放牧对典型草原生态系统磷循环的影响：生物过程与机制

Human activities have substantially altered the nitrogen (N) and phosphorus (P) cycling in many terrestrial ecosystems over the recent several decades, leading to widespread loss of biodiversity and degradation of ecosystem functioning and services throughout the world. Many previous studies have examined the patterns, mechanisms, and consequences of human-induced changes in N cycling. Few studies, however, have explored how human disturbance and climate change affect P cycling and consequently ecosystem functioning, particularly the biological processes affecting P dynamics and the interactions of P dynamics with carbon (C) and N. The proposed project is designed to study the effects of grazing and climate fluctuations on biological processes of P cycling in the typical steppe ecosystems on the Mongolia plateau, which is the representative of the largest contiguous grassland in the world―the Eurasian Steppe. Based long-term field experiments (e.g., grazing, P addition, biodiversity manipulation experiments) and combined with short-term laboratory incubations, and isotope labeling and high-throughput sequencing technology, our study is aimed to: (1) quantify the effects of grazing and precipitation fluctuations on P pools, flows, and transfer processes in the typical steppe ecosystems; (2) explore the relationships between plant biodiversity and mycorrhizal fungal diversity and plant nutrient capture; (3) examine responses of soil organic P mineralization and immobilization to different levels of grazing intensity and precipitation fluctuations; (4) investigate the effects of grazing on C: N: P stoichiometry of plant tissues and soil nematodes, bacteria, and fungi, and as well as the interactions between P dynamics with C and N; (5) elucidate key mechanisms controlling the grazing-induced changes in biological processes of P cycling. This study will have important implications for restoring the large areas of degraded grasslands and for providing guideline for sustainable ecosystem management in the semiarid Inner Mongolian grassland and beyond.

近几十年来，人类活动干扰以前所未有的速度和广度改变着陆地生态系统的氮、磷循环，引起了全球范围内生物多样性的丧失和生态系统功能与服务的退化。然而，国内外已有的相关研究以氮循环为主，人类活动干扰对磷环循影响的研究十分有限，特别是有关磷循环中生物过程及其机制的研究，亟待加强。有鉴于此，本项目以蒙古高原广泛分布的温带典型草原为模式生态系统，依托长期野外控制实验平台（放牧、养分添加），结合室内培养实验、同位素标记技术和土壤微生物高通量测序技术，研究放牧和气候波动对生态系统各组分磷库及其输入和输出的影响；植物多样性与菌根真菌多样性之间的内在联系；土壤有机磷矿化与固持对放牧和降水波动的响应机制；放牧对植物、土壤动物和微生物C:N:P化学计量关系的影响机制。在此基础上，阐明放牧对磷循环关键生物过程的调控机制，为生产中制定有效的草原管理措施，恢复大面积的退化草原，实现生态系统的可持续管理提供科学依据。

近几十年来，人类活动干扰以前所未有的速度和广度改变着陆地生态系统的氮、磷循环，引起了全球范围内生物多样性的丧失和生态系统功能与服务的退化。然而，国内外已有的相关研究以氮循环为主，人类活动干扰对磷环循影响的研究十分有限，特别是有关磷循环中生物过程及其机制的研究，亟待加强。为此，本项目以蒙古高原广泛分布的温带典型草原为研究对象，依托长期野外控制实验平台（放牧、养分添加），结合室内培养实验、同位素标记和土壤微生物组学技术，研究放牧和气候波动对生态系统各组分磷库及其输入和输出的影响；植物多样性与菌根真菌多样性之间的内在联系；土壤有机磷矿化与固持对放牧和降水波动的响应机制；放牧对植物、土壤动物和微生物C:N:P化学计量关系的影响机制。取得的主要研究结果包括：1）长期放牧改变了表层土壤的磷含量和磷库。重度放牧显著降低了地上生物量磷库、凋落物磷库、活根、死根、地下净初级生产力磷库；2）刈割和混合放牧显著降低了表层土壤不稳定磷含量，传统放牧加速了草原生态系统磷循环，促进了有机磷矿化；3）放牧结合资源添加有利于生物多样性和生态系统多功能稳定性的维持；4）放牧强度和土壤磷有效性共同决定了AMF对植物群落生产力和稳定性的影响。在轻度和中度放牧下，抑制AMF对无机氮库、氨基酸氮库以及相关氮转化过程具有负效应。在重度放牧下，抑制AMF对这些氮库和相关转化过程具有正效应。5）放牧对优势物种叶片氮、磷含量、重吸收和分配均具有显著的影响，但这些效应受土壤可利用P调控，因不同物种而异。6）高的根系凋落物多样性能够通过抑制CO2释放而促进土壤C储存，通过增加土壤氮矿化，加速氮循环。多数情况下，根系凋落物添加抑制了土壤磷的矿化，土壤磷矿化随物种丰富度的增加而降低。7）系统收集了放牧对磷循环影响的关键科学数据。这些成果为制定有效的草原管理措施，恢复大面积的退化草原，实现生态功能和生产功能的协同提供科学依据。