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Background and aims: Responses of grassland carbon (C) cycling to changes in precipitation and land use remain a major uncertainty in model prediction of future climate. It remains even far from clear on how does clipping regulate the response sensitivity of grassland C sequestration to precipitation change. Methods: We conducted a manipulative field experiment with five precipitation treatments (1/12P, 1/4P, 1/2P, 3/4P, P, and 5/4P, where P is the growing season precipitation) interacting with clipping treatments in an alpine meadow to examine net ecosystem C uptake and its associated processes in response to clipping and precipitation gradients as well as their interaction. Results: (1) clipping significantly increased soil temperature in 2016 and 2017, while had no effect on soil moisture; (2) clipping also stimulated the net ecosystem productivity (NEP) and gross ecosystem productivity (GEP) during the two growing seasons; (3) although there were no significantly interactive effects of clipping with precipitation changes, clipping increased the response sensitivity of C uptakes to soil moisture changes. NEP increased more with soil moisture under clipping (0.21 μmol m−2 s−1 v/v−1) than unclipping treatments (0.11 μmol m−2 s−1 v/v−1). This was due to the compensatory growth and the positive effect of clipping on soil temperature, which stimulated plant growth and productivity. Conclusions: Our findings suggest that clipping will aggravate grassland's sensitivity to precipitation change. These findings highlight the important role of land use change in regulating ecosystem C sequestration under further climate conditions in an alpine meadow. [ABSTRACT FROM AUTHOR]

Changes in precipitation have the potential to cause dramatic changes in ecosystem carbon (C) cycling; however, it remains unclear whether different components of the net ecosystem exchange (NEE) (e.g., C uptake vs. release, plant vs. microbe respiration, above‐ground vs. below‐ground plant respiration) have similar or differential sensitivity to precipitation gradients.We conducted a manipulative field experiment (from 2015 to 2017) with six precipitation treatments, including 1/12 annual precipitation (P), 1/4 P, 1/2 P, 3/4 P, P and 5/4 P in an alpine meadow to investigate the responses of the NEE components.Over the 3 years, all C fluxes showed a nonlinear response to the precipitation gradients, except for root respiration. The most extreme drought treatment (1/12 P) caused strong reductions in NEE by 15.57%, gross primary productivity by 17.26% and ecosystem respiration by 19.05%, in contrast to the control. Plant respiration was more sensitive to precipitation change than microbe respiration, and above‐ground plant respiration was more susceptible than below‐ground respiration. Structural equation models revealed that the response of C fluxes under precipitation changes was primarily due to changes in the soil water content and above‐ground net primary productivity.Our findings indicate that future precipitation changes, particularly extreme drought, will decrease ecosystem C fluxes with different magnitudes, leading to a consequent reduction of NEE. These emergent ecosystem properties are essential for the improved elucidation of carbon cycle dynamics and benchmarking models, to predict ecosystem responses to precipitation changes. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

To study the variation law of key components for water-salt balance of typical farmland in the central Shaanxi Plain, this paper calibrated and verified the SWAP model based on a field trial of summer maize growing season from 2013 to 2014. Variations of different components during different summer maize growth stages were analyzed and discussed according to the calculated results of water-salt balance by the verified SWAP model. Results showed that the calibrated and verified SWAP model could well simulate dynamic variation laws of water-salt content of different soil layers in the study area. Within the study period, transpiration of maize in 2013 and 2014 was restricted by water content in the surface layer. The accumulative actual transpiration accounted for 57.6% and 64.9% of potential transpiration, respectively. Soil water supply from underground water was mainly in the vigorous growth of maize. The accumulative underground water supplies in 2013 and 2014 were 10.3cm and 7.4cm, respectively. Water flux at bottom soil was highly sensitive to rainfall and irrigation, presenting the obvious deep leakage. The soil salinity changes at 0~120cm layer in 2013 and 2014 were -72.87 mg/cm2 and -81.32mg/cm2, respectively. Great desalination was observed in the rainy season of maize growth. [ABSTRACT FROM AUTHOR]

