Grasslands, comprising open grassland, grassy shrublands and savannas, cover nearly 40% of the world’s land area and provide a wide range of ecosystem services to humans (O'Mara 2012; Bardgett et al. 2021). They store approximately one-third of the total carbon (C) in terrestrial ecosystems and most of the C is stored within 1 m soil layers, which contributes significantly to the mitigation of global climate change (Wang et al. 2016; Xu et al. 2015). In addition, grasslands also play an important role in the global nitrogen (N) biogeochemical cycle (Chen et al. 2021; Risch et al. 2019). Grasslands are vulnerable terrestrial ecosystems due to overexploitation (Liu et al. 2020). C and N cycles in global grassland ecosystems are sensitive to global climate change and land-use change, especially extreme precipitation and global warming in the temperate zone (Wang et al. 2016). However, it is difficult to explore the C and N dynamics by directly measuring the change of C and N stocks because of their relatively slow change processes. Recently, with the rapid development of stable isotope ratio mass spectrometry, stable C (δ13C) and N (δ15N) isotope compositions, reflecting C and N transformation processes in plant-soil systems, have become an important tool to study the C and N biogeochemical cycles in terrestrial ecosystems (Dong et al. 2018; Han et al. 2020; Xia et al. 2021).
Nowadays, stable δ13C isotope has emerged as useful tool to assess the magnitude and distribution of plant productivity, water use efficiency and soil C turnover rate (Mcdowell et al. 2010; Wu et al. 2018). Previous studies have indicated that plant δ13C composition is mainly controlled by plant’s photosynthetic pathway, and soil δ13C composition mainly depends on the plant-derived organic C and SOC decomposition (An and Li 2015; Dixon et al. 2010). Plant community composition has a distinct influence on the plant δ13C (Chen et al. 2021; Luo et al. 2018). For example, the plant community composition with more C4 species will lead to higher stable δ13C values (Wu et al. 2019). In addition, forbs with higher water use efficiency also have relatively higher δ13C values compared to graminoids and sedges (Liu et al. 2018). Besides, the soil stable δ13C composition has become an important integrative measure of soil organic carbon (SOC) input and output (Bird et al. 1996; Wang et al. 2017). Soil stable δ13C composition depends not only on that of plant residuals, but also on synthetic action of abiotic and biotic factors (e.g., SOC decomposition, microbial mobilization and immobilization) (Wu et al. 2019; Yang et al. 2015). Besides, the stable δ13C composition can differ significantly among various layers within the same soil profiles (Brunn et al. 2014; Carvalhais et al. 2014; Wang et al. 2017). Up-to-date, studies in tropical, temperate and tundra regions have demonstrated that soil physicochemical properties (e.g., pH, C/N ratio and soil moisture) and climatic factors (e.g., mean annual temperature and mean annual precipitation) regulate biogeochemical processes in soil and influence interactions between soil and plants, and can shape the spatial and temporal distribution of stable δ13C composition in the terrestrial ecosystem (Nel et al. 2018; Yang et al. 2015).
Compared to the C cycle, the N cycle is more complex due to the various influencing factors along different environment gradients (Craine et al. 2015). Numerous studies have demonstrated that stable δ15N values in terrestrial ecosystems are positively correlated with mean annual temperature (MAT) but negatively related to mean annual precipitation (MAP) (Craine et al. 2015; Nel et al. 2018). Besides, stable δ15N composition is also influenced by soil C and N contents and other soil physicochemical properties (Craine et al. 2015; Yang et al. 2013). For example, ammonia (NH3) volatilization will accelerate when soil pH is high, which leads to an abiotic gaseous N loss and higher soil δ15N values (Booth et al. 2005; Chen et al. 2021; Yang et al. 2013). Generally, stable δ15N signals the openness of N biogeochemical cycle in terrestrial ecosystems (Boeckx et al. 2005). The N input in terrestrial ecosystems by livestock manure, biological N fixation and N deposition could alter the stable δ15N composition in plant-soil system (Fang et al. 2011). The stable δ15N composition of plant also depends on the various preferences of species to the available N forms and the fractionation during plant mycorrhiza transfer process (Chen et al. 2021; Wu et al. 2019; Xu et al. 2011). The soil stable δ15N is mainly controlled by plant N uptake and microbial mediated N-cycling processes (Golluscio et al. 2009).
Being one of the most widely-distributed terrestrial ecosystems, grasslands play a crucial role in the global terrestrial biogeochemical cycles of C and N (Yan et al. 2017; Yao et al. 2018). Grasslands in China are mainly distributed in arid and semi-arid regions, covering an area of approximately 4 × 108 ha and accounting for 41.7% of the country’s territory, and contain different grassland types adapted to various climatic conditions and altitudes. Previous studies have shown that the stable δ13C and δ15N compositions of plant and soil are mainly controlled by climatic variables and soil characteristics (Wu et al. 2018) and increased our understanding of C and N cycles at both regional and global scales. However, there are few studies on the role of climatic factors in regulating the stable δ13C and δ15N compositions in temperate grassland ecosystems. Therefore, this study focuses on the δ13C and δ15N of plants and soils in grasslands of northern China. We aimed to explore the spatial patterns of δ13C and δ15N in plant–soil system of grasslands in northern China and their driving factors. The results will provide scientific references for future research on the C and N biogeochemical cycles of grassland ecosystem.