摘要
过去的两个世纪,由于人类活动的影响,地球表面的温度和大气N沉降持续增加对植被,土壤及全球生态系统的碳循环产生深远的影响。陆地生态系统对全球气候的变化有重要的调控作用。作为陆地生态系统最重要的生物区系之一,草地占全球陆地植被面积的40%,但是人们对草地系统,尤其是荒漠草地生态系统在全球变化背景下的动态仍缺乏认识。本研究依托内蒙古四子王旗荒漠草原农牧科学院实验基地为研究平台,研究了增温和施氮处理下荒漠草原植物群落、土壤和生态系统碳交换的变化,探索了荒漠草原生态系统对全球气候变化的响应机制,为全球变化与陆地生态系统关系提供可靠的理论依据。该试验开始于2006年6月,设置有增温(增温、不增温)为主处理处理方式,施肥(施氮、不施氮)为副处理方式,采用裂区设计,共计24个试验小区。
实验期间(2006-2009),与对照相比,增温处理样地0cm,10cm,15cm,30cm平均增加了1.2℃,0.87。C,0.86。C和0.77。C,土壤温度随土壤土层的增加逐渐降低(P<0.05)。通过2007-2009年对土壤营养成分的调查,我们发现增温提高了土壤有机碳的含量,而氮的添加没有改变有机碳的含量。无论怎样,2007-2009年增温和氮的添加都没有影响土壤总氮的含量。铵态氮、硝态氮与不同土壤深度的土壤温度均成正相关关系,土壤温度升高使土壤中铵态氮和硝态氮含量均增加。增温,施氮以及增温施氮的互作效应没有影响土壤总氮铵态氮的含量。增温显著增加了2008年(P=0.02)和2009年(p=0.01)土壤硝态氮的含量,而没有影响2007年土壤硝态氮的含量(P=0.36)。
增温对各年内土壤呼吸没有产生显著影响,但土壤呼吸有明显的季节变化,而且与增温显著相关(P<0.05)。施氮对荒漠草地的影响程度与自然条件有密切关系,其中降雨量多少是施氮对荒漠草地土壤呼吸影响程度的重要制约因素,降雨量越高氮素对土壤呼吸作用愈加明显。施氮显著增加了植物群落的地下生物量(P<0.05),土壤呼吸与地下生物量呈显著的正相关(P<0.001)。
增温降低了几个物种的地上生物量从而降低了植物群落的地上生物量。同时,我们也发现在降雨量相对充足的2008年,氮素的作用更为明显,氮的添加显著增加了地上部分净初级生产力(ANPP)。通过主成分响应曲线分析(PRC),2008年氮的添加导致了占优势的植物物种转变,即一二年生的植物物种在植物群落中所占的比例增加。氮的添加增加了植物组织的地上部分的含氮量从而提高了牧草的品质。增温施氮的互作效应也影响植物群落的地上部分生产力和植物组织的地上部分的含氮量。季节降雨量的变化对群落地上部分生产力与施氮对群落的影响相一致,因此,水分和氮肥共同制约着荒漠草原群落地上部分的生长。
生态系统净CO2交换(net ecosystem CO2exchange, NEE)代表生态系统吸收或释放CO2的能力,它由生态系统总初级生产力(gross ecosystem productivity, GEP)与呼吸(ecosystem respiration, ER)二者共同决定。2007-2009年,我们研究了增温、氮的添加及增温施氮的互作效应对NEE、ER和GEP的影响,研究结果发现:氮的添加显著增加了NEE、ER和GEP,但在植物生长季节氮素对GEP的影响显著高于对ER的影响,因此整个荒漠生态系统表现为净碳吸收;2008年,增温显著增加了ER,但没有影响NEE和GEP;NEE、ER和GEP与生物(地上生物量)和非生物因素(土壤温度和土壤含水量)之间呈显著的线性相关。我们的调查结果证实氮的添加显著增加了荒漠生态系统净碳吸收,而短期的增温对生态系统的碳循环没有任何影响。
As consequences of human activities, the increasing of both Earth's surface temperature and atmospheric N deposition has impacted on plant community, soil and ecosystem C cycle over the past two centuries. As one of the most important terrestrial biomes, grassland accounts for approximately40%of the Earth's land area play critical roles in regulating global C balance, but human on the understanding of grassland ecosystem, especially on the desert grassland ecosystem, is still a lack of awareness under the global climate change. The study relies on the experimental base of Inner Mongolia Siziwangqi desert sreppe pastoral Academy as a research platform, the seasonal responses of on the desert grassland community, soil and ecosystem C exchange to warming and management regime were studied. According to the study of global change and terrestrial ecosystem relationship, a reliable theoretical basis was provided. The study was initiated in June2006with a split-plot experimental design with warming as the main-plot effect and N addition as the sub-plot effect.
