模拟增温对西藏高原高寒草甸土壤供氮潜力的影响

过去几十年青藏高原呈现显著的增温趋势,冬季增温幅度显著高于生长季的季节非对称特征。气候变暖会对生态系统氮素循环产生重要影响,但关于全年增温与冬季增温对高寒生态系统氮循环的不同影响仍缺乏研究。在青藏高原高寒草甸区开展模拟增温试验,研究季节非对称增温对高寒草甸生态系统氮循环的影响。该试验布设于2010年7月,设置3种处理(不增温、冬季增温与全年增温)。研究结果发现,开顶箱增温装置造成了小环境的暖干化:显著提高了地表空气温度和表层土壤温度,降低了表层土壤含水量。冬季增温会加剧土壤中氮素的流失,所以在经历了冬季增温后土壤氮含量显著降低;在生长季节,土壤氮素周转速率受土壤水分的调控,在降雨较少的季节,增温引起的土壤含水量降低会抑制土壤氮周转速率。对于土壤微生物量而言,高寒草甸土壤微生物量碳表现出明显的季节动态,在生长季旺盛期较低,在生长季末期和初冬季节反而较高,这说明为了降低对土壤养分的竞争,高寒草甸植物氮吸收与土壤微生物氮固持在时间上存在分离。研究结果表明,冬季增温导致的土壤养分含量变化会影响随后生长季植物群落的生产力、结构组成与碳氮循环等过程,对生态系统过程产生深远的影响。

Effects of simulated warming on soil nitrogen supply potential in an alpine meadow on the Tibetan Plateau

The temperature on the Tibetan Plateau has been shown a significant increasing trend since the recent decades. Moreover, the magnitude of warming showed an asymmetric trend with significant higher warming in non-growing seasons than in growing seasons. Climate warming could have significant impact on nitrogen (N) cycling in terrestrial ecosystems. The effects of asymmetric year-round warming on N cycling are still lacking in the alpine meadow ecosystem. We carried out a simulated warming experiment in an alpine meadow to compare the different effects on the N cycle between seasonal asymmetric warming and year-round warming in the alpine meadow on the Qinghai-Tibet Plateau. The experiment was set up in July 2010, and three kinds of warming treatments, i.e. no warming, non-growing season warming, and year-round warming, were implemented using open-top chambers. Our results showed that the warming devices induced warm and dry microclimate. Both air temperature and surface soil temperature were significantly increased, and soil water content was reduced in surface soil. Warming treatment in non-growing seasons exacerbated the soil N loss, so the soil N content was decreased significantly. the Soil inorganic N content decreased by 61%, 40%, and 60% in year-round warming treatment in the early growing seasons of 2012, 2013, and 2014, respectively. Meanwhile, winter warming decreased 65% and 25% of the soil inorganic N content in 2013 and 2014, respectively. During growing seasons, soil moisture controlled soil N turnover rate which increased in the seasons with less rainfall, especially in the early growing season. The decrease in soil water content caused by year-round warming could inhibit the soil N turnover rate. The treatment of year-round warming significantly reduced soil net N mineralization during early growing season in 2013 and 2014, but promoted the net N mineralization in July and August, the peak growing seasons. The soil microbial biomass carbon showed obvious seasonal dynamics in the alpine meadow, which was lower during the growing season and higher at the end of the growing season and in the early winter season. Such variations could indicated a seasonal partitioning in the soil N utilization between plants and soil microbes to reduce the nutrient competition. Soil water content showed a parabolic relationship with the net nitrogen mineralization rate, and the peak values occurred when the soil water content reached above 14%. Regression analysis showed that soil microbial biomass in cold seasons was significantly negatively correlated with the inorganic N content in the early growing season, indicating that warming in non-growing seasons has a carryout effect. Specifically, warming in non-growing seasons was not only exerting impact on soil N cycling during the warming stage, but also exerting impact on soil N cycling in the subsequent growing seasons or even more longer time. Our results demonstrate that the change in soil N content due to non-growing season warming could affect community species composition and productivity, as well as ecosystem soil carbon and N cycles in the following growing season. Therefore, warming in non-growing seasons would have profound impact on the ecosystem processes in the alpine meadow.