COMPARED CYCLING IN A SOIL-PLANT SYSTEM OF PEA AND BARLEY RESIDUE NITROGEN

Field experiments were carried out on a temperate soil to determine th e decline rate, the stabilization in soil organic matter and the plant uptake of N from N-15-labelled crop residues. The fate of N from fiel d pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) residu es was followed in unplanted and planted plots and related to their ch emical composition. In the top 10 cm of unplanted plots, inorganic N w as immobilized after barley residue incorporation, whereas the inorgan ic N pool was increased during the initial 30 days after incorporation (DAI) of pea residues. Initial net mineralization of N was highly cor related to the concentrations of soluble C and N and the lignin:N rati o of residues. The contribution of residue-derived N to the inorganic N pool was at its maximum 30 DAI (10-55%) and declined to on average 5 % after 3 years of decomposition. Residual organic labelled N in the t op 10 cm soil declined rapidly during the initial 86 DAI for all resid ue types. Leaching of soluble organic materials may have contributed t o this decline. At 216 DAI 72, 59 and 45% of the barley, mature pea an d green pea residue N, respectively, were present in organic N-forms i n the topsoil. During the 1-3 year period, residual organic labelled N from different residues declined at similar rates, mean decay constan t: 0.18 yr(-1). After 3 years, 45% of the barley and on average 32% of the pea residue N were present as soil organic N. The proportion of r esidue N remaining in the soil after 3 years of decomposition was most strongly correlated with the total and soluble N concentrations in th e residue. The ratio (% inorganic N derived from residues):(% organic N derived from residues) was used as a measure of the rate residue N s tabilization. From initial values of 3-7 the ratios declined to on ave rage 1.9 and 1.6 after 2 and 3 yrs, respectively, indicating that a ma jor part of the residue N was stabilized after 2 years of decompositio n. Even though the largest proportion of residue N stabilized after 3 years was found for barley, the largest amount of residue N stabilized was found with incorporation of pea residues, since much more N was i ncorporated with these residues. In planted plots and after one year o f decomposition, 7% of the pea and 5% of the barley residue N were rec overed in perennial ryegrass (Lolium perenne L.) shoots. After 2 years the cumulative recovery of residue N in ryegrass shoots and roots was 14% for pea and 15% for barley residue N. The total uptake of non-lab elled soil N after 2 years of growth was similar in the two residue tr eatments, but the amount of soil N taken up in each growth period vari ed between the treatments, apparently because the soil N immobilized d uring initial decomposition of residues was remineralized later in the barley than in the pea residue treatment. Balances were established f or the amounts of barley and mature pea residue N remaining in the 0-1 0 cm soil layer and taken up in ryegrass after 2 years of decompositio n. About 24% of the barley and 35% of the pea residue N were unaccount ed for. Since these apparent losses are comparable to almost twice the amounts of pea and barley residue N taken up by the perennial ryegras s crop, there seems to be a potential for improved crop residue manage ment in order to conserve nutrients in the soil-plant system.