| A principal focus of water quality management efforts in the United States is related to nutrients, specifically nitrogen (N) and phosphorus (P), export in runoff from croplands. This study investigated the influence of residue levels and manure addition on P delivery in particulate and dissolved forms in both rainfall and snowmelt runoff. Rainfall and snowmelt runoff samples were collected from three hydrologically isolated hillslope tracts in conservation tillage with the following treatments: corn-grain (CG); corn-silage (CS); and corn-silage with fall manure addition (SM). From May 2003 through Sep. 2005 the following events were sampled: two rain on frozen ground/snowmelt-runoff (frost melt, FM) events, (Jan.-Mar. 2004; 2005), and 18 rainfall-runoff (frost free, FF) events, (May-Nov 2003; May-Aug 2004; Sept 2005). Three intra-event storms were also sampled from sites with contrasting up-and-down and contoured row orientations. Runoff samples were analyzed for the following: P in the dissolved and particulate forms, sediment P-mass distribution in five different particle-size classes, particle and aggregate size distributions, and aggregate stability.; The median total P (TP), total dissolved P (TDP), dissolved reactive P (DRP), and volatile solids (VS) concentrations in FM runoff from SM treatment were greater than the CG and CS sites, attributable to manure P inputs the previous fall. The majority of P losses occurred in the particulate P form for all the monitored events. The FF period loading was greater than the FM period loading for all sites and constituents except for DRP from the SM treatment. Runoff from CG residue apparently had higher dissolved organic P concentration than runoff from the CS and SM treatments. A TP mass particle-size regression equation was developed to distribute P mass by sediment particle-size. Aggregates exported from the CG site were more stable than those exported from the CS and SM sites. The up-and-down row orientation exported more nutrients and solids than the contoured site.; This research highlighted how different management practices influence nutrient and sediment export from corn fields. These results should improve our understanding of P loss pathways and, consequently, help to develop better predictive tools to quantify agricultural nonpoint source pollution. |
