Dissolved and Particulate Runoff Phosphorus Loss As Affected By Tillage and Fertilizer or Swine Manure Sources in Corn and Soybean.

Phosphorus applied in excess of crop needs increases soil P levels and the risk of P loss from fields, and management practices may affect differently the loss of particulate and dissolved P. A field study (180-m² plots) was conducted under natural precipitation in Iowa from 2007 until 2012 in a Sac (Oxyaquic Hapludolls) soil with 2.7-3.9% slope. Treatments replicated three times were four sets applied to corn-soybean (CS) rotations harvested for grain and with each crop planted each year, and a fifth treatment was applied to continuous corn (CC) with grain and stover baling. For the CS plots, treatments were the combinations of no-till (NT), chisel-plow/disk tillage (CH), broadcast triple superphosphate fertilizer (TSP), and P-based liquid swine manure (SMP) injected for both tillage systems. Phosphorus was applied only before corn to supply the 2-year rotation amount (112 kg P ha^-1) to maintain an initially optimum level (17 mg Bray-1 kg^-1, 15-cm depth). Nonlimiting N rates were applied to corn of CS. The CC plots were managed with CH, N-based liquid swine manure, and no P application. Runoff was measured and analyzed for dissolved reactive P (DRP), total P (TP), and sediment. The last year, soil Bray-1 P was 24 mg kg^-1, 25 mg kg^-1, and 38 mg kg^-1 for CS-CH, CS-NT, and CC. There were no consistent soil P and runoff P differences between P sources for CS, but losses much higher with tillage. In the corn years, soil and TP losses were 4.59 Mg soil ha^-1 yr^-1 and 4.36 kg P ha^-1 yr^-1 for CH; and 1.27 Mg soil ha^-1 yr^-1 and 3.59 kg P ha^-1 yr^-1 for NT. In the soybean years, soil and TP losses were 0.78 Mg soil ha^-1 yr^-1 and 1.45 kg P ha^-1 yr^-1 for CH and 0.30 Mg soil ha^-1 yr^-1 and 0.72 kg P ha^-1 yr^-1 for NT. For CC, soil and TP losses were 1.9 Mg soil ha^-1 yr^-1 and 2.77 kg ha^-1 yr^-1. Although NT decreased TP loss, the proportion of DRP was higher with NT (57%) than with CH (22%) so DRP loss was 2.6 times higher with NT than with CH. In conclusion, runoff P loss did not differ for P-based application of fertilizer or swine manure and no-till significantly reduced both soil and TP loss but increased the DRP loss, which is the most readily-available P fraction for algae growth.