Carbon and nitrogen dynamics in submerged lowland soils.

Seasonal patterns of soil extractable inorganic N (EIN), extractable organic N (EON), soluble sugars (SS), total carbohydrates (T Cbh) and crop N uptake were compared in rice-rice and rice-maize systems at IRRI. These parameters were examined in the 1994 dry season in maize and rice main plots with N rate subplots of 0, 95, or 190 kg N/ha (N0, N95, N190). There were large differences in N and C dynamics due to cropping system. Fertilizer N rates had little effect on EIN and EON in rice plots. Regardless of N rate, EIN (mostly NH4+ -N) ranged from 12 kg N/ha at transplanting, to a maximum of 23 kg N/ha at 18 days after transplanting (DT) and decreased to 4 kg N/ha at 67 DT (flowering). Similarly, EON was 13 N/ha at transplanting then decreased to 6 kg N/ha at 67 DT. In the maize plots, N rate had a large impact in EIN (mostly N03- -N) and EON. EIN was greatest in N 190 plots with 16 kg N/ha at planting, a maximum of 34 kg N/ha at 18 days after emergence (DE), and 28 kg N/ha at 59 DE (flowering). Likewise, EON was 50 percent greater at flowering in N 190 (21 kg N/ha) than in N0 (11 kg N/ha), but similar in both treatments before flowering. N rate did not affect TCbh in either rice or maize system, although higher TCbh was observed in maize (1230 kg C/ha) than in rice (1100 kg C/ha). Neither cropping systems nor N rate influenced SS content. Rice grain yield increased from 4.2 t/ha in N0 treatments to 8.5 t/ha with N 190. Maize grain yields were 2.2 t/ha at N0 and 7.0 t/ha at N 190. At maturity, N accumulation was 49 kg N/ha for both rice and maize in N0 treatments. In N 190 treatments, N accumulation increased to 148 kg N/ha in rice, representing a recovery efficiency of 52 percent; maize N accumulation was 177 kg N/ha, for a recovery efficiency of 67 percent. These results suggest that the increased fertilizer use efficiency by maize reflected greater retention of applied N in EIN and EON pools throughout the growing season, and particularly in the period of greatest crop demand.