Impacts of Compost Addition on Sub-Surface Nitrate and Phosphorus Losses at the Farm Scale Under Two Different Tillage Systems.

Understanding phosphorus (P) and nitrate-nitrogen (N) leaching from soils receiving organic amendments is important to accurately estimate total soil P and N losses, and thereby develop best management practices to control non-point source pollution. An experiment was conducted to determine soil P and N losses in tile drainage as affected by addition of yard waste compost on a Brookston clay loam soil which had been under long-term either conventional tillage (CT) or no-tillage (NT). Compost was applied at 75 Mg ha^-1 in the fall of 1998, 1999, 2000 and 2001.  Tile drainage flows were monitored continuously from fall 1998 to fall 2001. Water samples were collected using auto-samplers and analysed for nitrate-N, total dissolved P (TDP), and particulate P. In the control plots (no compost), the flow-weighted mean concentrations of TDP and nitrate-N were averaged at 0.146 mg P L^-1 and 14.7 mg N L^-1  under CT, and at 0.0984 mg P L^-1  and 7.6 mg N L^-1 under NT, respectively. Compost addition increased TDP and nitrate-N concentrations to 0.265 mg P L^-1 and 27.5 mg N L^-1 under CT and 0.383 mg P L^-1 and 15.0 mg N L^-1 under NT, respectively. Total losses of TDP were similar between tillage regimes in the control plots, but increased by 140% under CT and by 177% under NT with compost addition. Compost application increased nitrate-N losses by 179% under CT and by 85 % under NT. Compost addition increased tile drainage losses of soil TDP, with a greater increase occurring under NT than under CT, while the effect on nitrate-N loss was greater under CT than under NT.