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gen addition amplifies the nonlinear drought response of grassland productivity to extended growing-season droughts. Ecology 102(11):e03483.

Global change factors, such as variation in precipitation regimes and nitrogen (N) deposition, are likely to occur simultaneously and may have profound impacts on the relative abundance of grasses differing in functional traits, such as C 3 and C 4 species. We conducted an extreme drought and re-watering experiment to understand differences in the resistance and recovery abilities of C 3 and C 4 grasses under different N deposition scenarios. A C 3 perennial grass ( Leymus chinensis ) and two C 4 grasses (annual species Chloris virgata and perennial species Hemarthria altissima ) that co-occur in Northeast China were selected as experimental plants. For both C 3 and C 4 grasses, N addition caused a strong increase in biomass and resulted in more severe drought stress, leading to a change in the dominant photosynthetic limitation during the drought periods. Although N addition increased antioxidant enzyme activities and protective solute concentrations, the carbon fixing capacity did not fully recover to pre-drought levels by the end of the re-watering period. N addition resulted in lower resilience under the drought conditions and lower resistance at the end of the re-watering. However, N addition led to faster recovery of photosynthesis, especially in the C 3 grass, which indicate that the effect of N addition on photosynthesis during drought was asymmetric, especially in the plants with different photosynthetic nitrogen use efficiency (PNUE). These findings demonstrated that nitrogen deposition may significant alter the susceptibility of C 3 and C 4 grass species to drought stress and re-watering, highlighting the asymmetry between resistance and resilience and to improve our understanding about plant responses to climate change.

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 Leymus chinensis 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 CO2 assimilation rate and ecosystem C exchange [gross ecosystem productivity (GEP), ecosystem respiration and net ecosystem CO2 exchange]. In contrast, the N addition treatment increased aboveground phytomass, GEP and net ecosystem CO2 exchange. The effects of N addition on the drought susceptibility of the L. chinensis meadow ecosystem varied with drought severity. The N addition treatment alleviated drought-induced suppression of CO2 exchange at the leaf and ecosystem levels in the middle of the drought period, whereas it exacerbated drought-induced suppression of the CO2 exchange and aboveground phytomass on the community level at the end of the drought period. Given that dominance by L. chinensis is a characteristic of the studied ecosystem, knowledge of the traits of L. chinensis 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 L. chinensis meadow ecosystem to further droughts by increasing carbon allocation to roots and therefore root–shoot ratios.

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