The sustainable development of efficient water-saving agricultural irrigation in Loess Plateau requires reasonable and quantitative planning, design, management, and strategies based on crop-water-salt response relationship. In the present paper, data were measured by using the soil-water-salt sensor CS655 to modify the simulation of root uptake in the original Soil Water Atmosphere Plant (SWAP) model. Spring wheat field experiment data in the salinized irrigation district of Fuping County, Shaanxi Province, China were used to test the feasibility of using the SWAP model in irrigation districts of Loess Plateau. The comparison of soil moisture and salt concentration in the root system, along with simulated and measured values of partial above- ground biomass showed the following results: the average relative error and root-square error of soil moisture in the root layer were close to 0; model R² tended to be 1; simulation accuracy of water module was high; salt concentration in the simulation varied, but the overall simulation consistency was good; crop growth parameters matched well; and simulated yield was close to the actual value with a relative error of 3.6%. The SWAP model can be well applied to simulate soil, water and salt transfer at field scale in salinized districts of Loess Plateau. [ABSTRACT FROM AUTHOR]

Droughts have significantly damaged the environment of the Inner Mongolia Autonomous Region, China. In this study, the region was divided into two subregions. Soil moisture was used as the basic parameter to analyze the characteristics of agricultural droughts. Based on a geographical detector, the spatial stratified heterogeneity in different seasons was discussed. Moreover, the copula joint functions of characteristics and dominant factors of agricultural droughts were constructed. Based on the Soil Moisture Anomaly Percentage Index (SMAPI), the results demonstrate that the climate tendency rate of droughts in the summer and in spring in Subregion I shows an increasing trend, while it decreases in the autumn and winter. In Subregion II, the climate tendency rate of droughts in different seasons has no significant change. Through geographical detection, the single factor detection illustrates that temperature and Precipitation Conversion Efficiency (PCE) show the highest explanatory power in different subregions. The interactive detection also demonstrates the explanatory powers of the combination of the PCE and temperature, respectively. The t-copula function describes the correlation coefficients of the SMAPI with the PCE and temperature, with the optimal tail dependence. In short, agricultural droughts are most significantly affected by temperature and the PCE, and their balance has a significant impact on agricultural droughts. [ABSTRACT FROM AUTHOR]

• N addition enhanced GEP and NEP only when volumetric soil moisture exceeded 15%. • GEP and NEP showed saturating responses to N addition only at high soil moisture. • N effects on C exchange were mediated by changes in species composition. • Temperature and soil moisture interactively regulate N effects on NEP, ER and GEP. Primary productivity in terrestrial ecosystems can be significantly altered by the predicted increases in nitrogen (N) deposition, but it is still unclear how N deposition influences the carbon (C) exchange processes especially in dryland ecosystems. In this study, a 3-year experiment with two types of fertilizers and five N addition levels was conducted in a semiarid steppe of Erguna, Inner Mongolia to assess the effects of exogenous N input on net ecosystem productivity (NEP), gross ecosystem productivity (GEP) and ecosystem respiration (ER). Our results showed that enhanced N input significantly increased NEP and GEP when the volumetric soil water content (VWC) was greater than 15% (v/v%), but had trivial and inconsistent effects when VWC was less than that threshold. Moreover, the NEP and GEP increased significantly with increasing N addition but leveled off when the N input reached 10 g N m–2 year–1 and the VWC was above 15%. However, the ER showed a significant increase with N addition rates, regardless of soil water content. Our main findings are as follows. First, the ecosystem C exchange can be significantly enhanced by addition of N only when soil moisture exceeds certain threshold, suggesting co-limitation of water and N in the grassland ecosystem that we studied. Second, in addition to concurrent consideration of water, temperature and N as well as their interactions also need to be accounted for. Third, the frequency of days with the VWC being higher than 15% accounted only around one third of days during the three growing seasons. Taken together, our findings imply that the expected increases of carbon sequestration should be minimal under N deposition in the Inner Mongolia grassland ecosystems. [ABSTRACT FROM AUTHOR]