We sieved soils from a Pinus massoniana plantation in the Three Gorges Reservoir area into four aggregate sizes, including aggregates of 2000-8000 μm (large macroaggregates), 1000-2000 μm (coarse aggregates), 250-1000 μm (small macroaggregates), and <250 μm (microaggregates). We analyzed the differences in the acidolyzable organic N components and net N mineralization of the aggregates under different N addition levels (30, 60, and 90 kg N·hm-2·a-1, representing by N30, N60 and N90, respectively). The results showed that net nitrification rate of the aggregates ranged from 0.30-3.42 mg N·kg-1 and accounted for more than 80% of net nitrogen mineralization. Compared with the control, addition of 30, 60, and 90 kg N·hm-2·a-1 increased total N by 24.1%-45.5%, 6.4%-34.3%, and 7.9%-42.4% in the large aggregates, coarse aggregate, small macroaggregates, and microaggregates, increased net N mineralization rate by 1.3-7.2, 1.4-6.6, and 1.8-12.9 times, but decreased the contents of available phosphorus by 9.3%-36.9%, 12.2%-56.7%, and 19.2%-61.9%, respectively. The contents of total acidolyzable N, soil organic matter, and rates of net ammonification, net nitrification, and net N mineralization increased as the aggregate size decreased, while available phosphorus contents showed an opposite trend. The levels of acid-hydrolyzable N components were ranked as acidolyzable amino acid N > acidolyzable ammonia N > acidolyzable unknown N> acidolyzable amino sugar N. Total N was the dominant contributor to the increases in acid-hydrolyzable N components. Results of stepwise multiple regression analyses showed that acidoly-zable amino acid N and acidolyzable amino sugar N were predictors of net ammonification rate. Acidolyzable amino sugar N, acidolyzable amino acid N, and acidolyzable ammonia N were predictors of net nitrification, net nitrogen mineralization rate, and net nitrogen mineralization accumulation. The physical structure of aggregates was associa-ted with soil net N mineralization. Addition of N increased the contents and bioavailability of acidolyzable organic N, a large amount of which contributed to soil organic matter levels and the decrease in available phosphorus.
以三峡库区马尾松人工林为对象,将土壤筛分为大团聚体(2000～8000 μm)、粗砂粒(1000～2000 μm)、小团聚体(250～1000 μm)和微团聚体(＜250 μm)4个粒径,研究低、中、高氮添加处理(氮添加量分别为30、60、90 kg N·hm-2·a-1)下土壤酸解性有机氮组分和净氮矿化的变化。结果表明: 不同处理下,团聚体净硝化速率为0.30 ～ 3.42 mg N·kg-1,占净氮矿化的80%以上。与对照相比,不同处理4个粒径的总氮含量分别提高24.1%～45.5%、6.4%～34.3%、7.9%～42.4%,净氮矿化速率分别提高1.3～7.2、1.4～6.6、1.8～12.9倍,而速效磷含量分别降低9.3%～36.9%、12.2%～56.7%、19.2%～61.9%。可酸解性有机氮组分、有机质含量以及净氨化、净硝化和净氮矿化速率均随着团聚体粒径的减小而增加,但速效磷含量变化呈相反的趋势。酸解性有机氮组分含量大小为:酸解氨基酸态氮＞酸解铵态氮＞酸解未知态氮＞酸解氨基糖态氮。总氮是提高酸解性有机氮组分含量的主导因子。多元逐步回归显示,酸解氨基酸态氮和酸解氨基糖态氮含量影响了净氨化速率;酸解氨基糖态氮、酸解氨基酸态氮和酸解铵态氮含量共同影响了净硝化、净氮矿化速率以及净氮矿化累积量。综上,团聚体的物理结构影响了土壤氮矿化,氮添加提高了土壤生物可利用性及易矿化态酸解性有机氮的含量,但大量氮添加导致土壤有机质和速效磷含量下降。.
Keywords: aggregate; mineralization; nitrogen addition; organic nitrogen.