东灵山油松纯林和油松-辽东栎针阔混交林土壤氮素矿化/硝化作用研究

利用PVC顶盖埋管原位培育法测定了北京东灵山地区一个油松纯林和一个油松-辽东栎落叶阔叶混交林生态系统土壤无机氮库、氮素净矿化/硝化速率的季节动态以及年度净矿化/硝化量。结果发现：1）两个生态系统的土壤无机氮库和氮素净矿化/硝化速率都存在比较明显且比较一致的季节动态，但个别时期也存在较大差异；2）纯林与混交林土壤NH+4-N浓度在各月都没有显著差异，而NO-3-N浓度，除了1995年11月和1996年8月纯林显著高于混交林外，其它月份也都差异不显著；3）无论是年度净矿化总量（纯林，22.7kg.hm-2；混交林，55.5kg.hm-2）及其占总全N量的百分比（纯林，0.694%；混交林，2.128%），还是年度净硝化总量（纯林，26.7kg.hm-2；混交林，44.6kg.hm-2）及其占总全N量（纯林，0.815%；混交林，1.707%）的百分比，油松针阔混交林生态系统均显著大于油松纯林，高达后者的两倍左右，而净硝化氮占净矿化氮的百分比则相反，油松纯林（100%）显著高于油松-辽东栎混交林（80.2%）。上述结果表明：油松-辽东栎针阔混交林生态系统土壤的氮素有效性（即土壤的供氮能力）以及维持土壤中植物可利用氮素的能力都显著高于油松纯林。物种构成及在其影响下所产生的林下微生境和人为干扰活动可能是造成这种差异的主要原因。

NET NITROGEN MINERALIZATION AND NITRIFICATION IN ONE PURE PINE (PINUS TABULAEFORMIS) FOREST AND ONE PINE-OAK MIXED FOREST IN DONGLING MOUNTAINOUS REGION

Soil net N mineralization and nitrification were determined usingthe PVC tube closed-top in situ incubation method in one pure pine stand and one pine-oak coniferous/deciduous broad-leaved mixed forest in Dongling Mountain region in SW Beijing City. Seasonal variation in the inorganic N pools and net mineralization/nitrification rates and the annual net N mineralization/nitrification were investigated. The results showed that: 1) there were obvious and consistently seasonal variations in soil inorganic N pools and the net N mineralization/nitrification in the two forest ecosystems except for some quite large differences in certain months; 2) there were no significant differences in NH+4-N concentration between the two ecosystems in any sampling dates, whereas NO-3-N concentration did differ, being higher in the pure pine stand than in the pine-oak mixed forest in Dec. 1995 and Aug. 1996; 3) both the annual net N mineralization (55.5 kg.hm-2) and the percentage of annually mineralized N of total N (2.13%) were much higher in pine-oak mixed forest than in pure pine stand (22.7 kg.hm-2 and 0.69%), respectively, with the former nearly being two times of the latter; 4) both the annual net N nitrification (44.6 kg.hm-2) and the percentage of annually nitrified N of total N (1.71%) in the pine-oak mixed forest were also much higher than in the pure pine stand (26.7 kg.hm-2 and 0.82%), while the percentages of nitrified N of mineralized N between the two forest ecosystems were quite different, with the pure pine stand (100%) being higher than pine-oak mixed forest (80.2%). These results indicate that the ability to supply available N to plants (i.e., soil N availability) and to prevent inorganic N loss was higher in the pine-oak mixed forest ecosystem than the pure pine forest ecosystem. Difference in species composition, microhabitats created by the different species and the intensity of human perturbation between the two forests might have accounted for the observed differences in our study.