The influence of straw incorporation and soil type on the losses of soil inorganic nitrogen and its use by winter wheat

The aim of this study was to measure the extent of immobilization caused by cereal straw incorporation in three widely differing soil types in southeast England. After harvesting the previous cereal crop, plots were treated with 15N in September in two different ways. One set of plots received 50 kg N ha-1 as K15NO3 at 4.6 atom% excess, representing a pre-sowing fertilizer application. Another set received 2.5 kg N ha-1 as K15NO3 at 80.7 atom% excess. The small highly labelled application was an attempt to label the existing pool of soil nitrate. Other treatments were established with either unlabelled N or no N addition in autumn. With and without straw incorporation treatments were established on all N treatments. The 15N-labelled plots were sampled in spring of the following year (April/May). Soil type, especially clay content, had a major effect on the quantity of 15N retained in soil when plots were sampled about 8 months after application. Retention increased with increasing clay content. For example, for the treatment receiving 50 kg ha-1 of labelled N without straw incorporation, the amounts retained were 4% at Woburn (14% clay), 14% at Rothamsted (26% clay) and 33% at Northfield (39% clay). In almost all cases, incorporation of straw increased the retention of 15N in soil, but the percentage increases were much smaller for the 2.5 kg ha-1 application than for the 50 kg ha-1 application. It seems likely that the small application is giving the best indication of the behaviour of soil-derived nitrate. With the 50 kg ha-1 application, any demand for N from increased immobilization due to straw incorporation will be met to an increased extent from the added 15N-labelled inorganic N; the amount of unlabelled soil-derived inorganic N in the cultivated layer (0-23 cm) at the time of autumn 15N application (between 19 and 35 kg N ha-1) was of the same order as the 50 kg ha-1 application. The overall effect of straw on over-winter loss of labelled N was fairly small. For example, the percentage of 15N unaccounted for in crop plus soil in spring at Woburn for the 2.5 kg N ha-1 application was 82% for 'straw removed' compared to 72% for 'straw incorporated'. The corresponding figures for Rothamsted were 53% and 49% and for Northfield 25% and 36%, respectively. For the 50 kg ha-1 application, the values for 'straw removed' and 'straw incorporated' were Woburn 95% and 82%, Rothamsted 74% and 50%, and Northfield 49% and 29%, respectively. The results demonstrate that, in the temperate maritime climate of the United Kingdom, over-winter losses of N by leaching can be large and that pre-sowing autumn applications of N fertilizer are largely wasted. Also, there was no yield benefit from autumn-applied N. Over-winter losses of N were greatest from the low-clay sandy soil at Woburn and least from the high-clay soil at Northfield. (author)