Availability of nitrogen in 15N-labelled mature pea residues to subsequent crops in the field

Abstract

The availability of N in 15N-labelled mature pea ( Pisum sativum L.) residues to subsequent autumn-sown or spring-sown crops was studied in two experiments in field microplots. The residues were incorporated in the soil in August or September. Winter barley ( Hordeum vulgare L.) and winter oilseed rape ( Brassica napus oleifera L.) recovered about 13% of the pea residue N during the initial 3 months of growth. At maturity the crops had recovered 15 and 13% respectively. This indicates that a major part of the residue N was mineralized and thus available to autumn-sown crops a few months after incorporation of the residues. At maturity, the pea residue N constituted <5% of total crop N, and the incorporation of residues in soil did not significantly influence the amount of non-labelled soil (+fertilizer) N accumulated by crops. The recovery of pea residue N in successive autumn-sown crops of barley, oilseed rape and wheat ( Triticum aestivum L.) was 14, 3 and 2% respectively; spring-sown barley, oilseed rape and wheat recovered only 6, 2 and 2% respectively. Similarly, the amount of non-labelled soil N accumulated was almost twice as high in autumn-sown crops as compared to spring-sown crops. The concentration of organic 15N in topsoils showed that 55% to 59% of the organic 15N had been released during 3 yr of decomposition in soils cropped to autumn- or spring-sown crops respectively. The total crop recovery of 15N was only 20% for autumn-sown and 9% for spring-sown crops. This indicates that of the labelled-N apparently mineralized only 36 and 15% were recovered in autumn-sown and spring-sown cropping systems respectively. Winter barley and winter oilseed rape established at the time of residue incorporation were found to utilize on average 60% of the mineralized N. This study suggests that in the first year after incorporation, pea residue N may only make a minor contribution to the N supply of autumn-sown crops. Significant amounts of pea residue N are mineralized mainly in the autumn following incorporation of the residues. It is therefore important to select and establish a crop which is very efficient in accumulating N during the autumn. However, pea residue N will increase the pool of soil organic N and in the long term contribute to the supply of available N by mineralization.