摘要:为深入了解降水格局改变和氮沉降增加对荒漠草原生态系统碳交换的影响机制，于2017年在宁夏荒漠草原设立了降水量变化（减少50%、减少30%、自然降水量、增加30%以及增加50%）和氮添加（0和5 g m^-2 a^-1）的野外试验，研究了2019年生长季（5-10月份）净生态系统碳交换（Net ecosystem carbon exchange，NEE）、生态系统呼吸（Ecosystem respiration，ER）和总生态系统生产力（Gross ecosystem productivity，GEP）的时间动态，分析了三者与植被组成以及土壤属性的关系。NEE、ER和GEP日动态和月动态均呈先增加后降低，NEE在整个生长季表现为净生态系统碳吸收。0和5 g m^-2 a^-1氮添加下，减少降水量显著降低了NEE、ER和GEP （P<0.05），增加30%降水量显著提高了三者（P<0.05）。相同降水量条件下，氮添加不同程度地提高了NEE、ER和GEP，且其效应在增加50%降水量时较为明显。净生态系统碳吸收（-NEE）、ER和GEP与群落生物量、牛枝子（Lespedeza potaninii）以及草木樨状黄芪（Astragalus melilotoides）生物量正相关。三者亦随Patrick丰富度指数和Shannon-Wiener多样性指数的增加而增加。本文结果意味着，减少降水量降低了土壤水分和养分有效性、抑制了植物生长，从而降低了生态系统碳交换。适量增加降水量则可能通过提高土壤含水量、刺激土壤酶活性、调节土壤C ： N ： P平衡特征等途径，促进了植物生长和物种多样性，从而提高了生态系统碳汇功能；氮添加亦促进了生态系统碳交换，但其与降水的交互作用尚不明显，需通过长期观测进行深入探讨。



Abstract:The change in precipitation pattern and increase in atmospheric nitrogen deposition are two important aspects of global change. Both of them closely relate to soil resource availabilities, plant growth, microbial activity, etc., and further posing profound influences on the carbon dynamics in plant-soil systems. Being one of main grassland ecosystem types in northwestern China, desert steppe is limited by soil water and nitrogen availabilities and thus is sensitive to the alterations in precipitation pattern and nitrogen deposition. However, the studies on how the carbon dynamics in desert steppes respond to the two global change aspects are still lacked, especially in those located in Ningxia, northwestern China. To deeply understand the influencing mechanisms of the alterations in precipitation pattern and nitrogen deposition on the ecosystem carbon exchanges in desert steppes, a field experiment was conducted in a desert steppe of Ningxia in 2017. The experiment involved five precipitation treatments (50% reduction, 30% reduction, natural, 30% increase, and 50% increase) and two nitrogen addition treatments (0 and 5 g m^-2 a^-1). The temporal dynamics of net ecosystem carbon exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP) were monitored from May to October of 2019. Their relationships with plant community composition and soil properties were analyzed as well. The daily and monthly dynamics of NEE, ER, and GEP increased first and then decreased. NEE was shown as net ecosystem carbon absorption during the whole growing season. Under 0 and 5 g m^-2 a^-1 of nitrogen addition, the decreasing precipitation significantly reduced NEE, ER, and GEP (P<0.05), while 30% increase in precipitation significantly promoted the three indices (P<0.05). Between the same precipitation treatments, nitrogen addition also greatly increased the three indices, especially under the treatment of 50% increase in precipitation. The net ecosystem carbon absorption (represented as-NEE), ER, and GEP were positively related with plant community biomass, Lespedeza potaninii population biomass, and Astragalus melilotoides population biomass. The three indices also enhanced with the increase of Patrick richness index and Shannon-Wiener diversity index. The results above indicate that decreasing precipitation reduces soil water and nutrient availabilities, inhibits plant growth, and thus limiting ecosystem carbon exchange. An appropriate increase in precipitation can promote plant growth and species diversity through increasing soil water content, stimulating soil enzyme activities, regulating soil C:N:P stoichiometric balance, etc., consequently improving the ecosystem carbon sink function. Nitrogen addition also promotes ecosystem carbon exchange. However, its interaction with precipitation is not clear after nearly 3-year experimental treatment. Therefore, a long-term observation is needed for further deeply exploring. The results of this paper will provide data supports for the global networking experiment on ecosystem carbon cycle under global change.





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