增温施氮对高寒草甸生产力及生物量分配的影响

在青藏高原高寒草甸区设置模拟增温和氮添加处理,研究长期增温与外源氮输入对高寒草甸群落生产及其分配的影响.结果表明:开顶箱增温装置造成小环境暖干化,即显著提高地表空气温度1.6℃,提高表层土壤温度1.4℃,降低土壤含水量4.7%.2012、2013和2014年不施氮处理下增温分别降低地上生物量61.5%、108.8%和77.1%,在高氮(40和80kg N·hm^-2·a^-1)处理下增温对群落地上生物量无显著影响,这说明增温的影响依赖于氮添加水平,且施氮补偿了增温导致的土壤氮损失.增温导致根冠比增加,2012、2013和2014年不施氮处理下增温分别增加根冠比98.6%、60.7%和97.8%.在不增温处理下,植物群落地上、地下生物量的变化率均表现出低氮(10、20 kg N·hm^-2·a^-1)促进、高氮抑制的趋势,达到饱和阈值时的氮添加剂量分别为56.0和55.5 kg N·hm^-2·a^-1;而在增温处理下,地上、地下生物量随施氮量增加呈线性增加趋势.这说明增温改变了高寒草甸生物量分配对外源氮输入的响应模式,增温导致的土壤无机氮含量变化是生物量分配模式改变的主要原因.由氮添加试验估算的高寒草甸氮饱和阈值表明,高寒草甸对氮输入的敏感性高于其他类型草地.

Effects of warming and nitrogen addition on community production and biomass allocation in an alpine meadow.

A long-term experiment, involving exogenous N addition and simulated warming, was conducted in an alpine meadow in Damxung, northern Tibet, to study how warming and N addition influence community production and biomass allocation. The results showed that warming resulted in a warm but dry microsite, that was, air temperature increased by 1.6 ℃ and soil surface temperature increased by 1.4 ℃, and soil water content decreased by 4.7%. Under no N addition treatments, warming significantly decreased plant aboveground biomass by 61.5%, 108.8% and 77.1% in 2012, 2013 and 2014, respectively. Under high N treatments (40 and 80 kg N·hm^-2·a^-1), warming had no significant effect on aboveground biomass. These findings indicated that the effect of warming might be dependent on N addition level, and N addition could compensate for soil N loss caused by warming. Warming led to an increase in root/shoot by 98.6%, 60.7% and 97.8% in 2012, 2013 and 2014 under no N addition treatments, respectively. Under the ambient condition, the change percentages of aboveground and belowground biomass of plant communities first increased and then decreased along an N gradient, with the saturation thresholds of above- and below-ground biomass for N addition 56.0 and 55.5 kg N·hm^-2·a^-1, respectively. Under the warming condition, above- and belowground biomass increased linearly with increasing N addition. These findings suggested that warming modulated the response patterns of alpine meadows to exogenous N input, which was mainly caused by decreased soil inorganic N under warming. The estimation of N thresholds highlights that alpine meadows are more sensitive to future N deposition than other types of grasslands.