Down Under: Organic Nitrogen Accumulations in Agricultural Landscapes

Considerable uncertainty is associated with the fate of anthropogenic nitrogen (N) inputs in agricultural watersheds, with numerous mass balance studies suggesting the existence of unaccounted for accumulations of N mass. Conventional wisdom dictates that terrestrial N accumulations are unlikely in agricultural landscapes, with intensive cultivation practices leading to a depletion of soil organic N. Our recent synthesis of soil core data from across the Upper Mississippi River Basin, however, demonstrates that while agriculture does lead to a depletion of organic N in surface soil, significant accumulations are occurring deeper in the soil profile. Soil core data collected in plots under intensive agriculture over the last ~50 years are suggestive of a unique process in agricultural landscapes analogous to podzolization in boreal forest soils, with organic nitrogen being transported by preferential flow pathways from surface soils to deeper layers, likely via the complexation of organic acids with metal ions. Changing conditions in agricultural soils, including increasing surface soil temperatures, decreases in pH related to the application of nitrogen fertilizer, and a lowering of the water table and increased oxygen availability associated with the installation of dense tile drainage networks, appear to initiate a process of "anthropodzolization," in which dissolved organic matter is translocated to subsoil layers. For preliminary hypothesis testing regarding various controls on this newly identified translocation mechanism, we have developed a carbon-nitrogen cycling model that extends beyond the plow layer, specifically taking into account vertical processes throughout the soil profile. Future research will involve rigorous testing of the model using field measurements and laboratory studies.