AVAILABILITY OF NITROGEN IN N-15-LABELED MATURE PEA RESIDUES TO SUBSEQUENT CROPS IN THE FIELD

The availability of N in N-15-labelled mature pea (Pisum sativum L.) r esidues 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.) a nd winter oilseed rape (Brassica napus oleifera L.) recovered about 13 % of the pea residue N during the initial 3 months of growth. At matur ity the crops had recovered 15 and 13% respectively. This indicates th at a major part of the residue N was mineralized and thus available to autumn-sown crops a few months after incorporation of the residues. A t maturity, the pea residue N constituted < 5% of total crop N, and th e incorporation of residues in soil did not significantly influence th e amount of non-labelled soil (+ fertilizer) N accumulated by crops. T he recovery of pea residue N in successive autumn-sown crops of barley , oilseed rape and wheat (Triticum aestivum L.) was 14, 3 and 2% respe ctively; spring-sown barley, oilseed rape and wheat recovered only 6, 2 and 2% respectively. Similarly, the amount of non-labelled soil N ac cumulated was almost twice as high in autumn-sown crops as compared to spring-sown crops. The concentration of organic N-15 in topsoils show ed that 55% to 59% of the organic N-15 had been released during 3 yr o f decomposition in soils cropped to autumn- or spring-sown crops respe ctively. The total crop recovery of N-15 was only 20% for autumn-sown and 9% for spring-sown crops. This indicates that of the labelled-N ap parently mineralized only 36 and 15% were recovered in autumn-sown and spring-sown cropping systems respectively. Winter barley and winter o ilseed rape established at the time of residue incorporation were foun d to utilize on average 60% of the mineralized N. This study suggests that in the first year after incorporation, pea residue N may only mak e a minor contribution to the N supply of autumn-sown crops. Significa nt amounts of pea residue N are mineralized mainly in the autumn follo wing incorporation of the residues. It is therefore important to selec t and establish a crop which is very efficient in accumulating N durin g the autumn. However, pea residue N will increase the pool of soil or ganic N and in the long term contribute to the supply of available N b y mineralization.