短期氮素添加和模拟放牧对青藏高原高寒草甸生态系统呼吸的影响

氮沉降和放牧是影响草地碳循环过程的重要环境因子,但很少有研究探讨这些因子交互作用对生态系统呼吸的影响。在西藏高原高寒草甸地区开展了外源氮素添加与刈割模拟放牧实验,测定了其对植物生物量分配、土壤微生物碳氮和生态系统呼吸的影响。结果表明:氮素添加显著促进生态系统呼吸,而模拟放牧对其无显著影响,且降低了氮素添加的刺激作用。氮素添加通过提高微生物氮含量和土壤微生物代谢活性,促进植物地上生产,从而增加生态系统的碳排放;而模拟放牧降低了微生物碳含量,且降低了氮素添加的作用,促进根系的补偿性生长,降低了氮素添加对生态系统碳排放的刺激作用。这表明,放牧压力的存在会抑制氮沉降对高寒草甸生态系统碳排放的促进作用,同时外源氮输入也会缓解放牧压力对高寒草甸生态系统生产的负面影响。

Interactive effects of short-term nitrogen enrichment and simulated grazing on ecosystem respiration in an alpine meadow on the Tibetan Plateau

Both nitrogen (N) deposition and livestock grazing are important factors influencing species composition, soil nutrient availability, plant productivity and allocation pattern in grassland ecosystems. However, little is known about their interactive effects on alpine ecosystem carbon cycling process such as ecosystem respiration. In 2010 we started a long-term factorial experiment in an alpine meadow in Damxung County in northern Tibet, China, to examine the interactive effects of N addition and grazing on plant biomass allocation, soil microbial carbon and N and ecosystem CO₂ fluxes. The experiment had two N treatments crossed with two grazing treatments, resulting in a total of four treatments coded as CK (control, no N addition and no simulated grazing), N (with N addition but no grazing), G (no N addition but with grazing) and NG (with both N addition and grazing). For N addition, we added 40 kg N hm^-2 a^-1, and to simulate livestock grazing, we selectively clipped palatable grasses and sedges. We analyzed the available data collected during the growing seasons in 2011 and 2012. Compared with CK, N significantly increased ecosystem respiration by 82% and 72% and soil respiration by 53% and 65% in August 2011 and 2012, respectively, but G had no significant effect on ecosystem respiration or soil respiration. Moreover, ecosystem respiration or soil respiration did not differ significantly between CK and NG, indicating that simulated grazing partially counteracted the stimulating effect of N addition on ecosystem CO₂ fluxes. Compared to CK, N addition stimulated plant aboveground growth by 82% and 84% in August 2011 and 2012, respectively, NG promoted plant root compensatory growth and increased plant allocation to belowground by 127% and 116% in August 2011 and 2012, respectively, and G had no effect on belowground biomass. G inhibited soil microbial carbon and did not affect soil microbial N, while N addition increased soil microbial N and soil microbial metabolic activity. Furthermore, compared to CK, both N and NG treatments increased dissolved organic carbon, while G had no such an effect. The correlation analysis showed that ecosystem respiration and soil respiration were mainly controlled by soil microbial metabolic activity and dissolved organic carbon. Additionally, aboveground biomass was significantly correlated with dissolved organic carbon but not with soil microbial metabolic activity. These results suggest that N addition promotes ecosystem respiration and soil respiration directly by the stimulating effects on soil microbial metabolic activity and aboveground plant production, and that photosynthetic substrate supplied by aboveground plant part may be of great importance for ecosystem respiration and soil respiration. The differential effects of N addition and simulated grazing on ecosystem CO₂ fluxes suggest that exogenous N input can relieve the negative effect of grazing pressure on plant production in the alpine meadow. On the contrary, livestock grazing will suppress the stimulated effects of increased N deposition on ecosystem CO₂ fluxes in the future global change scenarios.