Soil Aggregates and Their Mediation of Agroecosystem Carbon and Nitrogen Cycling Processes

Understanding carbon (C) and nitrogen (N) cycling in agroecosystems is essential to manage and predict soil ecosystem processes, including the storage of soil organic carbon (SOC) and emissions of nitrous oxide (N2O). Soil aggregates have been hypothesized to have a central role in mediating C and N cycling, however whether their study is necessary engages on-going debate. The goal of this thesis was to investigate the extent to which studying aggregates is useful for understanding SOC accumulation and freeze-thaw induced N2O emissions. Chapter 2 presents a quantitative review of the relationships between SOC, aggregate mass distributions, and aggregate C levels, and uses multiple strands of evidence to infer that macroaggregates promote the stabilization of C to occluded microaggregates. This finding is consistent with previous hypotheses about the importance of macroaggregates for C storage but offers the modification that microaggregate formation is independent of macroaggregate turnover. Chapter 3 present results from a 37-year field trial comparing different crop rotations, where I test agroecosystem design principles for their ability to promote SOC and aggregation. Crop rotation species diversity did not predict SOC, aggregation, or stabilization efficiency of crop C inputs to SOC. While high quality crop tissues have been theorized to be more efficiently stabilized to SOC, results from this site do not indicate that high quality structural plant inputs lead to preferential accumulation of bulk SOC. These results also highlight the need to use approaches other than assessing aggregate mass distributions and C concentrations to understand mechanisms of SOC persistence. Chapter 4 tests the hypothesis that aggregate disruption caused by freeze-thaw liberates organic substrates for N2O emissions. By freezing soil cores at different rates and durations, I show that aggregate disruption is not implicated in greater N2O emissions under longer freezing duration and that faster freezing rate is associated with a reduced extent of aggregate disruption. This thesis shows that studying aggregate disruption is not indicated as a means to understanding N2O induced by freeze-thaw, but that describing and quantifying the mechanisms through which aggregates mediate SOC accumulation has potential to inform understanding of agroecosystem C processes.