered to be the major reason for the increased rates of nitrification. Different nitrogen turnover rates in proximal stem areas had no influence on the nitrogen turnover rates in soil at the stand scale. Consequently, the observed high nitrate fluxes with seepage proximal to stems are attributed to the high nitrogen input by stemflow rather than to soil nitrogen turnover.