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Accueil du site → Doctorat → États-Unis → 1999 → Nitrogen retention in semiarid ecosystems of the United States central grasslands region

Colorado State University (1999)

Nitrogen retention in semiarid ecosystems of the United States central grasslands region

Barrett, John Edward

Titre : Nitrogen retention in semiarid ecosystems of the United States central grasslands region

Auteur : Barrett, John Edward

Université de soutenance : Colorado State University

Grade : Doctor of Philosophy (PhD) 1999

Human activities have increased the availability of nitrogen (N) to terrestrial and aquatic ecosystems. Since excess N often leads to NO3 - leaching, soil acidification, and changes in plant species composition, identifying natural sinks for anthropogenic N remains one of the greatest challenges in biogeochemistry. N retention in grasslands has received relatively little attention, due largely to the fact that N deposition in these systems is below levels of N input in most temperate forest ecosystems. However, semiarid grasslands of the U.S. provide good model systems in which to address questions of N retention because they occur in a region where broad climatic patterns influence large scale trends in soil organic matter (SOM) storage. SOM may have an important influence over N retention because it acts as a reduced carbon source for biochemical reactions that stabilize N. In this dissertation I address the influence of broad and local scale trends in SOM content and composition upon N retention in the laboratory and in the field. I used 15 N as a tracer to (1) estimate N immobilization for a series of soils collected from across a SOM gradient, and (2) to estimate N retention in field plots constructed across this gradient. I evaluated our understanding of these regional patterns in SOM content, with particular attention to soil C:N ratios, using the CENTURY SOM model. I found that regional patterns in soil C content accounted for the largest proportion of variance in N immobilization and N retention. Fine scale spatial variability associated with discontinuous plant cover and local scale differences in soil texture had significant, but much less clear, influences upon N immobilization and N retention than did soil C content. Soil C content alone accounted for 60% and 56% of the variance in estimates of N immobilization and N retention, respectively. Simulation exercises suggest that our understanding of SOM dynamics cannot explain observed trends in soil C:N ratios. This may be due to the CENTURY model’s overestimation of an ecosystem’s capacity to utilize excess N, and a lack of information regarding trends in SOM composition, belowground production and root turnover.

Mots clés : Ecology, Grasslands, Biogeochemistry, Soil sciences, Nitrogen retention, Earth sciences Semiarid, Biological sciences

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Page publiée le 23 février 2015, mise à jour le 19 novembre 2018