内蒙古放牧草地土壤碳固持速率和潜力

放牧是典型草地最重要的利用方式,弄清放牧对草地碳固速率的影响,将为我国内蒙古地区草地碳汇管理提供重要的科学依据。通过在平坦草地和斜坡草地设置相同的放牧梯度实验 (放牧强度0、1.5、3.0、4.5、6.0、7.5、9.0 羊/hm²),探讨了放牧和地形对草地土壤碳固持速率的影响。实验结果表明:轻度放牧草地表现为碳固持,重度放草地表现为碳流失;对放牧草地而言,存在碳源/碳汇的转化阈值(或放牧强度),且坡地阈值低于平地。为了实现草地碳增汇目的,平坦草地的放牧强度应低于 4.5羊/hm²(放牧期6-9月),斜坡草地应低于3 羊/hm²。地形因素(平地VS斜坡)使准确评估放牧草地土壤的碳固持速率变得更加复杂。总之,内蒙古地区放牧草地具有较大的碳固持潜力,通过控制放牧强度是实现其碳固持潜力的重要途径之一。     

火烧对长期封育草地土壤碳固持效应的影响

火烧是内蒙古典型草原的自然现象,对长期封育草地碳固持效应具有潜在的重要影响。基于野外控制火烧实验(未火烧、1a火烧1次、2a火烧1次、4a火烧1次),采用土壤和土壤粒级组分相结合的指标体系,分析了火烧以及火烧频率对长期封育草地土壤碳贮量的影响。结果表明:火烧会降低长期封育草地0—30 cm土壤碳贮量;其中,频繁火烧将显著降低草地土壤碳贮量,而4a 1次的火烧对土壤碳贮量影响较小。火烧对0—10 cm土壤碳贮量影响明显,而对10—30 cm土层影响较小。此外,火烧对长期封育草地土壤砂粒和粉粒碳氮贮量影响较大、对粘粒碳氮贮量影响较小。火烧后表层土壤砂粒和粉粒C∶N比下降,表层土壤有机质的稳定性有所提高。与自由放牧草地相比,连续遭受火烧处理的长期封育草地仍具有较高的碳贮量。总之,火烧会一定程度降低长期封育草地的碳贮量,但并不会彻底改变其显著的碳固持效应。     

不同放牧梯度下呼伦贝尔草甸草原土壤碳氮变化及固碳效应

放牧是内蒙古呼伦贝尔草甸草原主要的利用方式,弄清放牧情况下草地土壤碳氮变化和固碳效应,将为我国内蒙古地区草地碳汇管理提供重要的科学依据。通过小区控制放牧实验(放牧梯度0.00,0.23,0.34,0.46,0.69,0.92Au/hm2),探讨了不同放牧梯度下土壤碳氮变化及固碳效应,实验结果表明:放牧梯度对土壤碳氮含量、土壤碳氮贮量的变化有明显的影响,0—30 cm土层碳氮贮量随着放牧梯度增加呈显著的线性下降趋势。随着放牧时间的延续,轻度放牧利于提高草地的土壤有机碳密度和碳固持潜力,重度放牧显著降低了土壤碳氮含量、碳氮贮量和碳氮固持潜力。不同放牧梯度下土壤碳氮含量和碳氮贮量具有明显的垂直分布特征,随着土壤深度的增加土壤碳氮含量、碳氮贮量均呈明显的下降趋势。轻度放牧草地表现为碳固持,重度放草地表现为碳流失,对呼伦贝尔羊草草甸草原而言,土壤碳固持/碳流失的转化阈值为0.46Au/hm2,通过合理控制放牧梯度能够达到增加草地土壤固碳潜力,实现碳增汇的目的。     

