Biochar can be used as a strategy to facilitate C and N sequestration in less fertile soil. Surface soil samples from a 4 years long field experiment were collected to assess the effect of added biochar (0, 50, 90 and 130 t ha-1) and chemical fertilizers (viz., urea for N, single superphosphate for P and potassium chloride for K) on soil respiration and N mineralization during lab incubation experiment under wheat-mungbean and wheat-maize cropping systems. The rate of CO2 evolution increased significantly (P<0.05) with increasing level of added biochar during each incubation period. At day 2, the rate of CO2 evolution increased from 142 µg CO2 g-1 soil d-1 in the control to 259 µg CO2 g-1 soil d-1 in the treatment receiving 130 t biochar ha-1. The same trend continued during 4, 8 and 16 days of incubation periods. However, the rate of CO2 evolution declined over time in each treatment. The effect of mineral fertilizers and cropping systems on CO2 evolution were statistically non-significant (P<0.05). The response of cumulative CO2 production to biochar, mineral fertilizers and cropping systems was almost similar to rate of CO2 production. The data on net N mineralization revealed that maximum N mineralization was observed for the treatment which had received biochar at 90 t ha-1 over the last few years whereas the differences between the control (no biochar), lower biochar (50 t ha-1) treatments and highest biochar (130 t ha-1) treatment were statistically non-significant during 7, 14 and 28 days of incubation periods. Nitrogen mineralization did not vary significantly among the mineral fertilizers or cropping systems treatments. Moreover, the interactive effects of cropping systems x biochar, and cropping systems x mineral fertilizers were significant for CO2 efflux and N mineralization at least during the first 2 days of incubation period. These results suggest that biochar amendment enhanced CO2 evolution during short-term incubation period (i.e., 16 days) suggesting the availability of labile C pool in the added biochar which serve as a source of energy for promotion of microbial activity in the soil. The low net N mineralization in the high biochar treatment implies that biochar could be used as a strategy to minimize N losses from the easily available labile N pools through greater N retention in soil.
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