Extracellular enzymes (EEs) facilitated by microorganisms to acquire energy and nutrients (Sinsabaugh et al. 2009; Wallenstein and Burns 2011), are the primary mediators of biogeochemical cycling through soil organic matter (SOM) decomposition (Sinsabaugh et al. 2008; Burns et al. 2013; Luo et al. 2017). Soil hydrolytic extracellular enzyme activities (EEAs) can be used as indicators of change in soil function due to land-use practices (Trasar-Cepeda et al. 2008) and climate-linked environmental stresses (Schimel et al. 2007; Henry 2012); their activity can detect changes sooner than other soil analyses (Acosta-Martínez et al. 2007). Numerous studies have shown that EEAs were sensitive to changes in soil characteristics due to change in land-use practices (Bandick and Dick 1999; Acosta-Martínez et al. 2003a; Acosta-Martínez et al. 2003b; Wallenius et al. 2011; Stauffer et al. 2014; Tischer et al. 2015), fluctuating groundwater table depths (Pulford and Tabatabai 1988; Freeman et al. 1996; Wiedermann et al. 2017), and variations in salinity (Frankenberger and Bingham 1982; García et al. 1994; García and Hernández 1996; Pan et al. 2013; Shi et al. 2019). Therefore, EEAs have been suggested as potential indicators of soil quality, useful for understanding soil ecosystem functioning (Bandick and Dick 1999).
Wetland soils can serve as a reservoir of water, carbon, and nutrients, with fluctuations in water levels influencing the type and intensity of biogeochemical processes. The prairie pothole region (PPR) contains millions of small wetlands that support prairie grasses, habitat for migratory birds, productive agricultural land, and many further ecosystem services (Richardson and Arndt 1989; Mitsch and Gosselink 2000). With its semi-arid climate, the PPR is composed of a hydrologically distinct and highly sensitive wetland ecosystem that is vulnerable to land-use and climate change (Johnson et al. 2005; Johnson et al. 2010; Werner et al. 2013). Soils of this region experience both drought and deluge (Winter and Rosenberry 1998; Johnson et al. 2004). There is potential for future drier climatic conditions (Millett et al. 2009), jeopardizing the ecosystem services provided by the PPR due to the alteration of shallow groundwater induced by rapid evaporation and increased transpiration through wetland riparian zone land-use practices (Poiani and Johnson 1991; Poiani and Johnson 1993). Intensive agricultural land-use practices, including wetland drainage, have disturbed the native vegetation and soils throughout the PPR (Guntenspergen et al. 2002; Bartzen et al. 2010; McCauley et al. 2015). Hence, stresses related to land-use can diminish the functionality and capability of wetland ecosystems to sustain soil health and environmental quality (Rosen et al. 1995). Furthermore, hydrology research in Prairie wetlands suggests that surrounding land-use changes can significantly affect the water balance due to greater potential evapotranspiration vs. precipitation (Conly and van der Kamp 2001).
Short rotation willow (SRW) is a high biomass producing crop that was introduced in Canada during the early 1990’s as an environmentally sustainable land-use practice fulfilling multiple ecological benefits including the sustainable supply of bioenergy feedstock (Amichev et al. 2014b); however, this practice is relatively new to the PPR of Saskatchewan (Amichev et al. 2014a). Establishing fast-growing SRW within the riparian zones can impact shallow groundwater hydrology and the soil water balance (Mercau et al. 2016; Caldwell et al. 2018). During the growing season, shallow groundwater can be depleted by speedy evapotranspiration from the wetland vegetation in the riparian zones (Hayashi et al. 2016), which might become critical for agricultural production and wetland management in this region and globally (Fan et al. 2013).
