SEASONAL PHOSPHORUS LOSSES IN RUNOFF FROM A COASTAL-PLAIN SOIL

Nutrients from agricultural fields can contaminate runoff and groundwa ter and enhance eutrophication. Runoff, soil loss, soluble P (SP), and bioavailable P (BP) associated with runoff and soil loss were measure d from six 2 h simulated rainfall events (1 in./h) to determine amount of P transported during a corn (Zea mays L.) growing season and to de velop relationships that predict P amounts and forms leaving different size field plots. Two 60-sq-ft microplots and two 6500-sq-ft mesoplot s (3% slope) were located on a Tifton loamy sand (fine-loamy, siliceou s, thermic Plinthic Kandiudult). Phosphorus was broadcast at 31 Ib P/a cre, then incorporated to a depth of 6 in. Corn (conventional tillage) was planted after fertilizer application. Six simulated rainfall even ts occurred 5 d before and 1, 14, 29, 49, and 108 d after fertilizer a pplication. Total runoff and sediment yield from events 3 to 6 general ly decreased with time for all plots. Microplot runoff for each event was greater than that from mesoplots. Microplot soil loss was greater than that from mesoplots for events 3 and 4, but less for events 5 and 6. Runoff from mesoplots ranged from 0.14 to 0.65 in. and total soil loss ranged from 136 to 1097 lb/acre, whereas total runoff from microp lots ranged from 0.30 to 0.91 in. and total soil loss ranged from 152 to 2820 lb/acre. For the five events after fertilization, SP losses de creased with days since fertilization, and were greatest 1 d after fer tilizer application (0.70-0.99 Ib P/acre-129-181 g P/ha). Similar tren ds were obtained for microplots (event 2 SP losses were 1.21-1.56 Ib P /acre-222-287 g P/ha). Total BP, (1.22 Ib P/acre-225 g P/ha) and BP (1 .51-1.56 Ib P/acre-277-286 g P/ha) losses from mesoplots were greatest 14 d after fertilization. Microplot BPe values decreased with time, a nd BP values for events 2 to 6 decreased with time. Microplot BP value s were greatest for event 2 (1.89-2.64 Ib P/acre-347-485 g P/ha). Meso plot SP losses from each event were related to days since fertilizatio n and runoff volume for each event, whereas total BPE and BP losses fo r mesoplots were related to days since fertilization and total soil lo ss. Similar relationships were found for microplots. The P index (PI) was developed to identify site characteristics that may influence P mo vement. The PI for this site fell into the medium site vulnerability c lass, and was dominated by the soil P test category. The medium site v ulnerability class seemed to underestimate the total BP losses from th e six rainfall events.