封育对羊草草地土壤碳矿化激发效应和温度敏感性的影响

土壤碳矿化(或土壤异养呼吸)的温度敏感性和激发效应是深入揭示土壤呼吸控制机理及其对未来气候变化响应与适应的重要研究方向。该文以自由放牧(FG0)、封育11年(FG11)、封育31年(FG31)的羊草(Leymus chinensis)草地为研究对象, 通过0、5、10、15、20、25 ℃培养, 探讨了封育对羊草草地土壤碳矿化激发效应和温度敏感性的影响。结果表明: 封育年限、添加葡萄糖、培养温度和培养时间对土壤碳矿化速率均具有显著的影响, 不同因素间存在显著的交互效应(p < 0.000 1)。FG0的羊草草地土壤碳矿化累积量显著高于FG11和FG31的, 在添加葡萄糖处理下也呈现相同的趋势。长期封育降低了羊草草地土壤碳矿化的激发效应。在添加葡萄糖后, 培养前7天的土壤碳矿化的激发效应随温度增加而增加, 增加2.28-9.01倍; 在整个56天培养期间, 激发效应介于2.21-5.10倍, 最高值出现在10或15 ℃。土壤碳矿化速率可用经典的指数方程来表示, FG0草地的土壤碳矿化的温度敏感性指数(Q₁₀)大于长期封育草地(FG11和FG31); 与未添加处理相比, 添加葡萄糖显著增加了土壤碳矿化速率的温度敏感性, 即在添加葡萄糖后土壤微生物呼吸受温度的影响更大。长期封育会降低羊草草地土壤的碳矿化速率、温度敏感性和激发效应, 从而降低土壤碳周转速率和释放速率, 使内蒙古地区长期封育草地仍然具有碳固持能力。     

水热波动和土地覆盖变化对东北地区植被NPP的相对影响

研究水热波动和土地覆盖变化对植被净初级生产力（Net Primary Productivity,NPP）的影响对于估算陆地碳循环及其驱动机制具有重要意义。利用MODIS遥感影像获得的时间序列NPP和土地覆盖产品,结合气象观测数据（气温和降水）,采用相关分析、回归分析和空间分析相结合的方法,研究2000-2015年东北地区植被NPP的时空变化特征,并定量评估水热波动和土地覆盖变化对该地区植被NPP的相对影响。研究结果表明,2000-2015年东北地区植被NPP呈波动上升趋势,从2000年的369.24 g C m^-2 a^-1增加到2015年的453.84 g C m^-2 a^-1,平均值是412.10 g C m^-2 a^-1,年际增加速率为4.54 g C m^-2 a^-1。近16年来东北地区年均植被NPP空间上呈现南高北低、东高西低的分布格局,整体变化趋势以增加为主,其中轻微增加面积占该地区总面积的45.9%。不同土地覆盖类型的年均NPP差异明显,其中灌木最高为400.34 g C m^-2 a^-1,草地最低为300.49 g C m^-2 a^-1。东北地区植被NPP与气温的相关性不明显,而与降水量主要表现为正效应。水热波动对该地区不同土地覆盖类型NPP总量变化的贡献大于土地覆盖变化的贡献,其中对森林和农田的贡献最大,均达到70%以上。     

连续氮添加14年对温带典型草原土壤碳氮组分及物理结构的影响

全球氮沉降对生态系统造成了深远的影响,研究长时间氮沉降对草地生态系统土壤理化特征的影响有助于加强生态系统对氮沉降响应的长效机制的理解。通过连续14年长期施加N0（0 g N m^-2 a^-1）、N2（2 g N m^-2 a^-1）、N4（4 g N m^-2 a^-1）、N8（8 g N m^-2 a^-1）、N16（16 g N m^-2 a^-1）、N32（32 g N m^-2 a^-1）六种浓度尿素模拟氮沉降,并将土壤分成0-10、10-20和20-40 cm三个深度土层,研究温带草原生态系统土壤碳氮组分及物理结构对氮添加的响应及其相互关系,结果表明：（1）氮添加显著降低0-10 cm土壤酸碱度及土壤微生物量碳含量,N32相比N0分别下降了27.63%和58.40%（P<0.05）;各土层总有机碳和全氮含量对氮添加处理无显著响应,0-10 cm土层显著高于20-40 cm土层。（2）同一土层深度不同梯度氮添加处理显著增加土壤无机氮离子含量（P<0.05）,0-10 cm土层铵态氮含量N32相比N0增加了88.72%,20-40 cm土层硝态氮含量N32相比N0增加了19.55倍,土壤深度与氮添加对无机氮离子含量影响具有显著的交互效应。（3）同一土壤深度不同梯度氮添加处理土壤粒度分形维数及土壤团聚体差异不显著,相关分析表明土壤碳氮元素含量与土壤结构显著相关。土壤碳氮组分在适宜浓度氮添加的增加趋势说明氮添加在一定程度上可能促进土壤理化性质的改良,氮添加对土壤物理结构的影响还需要进一步的深入研究。     

