Nitrogen fertilization effects on soil organic carbon storage and aggregation mechanisms within continuous corn cropping systems
Soils have the potential to store carbon as soil organic carbon (SOC) and reduce atmospheric carbon dioxide (CO2), a common greenhouse gas. Agriculture can be thought of as the transfer of carbon (C) among different pools, and therefore, proper management can increase SOC storage. Unfortunately, effects on SOC from some of the most common agricultural practices are unknown or difficult to predict. Nitrogen (N) fertilization, for instance, is debated to cause both positive and negative effects on SOC. Effects of N fertilization on SOC levels are determined by the net balance between crop residue inputs (primary source of SOC) and the increase in mineralization. The fate of SOC in these systems is unclear, however, in situations where the soils have reached the upper limit of SOC equilibrium (C saturation). In addition, recent opinions have suggested that by increasing residue quality (lower C:N ratios) through N fertilization, the rate and amount of SOC stabilization will increase. This thesis aims to observe how SOC storage is affected within a continuous corn cropping system in Iowa, USA. Five N application rates were applied for 13 years in Iowa, which increased crop residue quantity and likely residue quality. Stable (protected) SOC pools did not increase, but labile (unprotected) SOC pools increased linearly with crop residue input, thus indicating soils were C saturated based on the two-pool saturation model. Nitrogen application affected the quality (C:N ratio) of coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), and fine intra-aggregate particulate organic matter (fiPOM), but most strongly affected the cPOM. In fact, cPOM displayed significantly greater regression coefficients when regressed against N application rates than both fPOM and fiPOM, confirming that N fertilization most strongly affected labile pools rather than stable pools. Once degraded to fPOM and fiPOM, C:N ratios become stable and are more reflective of the level of degradation than initial C:N ratios. Along with a change in residue quality, large macroaggregates displayed increases in the fiPOM:cPOM ratio. Elevated fiPOM:cPOM ratios indicate that microaggregates (inter-m) were more likely to form and store SOC as fiPOM. The increase in inter-m or fiPOM production may have been a result of rapid decomposition of higher quality cPOM. Large macroaggregates did not increase concurrently with the increase in fiPOM:cPOM ratio, and therefore, implies the stabilization did not increase. In addition, microbial biomass, including fungal populations, decreased with crop residue input meaning that microbial biomass or diversity did not influence aggregate dynamics. Not only did macroaggregates create more fiPOM, they also tended to increase the C concentrations of silt and clay (SC) that remained trapped inside macroaggregates. In this study, C saturation prevented new additions of SOC storage, and N fertilization mostly influenced cPOM quality and, in turn, may have shifted aggregation dynamics, allowing for SOC to be stored in more stable forms, such as fiPOM, inter- and intra-SC fractions.