Liebig, Mark (USDA-ARS, P.O. Box 459, Mandan, ND, 58554; Phone: 701-667-3079; Fax: 701-667-3054; Email: liebigm@mandan.ars.usda.gov)

Soil Response to Long-Term Grazing in the Northern Great Plains

M. A. Liebig*, S. L. Kronberg, J. R. Gross, J. D. Hanson, A. B. Frank, R. L. Phillips

Grazing management systems affect agroecosystem sustainability through impacts on soil condition. We investigated the effects of long-term (over 70 yr) grazing on soil properties and N₂O emission within a moderately-grazed pasture (MGP), heavily-grazed pasture (HGP), and a fertilized crested wheatgrass (Agropyron desertorum (Fisch. ex. Link) Schult.) pasture (FCWGP) near Mandan, ND. Grazing-induced changes in species composition and N fertilizer application contributed to differences in soil properties and N₂O emission among pastures. Soil organic C (SOC) was 5.7 Mg ha^-1 greater in FCWGP and HGP than MGP at 0 to 5 cm, whereas HGP had 2.4 Mg ha^-1 more SOC than FCWGP and MGP at 5 to 10 cm. At 30 to 60 cm, SOC in FCWGP was 4.0 and 7.5 Mg ha^-1 greater than in HGP and MGP, respectively. Particulate organic matter (POM) C and N in the surface 5 cm of FCWGP were three- and five-fold greater, respectively, than in HGP and MGP. Acidification from N fertilization in FCWGP decreased soil pH and exchangeable Ca and Mg compared to HGP and MGP in the surface 5 cm. Annual nitrous oxide emission was over three-fold greater in FCWGP compared to HGP and MGP, and was positively associated with POM-C across all pastures (r=0.92; P=0.0001). Results from this study suggest fertilized crested wheatgrass enhances deep storage of SOC, but contributes to surface acidification and greater N₂O emission relative to native vegetation pastures in the northern Great Plains.