1.Introduction
Litter plays an important role in maintains ecosystem productivity, enhancing soil fertility, improving soil texture, and optimizing the structure and metabolic characteristics of microbial communities in the system (Canessa et al., 2021, Huangfu and Wei, 2018). Litter decomposition is a crucial terrestrial ecosystem process which affects the nutrients cycling. (Chapin et al., 2002; Handa et al., 2014; Liu et al., 2016). Through material exchange and nutrient flow with soil, litter decomposition connects aboveground plants and soil and offers an essential way for plant-soil interaction (Chapin et al. 2002; Gregorich et al., 2016; Xiao et al., 2014). The changes of litter quality, including C, N, P concentration, cellulose and lignin can reflect the decomposition rate of litter matter to some extent (Yang et al., 2017). The decomposition rate is mainly controlled by climate, the plant properties, soil environment and microorganism activities, and most of these factors are affected by ecosystem nitrogen (N) deposition. (Van Diepen et al., 2015; Zhou et al., 2017; Liu and Greaver 2010). In addition, plant growth is usually limited by nitrogen or phosphorus, and is often even limited by both phosphorus concentration and effectiveness in the late stages of ecosystem regeneration, which will seriously affect the decomposition process of litter. (Vitousek et al. 2010; Jiang et al., 2016). Hence, understanding the effects of N and P input on litter decomposition is imperative.
The process of decomposition of litter by extracellular enzymes produced by soil microorganisms and plant roots is the key to promote biogeochemical cycling in terrestrial ecosystems (Yue et al., 2020). By changing soil physical and chemical properties, nitrogen and phosphorus can affect the effective use of soil nutrient by extracellular enzymes and regulate the nutrient uptake of plant, and ultimately affect the results of litter decomposition and feedback process (Song et al., 2014; Amend et al., 2015; Creamer et al., 2015; Waldrop et al. 2004). At the same time, litter decomposition is a continuous biodegradation process, the factors that dominate the mass loss of litter may change as the decay time of litter progresses (Huo et al., 2019). The addition of exogenous nutrients can usually accelerate the decomposition of litter in the early stage by providing nutrients necessary for microbial activities (Cuchietti et al., 2014), and slow the the decomposition of litter in later stage by change the microbial community and reduce enzyme activity (Cuchietti et al., 2014; Gill et al., 2021; Widdig et al., 2020). Therefore, it is essential to investigate the dynamic changes of litter decomposition and soil enzyme activity over time under N and P addition.
As a high altitude and unique type of grassland ecosystem in the world, alpine grasslands are extremely sensitive to global environmental changes and play an important role in regulating the global ecological environment (Chen et al., 2014). The alpine grassland of northwest Sichuan is located in the alpine semi-humid region on the southeastern edge of the Tibetan Plateau, with the world’s largest plateau peat swamp wetlands, and its regional ecological and environmental status is very important (Hoover et al., 2016; Suseela et al., 2018). Over the years, due to the intensity of human activities and overgrazing, the alpine grasslands of northwest Sichuan have faced important ecological problems such as the degradation of grassland functions and severe desertification (Jiang et al., 2018). Grassland desertification control has become an important task for regional ecological restoration. Because of its cold tolerance and drought resistance, Salix cupularis can prevent cold winds from fixing sand, improve the alpine ecological environment, promote the growth of other herbaceous plants, and gradually form a stable shrub ecosystem in the ecological restoration process (Zhang et al., 2016; Yang et al., 2017). Therefore, we conducted a N and P addition experiment in Salix cupularis ecosystem in alpine sandy land in northwest Sichuan. We aim to reveal the response of Salix cupularis litter decomposition and soil enzyme activities to different exogenous nitrogen and phosphorus additions and their inner relationship, and provide scientific basis for the formulation of ecological restoration measures as well as natural resource management and regional sustainable development in northwest Sichuan.