中国草地生态系统碳库及其变化

准确评估草地生态系统碳库及其年际变化, 对揭示草地在中国陆地生态系统碳循环中的作用以及合理利用有限的草地资源有着极为重要的意义. 虽然中国学者在研究草地碳库及其动态变化方面已开展了很多工作, 但目前仍缺乏对中国草地生态系统碳库及其动态变化特征的全面认识. 通过综述当前中国草地碳循环研究的最新进展, 结合本研究组的工作, 试图全面评价中国草地生态系统碳库(植被生物量碳库和土壤有机碳库)及其动态变化. 结果显示: (1) 不同研究得到的中国草地生物量碳密度(单位面积生物量)存在较大差异, 为215.8~348.1 g C/m², 平均值为 300.2 g C/m². 同样, 对中国草地土壤有机碳密度(单位面积土壤碳库)的估算也存在显著差异, 在8.5~15.1 kg C/m²之间变动, 但考虑到8.5 kg C/m²的估算值是基于近千个土壤剖面的实测数据计算得到, 全国平均水平的土壤碳密度一般不会超过此值. 因此, 若采用目前最广泛使用的草地面积(331×104 km²), 那么中国草地生态系统碳库约为29.1 Pg C(1 Pg=1×10¹⁵ g), 其中96.6%的碳储存于土壤有机质中. (2) 文献报道的近20年中国草地生物量和土壤有机碳库的变化方向和变化量均存在差异. 按照最新的估算, 中国草地生物量和土壤有机碳库在过去20年里没有发生显著变化, 即中国草地生态系统处于中性碳汇状态. (3) 中国草地生物量的时空变异与降水量的变化关系密切. 土壤有机碳库的空间变异主要受与降水量密切相关的土壤水分的影响, 但土壤质地等因素也起一定作用. 此外, 放牧与围封等人类活动将对草地生物量和土壤碳库及其动态变化产生强烈影响.     

淇河流域生态系统服务权衡及空间分异机制的地理探测

复杂的地形地貌使小流域具有生产、水源供给和调节等多种生态系统服务功能和显著的区域差异性,然而对小流域生态系统服务权衡关系及外部驱动机制集成测度一直是地理学、生态学和经济学研究的难题。以太行山南段淇河流域为研究对象,利用气象数据、土地利用/覆被数据、植被NDVI等多源数据,使用CASA模型和InVEST模型分别估算固碳、土壤保持及水源供给服务,运用空间叠置法和相关性分析测度生态系统服务权衡关系,进一步利用地理探测器中因子探测和交互探测分析自然环境和人类活动影响对生态系统服务权衡关系的空间影响。结果表明：①2000-2015年,淇河流域多年平均固碳量为375.55 gC m^-2 a^-1,且多年递减速率为18.20 gC m^-2 a^-1;多年平均土壤保持量为396.72 t hm^-2a^-1,多年递减速率为1.2 t hm^-2 a^-1;多年平均水源供给量为67.26 mm/a,以0.16 mm/a的速率递减。②研究时段内,生态系统服务关系以强权衡和低协同为主导,且存在显著的空间分异特征,低协同分布区域减幅为16.21%,强权衡增幅最多（11.85%）,表明流域生态系统服务能力呈升高趋势;生态系统服务两两关系中固碳与水源供给、水源供给与土壤保持表现为权衡关系,固碳与土壤保持呈协同态势。③地理探测器结果表明：固碳与土壤保持方面,植被NDVI（0.306） > 气温（0.241） > 土地利用程度（0.002）;固碳与水源供给方面,植被NDVI因子解释力最强（0.381）;水源供给与土壤保持方面,降水、气温和高程因子解释力均高于0.5;交互探测后各影响因子解释能力显著增强。     

