Crop rotation and residue management effects on carbon sequestration, nitrogen cycling and productivity of irrigated rice systems

The effects of soil aeration, N fertilizer, and crop residue management on crop performance, soil N supply, organic carbon (C) and nitrogen (N) conten t were evaluated in two annual double-crop systems for a 2-year period (199 4-1995). In the maize-rice (M-R) rotation, maize (Zea mays, L.) was grown i n aerated soil in the dry season (DS) followed by rice (Oriza sativa, L.) g rown in flooded soil in the wet season (WS). In the continuous rice system (R-R), rice was grown in flooded soil in both the DS and WS. Subplot treatm ents within cropping-system main plots were N fertilizer rates, including a control without applied N. In the second year, sub-subplot treatments with early or late crop residue incorporation were initiated after the 1995 DS maize or rice crop. Soil N supply and plant N uptake of 1995 WS rice were s ensitive to the timing of residue incorporation. Early residue corporation improved the congruence between soil N supply and crop demand although the size of this effect was influenced by the amount and quality of incorporate d residue. Grain yields were 13-20% greater with early compared to late res idue incorporation in R-R treatments without applied N or with moderate rat es of applied N. Although substitution of maize for rice in the DS greatly reduced the amount of time soils remained submerged, the direct effects of crop rotation on plant growth and N uptake in the WS rice crops were small. However, replacement of DS rice by maize caused a reduction in soil C and N sequestration due to a 33-41% increase in the estimated amount of mineral ized C and less N input from biological N fixation during the DS maize crop . As a result, there was 11-12% more C sequestration and 5-12% more N accum ulation in soils continuously cropped with rice than in the M-R rotation wi th the greater amounts sequestered in N-fertilized treatments. These result s document the capacity of continuous, irrigated rice systems to sequester C and N during relatively short time periods.