13CO2示踪不同化学形态氮素添加对高寒草甸植物光合碳分配的影响

外源氮素(N)输入陆地生态系统后会引起植物和土壤各碳库的变化,但是对不同化学形态氮素的长期输入如何影响光合碳在植物组织、土壤、土壤呼吸中的分配及转运知之甚少,尤其是对于氮输入引起光合碳分配变化进而作用于植物和土壤碳库的机制的认识还非常匮乏。基于在青藏高原矮嵩草草甸开展的不同化学形态氮素添加的长期实验,利用~(13)C示踪方法揭示了光合碳在植物地上、地下组织的分配,及其随时间在土壤中的滞留和随土壤呼吸的释放。研究结果表明,外源氮素添加10年后,与对照未添加氮素处理相比,氨态氮处理下的地上生物量增加了49.5%,氨态氮处理下的地下生物量增加了111.3%。土壤中滞留的~(13)C整体呈下降趋势,氨态氮处理下的土壤碳库显著高于硝态氮处理下的值。不同处理下的土壤呼吸中~(13)C的滞留量随时间呈指数衰减的变化趋势,其中,硝态氮处理下的~(13)C衰减最快。~(13)C同位素标记后第1天测定植物茎和叶内的~(13)C约占刚刚标定完茎和叶内~(13)C的80%,不同处理之间没有显著性差异。直至标记后的第30天,茎和叶内~(13)C的滞留量约占初始量的30%。硝态氮处理下的值在第21天和第30天显著低于对照和氨态氮处理下的值,表明硝态氮处理下,植物光合固定的碳在短期内迅速输入地下组织和土壤中。这些结果从机理上阐明了植物光合碳分配对不同化学形态氮素长期输入的响应,进而影响到土壤呼吸CO_2的释放,以及对土壤碳库动态的贡献。加深了对高寒草甸土壤有机碳库稳定性维持机制的认识,能够为高寒草地的科学管理以及资源的可持续利用提供理论指导。

Effect of nitrogen addition with different chemical forms on dynamics of photosynthate using 13CO2 tracing in an alpine grassland

Long-term exzogenous nitrogen (N) input into terrestrial ecosystems causes changes in carbon storage in plant and soil pools. However, little is known how input of N with different chamical forms could influence patterns of photosynthetic carbon (C) allocated into dofferent pools, such as above- and below-ground plant tissues, soil, and soil respiration. In particular, the mechanisms underlying changes in plant and soil carbon storage in responses to N input are not clear. A long-term fertilizaiton experiment of N with different chemical forms was carried out in an alpine meadow on the Tibetan Plateau. We tested the patterns of photosynthetic carbon allocated into plant tissues, soil, and soil respiration using ¹³C tracing experiment in the field. Our results showed that N input with different chemical forms caused changes in the patterns of photosynthetic C allocated into above- and below-ground plant tissues. Moreover, allocation of photosynthetic C into soil differed in response to different chemical forms of N, and the C-CO₂ fluxes were significantly different regarding to the N chemical forms. Specifically, significant differences in patterns of photosynthetic C allocated into different pools were found between ammonium-N and nitrate-N treatments. Comparison with ammonium-N, relatively faster transition of photosynthetic C was found in nitrate-N treatment, and photosynthetic C was fastly respired via soil respiration. The results from ¹³C labeling are corresponding to the significantly high carbon stored in plant root and soil in ammonium-N than in nitrate-N treatments. Our results showed that aboveground biomass in ammonia-N treatment were 49.5% higher than that in the control treatment as N addition treatments have been performed for 10 years. Meanwhile, belowground biomass in ammonia-N treatment were 111.3% higher than that in the control treatment. The ¹³C retained in the soil showed a downward trend during the 30 days, and the soil carbon pool in ammonia-N treatment was significantly higher than that in nitrate-N treatment. The ¹³C efflux from soil respiration with time followed an exponential decay function in each one of the treatments. Among them, decay rate of ¹³C in nitrate-N treatment was the fastest one. On the first day after ¹³C isotope labeling, ¹³C in plant shoots was ammounted to about 80% of the ¹³C in the shoots after the immidiate labeling. There was no significant difference in ¹³C in plant shoots on the first day after labeling between different treatments. On the 30th day, the retention of ¹³C in shoots was reduced to about 30% of the initial amount. The values in nitrate-N treatment were significantly lower than those in the control and ammonia-N treatments from day 21 to 30. The results showed that the photosynthetic C fixed by plants was rapidly transported into underground tissues and soil in a short period of time in the treatment of nitrate-N addition. Our results illustrate the responses of allocation in photosynthetic C to N input with different chemical forms. We also quantified the rate of C-CO₂ efflux via soil respiration, which is critical to understand clearly about the contribution of photosynthetic C to soil carbon stroage. The results can be used to develop more resonable policy in grassland management, which are benefial for sustable development of the alpine grasslands.