Soil response to long-term grazing in the northern Great Plains of North America

Grazing management affects ecosystem function through impacts on soil condition. We investigated the effects of long-term (over 70 years) grazing on soil properties and nitrous oxide (N2O) emission within a moderately grazed native vegetation pasture (MGP), heavily grazed native vegetation pasture (HGP), and a fertilized crested wheatgrass (Agropyron desertorum (Fisch. ex. Link) Schult.) pasture (FCWP) near Mandan, ND, USA. Grazing-induced changes in species composition and N fertilizer application contributed to differences in soil properties and N2O emission between pastures. Soil organic C (SOC) was 5.7 Mg ha−1 greater in FCWP and HGP than MGP at 0–5 cm, whereas HGP had 2.4 Mg ha−1 more SOC than FCWP and MGP at 5–10 cm. At 30–60 cm, SOC in FCWP 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 FCWP were three- and five-fold greater, respectively, than in HGP and MGP. Acidification from N fertilization in FCWP decreased soil pH and cation exchange capacity compared to HGP and MGP in the surface 5 cm. Annual N2O emission was over three-fold greater in FCWP compared to HGP and MGP, and was positively associated with POM-C across all pastures (P ≤ 0.0001; r2 = 0.85). 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 non-fertilized pastures in the northern Great Plains.