Nitrogen recovery by cover crops in relation to time of planting and growth termination

Abstract

Residual fertilizer nitrogen (N) in soil represents a potential environmental contaminant because of the risk of nitrate (NO₃) leaching into ground water. Winter annual grass cover crops can conserve residual soil N; however, their ability to recover N greatly varies with management. A two-year field experiment in the North Carolina Coastal Plain on a State fine sandy loam (fine-loamy, mixed, thermic Typic Hapludult) compared dry matter (DM), N accumulation, and soil inorganic N following rye (Secale cereale L.), winter wheat (Triticum aestivum L.), triticale (Triticum secale L.), black oats (Avena strigosa L.), and fallow (native weeds) in relation to planting (October, November, and December) and growth termination (early March, early April, and late April/early May) dates and levels of residual soil N. Cover crop DM and N accumulations decreased with successive planting date delays (November and December) for each growth termination date in the following spring. In 2000, cumulative DM yields of October- and November-planted cover crops were 3.17 Mg ha⁻¹ (1.41 tn ac⁻¹) and 3.74 Mg ha⁻¹ (1.66 tn ac⁻¹), respectively, which were significantly greater than those of December-planted cover crops (1.90 Mg ha⁻¹ [0.84 tn ac⁻¹]) at the second growth termination date (early April). Cover crop N accumulation increased with a delay in growth termination, although the interaction between cover crop species and the planting date varied. October planting of rye and triticale showed greater N accumulation at a March growth termination date, averaging 37.8 kg N ha⁻¹ (33.7 lb N ac⁻¹) for rye and 37.6 kg N ha⁻¹ (33.5 lb N ac⁻¹) for triticale; these values were 7.8% and 14.1% higher than those of black oat and wheat, respectively, at the same planting and growth termination dates. November-planted wheat showed in the highest N accumulation (average, 57.9 kg N ha⁻¹ [51.7 lb N ac⁻¹]) at the late April/early May termination date; this was 3.8% to 7.9% higher than that of other species. In contrast, black oat showed a greater ability to scavenge soil residual N with a late planting and growth termination date combination (average, 58.7 kg N ha⁻¹ [52.4 lb N ac⁻¹]). The cover crop planting date affected soil inorganic N distribution at each termination date, with an October planting date resulting in lower soil profile inorganic N levels than November and December planting dates. Lower soil inorganic N concentrations were strongly associated with increases in cover crop DM and N accumulation for each growth termination date. These results should help growers identify the best niche for a cover crop with respect to recovering residual soil N in their various rotations.