Nitrogen conservation and recycling and carbon management for the sustainability of intensive rice-based cropping system

Twelve out of twenty wells/pumps used for irrigation and drinking had near or above the WHO's nitrate-N limit for drinking water of 10/L. Soil mineral N (100 cm depth) ranging from 111 to 694 kg/ha was reduced by 10-68 percent in plots with DTW [dry-to-wet] transition crop, whereas in fallow plots, it was either maintained or increased across the seven sites. Unlike in planted plots, nitrate- in fallow plots moved from the 0- to 50- to 100-cm soil profile with up to 86 kg N/ha, demonstrating nitrate- leaching without a crop. Corn, with its past growth and shallow rooting system, was efficient in reducing nitrate- from the upper layers, whereas indigo, with initial slow growth and deep rooting, was efficient in the lower soil profile. On average, corn captured 176 kg N/ha and indigo 194 kg N/ha. Indigo acquired 20 percent N from biogical N2 fixation (BNF); this relatively low contribution was due to inhibition by high soil mineral N. In both fallow and planted plots, mineral N declined to low levels at 100 percent water-filled pore spaces (WFPS) before rice transplanting. A suggestion for developing indigo plus corn N catch crop technology is made to reduce nitrate leaching and maximize N use efficiency besides providing cash benefit to farmers from corn in rice-sweet pepper cropping system. In WS [wet season] rice, most of the ammonium - N (87 percent) was found in the upper layer (0-to 25-cm) whereas it was either absent or low at lower depths. Indigo, in combination with mungbean or alone grown in DTW transition and recycled in WS, showed an increasing trend of ammonium - N initially (15 DAT) and then decline thereafter. Corn residue started to mineralize after 29 DAT and ammonium - N peaked at 60 DAT. It declined thereafter. However, nitrate - N showed a trend opposite that of NH4 raised to the positive power-N. The high nitrate - N observed initially started to decline with an increase in WFPS of 60 percent or above and increased with a decrease in WFPS (60 DAT). The turnover of residue N from indigo mixed with mungbean was the highest (53 percent) and was comparable with indigo residue mixed with prilled urea (PU) (51 percent). Nitrogen released from corn residue was improved when it was mixed with tablet urea (TU) (43 percent) compared with corn residue alone (24 percent) or in combination with PU (38 percent). Catch crop residue not only increased organic N pool by 2-13 percent but also reduced nitrite emissions by 26-35 percent compared with without residue. The labile C pool was found to decrease with cropping and to increase with catch crop residue incorporation, but no significant change was observed in total C (CT) pool. This indicates that the labile C pool is sensitive to soil management practices, and hence is a more sensitive indicator of the C dynamics of the system. The labile C balance sheet indicated a loss of about 600 kg/ha in farmer's practice and a gain of 2500 kg/ha from catch crop residue especially indigo. The C management index, an indicator of a system's overall performance, was increased due to recycling of catch crop residue particularly indigo compared with without residue. The application of TU resulted in the highest N uptake and grain yield but N use efficiency was highest with indigo. Nitrogen uptake and grain yield obtained with PU was comparable with an equivalent amount of residue N. The N balance sheet demonstrated the ability of indigo and corn to reduce N leaching by 178-208 kg/ha and 215-244 kg/ha, respectively