Nitrogen management in wheat sown into rice crop residues

The sustainability of the rice-wheat cropping system of north-west India is under threat due to several constraints, of which burning crop stubbles, and delay in wheat sowing with the alternative of stubble incorporation, are the major issues. Burning stubble leads to emission of green house gases, loss of nutrients, diminished soil biota, and reduction in soil fertility. Government initiatives to halt the burning of stubble in India and elsewhere in South Asia have been assisted with purpose built machinery for establishing crops into stubble, for example, the Happy Seeder'. However, farmers may also need new guidelines for N management of crops established in stubble, because the addition of stubble alters the N mineralisation or immobilisation processes, which may influence crop growth and grain yield. Presently, there is insufficient information regarding the effect of rice stubble mulch on N immobilisation and N mineralisation, and subsequently on the growth and yield of crop. Therefore, in this thesis, the effect of rice stubble mulch on the availability of N from soil and applied fertiliser, and the growth, N uptake and grain yield of wheat has been studied. The studies were conducted in a controlled environment and in fields: in a rain fed dry environment (Wagga Wagga, Australia) and an irrigated environment (Ludhiana; Punjab, India).Under the controlled conditions of constant temperature (20 oC) and constant soil moisture, a rice stubble mulch on the soil surface resulted in N immobilisation that was sustained for 8 weeks longer than when no stubble was applied. Under these controlled conditions, higher rates of stubble mulch had a diminishing effect in decreasing available mineral N, for example, net immobilisation decreased from 17 to 9 kg Nt of stubble as the rate of stubble mulch increased from 4 to 12 tha.