Abstract
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 C3 and C4 species. We conducted an extreme drought and re-watering experiment to understand differences in the resistance and recovery abilities of C3 and C4 grasses under different N deposition scenarios. A C3 perennial grass (Leymus chinensis) and two C4 grasses (annual species Chloris virgata and perennial species Hemarthria altissima) that co-occur in Northeast China were selected as experimental plants. For both C3 and C4 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 C3 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 C3 and C4 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.
Highlights
The intensification of the hydrologic cycle (Knapp et al, 2008; Smith, 2011; IPCC, 2013; Knapp et al, 2015) is expected to increase the frequency and magnitude of climate extremes, such as severe drought and intense rainfall (Karl et al, 1995; Hoerling and Kumar, 2003; Reichstein et al, 2013)
The positive effects of N addition on water-use efficiency (WUE) disappeared at the end of the drought treatment, which may be attributed to greater biomass (Supplementary Figure 4) and more water consumption by transpiration associated relatively severe drought stress as compared to the unfertilized plants (Figures 1D–F)
We found a sharp reduction in Soil water content (SWC) during the drought treatment (Figures 1A–C) and the percent change in SWC in the fertilized plants was higher than in the unfertilized plants (Figure 6A), which mainly resulted from more plant transpiration caused by more biomass in the fertilized pots (Supplementary Figure 4)
Summary
The intensification of the hydrologic cycle (Knapp et al, 2008; Smith, 2011; IPCC, 2013; Knapp et al, 2015) is expected to increase the frequency and magnitude of climate extremes, such as severe drought and intense rainfall (Karl et al, 1995; Hoerling and Kumar, 2003; Reichstein et al, 2013). C3 and C4 plants may respond differently to altered precipitation and N deposition due to functional differences in anatomical structure, photosynthetic processes, responses to temperature, and water and N use efficiency (Long, 1999; Yuan et al, 2007; Niu et al, 2008a; Ripley et al, 2010; Taylor et al, 2014) Such differences could cause substantial changes in vegetation composition and ecosystem functions (Still et al, 2003). Other studies found that leaf carbon assimilation in C4 species is sensitive to variation in soil water content, such that they can sometimes even lose their photosynthetic advantage (Ripley et al, 2007, 2010; Ibrahim et al, 2008; Taylor et al, 2011)
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