Labile soil organic matter in response to long-term cattle grazing on sloped rough fescue grassland in the foothills of the Rocky Mountains, Alberta

Globally, livestock grazing has pronounced effects on soil carbon (C) and nitrogen (N) cycling, but how labile soil organic matter responds to grazing at different slope positions is uncertain. This study investigated how labile soil organic matter responded to cattle stocking rates and slope position on a rough Fescue grassland. Cattle have been grazing at four stocking rates, 0, 1.2, 2.4 and 4.8 animal unit months (AUM) ha(-1) since 1949, representing Control (CK), Light, Heavy and Very Heavy grazing intensities, respectively. Soil samples were collected from the bottom and top slope landscape positions for each grazed paddock (but only from the top position for Control) in May and September 2016. Soil samples were analyzed for organic carbon (OC), total nitrogen (TN), active C (permanganate-oxidizable C), microbial respiration C, available N, potentially mineralizable N (PMN), soil protein, water extractable organic carbon (WEOC) and nitrogen (WEN). Most variables responded to cattle stocking rate (P < 0.05) and the responses depended on slope position. Specifically, in the bottom position, OC, TN, active C, NH4+-N, soil protein and WEN contents were higher with Light grazing than with Heavy or Very Heavy grazing. In contrast, for the top position, no significant differences were found among the three grazing treatments for OC, active C, NH4+-N and PMN and all variables were lower than with CK except for PMN with Heavy grazing. As expected, soil OC and TN contents were similar between sampling dates in May and September (P > 0.05) while active C, NH4+-N, WEOC and WEN contents were higher (P < 0.05) in May than September. Our study suggests that grazing may detrimentally affect grassland soil C and N and slope position plays an important role in regulating the response of labile soil organic matter to grazing. Therefore, there is a need for researchers and range managers to consider developing 'slope-specific' grazing strategies to optimize grazing effects.