黄土高原不同生态治理小流域碳氮循环的差异及内在机制

Carbon and nitrogen cycling of small watershed is the current weakness of terrestrial ecosystem carbon and nitrogen cycling research. Taking small watershed as a unit in carbon and nitrogen cycling research has the advantage of comprehensively exploring the interactions of hydrological and carbon/nitrogen cycles. The Loess Plateau is a typically ecological fragile region in China and has a unique landscape that is incised by numerous gullies. Consequently, small watersheds are densely distributed in this area. Tree plantation and natural vegetation restoration are the two most important measures for the remediation of degraded ecosystems on the Loess Plateau. However, there is a lack of full understanding on the effects of the two contrasting measures of small watersheds on carbon and nitrogen cycling. This study is planned to examine two neighboring small watersheds in Nanxiaohegou, Xifeng city, Gansu province. Since 1954, natural vegetation restoration has been conducted in one of these watersheds and tree plantation in the other. The two watersheds have now formed completely different vegetation landscapes: naturally restored grassland and artificial forestland. In the two watersheds, we will conduct four aspects of research: first, soil physical, chemical and microbial parameters will be investigated; Second, soil water dynamics in different landforms will be observed in situ and stable isotopic values of carbon and nitrogen in surface soils will be analyzed; third, flux of carbon and nitrogen in stream water, sediment, and atmospheric deposition will be observed and stable isotopic values will be analyzed; fourth, all of the observed and analyzed data will be synthesized and the difference in carbon/nitrogen cycling and internal mechanisms between the two watersheds will be explored. This study has important scientific significance in evaluating the effects of natural and human vegetation measures on carbon and nitrogen cycling of small watersheds on the Loess Plateau, which is also the cutting edge of earth surface process research.

小流域碳氮循环是当前陆地生态系统碳氮循环研究的薄弱环节。以小流域为单元开展碳氮循环研究，能够从地球系统科学角度充分考虑水文循环与碳氮循环的相互作用。黄土高原是我国典型的生态脆弱区，地形破碎，小流域分布密集。人工植树造林和植被自然恢复是黄土高原两种最为重要的植被恢复方式。然而，这两种完全不同的生态治理方式，在小流域尺度上碳氮循环规律有何差异，机制是什么，一直缺乏充分的认识。基于此，本项目以甘肃西峰南小河沟植被自然恢复和人工植树造林小流域为研究对象，通过土壤物理、化学和微生物性质调查、土壤水分观测以及碳氮稳定同位素分析，获得对比小流域土壤环境的差异；进一步通过径流泥沙和大气沉降的观测和稳定同位素分析，获得对比小流域径流输出和大气输入的碳氮通量及同位素信号；在此基础上，综合分析对比小流域碳氮循环的差异及内在机制，科学认识黄土高原自然和人为生态措施对小流域碳氮循环的影响。

近60年来，黄土高原先后进行了大规模的生态治理工作，特别是近10年来实施的退耕还林（草）工程，取得了十分显著的效果：植被覆盖度明显增加，生态服务功能提高，水土流失减弱。人工植树造林和植被自然恢复是黄土高原两种最为重要的植被恢复方式。然而，这两种完全不同的生态治理方式，在小流域尺度上碳氮循环规律有何差异，机制是什么，一直缺乏充分的科学认识。系统研究这两种不同生态治理方式下小流域碳氮循环的差异及内在机制，对深入认识黄土高原大规模植被建设背景下小流域物质循环的变化、科学预测其未来发展趋势，具有重要的理论和实践意义。针对上述科学问题，本项目选择甘肃西峰南小河沟植被自然恢复和人工植树造林小流域为研究对象，通过两条小流域土壤物理、化学和生物学性质调查、土壤-植被碳氮稳定同位素分析及土壤水分观测、径流输出的碳氮通量及同位素比值分析、大气输入的碳氮通量及同位素比值分析和两个小流域碳氮循环的差异及内在机制综合分析五个方面研究内容，揭示了黄土高原植被自然恢复和人工植树造林小流域碳氮循环的差异及驱动机制，发现草地小流域由于具有更好的水文环境和更高的径流输出，类似于“大浪淘沙”，不断淋洗流域表层可溶性物质，包括碳氮元素等，使得草地小流域发生了更多的横向物质迁移和流动；这一认识拓展了传统生态学（主要关注垂直界面）的学科范畴，将生态-水文-地貌-生物地球化学循环更好地耦合在一起。此外，进一步向水文过程研究进行了拓展，阐明了植被自然恢复和人工植树造林小流域降雨补给土壤水分阈值的差异；探讨了人工植树造林对小流域地表能量平衡的影响。通过地温和气温的对比观测表明，植树造林通过改变地表反照率和蒸散发，显著改变了流域地表的能量平衡，并且局部地形具有重要影响。在冬春季节，地形对地表能量的影响起主导作用；夏季，ET增强，植树造林的影响超过地形起主导作用；秋季，植被的影响减弱，地形的影响逐步增强。