1 INTRODUCTION
Climate change may amplify inter-annual precipitation fluctuation in
terrestrial ecosystems (Alan K. Knapp, Avolio, et al., 2017; Sloat et
al., 2018; W. Zhang & Zhou, 2019), and interactions between
precipitation fluctuation and anthropogenic activities (e.g.
overgrazing, land use change and fertilisation) could have significant
impacts on grassland ecosystem structure and functioning (Ahlborn et
al., 2021; Brambila, Chesnut, Prugh, & Hallett, 2020; Cui et al.,
2020a; Han et al., 2021; L. Huang et al., 2019). Arid and semi-arid
grassland ecosystems generally experience high inter-annual
precipitation variation (Carmona, Mason, Azcarate, & Peco, 2015; Alan
K. Knapp, Ciais, & Smith, 2017; A. K. Knapp & Smith, 2001; Kong et
al., 2013) and anthropogenic activity–induced soil nutrient enrichment
(Ma et al., 2020; Yahdjian, Gherardi, & Sala, 2011). In grassland
ecosystems, arbuscular mycorrhizal (AM) fungi can colonize over 80% of
terrestrial plants, which can improves host plant elements absorption
(Smith & Read, 2008). However, much of our current understanding of
plant community performances in desert steppe in relation to
inter-annual precipitation fluctuation is based on the changes of plant
productivity and diversity (Ahlborn et al., 2021; Han et al., 2021; Alan
K. Knapp, Avolio, et al., 2017) and assessed independently of the
effects of added nutrients and AM fungi (J. Huang et al., 2021; J. Huang
et al., 2016; Qiao et al., 2019). As such, it is critical that we
clarify plant community responses to the potential interactive impacts
of multiple factors such as inter-annual fluctuation in precipitation,
soil nutrient availability and AM fungi. Additionally, a reliable
benchmark is needed to develop a robust understanding of arid grassland
sensitivity using terrestrial ecosystem models (Püschel, Bitterlich,
Rydlová, & Jansa, 2021; Xin Yang, Shen, Badgery, Guo, & Zhang, 2018).
Precipitation in growing season is a vital driver affecting variation in
ANPP and plant community composition in grasslands (Bai, Han, Wu, Chen,
& Li, 2004; Irisarri et al., 2016). However, plant communities can vary
drastically in response to precipitation fluctuation (Hsu, Powell, &
Adler, 2012; Alan K. Knapp, Avolio, et al., 2017; Koerner, Collins,
Blair, Knapp, & Smith, 2014; Reichmann et al., 2018). For example,
annual primary production is more sensitive to inter-annual fluctuation
in precipitation in arid grassland than in humid grassland (Maurer,
Hallmark, Brown, Sala, & Collins, 2020). Furthermore, evidence from
recent field studies shows that environmental factors (e.g. soil
nutrient availability and soil microorganisms) may affect plant
communities in response to inter-annual precipitation fluctuation
(Felton, Slette, Smith, & Knapp, 2020; J. Huang et al., 2016). Thus,
our current konwledge of the responses of plant communities to
inter-annual precipitation fluctuation may be impeded by a limited
understanding of multiple factors.
Anthropogenic input of nitrogen (N) and phosphorous (P) can
synergistically promote aboveground net primary productivity (ANPP),
reduce plant species richness and change the dominance of plant species
in nutrient-limited grasslands (Suding et al., 2005). The impacts of
nutrients input on ANPP and plant diversity rely on the abundance and
nutrient uptake strategy of plant species; N input favours the dominance
of perennial grasses rather than other plant species (Avolio et al.,
2014; Ma et al., 2020; Stevens et al., 2020). Nevertheless, P input
alone promotes high abundance and shoot biomass in legumes because they
have higher P demands than other plant species (Benner & Vitousek,
2007). Thus, it is necessary to consider the nutrient responses of
various plant species to effectively interpret how the addition of
nutrients changes plant communities.
Nutrient supply to plants can also alter the structure and functioning
of AM fungi via changing the exchange of carbon (C) and P (Johnson,
Wilson, Wilson, Miller, & Bowker, 2015; Kiers et al., 2011). Field
studies found that N and/or P input could weaken the impacts of AM fungi
on ANPP and plant species richness (G. W. Yang et al., 2014; T. Zhang &
Feng, 2021). In general, AM fungi promote ANPP when soil available P is
limited and vice versa (Collins & Foster, 2009; Johnson et al.,
2015; G. W. Yang et al., 2014). In contrast, AM fungi have a neutral
impact on ANPP when N is limited but reduce it when N is enriched (Jiang
et al., 2018). Relatively high soil nutrient availability, especially
high P, can decrease belowground C allocation in plants, thus directly
reducing C supply for AM fungi (Zheng et al., 2018) and changing this
symbiotic relationship from mutualism to parasitism (Jiang et al., 2018;
Johnson, 2010).
Desert steppe generally has low ANPP and plant diversity because of soil
water and nutrients (N and P) are strangely limited (Alan K. Knapp,
Ciais, et al., 2017). Precipitation in the growing season in the desert
steppe in China varies drastically between years, ranging from 111.4 to
382.4 mm (Zhao et al., 2019). AM fungi can still benefit plant N and P
nutrition in this water- and nutrient-limited conditions (Qiao et al.,
2019). Thus, we conducted a two-year field study to evaluate the
inter-annual response of ANPP and plant species composition to nutrient
addition (N, P and N+P) and AM fungi suppression in desert steppe. We
examined the following three hypotheses: (1) inter-annual precipitation
fluctuation alters ANPP and plant species composition response to N and
P addition and AM fungal suppression; (2) the addition of nutrients
enhances ANPP by changing the plant growth response in combination with
AM fungi; and (3) inter-annual precipitation fluctuation changes the
dominant species in a community regardless of nutrient addition and
mycorrhizal suppression.