Soil nitrogen cycling in agroecosystems as modified by biochar amendment and plant processes
Date
2019
Authors
Rocci, Katherine, author
Cotrufo, M. Francesca, advisor
Fonte, Steven, advisor
von Fischer, Joseph, committee member
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Abstract
Ecosystem productivity is dependent upon cycling of nutrients, such as nitrogen (N). In agricultural systems, humans have greatly altered N cycling through the application of synthetic fertilizers such that soil N in agroecosystems is lost at higher rates than N in unmanaged systems. A variety of strategies have been assessed to reduce losses of soil N through nitrous oxide (N2O) emissions and leaching, which can negatively impact climate and water quality, respectively. The application of biochar, a carbon-rich soil amendment, has shown promise for increasing N retention in agricultural systems, but field and greenhouse studies often present less dramatic and often conflicting effects, suggesting the need for greater study in these environments. Further, the effects of biochar do not occur in isolation, but rather depend on plant processes that may affect soil N dynamics. This thesis explores these ideas through: (1) a greenhouse study considering the effects of different biochar types on N cycling with and without plants and (2) a field study looking at seasonal patterns of N cycling and fixation in alfalfa as altered by strategically-placed, low rates of biochar application. Study 1 sought to determine differential effects of biochar and plants, and raw and engineered biochar, on both fertilizer and innate soil N cycling using isotopically labelled fertilizer. While biochar effects on soil-derived N were minimal, we found that engineered biochar led to significantly higher leaching losses of fertilizer N. Plants, in contrast, were found to reduce N loss and increase overall recovery of fertilizer N. Study 2 focused on the effects of low and economically feasible application rates of two different biochars on N fixation, N loss, and mineral N availability over a growing season. We found no biochar effects on any N cycling parameter and, rather, found significant temporal effects in all N pools. Seasonal dynamics suggest connections between SIN availability and N fixation and loss. Indications of increased N loss with engineered biochar in Study 1 urge the need for greater study of biochars in combination with a variety of fertilizer types in order to provide the best recommendations to farmers. Lack of effects with biochar in Study 2 indicate that low application rates of biochar may not be useful for increasing N retention, suggesting the need to find a balance between economic and effective biochar application rates. Since both studies suggest that plant processes have more substantial impacts on N cycling than biochar amendment, via reduced N loss (Study 1) or increased symbiotic N input (Study 2), it is important that plants are included in more biochar studies such that the strength of biochar effects can be more realistically evaluated.
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Subject
biochar
plants
nitrogen
agroecology