Arid and semiarid ecosystems around the world are experiencing sporadic rain events, which play underlying roles in determining microbial activity and resulting effects on biogeochemical cycling (Collins et al., 2008; Blazewicz et al., 2014). Soil microbial activities decrease during dry conditions with rewetting episodes, causing a sharp rise in microbial respiration and activity (Birch, 1958). Soil organic matter decomposition rates are linked to microbial activity (Moorhead and Sinsabaugh, 2000) and facilitate nutrient cycling and organic matter turnover in dryland environments. The use of soil microbial extracellular enzyme activities (EEAs) could play an important role in monitoring the response of microbial organisms to climate change under field conditions, as EEAs reflect the metabolic requirements of the microbial community (Caldwell, 2005).
Soil moisture is a key factor controlling microbial activity and thus carbon cycling (Borken and Martzner, 2009). At global scales, temperature seems to be more influential than moisture in determining EEA (German et al., 2012); however, at regional scales, changes in soil moisture may become more important (Brockett et al., 2012). EEA normally increases with temperature up to an optimum of 40 °C (Stone et al., 2012) and can also be influenced by soil water contents through changes in the diffusion rate of substrates and inhibitory compounds (Zak et al., 1999; Toberman et al., 2008).
Increased precipitation can enhance soil moisture availability, thereby stimulating plant productivity and microbial-driven processes (Vargas et al., 2012). Studies have shown that the frequency and magnitude of rain events alter nitrogen mineralization, transformation of organic matter, and carbon loss from dryland soils (Austin et al., 2004; Bell et al., 2009; Vargas et al., 2012). Therefore, small precipitation events may differentially influence plant and microbial activities, thereby affecting soil EEA.
Shifts in plant species composition could also lead to additional, indirect effects of shifting rainfall regimes on ecosystem functioning and biogeochemical cycling (Smith et al., 2009). For example, plant communities and belowground properties might be affected (Briggs et al., 2005), which might trigger shifts in soil faunal composition (Yannarell et al., 2014), subsequently affecting biogeochemical processes (McKinley et al., 2008). Because soil EEAs are highly sensitive to environmental changes and are strongly regulated by the presence of plants, they could serve as indicators of various changes in the plant-soil system (Burns et al., 2013). Despite the important roles these microbial enzymes play in maintaining ecosystem functioning in terrestrial environments (Bardgett and van der Putten, 2014), little is known about how they will respond to precipitation change in shrub-encroached grasslands. Shrub encroachment has caused a vegetation shift in arid and semiarid grasslands around the world, leading to marked changes in ecosystem structure and function (Eldridge et al., 2011).
For example, soil resources and vegetation cover in Inner Mongolian grasslands have become heterogeneous because more than 5.1 million hectares of grasslands have been impacted by shrub encroachment (Peng et al., 2013; Chen et al., 2015). This has led to a mosaic landscape with shrub patches interspersed with grass patches (Chen et al., 2015). Shrubs generally have wider canopies and more leaf biomass, thus serving as nutrient pumps, with greater deposition of root exudates within the rhizosphere (Eldridge et al., 2011). The combination of greater litter mass and increased resource capture would likely promote higher levels of microbial decomposition, leading to enhanced carbon and nitrogen pools (Throop and Archer, 2008). Few studies have examined the response of soil EEA to altered precipitation (Bell and Henry, 2011; Ren et al., 2017) and shrub encroachment (Maestre et al., 2011), but it is still unclear how soil EEA will respond under future patterns of precipitation and shrub encroachment. In this study, we investigated in situ the response of EEA in a four-year experiment to altered precipitation and shrub encroachment in a temperate grassland in Inner Mongolia, China.
The specific questions of this study are as follows: (1) What is the response of soil EEA to precipitation change in shrub-encroached grasslands? (2) What are the driving factors controlling the soil EEA response to precipitation change in shrub-encroached grasslands? and (3) is soil EEA related to changes in carbon and nitrogen as a result of shrub encroachment?