Watershed-Scale Conservation, Through Changing Land Cover, Reduces Nutrient Export From Agroecosystems Even Under Changing Hydrology.

Abstract:

In the Midwestern US, excess fertilizer nutrients leave farm fields and enter agricultural streams, degrading local water quality. Further downstream, nutrient runoff fuels algal blooms that result in periods of hypoxia in sensitive downstream ecosystems such as the Gulf of Mexico. We are examining controls on nutrient retention and export from agroecosystems using an experimental watershed-scale change in vegetative land cover, through the planting of winter cover crops after cash crop harvest, and how responses might vary under changing hydrological conditions. After a pre-treatment year of data collection (2013 water year), we have planted winter cover crops on ~70% of croppable land area in the Shatto Ditch Watershed (Indiana, USA), and have quantified nutrient loss from fields by sampling nitrogen (N) and phosphorus (P) from 25 distributed subsurface tile drains and longitudinally along the stream channel throughout the watershed. Cover crops consistently reduced dissolved inorganic N and P fluxes leaving tiles from cover cropped fields compared to those without. Compared to the pre-treatment year, annual watershed nutrient export also decreased, and reductions in N and P loss (~30-40%) exceeded what was expected based on the <10% reduction in water yields resulting from increased watershed water holding capacity. We also explored whether increased nutrient retention due to cover crops was due to a change in baseflow export or responses to storm-induced losses. For example, daily nitrate-N export during baseflow conditions (~75% of the year) were not different before and after cover crops, averaging ~27kgN/d. The number of storm days (defined as Q= >95% baseflow) for each water year was similar before (2013=91d) and after cover crops (2014=88d, 2015=96d), but daily nitrate-N export on storm days was highest during pre-treatment (2013=227 kgN/d), and decreased after cover crops (2014= 140kgN/d, 2015=149kgN/d). Moreover, before cover crops, ~75% of the annual nitrate-N export occurred during storms, but this declined to ~63% in the two years with winter cover. In summary, watershed-scale cover crops may significantly reduce nutrient export from agricultural watersheds while simultaneously buffering the impacts of increased storm frequency predicted for the Midwestern US under a changing climate.