Nitrogen uptake by cover crops and inorganic nitrogen dynamics in Andisol paddy rice [Oryza sativa] field

Paddy field rice can conserve N in the soil under flooded conditions. However, residual soil N represents a potential environment concern when fields are no longer flooded. Winter annual grass cover crops may provide an alternative means to conserve residual soil N following rice harvest. A two years field experiment was conducted at the Ibaraki University of Experimental Farm, to compare dry matter and N uptake by rye (Secale cereale L.), oat (Avena sativa L.), triticale (Triticum secale L), wheat (Triticum aestivum L.) and fallow (no cover) in relation to soil residual N level. Dry matter and N accumulation by the following April were in the descending order of ryetriticalewheat=oatfallow, while residual soil N levels followed the reverse order. Residual soil N level exerted the greatest influence on cover crop DM accumulation, with differences in N levels becoming more pronounced by the April sampling date. On 17 April, DM differences between the low and high residual soil N levels were 3.45 vs 6.82 Mg/ha for rye (98% increase), 1.15 vs 1.45 Mg/ha for oat (26% increase), 1.49 vs 1.99Mg/ha for wheat (34% increase), and 1.70 vs 2.98 Mg/ha for triticale (75% increase), respectively. Cover crop N accumulation followed patterns similar to those for DM, but was mainly influenced by main effect factors. Residual soil N level again exerted the greatest influence on N accumulation. Between species, N accumulation for rye was greater than oat and wheat across all planting dates. By 8 March, the greatest N accumulation occurred with rye (14.0 kg N/ha), with other species accumulating 5.4 to 7.5 kg N/ha. Cover crop N accumulation increased appreciably from 10 March to 17 April. These results demonstrated that grass cover crops have a great potential for controlling soil residual N. However, additional research will be needed to determine the contribution of cover crop N to subsequent rice growth.