氮输入对东北土壤碳蓄积氮素利用效率的影响

由于人类活动影响,通过沉降和施肥方式进入生态系统的活性氮显著增加,其对土壤有机碳库产生重要影响。氮素利用效率(NUE)作为深入理解陆地生态系统碳氮耦合关系的重要参数,对NUE时空规律的研究不仅可以评估目前氮输入对陆地生态系统碳汇增加的贡献,同时也有助于预测未来氮输入情况下陆地生态系统的碳平衡。利用生态系统过程模型——CEVSA2模型的模拟结果,分析了东北地区氮输入情况下,土壤碳的氮素利用效率(SNUE)的时空变化规律及其影响因素,结果表明:(1)1961—2010年,氮输入的显著增加促进了土壤碳的蓄积,但SNUE显著下降;(2)森林的平均SNUE最高,农田最低;灌丛的下降速率最大,森林的SNUE变化趋势最不显著;(3)三江平原和长白山地区以及大小兴安岭的部分地区SNUE最大,其次是辽河平原、松嫩平原地区;内蒙古高原、呼伦贝尔高原地区以及大、小兴安岭的部分地区SNUE出现负值,说明在这些地区,外援氮输入抑制了土壤碳的蓄积;(4)氮输入的空间分异和不同生态系统响应氮输入的差异共同决定了SNUE及其变化的空间格局。该研究结果可为进一步分析不同区域氮促汇潜力和预测未来氮输入情景下的区域碳平衡提供参考。

Effects of nitrogen input on nitrogen use efficiency of soil carbon storage in Northeast China

Anthropogenic activities have altered the global nitrogen (N) cycle, leading to increased N input into the ecosystem through N deposition and the use of N fertilizers. Nitrogen is considered to be the limiting factor of plant growth in many ecosystems; therefore, the increased N input into ecosystems has an important impact on the carbon cycle and carbon accumulation. N use efficiency (NUE) can be an important indicator for the coupling relationship between ecosystem carbon and nitrogen cycles. Studies on the temporal and spatial patterns of NUE will aid in assessing the contribution of N input to increases in the terrestrial ecosystem carbon sink, and to predict the carbon balance of terrestrial ecosystems under different N input increase scenarios. We use a newly developed process-based ecosystem model, CEVSA2, to study the spatial and temporal variation in N use efficiency of soil carbon storage (SNUE) in Northeast China during 1961-2010, and to examine the effects of N input and vegetation types on SNUE. Our model simulations show that: (1) enhanced N input induced more soil carbon storage in the Northeast, but it decreased SNUE greatly. (2) Different vegetation types led to different SNUEs; forest had the highest SNUE and cropland had the lowest SNUE. The SNUEs of all vegetation types decreased with an increase in N input; however, the forest had the most stable SNUE compared to other vegetation types. The SNUE of shrubland decreased significantly during the early period of the past 50 years. (3) SNUE was the highest in the Sanjiang Plain, the Changbai Mountains, and parts of the Daxinganling and Xiaoxinganling regions. SNUE was negative in the Inner Mongolia Plateau, Hunlun Buir Plateau, and parts of the Daxinganling and Xiaoxinganling regions. This indicates that exogenous N input decreased the storage of soil carbon. (4) The variations in N input and the different responses of different vegetation type to N input impacted the spatial pattern of SNUE. The effects of N input on soil carbon accumulation are also affected by complex ecological processes. The complex responses and adaptation of soil processes to N addition will result in uncertain predictions for long-term changes if the model is based on short term observation results. Further efforts are required on the mechanisms of responses of soil processes to all environmental changes in order to accurately predict the effects that continued N addition will have on soil carbon storage in the future.