Compared with CK, soil temperatures at depths of0cm,10cm,20cm and30cm increased by2℃,0.87℃,0.86℃and0.77℃in the warmed plots, respectively. With the increasing of soil depth, the soil temperature was decreased. The survey of soil nutrients in2007-2009, we found the warming and N addition didn't change soil organic carbon content. However, warming and N addition didn't soil total N content. The positive linear interaction was found between NH4+and the depth of temperature, and between NO3-and the depth of temperature. With the increasing of temperature, both NH4+and NO3-were also increased. No significantly were detected on the effect of total NH4+by warming, N addition or their interactions from in2007to in2009. Warming significantly increased soil NO3-concentration both in2008(P=0.02) and in2009(P=0.01). whereas no effects of warming was detected in2007(P=0.36).
Three years' study (2006-2008) showed the evident temperature was linearly and positively correlated with mean soil respiration in three years, however, elevated temperature had not impact on soil respiration. A close relationship between the natural conditions and the effect of the nitrogen fertilizer to the desert grassland among which the rainfall was the main factor which had a positive effect to the nitrogen fertilizer, while in high precipitation season. N addition promoted soil respiration. N addition significant increased plant below-ground biomass,and a significantly positive correlation was detected between belowground biomass and soil respiration (P<0.001).
We found that warming decreased biomass production of several species and consequently the total aboveground biomass. Similarly, divergent species responses to N addition were aggregated to an increase in aboveground net primary productivity (ANPP) under N addition in2008with more rainfall in the growing season. Based on principal response curves (PRC) analysis, N addition lead to a shift in dominance from perennials to annuals by the third year, but warming did not affect species composition. Nitrogen (N) addition also increased [N] and total N content of aboveground plant tissues, suggesting enhanced forage quality by N addition. We also found an interactive effect of warming and N addition on ANPP and total plant N content. Seasonal precipitation patterns strongly affected the temporal changes of community productivity as well as its response to N addition, indicating that the desert steppe community was co-limited by water and N.
Net ecosystem CO2exchange on behalf of the ecosystem to absorb or release of CO2capacity, both ecosystem gross primary productivity and ecosystem respiration decided on net ecosystem CO2exchange. The unprecedented global warming and nitrogen (N) deposition impacts ecosystem carbon (C) cycling. From2007to2009, a field study was conducted to evaluate the effects of warming, N addition, and their compound effect on net ecosystem exchange (NEE), ecosystem respiration (ER) and gross ecosystem productivity (GEP), in the Desert Steppe in Inner Mongolia, China. N addition increased NEE, ER and GEP, but the increase of GEP was higher than increase of ER during the growing season, leading a net C sink status during the study. Warming increased ER, but had no effect on NEE and GEP during the study. Seasonal mean NEE, ER and GEP had positive linear correlations with both soil moisture and peak aboveground biomass. In this dry desert steppe of Inner Mongolia, N addition increased net C storage during the growing season, but the warming had no significant effect on NEE in a short term.
引文
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