Land-use practices that supply elevated levels of crop residues can significantly increase the soil EEAs (Jordan et al. 1995; Bandick and Dick 1999). In cultivated soil systems, EEAs were higher where organic residues were added as compared to treatments without organic amendments (Bandick and Dick 1999); and showed that the soil β-glucosidase (BG) activities best reflect the management effects on soil quality. Soil EEAs (except α- and β-glucosidase, and α- and β-galactosidase) remained higher in the adjacent pasture (PA) compared to annual crop (AC) production (Bandick and Dick 1999). In a study with SRW compared to forestry, pasture, and agroecosystem, high laccase and phosphatase activities were observed in the forest soil compared to the other land-uses and did not significantly differ between the SRW and the other land-uses (Stauffer et al. 2014).
Salinization is a pressing environmental challenge globally (Rengasamy 2006) and a significant threat to agricultural productivity across the PPR (Eilers et al. 1997; Nachshon et al. 2014). Precipitation events contribute to shallow groundwater fluctuations and dilution of soil salinity (LaBaugh et al. 1995), whereas drought periods can concentrate salts in riparian soils (Levy et al. 2018). This oscillation in salinity (Euliss and Mushet 1996; LaBaugh et al. 2018) can also potentially affect soil biogeochemical cycling (Holloway et al. 2011; Evenson et al. 2018) through nutrient imbalances and the lower osmotic potential of the soil solution.
Elevated soil salinity can reduce EEAs directly via the effects of osmotic potential and specific ions on enzymes and indirectly by lowering microbial biomass (Rath and Rousk 2015). For example, in a laboratory experiment, Frankenberger and Bingham (1982) found that soil β-glucosidase, phosphatase, sulfatase, amylase, and dehydrogenase activity decreased with increasing electrical conductivity (EC); however, the degree of inhibition varied among the EEAs, and the nature and amounts of salts added. Egamberdieva et al. (2011) observed that soil glucosidase, alkaline phosphatase (AP), phosphodiesterase, urease, and protease activity were inhibited by higher soil salinity treatments (5.6 and 7.1 mS cm− 1) compared to non-saline soil (1.3 mS cm− 1). Additionally, high salt concentrations are often combined with low availability of soil water and have different effects on microorganisms (Kakumanu and Williams 2014).
During drought conditions, water is held more tightly to soil aggregates as matric potential decreases (Kakumanu et al. 2013). Drought conditions created due to the decline in water table depth have shown to affect soil EEAs. For instance, β-glucosidase and phenol oxidase activities decreased with declining water table depth in a mesocosm experiment with alpine wetland (Wang et al. 2017). In a mesocosm experiment with peat monoliths in Michigan, USA, Wiedermann et al. (2017) measured hydrolytic EEAs at intermediate depth and found the reduced activity of β-glucosidase, N-acetyl glucosaminidase (NAG), alkaline phosphatase, and sulfatase except for cellulase. However, there are also conflicting results exist in literature with the water table drawdown experiment. In a field-based experiment in Welsh peatland Freeman et al. (1996) found a 31 to 67% increase in β-glucosidase, phosphatase, and sulphatase activities upon water table drawdown, suggesting that drought condition increased mineralization rate through direct stimulation of enzymes.
Understanding the interactions among climatic conditions, shallow groundwater hydrology, salinity, and biogeochemical cycling associated with prevailing land-use practices within the PPR is complex and vital. Individual effects of land-use, salinity, and groundwater table variation due to climatic variability on soil EEAs have been well documented in the literature. However, their combined effects on soil hydrolytic EEAs, especially in mineral wetlands, has not been studied before. We conducted a microcosm experiment with controlled groundwater table levels at two levels of salinity with intact soil cores collected from three adjacent riparian land-use practices from the PPR to evaluate the effects on three hydrolytic soil EEAs, i.e., β-glucosidase, N-acetyl glucosaminidase, and alkaline phosphatase. We hypothesized that soil EEAs would be: 1) higher in soils from pasture compared to annual crop and short rotation willow land-use practices, irrespective of groundwater table depths or salinity levels; 2) lower under higher salinity (groundwater EC = 12 mS cm− 1) due to microbial stress from increased osmotic potential; 3) lower under a reduced groundwater table level because of slower SOM decomposition resulting from a decrease in soil moisture.