氮添加对退化高寒草地土壤微生物量碳氮的影响

若尔盖高寒草地生态系统脆弱,对环境因子的改变响应敏感。本试验以若尔盖高寒退化草地为研究对象,在2015-2016年每年返青期,以尿素作为氮源在野外开展控制试验,4个氮处理分别为CK(0g·m^-2·a^-1)、N5(5g·m^-2·a^-1)、N10(10g·m^-2·a^-1)、N20(20g·m^-2·a^-1),分析了氮添加下4个不同退化程度的高寒草地土壤微生物量碳氮以及土壤理化性质的变化规律,探讨若尔盖高寒草地对氮添加的响应机制,旨在为脆弱生境草地的治理与恢复提供参考。结果表明,不同退化草地的土壤微生物量碳氮对氮添加的敏感性随退化程度加剧而逐渐降低。氮浓度20g·m^-2·a^-1处理下土壤微生物量碳氮含量变化趋势发生显著变化:轻度退化草地>未退化草地>中度草地>重度退化草地。相关分析表明,土壤微生物量碳氮与速效磷、硝态氮、全氮、全磷、有机碳具有显著正相关,可在一定程度上表征土壤养分状况。氮添加下,土壤微生物量碳氮与土壤理化性质的相关关系发生变化,尤其在N20处理下土壤微生物量碳、氮与其他理化因子间无显著相关关系,需要进一步从土壤微生物对土壤养分的吸收利用方面解释其原因。氮浓度变化显著改变土壤微生物C/N:CK重度退化草地的土壤微生物量碳氮比显著高于其他3个退化样地。N5和N10条件下不同退化草地土壤微生物C/N无显著差异,而N20处理下未退化草地土壤微生物C/N与CK比显著提高33.7%,而重度退化草地与CK比下降了62.5%,说明氮添加在一定程度上对土壤微生物的组成和群落结构产生了影响。

Effects of nitrogen addition on the soil microbial biomass C and microbial biomass N in degraded alpine grassland in Zoige County

Alpine meadow is a fragile and vulnerable ecotone in northern Tibet, characterized by high altitude, low temperatures and limited precipitation. Low soil nitrogen availability in these meadows is a key factor limiting the productivity of the grassland. Increasing nitrogen deposition can change C and N cycling dynamics in this ecosystem. The research reported in this paper explored the impact of N addition on soil microbial biomass C and N for four grassland categories in the alpine meadow region of Zoige County: non-degradation grassland (NDG), light degradation grassland (LDG), moderate degradation grassland (MDG), and serious degradation grassland (SDG). In 2015 and 2016, urea was applied to plots during the return of green growth in spring, at rates of nitrogen concentrations 0, 5, 10 and 20 g·m^-2 (denoted CK, N₅, N₁₀ and N₂₀, respectively), in which the nitrogen concentrations of 5, 10 and 20 g·m^-2 were equivalent to urea application amounts of 10.78, 21.55, 43.10 g·m^-2. The results showed that soil microbial biomass C and N were sensitive to N addition but this response diminished with incrfeasing level of grassland degradation. N addition changed the relationship between soil microbial biomass C, microbial biomass N, and also other soil parameters, such as available phosphorus, nitrate nitrogen, total phosphorus, total nitrogen, total organic carbon, pH, and soil moisture content. Correlation of soil microbial biomass C or N with and other environmental factors was not significant, especially for the N₂₀ treatment. Grassland degradation and nitrogen addition can also greatly impact on the ratio of soil microbial biomass C and soil microbial biomass N (SMBC∶SMBN). For the CK treatment, SMBC∶SMBN was significantly higher in serious degradation grassland than in the other three grassland degradation categories. Under the N₅ and N₁₀ treatments, SMBC∶SMBN showed no significant difference between plots of the four degradation categories. However, in non-degradation grassland, the SMBC∶SMBN for the N₂₀ treatment was 33.7% higher than in the CK treatment. In contrast, in serious degradation grassland, SMBC∶SMBN decreased 62.5% for the N₂₀ treatment, compared to the CK treatment. The results indicate that the structure and composition of the soil microbial community is significantly changed following N fertilizer addition to the alpine meadow grassland.