Michael Castellano1, Charles Walker1, John Schmidt2, Curtis Dell3, Jason Kaye1, and Henry Lin1. (1) Crop and Soil Sciences, The Pennsylvania State University, 116 ASI Building, University Park, PA 16802, (2) USDA-ARS, USDA-ARS-PSWMRU, Curtin Road Building 3702, University Park, PA 16802-3702, (3) USDA-ARS Pasture Systems & Watershed Mgmt Research Unit, USDA/ARS/PSWMRU Bldg 3702, Curtin Road, University Park, PA 16802
Artificial drainage systems can effectively transport excess water from agricultural soils. However, they can also transport large amounts of biologically available nitrogen (N) to open waters, potentially fueling eutrophication. Using 12 large (30 x 33 cm) undisturbed soil columns, we examined three landscape positions’ ability to remove biologically available N during rapid water table fluctuations (middle-field, near-ditch, in-ditch). Simulating field data, we raised the columns' water table to the surface and then lowered it within 24 hours while monitoring volumetric soil moisture, matric potential and CO2 and N2O emissions. During water table rise and fall, soils from the middle-field location consistently emitted more N2O than the near-ditch location. The in-ditch soils’ N2O emissions were intermediate between the middle-field and near-ditch locations except when the water table was at the soil surface; at this time the in-ditch location was the highest N2O emitter at 10.2 mg N2O-N m-2 h-1 compared to the middle-field (3.63 mg N2O-N m-2 h-1) and near-ditch (1.75 mg N2O-N m-2 h-1) locations. At approximately 40 hours after the onset of the water table manipulation all landscape positions converged upon similar N2O emission rates. Over the 40 hours prior to convergence, the in-ditch emitted more N2O (167.71 mg N2O-N m-2 40h-1) than the middle-field (111.62 mg N2O-N m-2 40h-1) or near-ditch (62.86 mg N2O-N m-2 40h-1) locations. However during this 40h period, in-ditch soils’ low water filled pore space (WFPS) suggest that some of these N2O emissions were due to nitrification. Conversely, the middle-field and near-ditch locations’ high WFPS throughout the 40h period indicate that all N2O emissions at these sites were due to denitrification. As management efforts focus on the removal and retention of N in agro-ecosystems, our results suggest that management practices promoting in-ditch soil water retention could increase denitrification.
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