Nitrogen Removal and Nitrate Leaching for Forage Systems Receiving Dairy Effluent

ABSTRACT Florida dairies need year‐round forage systems that prevent loss of N to ground water from waste effluent sprayfields. Our purpose was to quantify forage N removal and monitor nitrate N (NO−3–N) concentrations in soil water below the rooting zone for two forage systems during four 12‐mo cycles (1996–2000). Soil in the sprayfield is an excessively drained Kershaw sand (thermic, uncoated Typic Quartzipsamment). Over four cycles, average loading rates of effluent N were 500, 690, and 910 kg ha−1 per cycle. Nitrogen removed by the bermudagrass (Cynodon spp.)–rye (Secale cereale L.) system (BR) during the first three cycles was 465 kg ha−1 per cycle for the low loading rate, 528 kg ha−1 for the medium rate, and 585 kg ha−1 for the high. For the corn (Zea mays L.)–forage sorghum [Sorghum bicolor (L.) Moench]–rye system (CSR), N removals were 320 kg ha−1 per cycle for the low rate, 327 kg ha−1 for the medium, and 378 kg ha−1 for the high. The higher N removals for BR were attributed to higher N concentration in bermudagrass (18.1–24.2 g kg−1) than in corn and forage sorghum (10.3–14.7 g kg−1). Dry matter yield declined in the fourth cycle for bermudagrass but N removal continued to be higher for BR than CSR. The BR system was much more effective at preventing NO−3–N leaching. For CSR, NO−3–N levels in soil water (1.5 m below surface) increased steeply during the period between the harvest of one forage and canopy closure of the next. Overall, the BR system was better than CSR at removing N from the soil and maintaining low NO−3–N concentrations below the rooting zone.