This study set out to investigate the effects of level of nitrogen supply on the root dynamics, carbon (C) and nitrogen (N) return to soil from root turnover, decomposition, and stability of improved grasslands. Field studies involved a two-year field experiment, where plant species composition, above ground biomass production, root production, litter decomposition and short term soil organic matter (SOM) dynamics, as a response to varying levels of inorganic fertiliser nitrogen supply were studied. The results showed that root dynamics are strongly affected by N fertilisation, with an increase in root production and death with increasing N supply. Reduced water availability lead to a greater root disappearance rate and the production of roots with a shorter lifespan. Root decomposition was strongly influenced by the age of the grass ley, which also exerted a strong influence on the structure of the soil microbial communities (SMC). Rate of litter decomposition and SMC structure were also influenced by the level of N supply. Level of mineral N supply and age of the grass ley also influenced the formation and stability of water stable aggregates, the microbial community structure and microbial community function within the aggregates. Mineral N applications are likely to influence SOM and soil nutrient dynamics. The finding that climatic conditions, and in particular water availability, had the strongest impact on both above and below ground productivity, and strongly influenced the amount of C, N and root/shoot biomass returned to soils, is of great importance in helping to make more accurate predictions of the response of plant communities to projected changes in the global climate.

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Typescript (photocopy) / viii, 212 leaves, [7] leaves of plates : ill. (part col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.) Dept. of Soil Science, University of Adelaide, 1982

Thesis (M.S.)--Oregon State University, 1963. / Typescript. Includes bibliographical references (leaves 89-93). Also available on the World Wide Web.

Thesis (M.S.)--State University of New York at Binghamton, Biological Sciences Department, 2005. / Includes bibliographical references.

Copies of author's previously published articles inserted. Bibliography: leaves 150-166.

Plant roots provide important soil reinforcement and improve the stability of slopes. From an engineering view, roots mechanically increase soil shear strength by transforming shear stress from soil into tensile forces of roots themselves via interface friction thus acting like soil nails. However, due to the complex spatial geometries and mechanical heterogeneities of natural root systems, more specific considerations are needed when analyzing the root reinforcement issues. According to a literature review, most studies on slope stability consider root reinforcement as an apparent cohesion by upscaling the behaviour of static individual roots. However, recent studies have shown that better predictions can be made if the progressive failure of roots is considered, thus highlighting the importance of load-displacement relations of soil-root interaction. Therefore, numerical pull-out tests considering the progressive friction interface relationship were carried out in this study to investigate the mechanism and the influences of various factors on plant root pull-out behaviour, which is of great importance to evaluating the stabilization effect of roots. In this study, the classic Coulomb friction model was adopted to simulate the interaction along the root-soil interface with the surface to surface modelling technique available in ABAQUS. The numerical investigations could be mainly divided into three parts: study of single straight root pull-out behaviour, study of the branched root segments with only first-order lateral branches (herringbone system), and study of the root segments with second-order branches (dichotomous system). Conclusions regarding the mechanism of the root pull-out process, the influence of geometry-related factors and the influence of the intrinsic factors related to mechanical properties and root-soil interaction were drawn based on the observations of the numerical pull-out tests. Progressive stick-to-slip behaviours along the root-soil interface were observed in the numerical models. Generally, the pullout resistance of roots increased with the branch depth and branch length. It was also observed that the pullout resistance had strong regression with the weight of the soil potentially lifted by the root system. The most efficient branch angle for providing pull-out resistance for the herringbone system was between 60 and 90 degrees and the most efficient branch angle for the dichotomous root system was shown to be around 45. The central symmetrical branch arrangement pattern was observed to be the most efficient in providing pull-out resistance compared with the plate symmetrical and asymmetrical patterns. / published_or_final_version / Civil Engineering / Master / Master of Philosophy

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