间伐对杨树人工林土壤微生物量和氮含量的影响

针对生产中杨树大径材培育存在的问题，设计了4种间伐处理(对照、30％下层伐、50％下层伐和50％机械伐)，研究了不同间伐处理对杨树人工林土壤微生物量碳、微生物量氮、土壤无机氮和土壤溶解性有机氮含量的影响及其时间和空间格局。结果表明，由于间伐处理改变了林地土壤的温度和水分条件，导致在生长季高强度间伐处理(50％下层伐和50％机械伐)显著提高了土壤微生物量碳和微生物量氮含量，表现为50％机械伐>50％下层伐>30％下层伐>不间伐，而在非生长季则表现相反的趋势。高强度间伐显著提高了土壤无机氮含量，但50％下层伐和50％机械伐的土壤无机氮含量差异不显著；土壤无机氮( NH4＋－N和NO3－－N)主要以硝态氮的形式存在，并表现出明显的季节动态。高强度间伐提高了土壤溶解性有机氮含量，特别在夏季和秋季这种差异达到显著水平。从不同土壤层次看，土壤微生物量、土壤无机氮和溶解性有机氮均表现出明显的表聚性。研究结果可为杨树人工林的可持续经营提供理论依据和技术支撑。

Effects of thinning intensity and method on soil microbial biomass and nitrogen content in the poplar plantations

Based on the existed problems for large diameter timber cultivation of poplar plantations, four thinning treatments ( CK, 30% lower thinning, 50% lower thinning and 50% mechanical thinning ) were designed, and the dynamics in the contents of inorganic nitrogen, dissolved organic nitrogen (DON), microbial biomass nitrogen (MBN) and carbon (MBC) under different thinning treatments were monthly investigated at two soil depths. The results indicated that thinning treatments modified the soil temperature and moisture, and consequently intensive thinning treatments ( 50% lower thinning and 50% mechanical thinning ) significantly improved soil MBC and MBN contents, with the order of 50% mechanical thinning>50% lower thinning>30% lower thinning>CK during the growing season, while showing the opposite trend during the non￣growing season. Intensive thinning treatments also significantly increased inorganic nitrogen content of the soil, but no significant difference was observed between 50%lower thinning and 50% mechanical thinning. Soil inorganic nitrogen was mainly in the form of nitrate nitrogen, and showed an obvious seasonal variation. Intensive thinning treatments enhanced DON content, but a significance was only detected in summer and fall. From the view of soil depths, the contents of soil microbial biomass, inorganic nitrogen and dissolved organic nitrogen were higher at 0-10 cm than at 10-20 cm soil, showing a clear surface aggregation characteristics. Results from this study would provide some theoretical and technical supports for the sustainable management of the poplar plantations.