凋落物添加和模拟氮磷沉降对红松凋落物木质素降解和碳释放的影响

通过对阔叶红松林和红松人工林2种林型凋落物处理（分别为不添加凋落物（原样组）、添加凋落物（双倍组）和去除凋落物（去除组）等3个处理）与模拟氮磷沉降（分别为对照CK （0 g N m^-2 a^-1、0 g P m^-2 a^-1）、低浓度氮磷（5 g N m^-2 a^-1、5 g P m^-2 a^-1）、中浓度氮磷（15 g N m^-2 a^-1、10g P m^-2 a^-1）和高浓度氮磷（30 g N m^-2 a^-1、20 g P m^-2 a^-1）等4个强度）原位培养试验,研究凋落物质量的增加与氮磷沉降及两种处理的耦合作用对碳（C）和木质素分解释放的影响。结果表明：凋落物添加在试验前期（6月）抑制人工林L层的C释放,促进H层的C释放;试验后期（10月）促进人工林L层C释放,而抑制H层的C释放。凋落物添加在前期（6月）是促进天然林L层C释放的,但在后期（10月）产生抑制作用。与L层相反,凋落物添加持续促进天然林H层的C释放。低、中浓度氮磷沉降显著促进了红松人工林和阔叶红松林L、H层C释放和木质素降解,但高浓度的氮磷添加会抑制C释放和木质素的降解,两种处理之间无交互作用。     

中国亚热带-暖温带过渡区锐齿栎林净生态系统碳交换特征

在2017年1月1日-2017年12月31日期间,采用涡度相关法对位于亚热带-暖温带气候过渡区的河南宝天曼国家级自然保护区的65年生锐齿栎（Quercus aliena）天然次生林的碳通量进行了连续观测。结果表明：在观测期间,该森林生态系统在生长季5-10月份为碳汇,非生长季各月为碳源,净碳吸收量与释放量分别在7月和4月达到最大。净生态系统生产力为569.4 g C m^-2a^-1,生态系统呼吸为529.9 g C m^-2a^-1,总生态系统生产力为1099.3 g C m^-2a^-1。30min尺度上夜间净生态系统碳交换量与5cm深度土壤温度的关系可用指数方程表示（R²=0.21,P < 0.001）,其温度敏感性系数（Temperature sensitivity coefficient,Q₁₀）为2.2。如果排除夜间通量观测的误差,处在海拔较高地区的夜间低温和非生长季的低温抑制了生态系统呼吸排放,可能导致全年生态系统呼吸量较低。在生长季5-10月份,各月的白天净生态系统碳交换量对光合有效辐射的响应符合直角双曲线模型,初始光能利用效率、平均最大光合速率和白天平均生态系统呼吸强度呈明显的季节变化,范围分别是0.06-0.12 μmol CO₂ μmol^-1 photon、0.44-1.47 mg CO₂ m^-2s^-1和0.07-0.19 mg CO₂ m^-2s^-1。夏季7、8月份,较高的饱和水汽压差对白天锐齿栎林的碳吸收有明显的抑制作用;生长季末期9月份较高的土壤含水量对白天锐齿栎林的碳吸收也产生了抑制作用,表明生长末期降水过多影响森林的碳吸收。     

Effects of fencing and grazing on the temporal dynamic of soil organic carbon content in two temperate grasslands in Inner Mongolia,China

