Nitrogen-15 balance as affected by rice straw management in a rice-wheat rotation in northwest India

The sustainability of the productive rice-wheat systems of Northwest India is being questioned due to the complete removal of straw for animal consumption and fuel, or the burning of straw which has reduced the soil organic matter contents. However, straw incorporation at planting can temporarily reduce the availability of fertilizer-N and reduce crop yields. In a field study on a loamy sand soil, the effect of 6 mg ha(-1) rice straw incorporated into the soil 20 or 40 days before sowing (DBS) the wheat was compared with removal or burning of rice straw on the fate and balance of 120 kg ha(-1) of 5 atom% N-15-urea applied to wheat and to a following crop of rice. Wheat grain yield and agronomic efficiency (AE) of applied N (kg grain/kg N applied) were not influenced by rice straw management. However. N uptake (NU), and recovery efficiency (RE) of N by the difference method were lower with rice straw incorporation than with burning. Nitrogen-15 recovery by wheat was highest (41%) when the rice straw was removed or burned and lowest (30.4%) when 30 of the 120 kg N ha(-1) was applied at the time of straw incorporation at 20 DBS of wheat. However, this strategy of adding 25% of the urea-N dose at the time of straw incorporation resulted in the highest N-15 losses (45.2%). Inorganic N remaining at harvest in the 0 to 60 cm soil profile, mostly NO3-, was 5.5% after wheat and 4.2% after rice. Rice grain yields, NU, and RE were not influenced by rice straw management. Nitrogen-15 losses were similar in rice and wheat (31% with straw removed) despite total irrigation and rainfall inputs of 340 and 32 cm to rice and wheat, respectively. These results suggest to the farmers of northwest India that straw incorporation does not necessarily hurt grain yields, and indicates to researchers that work is still needed to improve N use efficiency in rice and wheat.