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.