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Crossing certain aridity thresholds in global drylands can lead to abrupt decays of ecosystem attributes such as plant productivity, potentially causing land degradation and desertification. It is largely unknown, however, whether these thresholds can be altered by other key global change drivers known to affect the water-use efficiency and productivity of vegetation, such as elevated CO2 and nitrogen (N). Using >5000 empirical measurements of plant biomass, we showed that crossing an aridity (1–precipitation/potential evapotranspiration) threshold of ∼0.50, which marks the transition from dry sub-humid to semi-arid climates, led to abrupt declines in aboveground biomass (AGB) and progressive increases in root:shoot ratios, thus importantly affecting carbon stocks and their distribution. N addition significantly increased AGB and delayed the emergence of its aridity threshold from 0.49 to 0.55 (P  < 0.05). By coupling remote sensing estimates of leaf area index with simulations from multiple models, we found that CO2 enrichment did not alter the observed aridity threshold. By 2100, and under the RCP 8.5 scenario, we forecast a 0.3% net increase in the global land area exceeding the aridity threshold detected under a scenario that includes N deposition, in comparison to a 2.9% net increase if the N effect is not considered. Our study thus indicates that N addition could mitigate to a great extent the negative impact of increasing aridity on plant biomass in drylands. These findings are critical for improving forecasts of abrupt vegetation changes in response to ongoing global environmental change. [ABSTRACT FROM AUTHOR]

Understanding under which climate and soil conditions the plant productivity–precipitation relationship is linear or nonlinear is useful for accurately predicting the response of ecosystem function to global environmental change. Using long-term (2000–2016) net primary productivity (NPP)–precipitation datasets derived from satellite observations, we identify > 5600 pixels in the North Hemisphere landmass that fit either linear or nonlinear temporal NPP–precipitation relationships. Differences in climate (precipitation, radiation, ratio of actual to potential evapotranspiration, temperature) and soil factors (nitrogen, phosphorous, organic carbon, field capacity) between the linear and nonlinear types are evaluated. Our analysis shows that both linear and nonlinear types exhibit similar interannual precipitation variabilities and occurrences of extreme precipitation. Permutational multivariate analysis of variance suggests that linear and nonlinear types differ significantly regarding to radiation, ratio of actual to potential evapotranspiration, and soil factors. The nonlinear type possesses lower radiation and/or less soil nutrients than the linear type, thereby suggesting that nonlinear type features higher degree of limitation from resources other than precipitation. This study suggests several factors limiting the responses of plant productivity to changes in precipitation, thus causing nonlinear NPP–precipitation pattern. Precipitation manipulation and modeling experiments should combine with changes in other climate and soil factors to better predict the response of plant productivity under future climate. [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]