This study aimed to investigate the impact of long-term grassland management on the temporal dynamic of SOC density in two temperate grasslands. The top soil SOC density, soil total nitrogen density and soil bulk density (0–20 cm) under long-term fencing and grazing treatments, the aboveground net primary productivity of fenced plots and the associated climatic factors of Leymus chinensis and Stipa grandis grasslands in Inner Mongolia were collected from literatures and analyzed. The results showed that the SOC density increased linearly with fenced duration but was insensitive to grazed duration in both grasslands. Compared with long-term grazing, fenced plots had larger potential for carbon sequestration, and the accumulation rate of SOC density was 29 and 35 g Cm^–2y^–1 for L. chinensis and S. grandis grasslands. Fenced duration and mean annual temperature jointly contributed large effect on temporal pattern of SOC density. Climate change and grazed duration had little influence on the inter-annual variance of SOC density in grazed plots. Our results confirmed the enhancement effect of long-term fencing on soil carbon sequestration in degraded temperate grassland, and long-term permanent plot observation is essential and effective for accurately and comprehensively understanding the temporal dynamic of SOC storage.     

Carbon,nitrogen, and phosphorus storage in alpine grassland ecosystems of Tibet: effects of grazing exclusion

In recent decades, alpine grasslands have been seriously degraded on the Tibetan Plateau and grazing exclusion by fencing has been widely adopted to restore degraded grasslands since 2004. To elucidate how alpine grasslands carbon (C), nitrogen (N), and phosphorus (P) storage responds to this management strategy, three types of alpine grassland in nine counties in Tibet were selected to investigate C, N, and P storage in the environment by comparing free grazing (FG) and grazing exclusion (GE) treatments, which had run for 6–8 years. The results revealed that there were no significant differences in total ecosystem C, N, and P storage, as well as the C, N, and P stored in both total biomass and soil (0–30 cm) fractions between FG and GE grasslands. However, precipitation played a key role in controlling C, N, and P storage and distribution. With grazing exclusion, C and N stored in aboveground biomass significantly increased by 5.7 g m⁻² and 0.1 g m⁻², respectively, whereas the C and P stored in the soil surface layer (0–15 cm) significantly decreased by 862.9 g m⁻² and 13.6 g m⁻², respectively. Furthermore, the storage of the aboveground biomass C, N, and P was positively correlated with vegetation cover and negatively correlated with the biodiversity index, including Pielou evenness index, Shannon–Wiener diversity index, and Simpson dominance index. The storage of soil surface layer C, N, and P was positively correlated with soil silt content and negatively correlated with soil sand content. Our results demonstrated that grazing exclusion had no impact on total C, N, and P storage, as well as C, N, and P in both total biomass and soil (0–30 cm) fractions in the alpine grassland ecosystem. However, grazing exclusion could result in increased aboveground biomass C and N pools and decreased soil surface layer (0–15 cm) C and P pools.     

Enhancement of Carbon Sequestration in Soil in the Temperature Grasslands of Northern China by Addition of Nitrogen and Phosphorus

Increased nitrogen (N) deposition is common worldwide. Questions of where, how, and if reactive N-input influences soil carbon (C) sequestration in terrestrial ecosystems are of great concern. To explore the potential for soil C sequestration in steppe region under N and phosphorus (P) addition, we conducted a field experiment between 2006 and 2012 in the temperate grasslands of northern China. The experiment examined 6 levels of N (0–56 g N m^-2 yr^-1), 6 levels of P (0–12.4 g P m^-2 yr^-1), and a control scenario. Our results showed that addition of both N and P enhanced soil total C storage in grasslands due to significant increases of C input from litter and roots. Compared with control plots, soil organic carbon (SOC) in the 0–100 cm soil layer varied quadratically, from 156.8 to 1352.9 g C m^-2 with N addition gradient (R² = 0.99, P < 0.001); and logarithmically, from 293.6 to 788.6 g C m^-2 with P addition gradient (R² = 0.56, P = 0.087). Soil inorganic carbon (SIC) decreased quadratically with N addition. The net C sequestration on grassland (including plant, roots, SIC, and SOC) increased linearly from -128.6 to 729.0 g C m^-2 under N addition (R² = 0.72, P = 0.023); and increased logarithmically, from 248.5 to 698 g C m^-2under P addition (R² = 0.82, P = 0.014). Our study implies that N addition has complex effects on soil carbon dynamics, and future studies of soil C sequestration on grasslands should include evaluations of both SOC and SIC under various scenarios.     

