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Accueil du site → Doctorat → États-Unis → 1994 → Carbon and nitrogen dynamics in grazed and protected semiarid shortgrass steppe

Colorado State University (1994)

Carbon and nitrogen dynamics in grazed and protected semiarid shortgrass steppe

Vokhiwa, Zipangani Mauru

Titre : Carbon and nitrogen dynamics in grazed and protected semiarid shortgrass steppe

Auteur : Vokhiwa, Zipangani Mauru

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (PhD) 1994

This study investigated patterns of carbon and nitrogen dynamics in shortgrass steppe as affected by grazing and protection from grazing by domestic herbivores. The primary objective was to provide information on the interaction between grazing and protection from grazing disturbances on vegetation and soil. Community variables were investigated under each type of disturbance from clay loam (CL) and sandy loam (SL) soils beneath and between plants at 0-5, 5-10 and 10-20 cm soil depth and from above-ground plant biomass. The hypothesis tested in this study was that mineralization and decomposition potential of soil organic matter (SOM) is within a normal range of variation on both grazed and protected rangeland represented by clay loam and sandy loam sites. Another hypothesis that was tested was that the distribution of total organic carbon (TOC) and total nitrogen (TN) is similar beneath and between plants in a steppe ecosystem dominated by shortgrasses. Bulk density was 12 and 4 percent higher (P = 0.0283) in surface soils (0-5 cm) in grazed than protected sites in both CL and SL soils. Soil porosity was greater (P = 0.0283) in protected than grazed sites. TOC and TN amounts, expressed by C:N ratio, were similar whether the site had been grazed or protected. The surface 0-10 cm contained most of the roots, TOC and TN decreasing with depth. Below-ground C concentrations were greater (P = 0.0095) in grazed than protected sites ; N concentrations were similar. Mean total root biomass and mean total above-ground biomass were similar in grazed and protected sites. Above-ground C pools were similar between grazed and protected sites, while N concentrations were greater (P = 0.0294) in grazed than protected sites. The C:N ratio of above-ground biomass was higher in protected than grazed sites. Mean total litter crop was higher on protected than grazed sites. Litter from protected sites had 2.8 and 3.0 times more total C and N, respectively. The spatial distribution of TOC, TN and roots was not even. More C, N and roots were found beneath than between plants. Mean above-ground biomass was greater (P = 0.0448) in sandy loam soil than clay loam soils, probably due to pedogenic and historical geomorphic processes that impacted the contemporary rates of biogeochemical organic C and N transformation in this shortgrass steppe. Long-term mineralization could have been slowly modified by the impact of grazing and weather patterns and on consequent soil-plant-water relations.

Mots clés : Ecology, Range management, Biological sciences, Botany

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