Seasonal timing regulates extreme drought impacts on CO2 and H2O exchanges over semiarid steppes in Inner Mongolia, China

Authors: HAO, Y.B. - Chinese Academy Of Sciences; ZHANG H. - Chinese Academy Of Sciences; Biederman, Joel; LI, L.F. - Chinese Academy Of Sciences; CUI, X.Y. - Chinese Academy Of Forestry; XUE, K. - Chinese Academy Of Sciences; DU, J.Q. - Chinese Academy Of Sciences; WANG, Y.F. - Chinese Academy Of Sciences

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2018
Publication Date: 8/16/2018
Citation: Hao, Y., Zhang H., Biederman, J.A., Li, L., Cui, X., Xue, K., Du, J., Wang, Y. 2018. Seasonal timing regulates extreme drought impacts on CO2 and H2O exchanges over semiarid steppes in Inner Mongolia, China. Agriculture, Ecosystems and Environment. 266:153-166. https://doi.org/10.1016/j.agee.2018.06.010.
DOI: https://doi.org/10.1016/j.agee.2018.06.010

Interpretive Summary: As global climate continues to change, it is expected that drought will become more common. This is of concern for semiarid grasslands, which are important resources for grazing and habitat. It is unknown how the timing of drought at different periods of the growing season may alter drought impacts. In this work, we used plastic shelters to exclude rainfall for 30 days in the early, middle, or late portions of the growing season. We found that in a grassland which was fenced to exclude grazing, a 30-day drought reduced productivity and water usage regardless of seasonal timing. In a grazed grassland, however, drought early in the growing season had stronger negative impacts on the ecosystem. Measurements of soil moisture and soil temperature suggest the moderating impact of grazing was related to how grazing prevents accumulation of a litter layer of dead grasses on the soil, which can act as a buffer to evaporation. These results show that seasonal timing of drought and land management can interact to regulate how drought impacts semiarid grasslands.

Technical Abstract: Climate models predict a substantial increase in the frequency of extreme drought, suggesting subsequent impacts on the carbon (C) and water cycles. Although many studies have investigated the impacts of extreme drought on ecosystem functioning, it remains unknown how the timing of extreme drought within a growing season may a'ect carbon and water cycling. Here we conducted a 3-year 'eld experiment to investigate the in'uence of seasonal drought timing on ecosystem carbon and water exchange by excluding rainfall (for con- secutive 30 days) during three periods of the growing season (May–June, July–August and August–September) in fenced and grazed sites of a semiarid temperate steppe in Inner Mongolia, China. In the fenced steppe, extreme drought reduced growing-season net CO2 uptake regardless of drought timing, while in the grazed steppe, early-growing season drought caused relatively larger reductions to net CO2 uptake than drought imposed later in the season. The e'ect of extreme drought on evapotranspiration (ET) was similar to that of CO2 exchange at the fenced site, with consistent reductions of seasonally-integrated ET for all treatments compared with the ambient condition. In contrast, at the grazed site, the response of ET to extreme drought was more variable, possibly due to the absence of litter and greater bare ground. Surprisingly, both gross and net carbon uptake declined with increasing ET at the grazed site, while the fenced site showed the positive water-carbon linkage typically seen in semiarid ecosystems. The di'erent responses of CO2 and water exchanges for the fenced and grazed sites were regulated predominately by soil temperature and soil water content. Together, our results show that drought timing within the growing season can signi'cantly alter drought impacts on ecosystem water and CO2 ex- changes, and that grazing management may further mediate the response.