Climate variability has a tremendous impact on natural ecosystems and human society. The Intergovernmental Panel on Climate Change (IPCC 2018) predicts that a minor difference between mean global temperature increase of 2°C above pre-industrial ranges, compared to 1.5°C would lead to more extreme heatwaves, more intense precipitation, more frequent droughts, increased sea-level and huge loss of species and ecosystems. Researches also corroborate that precipitation has witnessed extreme variations during the last hundred years (Alexander et al. 2006; Trugman et al. 2018; Paschalis et al. 2020). Changes in water availability triggered by variation in precipitation regulate plant community dynamics and ecological characteristics (Peralta et al. 2019; Wu et al. 2016; Yang et al. 2011). For instance, modifications in the rainfall pattern impact species richness, species composition (Libalah et al. 2020; Báez et al. 2013; Cleland et al. 2013) and net primary productivity of the ecosystem (Heisler-White et al. 2009; Fay et al. 2003).
Nitrogen (N) deposition in interaction with rainfall variability has a profound impact on terrestrial ecosystems (Karl and Trenberth, 2003; Xu et al. 2012). Rainfall and N are vital ecological components in defining the structure and function of the ecological community, specifically in water and N-constrained grasslands (Epstein et al. 2002). N deposition in terrestrial ecosystems is predicted to boom to 200 Tg N yr − 1 by 2050 because of increased N-uses in industries and agriculture (Basto et al. 2018). Nitrogen enrichment will probably affect biomass production (Stevens 2019; Epstein et al. 2002; Poorter et al. 2012b). The consequences of N augmentation on forest biomass were evaluated and analyzed in preceding research (Huang et al. 2013; Qin et al. 2018). Previous studies also emphasized the effect of rainfall variability on aboveground biomass (AGB) of grasslands (IPCC 2014; Sala et al. 2015), but the consequences of N addition under different rainfall scenarios on grassland biomass still stays unknown.
In a terrestrial ecosystem, grasslands approximately cover 25% of the land surface on earth (Easterling, 2000). In comparison to forests, grasslands are more sensitive to climatic variations, even for a short duration (Maurer et al. 2020; Eziz et al. 2017), and therefore they become crucial to study in the situation of ongoing global climate change. In grasslands, biomass allocation plays a key role in determining the response of plant growth, and variation in it would lead to a substantial change in the structure and function of the grassland ecosystem (Poorter et al. 2012a,b). Changes in precipitation could affect plant productivity by altering plant physiological responses and nutrient availability over relatively short-time scales (Xu et al. 2010). Many studies have focussed on understanding the effects of N-addition or rainfall variability on aboveground biomass in grasslands (Su et al. 2013; Lu et al. 2014; Xu et al. 2015). For a long time, plant community ecologists have been interested in determining that how different plant functional groups (grasses, forbs and legumes) impact ecosystem productivity. Yet there are fewer studies that have focussed on the effect of both rainfall variability and N- addition on relative changes in AGB of different plant functional groups in grassland ecosystems. A study by Mowll et al. (2015) in a semiarid temperate grasslands report that aboveground net primary productivity (ANPP) is restrained due to N availability and climatic variables, which include rainfall and air temperature. On that account, N enrichment was found useful in increasing ANPP, but at the same time increase in ANPP with the aid of N enrichment also causes a lack of plant richness (Midolo et al. 2019; Clark and Tilman, 2008). Species which shared common functional traits also manifested similar responses to rainfall variability and N enrichment (Tian et al. 2020; Cleland et al. 2013). Nitrogen deposition increased the growth of grasses, but reduced the growth of forbs, leading to a decline in forbs richness in the grassland ecosystem (Tian et al. 2016; Tian et al. 2020). Certain plant functional groups can increase the aboveground and belowground biomass by 300% in comparison to monoculture species (Tilman et al. 2001). Low precipitation and high temperature caused the shallow and fibrous-rooted grasses to suffer (Grant et al. 2017), while deep-rooted forbs and legumes maintained their productivity. Hence, in the light of prevailing global climate change driven by rainfall variability and N- deposition, it becomes important to understand the adaptive mechanisms of different plant functional groups in tropical grassland ecosystems. Therefore, we conducted a coupled rainfall-nitrogen experiment to investigate the changes in aboveground biomass (AGB) and belowground biomass (BGB) of the grassland community along with the changes in AGB of three different plant functional groups viz. grasses, forbs and legumes due to nitrogen (N) treatments under different rainfall conditions. We addressed the subsequent questions: (1) how does the allocation of AGB and BGB vary with nitrogen treatments under different rainfall conditions? (2) How does the AGB of grasses, forbs and legumes differ in their response to nitrogen treatments under different rainfall conditions?