Grazing Management, Season, and Drought Contributions to Soil Property Dynamics and Greenhouse Gas Flux in Semiarid Rangeland.

Semiarid rangelands provide an array ecosystem services, yet the role of grazing management and environmental conditions to affect rangeland soil function is poorly understood.  A study was conducted to assess effects of grazing management, season, and drought on soil property dynamics and greenhouse gas (GHG) flux within semiarid rangeland.  Grazing management treatments evaluated in the study included two native vegetation pastures differing in stocking rate [moderately grazed pasture (MGP), heavily grazed pasture (HGP)] and a fertilized, heavily grazed crested wheatgrass [Agropyron desertorum (Fisch. ex. Link) Schult.] pasture (CWP) near Mandan, ND, USA.  Over a period of three years, soil bulk density, electrical conductivity, soil pH, and extractable NO₃-N and NH₄-N were measured in the spring, summer, and fall at depths of 0-5, 5-10, 10-20, and 20-30 cm, while soil-atmosphere fluxes of CO₂, CH₄, and N₂O were measured annually on one to two week intervals.  All soil properties were significantly affected by grazing management, season, and year.  High stocking rate and fertilizer N application within CWP contributed to increased soil bulk density, electrical conductivity, and extractable NO₃-N and NH₄-N, and decreased soil pH compared to native vegetation pastures.  Extractable NO₃-N tended to be greatest at peak aboveground biomass, whereas extractable NH₄-N was greatest in early spring.  Drought conditions during the third year of the study contributed to nearly two-fold increases in extractable NO₃-N and NH₄-N at 0-5 and 5-10 cm.  Stepwise regression found select soil properties to be moderately-related to soil-atmosphere fluxes of CO₂, CH₄, and N₂O, with model R-squares ranging from 0.09 to 0.76.  Among soil properties, electrical conductivity was included most frequently in stepwise regressions.  Soil attributes measured in this study suggest high stocking rate combined with long-term fertilizer N application may compromise soil functions necessary to support and regulate key ecosystem services in semiarid rangeland.  Moreover, easily measured soil properties (e.g., electrical conductivity) may serve as useful indicators for identifying GHG hotspots in rangeland.