外源氮输入对典型喀斯特草地生态系统碳固定的影响

The input of exogenous nitrogen (N) plays a key role in driving carbon (C) sequestration in karst grassland ecosystems. Because N is the limiting nutrient to productivity of karst grasslands, increased exogenous N input either through atmospheric N deposition or symbiotic nitrogen fixation is expected to stimulate grassland productivity, and subsequently increase C sequestration in these ecosystems. However, the contributions of exogenous N input to C sequestration have rarely been tested empirically. This kind of studies is undoubtedly important for understanding the C sequestration potential under the background of elevated atmospheric N deposition and widespread introduction of N-fixing plants. In the present study, responses of ecosystem C cycling to N input will be systematically assessed in two types of karst grassland, i.e., natural grassland regenerated from abandonment cropland and artificial grassland planted with Guimu No 1 hybrid elephant grass. Both grasslands receive two kinds of N input forms, i.e., simulated N deposition and biological N fixation via introduction of N-fixing plants. The allocation of input N among different compartments of the ecosystem will be determined using 15N tracer method and 15N natural abundance method, respectively. Plant productivity and soil carbon cycling processes will be investigated. The objectives of this project include: 1) to analyze the responses of the C-N stoichiometry and C sequestration to anthropogenic nitrogen addition and symbiotic nitrogen fixation, 2) to quantify the effect of exogenous nitrogen on grass growth and carbon sequestration, 3) to reveal the mechanisms underlying the impacts of exogenous nitrogen input on carbon cycling of karst grassland ecosystems. The results will be useful for understanding the potential and mechanisms of carbon sequestration, and providing scientific basis for guiding ecological recovery and management of degraded karst ecosystems.

外源氮输入是驱动喀斯特草地生态系统持续碳固定的关键控制因子。在区域氮沉降水平日益增加和豆科植物被广泛种植的背景下，我们对喀斯特草地生态系统的固碳潜力及其未来变化规律还缺乏明确认识。本项目基于模拟氮沉降和固氮植物引种，利用15N示踪法和15N自然丰度法，通过对草地生态系统碳循环关键组分进行连续监测，研究氮输入（模拟氮沉降与生物固氮）对自然草地生态系统（撂荒后自然恢复）和人工草地生态系统（桂牧一号牧草）碳固定的影响，旨在阐明：1）喀斯特草地生态系统碳氮耦合和碳固定对不同形态氮输入的响应特征；2）量化氮输入驱动的草地生态系统碳固定率；3）解析生态系统碳循环对氮输入的响应机制。本研究不但能为喀斯特“退耕还林还草工程”的碳汇生态补偿机制的制定奠定基础，还可以为退化生态系统可持续恢复和科学管理提供理论支撑。

系列生态工程实施后，喀斯特地区退化生态系统的恢复演替具有巨大的C固定潜力。由于碳氮循环密切耦合，在N限制作用下生态系统持续C固定依赖于N素限制的消减。然而在该区域氮沉降水平日益增加和豆科植物被广泛种植的背景下，我们对喀斯特草地生态系统的固碳潜力及其未来变化规律还缺乏明确认识。本项目基于模拟氮沉降和固氮植物引种，利用15N示踪法和15N自然丰度法，通过对草地生态系统碳循环关键组分进行连续监测，研究氮输入（模拟氮沉降与生物固氮）对自然草地生态系统（撂荒后自然恢复）和人工草地生态系统（桂牧一号牧草）碳固定的影响。研究结果发现豆科灌木引种比模拟氮沉降更能缓解喀斯特自然草地“氮限制”，且能够维持人工草地牧草产量，引种豆科灌木增加了草本地上生物量的40.5~80.4%；模拟氮沉降增加了草本地上生物量的21.3~64.8%；氮沉降对喀斯特草地生态系统生物固氮速率抑制作用，施用磷肥对豆科灌木的共生固氮速率有促进作用，为豆科灌木接种丛植菌根真菌并适量添加磷肥，可以更好发挥豆科灌木为喀斯特草本群落提供氮素营养优势；模拟N沉降驱动的生态系统固C速率为0.19~0.31 Mg C hm-2 yr-1，豆科灌木共生固氮驱动的生态系统固C速率为0.87~1.62 Mg C hm-2 yr-1，豆科灌木共生固氮驱动的固碳效应远高于模拟N沉降；揭示了豆科灌木引种促进生态系统碳固定的作用机制，发现豆科灌木相比氮沉降（氮肥）能有效提高AMF和固氮菌多样性及丰度，降低AMF群落间竞争，强化AMF优势物种对养分吸收和运输的重要性。以上研究结果为喀斯特脆弱生态系统可持续恢复和科学管理提供了重要理论支撑。