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.