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Title: RESPONSE OF ORGANIC AND INORGANIC CARBON AND NITROGEN TO LONG-TERM GRAZING OF THE SHORTGRASS STEPPE

Author
item Reeder, S
item Schuman, Gerald
item Morgan, Jack
item Lecain, Daniel

Submitted to: Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2003
Publication Date: 4/1/2004
Citation: Reeder, S.J., Schuman, G.E., Morgan, J.A., Lecain, D.R. 2004. Response of organic and inorganic carbon and nitrogen to long-term grazing of the shortgrass steppe. Environmental Management. 33(4):485-495.

Interpretive Summary: Evaluations of rangeland grazing management strategies generally have focused on the effects of grazing on livestock production, plant community composition and forage production. However, grazing also affects the distribution and cycling of nutrients between the soil and plants. Soil organic matter (SOM) is the primary source of nutrients for plant growth, and is the largest reservoir of carbon (C) and nitrogen (N) in rangeland ecosystems. As such the quality and quantity of SOM are major factors determining forage production and quality, as well as ecosystem health and stability. Knowledge of how grazing affects C and N cycling is necessary for developing grazing management strategies that enhance livestock production while sustaining the desired plant community. We investigated the influence of long-term (56 years) grazing at three stocking rates on the C and N contents of the plant-soil system of a shortgrass steppe in northeastern Colorado. A heavy stocking rate resulted in a plant community shifts the plant community to one dominated by the warm-season grass Blue Grama, whereas excluding livestock grazing entirely resulted in a plant community dominated by cactus, cool season grasses and weedy species. Light-to-moderate grazing resulted in a stable plant community dominated by grasses. Higher levels of organic C and N were observed in the root zone of the soil of the heavily grazed treatment compared to the light and no grazing treatments. This result reflects the dominance of blue grama in the heavily grazed treatment, which has a high root-to-shoot ratio and transfers most of its photosynthate below ground. Heavy grazing strongly increased the amount of inorganic C (lime) in the soil. We are currently investigating the source of this additional lime in the soil profile.

Technical Abstract: We investigated the influence of long-term (56 years) grazing on organic and inorganic carbon (C) and nitrogen (N) contents of the plant-soil system (to 90 cm depth) in shortgrass steppe of northeastern Colorado. Grazing treatments included continuous season-long (May - October) grazing by yearling heifers at heavy (60-75% utilization) and light (20-35% utilization) stocking rates, and non-grazed exclosures. The heavy stocking rate resulted in a plant community that was dominated (75% of biomass production) by the C4 grass blue grama (Bouteloua gracilis,), whereas excluding livestock grazing increased the production of C3 grasses and prickly pear cactus (Opuntia polycantha). Soil organic C (SOC) and organic N were not significantly different between the light grazing and non-grazed treatments, whereas the heavy grazing treatment was 7.5 Mg ha-1 higher in SOC than the non-grazed treatment. Lower ratios of net mineralized N to total organic N in both grazed compared to non-grazed treatments suggest that long-term grazing decreased the readily mineralizable fraction of soil organic matter. Heavy grazing affected soil inorganic C (SIC) more than the SOC. The heavy grazing treatment was 23.8 Mg ha-1 higher in total soil C (0-90 cm) than the non-grazed treatment, with 68% (16.3 Mg ha-1) attributable to higher SIC, and 32% (7.5 Mg ha-1) to higher SOC. These results emphasize the importance in semi-arid and arid ecosystems of including inorganic C in assessments of the mass and distribution of plant-soil C and in evaluations of the impacts of grazing management on C sequestration.