内蒙古草甸草原与典型草原地下生物量与生产力季节动态及其碳库潜力

地下根系是草原生态系统的重要组成部分,其生物量及其净生产力对地下碳库具有直接与间接作用,分析地下生物量季节动态与周转对深入揭示草原生态系统碳库动态及其固碳速率与潜力具有重要意义。应用钻土芯法对不同利用方式或管理措施下内蒙古草甸草原、典型草原地下生物量动态及其与温度、降水的相关性研究表明:草甸草原和典型草原地上生物量季节动态均为单峰型曲线,与上月降水显著正相关(P0.05),但地下生物量季节动态表现为草甸草原呈"S"型曲线,典型草原则是双峰型曲线,与温度、降水相关性均不显著(P0.05);两种草原根冠比和地下生物量垂直分布均为指数函数曲线,根茎型草原地下生物量集中在土壤0—5 cm,丛生型草原地下生物量集中于土壤5—10 cm,根冠比值在生长旺季(7—8月份)最小。草甸草原地下净生产力及碳储量范围分别为2167—2953 g m-2a-1和975—1329 gC m-2a-1,典型草原为2342—3333 g m-2a-1和1054—1450 gC m-2a-1,地下净生产力及其碳储量约为地上净生产力及其碳储量的10倍,具有较大的年固碳能力,且相对稳定;地下净生产力与地上净生产力呈显著负相关性(P0.05);地下生物量碳库是地上生物量碳库的10倍左右,适度放牧可增加地下生产力,但长期过度放牧显著降低其地下生物量与生产力,并使其垂直分布趋向于浅层化。     

Relationship of plant diversity with litter and soil available nitrogen in an alpine meadow under a 9-year grazing exclusion

Grazing exclusion is widely used globally to restore degraded grasslands. Plant diversity has important impacts on grassland ecosystem functions, including grassland productivity and carbon storage. In this study, we selected a Kobresia meadow on the Qinghai–Tibetan Plateau to investigate how grazing exclusion affects plant diversity. Inorganic nitrogen (NH₄ ⁺ and NO₃ ^?) was also measured because its availability impacts plant growth. We found that plant diversity in the meadow was significantly lower under grazing exclusion (fenced meadow) for 9 years compared with moderate grazing. Accumulated litter was significantly higher under grazing exclusion (386.41 g m^?2) compared with grazing (58.77 g m^?2). Soil inorganic nitrogen at 0–5 cm depth was significantly higher under grazing exclusion (13.60 × 10^?2 g kg^?1) than under grazing (9.40 × 10^?2 g kg^?1). The composition of the four functional groups (grasses, sedges, legumes, and forbs) might alter in response to significant changes in the amount of litter and soil available nitrogen content under grazing exclusion compared with grazing. However, the enhanced soil available nitrogen content showed weak feedbacks on plant diversity. In conclusion, light limitation induced by increased amounts of litter may be the main factor causing decreased plant diversity in grazing-excluded meadows compared with moderately grazed meadows.     

