Fertiliser use by winter wheat when straw is burnt or incorporated: A study with isotopically-labelled fertiliser

Abstract

The response of winter wheat to fertiliser nitrogen, when straw was burnt or incorporated during three successive seasons, was measured on a clay soil at ADAS Boxworth Experimental Husbandry Farm.

Results support the evidence from other ADAS-AFRC experiments in confirming both the smaller recovery of autumn fertiliser nitrogen and the lack of a unique yield response to it. The results also demonstrate the sufficiency of the indigenous normal soil nitrogen supply to sustain early crop growth, dispelling reservations about straw decomposition using up soil mineral nitrogen to the detriment of crop needs.

A total of 120, 200 or 280 kg/ha of fertiliser nitrogen was applied at timings nil or 40 kg/ha N shortly after crop emergence, 40 or 80 kg/N in early spring and the remainder at the start of stem extension.

Recovery of fertiliser nitrogen by the crop, and the amount remaining in soil were determined by distinguishing between soil and fertiliser derived nitrogen by using 15-N enriched or depleted ammonium nitrate. The levels of nitrogen used in this experiment were part of a larger experiment designed to produce full response curves using 0, 80, 120, 160, 200, 240, 280 and 320 kg/ha N applied in the manner described.

Optimum yields and fertiliser nitrogen requirement were both similar when straw was burnt or shallowly incorporated with tines, provided chemical weed control was successful. In contrast, ploughing in straw resulted in the largest optimum yields with notable reduction in fertiliser nitrogen requirement.

When uncontrolled grass weeds (and volunteers) were competing, optimum yield was greatly reduced and fertiliser nitrogen requirement increased, especially after the first year of non- plough incorporation.

In no case was there a unique benefit from the 40 kg/ha N added in the autumn that could not be realised by an adequate spring top dressing. Addition of 80 kg/ha instead of 40 kg/ha N in early spring was not consistently beneficial, irrespective of whether straw was burnt or incorporated.

Recovery of autumn fertiliser nitrogen by the crop in the following spring was small, between 0.9 and 9.2% depending strongly on early dry matter growth achieved up to that stage. 28 to 65% of the autumn fertiliser nitrogen survived in the top 30 cm layer of soil after winter. Marginally more of the autumn applied nitrogen remained in the topsoil after winter 1986 where straw was incorporated rather than burnt.

Recovery of fertiliser nitrogen by the crop at harvest was less when 40 kg/ha of the total nitrogen had been added in the autumn but recovery was independent of straw disposal method. Total dry matter growth achieved by the crop was of major importance in accounting for fertiliser nitrogen recovery. Seedbed conditions and competition from volunteers and grass weeds, influenced fertiliser nitrogen recovery far more than whether straw was burnt or incorporated.