Effect of stubble management technique on soil and fertiliser nitrogen recovery by wheat sown after rice

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Abstract
Effects of rice stubble and nitrogen (N) fertiliser management strategies on soil and 15N labelled fertiliser recovery by wheat were studied in 2 microplot experiments in the field. In 1 experiment, rice stubble which received 113 kg N ha-1 as 15N labelled (NH4)2SO4 was treated in 1 of 4 ways: (i) burnt, no till; (ii) incorporated 6 weeks before wheat sowing; (iii) incorporated at wheat sowing; or (iv) retained on the soil surface, no till. Burning, instead of incorporating rice stubble at wheat sowing, increased N uptake by wheat by 47% and the uptake of residual 15N applied to the previous rice crop from 1.2 to 2.1 kg N ha-1. Retaining rice stubble on the soil surface increased recovery of residual 15N to 2.5 kg ha-l. In a second experiment, 60 plots within a rice paddy were fertilised with 100 kg N ha-l while another 60 were not fertilised. Fertilised plots produced 21 t ha-l of stubble while unfertilised plots produced 11.5 t ha-1. The 2 stubble levels were factorially combined with 5 stubble management treatments - the 4 treatments mentioned above plus a burn with tillage treatment. 15NH415NO3 was applied at sowing, tillering, or as a 50: 50 split between sowing and tillering. Tillage, with or without stubble burning, reduced yield (by 0.7 t ha-l), and uptake of N from soil (by 12 kg N ha-1) and from fertiliser (by 10 kg N ha-1) compared with the no till treatments. Increasing quantities of rice stubble on plots where stubble was incorporated at wheat sowing significantly reduced wheat performance and increased the proportion of N derived from fertiliser. Delaying N application also increased the proportion of wheat N derived from fertiliser. In the intensive rotations reported in the study, management strategies with no till, and with or without stubble burning, increased wheat uptake of both soil and fertiliser N, and this maximised yield.