The steppe ecosystem is the largest terrestrial ecosystem in China, accounting for about 40% of the land area (Galloway et al. 2008). Though the steppe represents a vast carbon pool important for the sequestration of atmospheric carbon, the steppe ecosystem is highly vulnerable to climate change and human activities (Wang et al. 2022). As perhaps the paradigmatic steppe vegetation, Stipa breviflora communities are generally sparse, leading to low grass cover, fragile ecological environment, large annual fluctuations in productivity, and a particular sensitivity to disturbance (Zhao, 2004). With increasing pressure from grazing and intensified land management, these fragile ecosystems have been seriously degraded, raising concerns about their sustainability in the Anthropocene. The carbon pool of the steppe ecosystem is about twice that of the atmospheric carbon pool (Fang et al., 2007). Plants absorb CO2 from the atmosphere through photosynthesis and convert it into organic matter fixed in the steppe, a process that can be estimated using gross ecosystem productivity (GEP). This carbon is stored as biomass in grasslands to provide material and energy for the steppe’s main consumers (Cleveland et al., 2006; Xu et al., 2008). However, not all of the carbon that is absorbed from the atmosphere is converted to biomass. Part of this fixed carbon is released into the atmosphere by the respiration of plants, animals, and microorganisms, the sum of which is often referred to as ecosystem respiration (ER). the other part the net ecosystem carbon exchange (NEE) represents the carbon stored or released by the ecosystem and its positive or negative value depends on the balance of GEP and ER (Christensen et al., 2004; Baldocchi, 2008).
Precipitation is an important factor in the carbon exchange of steppe ecosystems. According to the United Nations Intergovernmental Panel on Climate Change (IPCC), extreme precipitation events are likely to be more intense and more frequent, and precipitation changes will directly affect carbon fluxes (IPCC, 2014; Lü et al., 2007). Many studies have examined the response of ecosystem CO2 fluxes to precipitation, and in general suggest that increased precipitation tends to increase NEE, ER and GEP (Cheng et al., 2010). The change range of GEP, however, may not be equal to or even proportionate to the change range of ER as precipitation increases. The direction of changes in NEE hence isn't always the same as the direction of GEP (Yan et al., 2011). For example, some studies have shown that as precipitation increased, GEP increased more than ER, leading to a negative change in NEE (Huxman et al., 2004; Patrick et al., 2007; Chen et al., 2009); however, other studies have shown that the increase in GEP was offset by a corresponding increase in ER, resulting in no change or a positive change in NEE (Verma et al., 2009; Risch et al., 2007). Hao (Hao et al., 2017) found that ER increased significantly with increasing precipitation, resulting in a significant decrease in NEE. Ru (Ru et al., 2017) believes that during the plant growing season, the increase in precipitation can lead to a significant increase in both NEE and ER. In general, precipitation (and thus soil moisture) regulate seasonal and interannual variations in GEP and ER, and subsequently NEE (Yan et al., 2011). In at least one setting, the interannual variation of NEE could be estimated by measuring the difference in mean annual precipitation (Yan et al., 2011). In another, seasonal mean soil CO2 flux was found to decrease by 8% under reduced precipitation (Harper et al., 2005). Clearly, precipitation plays a significant role in regulating NEE.
The steppe ecosystem can be divided into C3 and C4 plants according to the different photosynthetic pathways. The growth of the two has the differentiation of time niche. Due to the different physiological processes and the different structure of the solution, the utilization efficiency of the two is different, especially the water resources. Drought and heat, when applied alone, lowered plant performance (Judith et al., 2024). Aboveground net primary productivity of steppe exhibited consistent evidence for a linear response to water availability (Andrew et al., 2018). The increase of precipitation significantly increased the biomass of plants (Ma et al., 2017). Niu (Niu et al., 2005) found that the seasonal distribution of precipitation changed photosynthetic characteristics and interspecific competition of C3 and C4 plants. Seasonal variation in precipitation will increase the risk of uncoupling between water availability and plant community water requirements, and ultimately affect ecosystem carbon fluxes by reducing plant water use efficiency (Zeppel et al., 2014). But the greater sensitivity of aboveground net primary productivity to drought support has not been proven (Daniel et al,. 2020).
The water cycle has been significantly perturbed at both regional and global scales by climate change and intensified land use (Zika et al., 2015; Guo et al., 2020). Evapotranspiration (ET) rates reflect the local water cycle, and can be a measure of the actual water use of steppe ecosystems (Zhang et al., 2015; Xin et al., 2014; Mi et al., 2017). The surface ET of the steppe ecosystem is known to vary greatly, both a possible explanation and reflection of the fragility of the ecosystem (Xin et al., 2014). Water use efficiency (WUE) is one way to measure the coupling of the ecosystem carbon-water cycle. WUE refers to the amount of CO2 fixed or dry matter produced by plants that consume a unit mass of water. As a consequence, WUE can be used to evaluate theimpact of climate change on ecosystem-level water relations (Hu et al., 2009; Shi et al., 2020). It is a key factor and (Du et al., 2018). Climate change willobviously precipitation, interception, soil evaporation and vegetation transpiration, all directly or indirectly affecting WUE (Chang et al., 2020). It has already been established that spatial variability of climatic factors is one of the important factors affecting the spatial distribution of WUE (Shi et al., 2020). In a similar vein, Xu (Xu et al., 2019) has already identified that the regional distribution of WUE in arid regions can be predicted using a drought index, further reinforcing that climate (re: drought) can regulate WUE.
At present, studies on ecosystem carbon and water exchange have been carried out in tropical savanna (Xuanlong et al., 2020), Mediterranean steppe (Lafuente et al., 2020), alpine meadow (Zhu et al., 2016), typical steppe (Zhu et al., 2016), desert steppe (Jin et al., 2018) and many other regions. Global warming and changing precipitation regime are expected to profoundly impact water-limited systems like the arid and semiarid steppe (Niu et al., 2008). Low annual rainfall, sparse vegetation, and poor management, have significantly reduced the carbon sequestration potential of the steppe (Zhou et al., 2011; Zhao, 2004).
A potential management solution is increasing water use efficiency in the steppe. survival, growth and reproduction of steppe vegeation is intensely limited by water, which means that increasing precipitation will lead to increased carbon storage in biomass (Wu et al., 2016). However, as we have establishes, precipitation can also lead to changes in the soil water status of the ecosystem, and control the carbon source-carbon sink function of terrestrial ecosystems (Wei et al., 2021; Wu et al., 2022). Differences in annual growing season precipitation and precipitation distribution are known to impact soil moisture availability and influence vegetation productivity, leading to changes in C exchange and WUE (Zhao et al., 2014; Zhang, 2019). Given the sensitivity of growth to rainfall on the semi-arid steppe, increased precipitation may stimulate carbon exchange more than it does ET, which would be evidenced by overall higher WUE (Niu et al., 2008).
The ecological balance of the steppe has been destroyed by drought and intense land use (Jin et al., 2013). Exploring the influence of precipitation on the dynamic balance of carbon source and carbon sink in the steppe and analyzing the variation of aboveground biomass of C3 and C4 species, carbon and water fluxes under different precipitation gradients at different times play a very important role in improving the carbon exchange efficiency of the steppe ecosystem.