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Keywords:

  • land-use history;
  • litter decomposition;
  • nitrogen addition;
  • nitrogen deposition;
  • phosphorus availability;
  • phosphorus release;
  • reforested forest;
  • tropical forests

Abstract

Scant information is available on how soil phosphorus (P) availability responds to atmospheric nitrogen (N) deposition, especially in the tropical zones. This study examined the effect of N addition on soil P availability, and compared this effect between forest sites of contrasting land-use history. Effects of N addition on soil properties, litterfall production, P release from decomposing litter, and soil P availability were studied in a disturbed (reforested pine forest with previous understory vegetation and litter harvesting) and a rehabilitated (reforested mixed pine/broadleaf forest with no understory vegetation and litter harvesting) tropical forest in southern China. Experimental N-treatments (above ambient) were the following: Control (no N addition), N50 (50 kg N ha−1 yr−1), and N100 (100 kg N ha−1 yr−1). Results indicated that N addition significantly decreased soil P availability in the disturbed forest. In the rehabilitated forest, however, soil P availability was significantly increased by N addition. Decreases in soil P availability may be correlated with decreases in rates of P release from decomposing litter in the N-treated plots, whereas the increase in soil P availability was correlated with an increase in litterfall production. Our results suggest that response of soil P availability to N deposition in the reforested tropical forests in southern China may vary greatly with temporal changes in tree species composition and soil nutrient status, caused by different land-use practices.

在氮沉降全球化的背景下,人们对大气氮沉降如何影响森林生态系统土壤有效磷水平知之甚少,特别是在热带地区。本研究通过研究氮沉降对土壤理化性质、凋落物生产、凋落物磷的释放以及土壤有效磷水平的影响,来探讨氮沉降增加如何影响热带地区森林土壤的有效磷水平。为此,我们选择了位于中国南方两种典型的热带森林生态系统:即干扰林(马尾松林,马尾松种植后收获林下层植物和凋落物)和恢复林(针叶阔叶混交林,马尾松种植后不收获林下层植物和凋落物)。该区域的大气氮湿沉降水平约为30 kg N ha−1 yr−1,在此基础上,我们进行模拟氮沉降增加试验。试验设计分为三个处理水平:即对照(不加氮)、低氮(N50,50 kg N ha−1 yr−1)和高氮(N100,100 kg N ha−1 yr−1),每个处理三个重复。结果表明,为期四年的氮沉降增加试验显著降低了干扰林的土壤有效磷水平,但却增加了恢复林的有效磷水平。进一步研究发现,干扰林中氮处理可能通过降低凋落物分解过程中磷释放速率从而导致土壤有效磷减少,而恢复林可能通过增加凋落物生产量来提升有效磷水平。我们的研究表明:热带再造林(reforested forests)土壤有效磷水平对氮沉降增加的响应依赖于因土地利用历史不同而导致的物种组成和土壤营养状态的变化。