Extracellular enzyme activity response to defoliation and water addition in two ecosites of the mixed grass prairie

Abstract

Grassland composition is affected by livestock grazing and moisture availability, yet little is known about how defoliation and soil moisture interact to affect belowground processes. In particular, microbial activity, the proximate driver of decomposition, may be affected by plant responses and environmental variability. We hypothesize that grassland soils with different defoliation and moisture treatments will differ in soil biogeochemical cycling in response to both physical environment (i.e. soil moisture) and biotic (i.e. plant community) shifts. To understand how microbial function is affected by defoliation and moisture, we measured extracellular enzyme activity (EEA) at two mixed grass prairie ecosites (mesic lowland and xeric upland) in Alberta, Canada using a fully factorial experiment that manipulated growing season defoliation and water addition over 4 years. The defoliation treatments were: high intensity – high frequency (HIHF), high intensity − low frequency (HILF), or low intensity – high frequency (LIHF) from May through August each year, and a treatment where defoliation was deferred until the end of each summer. The watering treatments were: ambient and water addition (150mm month−1 above ambient), which was intended to eliminate moisture limitations. In the fourth year of treatment, we measured the activities of 5 hydrolytic extracellular enzymes responsible for carbon (C), nitrogen (N) and phosphorous (P) release. We observed that water addition reduced activity of most EEAs across both sites, although these effects were distinctly mitigated by defoliation at the lowland site. Within the lowland, water addition reduced C cycling enzyme activities under deferred and HILF defoliation, with a similar pattern in the activity of the P cycling enzyme in response to HILF and LIHF defoliation. Defoliation effects on EEA in the lowland were limited largely to ambient moisture conditions, where severe (HIHF) defoliation reduced C cycling EEA, with a similar pattern in the upland, though only for β-d-cellobiosidase activity. Independent of moisture treatment, deferred defoliation reduced activity of phosphatase in the lowland, as well as the activity of one enzyme responsible for C cycling in the upland site. There was no effect of defoliation or water addition on N-acetyl-β glucosaminidase activity at either site. Overall, we demonstrate that while EEAs are strongly affected by environmental conditions, defoliation during the growing season may interact with the physical environment and regulate biogeochemical cycling.