季节性冻融格局变化对高山森林土壤氮素淋溶损失的影响

气候变化情景下冻融格局的改变可能导致寒冷生物区土壤氮的淋溶损失,从而改变土壤养分循环和森林溪流的水体环境.因此,为了解季节性冻融循环及其变化对高山森林土壤氮淋溶损失的影响,于2010年5月到2012年4月期间,采用土柱培养实验,利用海拔形成的温度差异模拟气候变暖过程,将高山森林(海拔3600 m)土壤分别培养在海拔3600 m(A1)、3300 m(A2)和3000 m(A3)的森林地表,研究了生长季节与冻融季节不同关键时期的土壤氮淋溶特征.结果表明:川西高山森林土壤氮素淋溶损失随着海拔增加而增加,其损失量为(1.85±0.39)kg·hm-2·a-1(A3)(1.87±0.34)kg·hm-2·a-1(A2)(2.94±0.73)kg·hm-2·a-1(A1),其中,62%以上的氮损失发生在季节性冻融期间.冻融季节高山森林土壤氮素淋溶流失的主要形式为铵态氮和硝态氮,且铵态氮的淋溶损失量高于硝态氮,而生长季节土壤氮素淋溶损失的主要形式是可溶性有机氮.这意味着冻融循环格局在很大程度上控制着高山森林土壤氮素淋溶损失特征,未来气候变暖可能降低高寒森林土壤氮素肥力,增加森林溪流中的氮含量.

Effect of changes in seasonal freeze-thaw pattern on nitrogen loss from leaching in the alpine forest soil

Changes in seasonal freeze-thaw pattern under climate change scenarios might lead to nitrogen loss via leaching in cold biomes, and in turn alter the cycles of soil nutrients and water environment of forest streams. In order to understand the effects of seasonal freeze-thaw cycles and their changes in seasonal freeze-thaw pattern on the soil nitrogen loss from leaching in the alpine forest, soil columns were sampled from an alpine forest with 3600m altitude. The soil columns were then incubated in the forest floor at the sites of 3600 m (A1), 3300 m (A2) and 3000 m (A3) altitudes to simulate climate warming induced by elevation decreasing. Soil nitrogen loss from leaching in three sites was measured at four critical periods as temperature changed in the freeze-thaw season and the growing season from May 2010 to April 2012. The results indicated that the loss of soil nitrogen via leaching increased with the increase of altitudes, and showed the order as: (1.85±0.39) kg·hm^-2·a^-1(A3) < (1.87±0.34) kg·hm^-2·a^-1(A2) < (2.94±0.73) kg·hm^-2·a^-1 (A1) in the alpine forest of Western Sichuan. More than 62% of annual nitrogen loss from leaching was observed in the freeze-thaw season. Moreover, the lost nitrogen in the freeze-thaw season was mainly in the form of ammonium and nitrate, and more ammonia lost from leaching was detected compared with nitrate. In contrast, soluble organic nitrogen was the main component in the lost nitrogen in the growing season. The results demonstrated that seasonal freeze-thaw cycles greatly controlled the soil nitrogen loss from leaching in the alpine forest. The results here also imply that nitrogen content will decrease in forest soil, but increase in the forest stream in this cold alpine forest in the scenarios of climate warming.