Abstract
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.
Highlights
Anthropogenic activities strongly influence environmental change, including discrete climate extremes and chronic environmental changes (e.g., N deposition and increasing CO2 concentration) (IPCC, 2013; Zhu et al, 2016)
The N addition, drought and combination of N addition and drought treatment had a strong effect on the soil water content, and the soil water contents in the N, D, and DN plots were 28, 43, and 52% lower than that in the C plots on day 196, respectively
Our results show that N addition exacerbated drought-induced suppression of aboveground phytomass under extreme drought stress, but may enhance drought resistance to further drought events due to increased root phytomass and an increased root–shoot ratio
Summary
Anthropogenic activities strongly influence environmental change, including discrete climate extremes (e.g., drought and torrential rainfall) and chronic environmental changes (e.g., N deposition and increasing CO2 concentration) (IPCC, 2013; Zhu et al, 2016). Drought induces decreases in the soil water content and increases in plant water deficit, which cause subsequent decreases in the leaf carbon assimilation rate and soil available nutrients, leading to plant N limitation and earlier senescence and mortality of tissues, further reducing plant aboveground productivity and net ecosystem CO2 exchange (NEE) (Durand et al, 2010; Yi et al, 2015; Hofer et al, 2016; Hoover et al, 2017) Grassland ecosystems face both extreme drought events and other global change drivers, of which increasing N deposition strongly influences aboveground productivity and carbon cycles (Niu et al, 2010; Ladwig et al, 2012).
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