Drought has been one of the most important limiting factors for crop production, which deleteriously affects food security worldwide. The main objective of the present study was to quantitatively assess the effect of drought on the agronomic traits (e.g., plant height, biomass, yield, and yield components) of rice and wheat in combination with several moderators (e.g., drought stress intensity, rooting environment, and growth stage) using a meta-analysis study. The database was created from 55 published studies on rice and 60 published studies on wheat. The results demonstrated that drought decreased the agronomic traits differently between rice and wheat among varying growth stages. Wheat and rice yields decreased by 27.5% and 25.4%, respectively. Wheat grown in pots showed greater decreases in agronomic traits than those grown in the field. Rice showed opposite growing patterns when compared to wheat in rooting environments. The effect of drought on rice increased with plant growth and drought had larger detrimental influences during the reproductive phase (e.g., blooming stage, filling stage, and maturity). However, an exception was found in wheat, which had similar decreased performance during the complete growth cycle. Based on these results, future droughts could produce lower yields of rice and wheat when compared to the current drought.

China is a global hotspot of atmospheric ammonia (NH) emissions and, as a consequence, very high nitrogen (N) deposition levels are documented. However, previous estimates of total NH emissions in China were much lower than inference from observed deposition values would suggest, highlighting the need for further investigation. Here, we reevaluated NH emissions based on a mass balance approach, validated by N deposition monitoring and satellite observations, for China for the period of 2000 to 2015. Total NH emissions in China increased from 12.1 ± 0.8 Tg N yr in 2000 to 15.6 ± 0.9 Tg N yr in 2015 at an annual rate of 1.9%, which is approximately 40% higher than existing studies suggested. This difference is mainly due to more emission sources now having been included and NH emission rates from mineral fertilizer application and livestock having been underestimated previously. Our estimated NH emission levels are consistent with the measured deposition of NH (including NH and NH) on land (11-14 Tg N yr) and the substantial increases in NH concentrations observed by satellite measurements over China. These findings substantially improve our understanding on NH emissions, implying that future air pollution control strategies have to consider the potentials of reducing NH emission in China.

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Persistent drought severely inhibits plant growth and productivity, which negatively affects terrestrial primary productivity worldwide. Therefore, it is important to investigate the impacts of drought on plant leaf CO 2 /H 2 O exchange and water use efficiency. This study assessed the responses of net photosynthesis (Pn), stomatal conductance (Gs), transpiration (Tr), and instantaneous water use efficiency (WUE) to drought based on a worldwide meta-analysis of 112 published studies. The results demonstrated that drought decreased Pn, Tr, and Gs significantly and differently among different moderators. C 4 plants had smaller Pn reduction than C 3 plants, which gives C 4 plants an advantage in Pn. But their WUE decreased under drought conditions, indicating a great flexibility in C 4 WUE. Annual herbs sacrificed WUE (−6.2%) to maintain efficient Pn. Perennial herbs took a different strategy in response to drought with an increased WUE (25.1%). Deciduous tree species displayed a greater increase in WUE than conifers and evergreen species. Additionally, Gs had a significant correlation with Pn and Tr, but an insignificant correlation with WUE, which could be because WUE is affected by other factors (e.g., air flow, CO 2 concentration, and relative humidity). These findings have significant implications for understanding the worldwide effects of drought on plant leaf CO 2 /H 2 O exchange and water use efficiency.

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