Turnover of 15n-straw and nh 4no 3 in a sandy loam soil: effects of straw disposal and n fertilization

Abstract

Soil was sampled from a field experiment where spring barley had been growing continuously for 10 yr. Straw had either been baled and removed or incorporated annually at a rate of 121 ha −1. Plots were unfertilized or received recommended dressings of fertilizer N. 15N-labelled barley straw was incubated with the soil at 25°C under moisture regimes similar to those prevailing in early autumn when straw is incorporated (termed moist) or at moisture contents found during late autumn, winter and early spring (termed wet). After 7 weeks of incubation with 15N-straw in moist soil, total N in the microbial biomass increased by up to 82%. Significant increases were observed after 2 weeks. Biomass 15N decreased between weeks 2 and 7 despite an increase in biomass total N during this period. When soils were incubated for 7 weeks under moist conditions, 15N derived from the straw accounted for 41–49% of the increase in biomass total N. The increase in microbial biomass N induced by addition of 15N-straw was slightly higher in soil exposed to straw incorporation. Unlabelled NH 4NO 3 was incubated with soil sampled in autumn before straw incorporation, and with soil taken in spring from plots where straw had been incorporated in the previous autumn. Biomass N was reduced considerably after 7 weeks of incubation with NH 4NO 3. N mineralization in the spring-sampled soil with annual straw incorporation was about half that found in soil from plots with straw removal. In autumn-sampled soil, N mineralization rates in unfertilized soils were similar irrespective of previous straw disposal. The effect of straw incorporation on N turnover in the field, therefore, seems to be very small after 1 yr.