From Soil Stability to Nitrogen Management: Using a Plant Development Perspective to Investigate Effects of Root and Shoot Systems on Soil Stability, Soil Erosion, and Soil Nitrate Management.

摘要

Plant root systems are central to many important plant functions, including anchorage, mechanical support for the growing shoot system and acquisition of nutrients and water. Through their actions, root systems provide many important ecosystem services. The mechanical support strengthens the soil and prevents erosion. Also, root systems drive nutrient cycling through plant uptake and complex rhizosphere processes. This dissertation includes three studies that examined interactions between developing shoot and root systems and soil stability and nitrate dynamics.;In the first chapter, root reinforcement of soil was studied using Avena fatua (wild oat), a grass that produces a fibrous root system. During development, changes in root abundance and root mechanical properties were observed. Root density increased over time as more roots were produced by the root system. Less conspicuously, root mechanical strength (i.e., root tensile strength) was also discovered to increase during development. The increase could not be explained entirely by changes in root diameter, suggesting biochemical and anatomical changes are responsible for the increase in tensile strength. The results call attention to the value of incorporating developmental changes in root mechanical properties into models of soil stability.;In the second chapter, the effects of a developing fibrous root system on surface soil erosion were investigated. Avena fatua was grown in wooden boxes and subjected to simulated rainfall at three important stages of development: seedling emergence, initiation of tiller formation, and initiation of flowering. At each stage, aboveground cover was clipped at the surface in order to isolate the effects of only the root system to soil erodibility. A series of three, 1h simulated rainfall events were applied to planted and unplanted treatments. Soil erosion was found to be highest during the one week stage, when coleoptile emergence loosened the soil. Over time, as the root system grew, the soil became less susceptible to soil loss during the rainfall events. These results suggest developing root systems may actually increase the erodibility of the soil during the early stages of growth before providing a stabilizing effect when mature.;In addition to the effects of root development on soil stability and erodibility, developing root systems can influence nutrient dynamics. In the third chapter, winter cover crops were investigated in a field experiment to better understand nitrate uptake and nitrate distribution in the soil throughout plant development. Two contrasting cover crops were studied (triticale, a cereal, and bell bean, a legume). Triticale was found to produce a deeper, denser root system than bell bean, although differences in soil nitrate profiles between the two treatments were minimal. For triticale, a model based on N uptake and growing degree-days was developed to investigate the effect of different planting dates on N uptake. Model predictions suggest planting earlier in the autumn would enable extraction of N from the soil prior to the heaviest winter rains.