Effects of Nitrogen Addition on the Drought Susceptibility of the Meadow Ecosystem Vary with Drought Duration.

It is not clear yet how extreme drought and nitrogen (N) deposition influence grassland ecosystem functions when they are considered together, especially in complex field conditions. To explore the response of the meadow ecosystem to manipulated extreme drought (45 days), N addition and their interaction, we measured leaf photosynthetic characteristics, aboveground phytomass on the community level and ecosystem C exchange in different treatments at the middle and the end of the drought period. The extreme drought treatment decreased the leaf net CO assimilation rate and ecosystem C exchange [gross ecosystem productivity (GEP), ecosystem respiration and net ecosystem CO exchange]. In contrast, the N addition treatment increased aboveground phytomass, GEP and net ecosystem CO exchange. The effects of N addition on the drought susceptibility of the meadow ecosystem varied with drought severity. The N addition treatment alleviated drought-induced suppression of CO exchange at the leaf and ecosystem levels in the middle of the drought period, whereas it exacerbated drought-induced suppression of the CO exchange and aboveground phytomass on the community level at the end of the drought period. Given that dominance by is a characteristic of the studied ecosystem, knowledge of the traits of and its response to multiple global change drivers will be crucial for predicting future ecosystem functions. Furthermore, increasing N deposition may affect the response of the meadow ecosystem to further droughts by increasing carbon allocation to roots and therefore root-shoot ratios.