Short-term rather than long-term exclusion of grazing increases soil bacterial diversity in an Inner Mongolian steppe

In the Three Gorges Reservoir Region of China, periodic flooding has led to plant destruction, causing much ecological damage. Re-vegetation with submergence-tolerant species is a possible solution to this problem. At present, many submergence-tolerant species have been selected for such restoration efforts, but it is unclear why these species can survive complete submergence while other species cannot. In this study, we investigated the response of two species – submergence-tolerant Salix variegata Franch. and submergence-intolerant Cinnamomum camphora (L.) Presl. – to flooding. Plants were submerged to 2 m for 3, 9, 15, and 30 days, after which malondialdehyde (MDA) (a membrane injury product) and superoxide anion content, as well as superoxide dismutase (SOD) and peroxidase (POD) activity, was measured. We found that (1) MDA levels increased in submerged C. camphora seedlings but remained constant in S. variegata; (2) superoxide anion content and SOD activity in the two species responded similarly to submergence; and (3) POD activity in S. variegata seedlings was much higher than in C. camphora. These results demonstrate that plant tolerance to submergence is related to membrane stability, and that POD activity is an important factor in this tolerance.     

Grazing intensity significantly affects belowground carbon and nitrogen cycling in grassland ecosystems: a meta‐analysis

Livestock grazing activities potentially alter ecosystem carbon (C) and nitrogen (N) cycles in grassland ecosystems. Despite the fact that numerous individual studies and a few meta‐analyses had been conducted, how grazing, especially its intensity, affects belowground C and N cycling in grasslands remains unclear. In this study, we performed a comprehensive meta‐analysis of 115 published studies to examine the responses of 19 variables associated with belowground C and N cycling to livestock grazing in global grasslands. Our results showed that, on average, grazing significantly decreased belowground C and N pools in grassland ecosystems, with the largest decreases in microbial biomass C and N (21.62% and 24.40%, respectively). In contrast, belowground fluxes, including soil respiration, soil net N mineralization and soil N nitrification increased by 4.25%, 34.67% and 25.87%, respectively, in grazed grasslands compared to ungrazed ones. More importantly, grazing intensity significantly affected the magnitude (even direction) of changes in the majority of the assessed belowground C and N pools and fluxes, and C : N ratio as well as soil moisture. Specifically,light grazing contributed to soil C and N sequestration whereas moderate and heavy grazing significantly increased C and N losses. In addition, soil depth, livestock type and climatic conditions influenced the responses of selected variables to livestock grazing to some degree. Our findings highlight the importance of the effects of grazing intensity on belowground C and N cycling, which may need to be incorporated into regional and global models for predicting effects of human disturbance on global grasslands and assessing the climate‐biosphere feedbacks.     

Changes in the temperature sensitivity of SOM decomposition with grassland succession: implications for soil C sequestration

Understanding the temperature sensitivity (Q₁₀) of soil organic matter (SOM) decomposition is important for predicting soil carbon (C) sequestration in terrestrial ecosystems under warming scenarios. Whether Q₁₀ varies predictably with ecosystem succession and the ways in which the stoichiometry of input SOM influences Q₁₀ remain largely unknown. We investigate these issues using a grassland succession series from free‐grazing to 31‐year grazing‐exclusion grasslands in Inner Mongolia, and an incubation experiment performed at six temperatures (0, 5, 10, 15, 20, and 25°C) and with four substrates: control (CK), glucose (GLU), mixed grass leaf (GRA), and Medicago falcata leaf (MED). The results showed that basal soil respiration (20°C) and microbial biomass C (MBC) logarithmically decreased with grassland succession. Q₁₀ decreased logarithmically from 1.43 in free‐grazing grasslands to 1.22 in 31‐year grazing‐exclusion grasslands. Q₁₀ increased significantly with the addition of substrates, and the Q₁₀ levels increased with increase in N:C ratios of substrate. Moreover, accumulated C mineralization was controlled by the N:C ratio of newly input SOM and by incubation temperature. Changes in Q₁₀ with grassland ecosystem succession are controlled by the stoichiometry of newly input SOM, MBC, and SOM quality, and the combined effects of which could partially explain the mechanisms underlying soil C sequestration in the long‐term grazing‐exclusion grasslands in Inner Mongolia, China. The findings highlight the effect of substrate stoichiometry on Q₁₀ which requires further study.