短花针茅草原凋落物分解对氮沉降和降水变化的响应机制

Human activities have significantly altered global nitrogen(N) cycling, doubling N input into terrestrial ecosystem over the past century. Global climate models also predicted more extreme rainfall events throughout the world, combined with precipitation increase at mid-latitude regions. As two important components of anthropogenic-driven global changes, N deposition and precipitation change have dramaticly affected ecosystem processes and functioning, especially for temperate semiarid steppe which are inevitably co-limited by both N and water availability. .Litter decomposition is a key process of nutrients cycling and energy flow in grassland ecosystem. The effects of N deposition and precipitation change on grassland litter decomposition has become one of key research areas in grassland ecology. Global change factors may affect ecosystem interactly, for example, elevated N inputs have compensation effect on water deficiency to some extent, increased soil water availability can enhance the nutrient concentrations of senesced leaves and soil N availability, which all have great implications for litter decomposition and consequently for C and N cycling. However, extensive work to date have been conducted to determine how single factor (N deposition or altered precipitation regime) would affect decomposition of single-species aboveground litter. Limited studies have been conducted to address the combined effects between the two factors on decomposition of above/belowground litter, and single/mixed-species litter together..In order to well investigate the effects of N deposition and precipitation increase and their interaction on litter decomposition, a field-manipulated experiment with N and water additions should be conducted in Stipa baicalensis steppe in Inner Mongolia of northern China. In the experiment, 5 typical grassland species of Stipa baicalensis, Leymus chinensis, Carex pediformi, Medicago ruthenica and Artemisia frigid will be selected, the split block design will be employed with 2 water treatments (0, 10 mm every week throughout growing season) as main factor and 8 N treatments (0, 15, 30, 50,100, 150, 200, 300 kg N ha-1) as sub-factor, and then litter samples decompose under varying conditions. The main objectives of current project are to systematically illustrate decomposition rate and nutrient dynamics of above/belowground litter, and single/mixed-species litter in the context of N deposition and precipitation change, providing a better understanding of the interrelation and associated feedback between litter decomposition and plant diversity, litter quality (tissue chemistry), soil microorganism, and other soil environment factors. The research results should have significant implications for comprehensively analysizing and evaluating the global change effects on Stipa breviflora steppe, and lay theoretical foundations for scientificly making grassland management countermeasures.

凋落物分解是草原生态系统养分循环和能量流动的主要途径。氮沉降和降水变化是全球变化的两个主要方面。全球变化背景下，氮沉降和降水变化对草原凋落物分解的作用与影响机制成为草地生态学研究的热点之一。已有研究多集中于单因素对某一植物地上部凋落物分解的影响，多因素复合对植物地上部和地下部凋落物、单一和混合凋落物分解影响的系统研究十分缺乏。本项目以内蒙古短花针茅草原为研究对象，通过外源氮素和水分输入，模拟氮沉降增加和夏季增雨减雨的控制试验，探讨氮沉降和降水变化及其复合作用对凋落物分解的影响。阐明氮沉降和降水变化背景下短花针茅草原主要植物地上部和地下部凋落物、单一和混合凋落物的分解动态，揭示凋落物分解与植物多样性、植物化学组成、土壤微生物和其他土壤环境因子之间的相互关系，为全面分析和评估全球变化对短花针茅草原的影响，制定科学的草原生态系统管理对策提供理论依据。

全球气候变化背景下，大气氮沉降和降水变化日益显著，其对荒漠草原凋落物分解的影响存在很大的不确定性。为探究氮沉降及降雨变化对荒漠草原凋落物分解的影响，采用凋落物分解网袋法，研究水、氮及其互作对荒漠草原凋落物分解的影响。本试验采用裂区设计，主区为自然降雨(CK)、增雨30%(W)和减雨30%(R)3个水分处理，副区为0(N0)、30(N30)、50(N50)和100 kg·hm-2·a-1(N100)4个氮素添加水平，(其中不包括大气氮沉降)，结果表明：(1)自然状态下短花针茅荒漠草原植物地上部凋落物分解50%所需时间为34.39~63.90个月；分解95%所需时间为187.14~356.27个月。群落地下部混合根系分解50%所需时间为60.62个月，分解95%所需时间为377.27个月。增雨有助于凋落物分解，减雨则抑制凋落物分解，适量氮沉降(50kg N·hm-2·a-1)促进凋落物分解，增雨可以减缓过量氮沉降对凋落物分解造成的抑制作用，在氮沉降量为100kg N·hm-2·a-1时仍对凋落物分解产生促进作用。影响荒漠草原凋落物分解的主要因子是水分氮素为次之。(2)水氮交互对凋落物分解的促进作用要大于单一水分或氮素的作用。(3)凋落物分解与底物自身C、N、C/N、纤维素、木质素含量相关。C/N、纤维素与木质素越低越有利于凋落分解，分解过程中C、N、纤维素、木质素含量均是影响分解的关键因素。(4)中小型土壤动物中疣䖴科(Neanuridae)和阿斯甲螨科(Astegistidae)数量越多凋落物分解越快，是参与凋落物分解的主要类群，且对氮沉降与降水变化响应敏感。(5)细菌对短花针茅荒漠草原凋落物分解的作用强于真菌，且细菌与凋落物分解相关性更强。(6)自然状态与减雨处理下，氮素添加对短花针茅荒漠草原凋落物分解具有正效应，但氮沉降量超过50kg N·hm-2·a-1将减缓甚至抑制凋落物的分解效率，而水分增多可消除过量氮素添加的负效应。