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Responses of soil respiration (R s ) to increasing nitrogen (N) deposition and warming will have far-reaching influences on global carbon (C) cycling. However, the seasonal (growing and non-growing seasons) difference of R s responses to warming and N deposition has rarely been investigated. We conducted a field manipulative experiment in a semi-arid alfalfa-pasture of northwest China to evaluate the response of R s to nitrogen addition and warming from March 2014 to March 2016. Open-top chambers were used to elevate temperature and N was enriched at a rate of 4.42 g m − 2 yr − 1 with NH 4 NO 3 . Results showed that (1) N addition increased R s by 14% over the two-year period; and (2) warming stimulated R s by 15% in the non-growing season, while inhibited it by 5% in the growing season, which can be explained by decreased plant coverage and soil water. The main effect of N addition did not change with time, but that of warming changed with time, with the stronger inhibition observed in the dry year. When N addition and warming were combined, an antagonistic effect was observed in the growing season, whereas a synergism was observed in the non-growing season. Overall, warming and N addition did not affect the Q10 values over the two-year period, but these treatments significantly increased the Q10 values in the growing season compared with the control treatment. In comparison, combined warming and nitrogen addition significantly reduced the Q10 values compared with the single factor treatment. These results suggest that the negative indirect effect of warming-induced water stress overrides the positive direct effect of warming on R s . Our results also imply the necessity of considering the different R s responses in the growing and non-growing seasons to climate change to accurately evaluate the carbon cycle in the arid and semi-arid regions. [ABSTRACT FROM AUTHOR]

Human activities are critical factors influencing ecosystem sustainability. However, knowledge on regarding the mechanisms underlying the response of vegetation dynamics to human activities remains limited. To detect the driving factors and their individual contribution to the grassland vegetation dynamics in China's Loess Plateau, a structural equation model (SEM) and a principal component regression model were built. The SEM showed that population change and urbanization, temperature and humidity, and agriculture and economy accounted for 62.5%, 31.2%, and 7.7%, respectively, of the overall impact directly affecting grassland vegetation dynamics. Furthermore, the principal component regression model demonstrated that the effects of the urbanization rate on the grassland above-ground biomass exceeded those of the other factors. The agriculture population had the maximum negative effect on grassland area. The higher the urbanization rate means the higher the number of residents migrates from rural to urban areas. Following this argument, the disturbances of human activities to grassland vegetation were expected to gradually decrease in rural areas, where the vast majority of the Loess Plateau is located. The migration of rural residents to urban areas promoted the increase in biomass and areas of grassland vegetation. Our findings suggest that the effect of urbanization should be considered when assessing vegetation change. [ABSTRACT FROM AUTHOR]

Abstract China initiated the "Grain for Green Project" in 1999 to mitigate soil erosion. The vegetation cover of the Chinese Loess Plateau, one of the most erosive regions in the world, has been greatly increased. However, studies on quantitatively investigating the climate change and human activities on vegetation coverage change were rare. In this study, spatio-temporal changes in vegetation coverage were investigated using MODIS normalized difference vegetation index (NDVI) data over 2000–2016. And a new method was introduced using Net Primary Productivity (NPP) model and relationship between NPP and NDVI to quantitatively and spatially distinguish the NDVI affected by climate change and human activities. Results showed that mean NDVI value over 2009–2016 were 14.46% greater than that over 2000–2007. In order to quantify the contribution of climate change and human activities to vegetation change, an NPP model suitable for the grassland of the Chinese Loess Plateau was identified using biomass observations from field survey and literature. The NDVI affected by climate change (NDVI climate) was estimated by the NPP model and the relationship between NPP and NDVI. And the NDVI affected by human activities (NDVI human) was calculated by actual NDVI minus NDVI climate. Comparison of the two stages showed that human activities and climate change contributed 42.35% and 57.65% respectively to the ΔNDVI on grassland in the Loess Plateau. After analysis of numerous NDVI human related factors, the slopes restored by the "Grain for Green Project" was considered the main influence factor of human activities. Graphical abstract Unlabelled Image Highlights • Novel method was developed to distinguish climate change and human activities. • Human activities contributed 42.35% to the vegetation change. • Climate change contributed 57.65% to the vegetation change. • The "Grain for Green Project" was the main human influence on vegetation. [ABSTRACT FROM AUTHOR]

Evaluating the responses of soil respiration and its components to global environmental change is crucial for predicting future terrestrial carbon cycle. However, the effects of warming and nitrogen (N) addition on them remain unclear. A field manipulative experiment was conducted in a semi-arid grassland on the Loess Plateau to evaluate the responses of soil total respiration (Rt), autotrophic respiration (Ra) and heterotrophic respiration (Rh) to warming and N addition from April 2015 to December 2016. Open-top chambers were used to elevate temperature and N was added as NH 4 NO 3 at a rate of 4.42 g N m −2 yr −1 . Warming significantly decreased Rt and Rh by 7.4% and 9.5%, respectively, but had no significant effect on Ra. N addition significantly stimulated Ra by 34%, whereas it inhibited Rh by 11% and had no significant effect on Rt. The maximum N-inducing stimulation of Rt and Ra were observed in the month with the most rainfall events in 2015. N addition significantly increased the contribution of Ra to Rt by 10%. Warming decreased the Q10 values of Rt and Rh but had no significant effect on Q10 values of Ra. N addition significantly increased the Q10 values of Rt and Rh, whereas it decreased the Q10 values of Ra. The combination of warming and N addition had a synergistic effect on the cumulant of Rh, whereas it had an antagonistic effect on Ra. No interactive effect between warming and N addition was observed on Rt. Our results emphasized that Ra and Rh responded differently to warming and/or N addition, and the extreme rainfall frequency influenced the responses of Rt and its components to N addition. Our findings suggested that Rt has the potential to resist climate warming and increasing N deposition by differentiating the responses of its inherent components. [ABSTRACT FROM AUTHOR]