The influence of watershed land use on lake N:P in a predominantly agricultural landscape

This study tests the hypothesis that lakes in watersheds dominated by row-crop agriculture (e.g., maize or soybeans) have systematically higher N:P than lakes in watersheds with large tracts of pasturelands. Current biogeochemical models of eutrophication suggest that agricultural nitrogen and phosphorus fluxes lead to a systematic decline in the N:P of receiving waters. In contrast, different agricultural activities (i.e., row-cropping vs. animal agriculture) use greatly divergent N and P amendments, and fluxes from agricultural watersheds diverge through a broad range of observed N:P (i.e., sub-Redfield to > 100). Animal agriculture leads to low N:P fluxes and row-cropping to high N:P. The connection between agricultural watershed land use and lake nutrient stoichiometry was tested in a highly agricultural region of the United States (Iowa) on 113 lakes in watersheds with different amounts of row-crop (0%-95%) and pastureland (0%-36%). Multiple regression analysis shows that lakes in watersheds with large areas in pasturelands have low N:P, whereas lakes in watersheds dominated by row-cropping have systematically high N.P Lakes in watersheds with > 30% pasture had the lowest N:P. approaching Redfield levels. N:P was most frequently high (> 50 as atoms) in lakes with > 90% of their watersheds in row-crop agriculture. The dynamics of agricultural practice necessitates the inclusion of real-world differences among agricultural systems in nutrient stoichiometric models. Intensive row-crop agriculture yields N:P stoichiometry at high levels usually observed in pristine headwaters and open oceans, whereas increased animal agriculture will drive N:P to low levels usually associated with cyanobacterial blooms.