Influence of the N and P status of plant material and of added N and P on the mineralization of C from 14C-labelled ryegrass in soil
1996
Cheshire, M.V. | Chapman, S.J.
Ryegrass was grown under conditions of low N, low P, or high N and P nutrient supply in an atmosphere containing 14CO2 and then incubated in soil supplemented with or without N or P fertilizer. Determined in fresh plant tissue, the persistency of residual labelled C after 6 months was in the order low-N plants > low-P plants > high-N and -P plants. The addition of N conserved C, particularly when there was additional P present. Hydrolysable labelled C (12 M/0.5 M H2SO4) showed similar trends. In analyses of freeze-dried plant tissue, the main effect was also the increased persistency of C from low-N plants compared to high-N plants. The addition of N fertilizer increased the persistence of plant residue C, but only with grass containing low P. The addition of P fertilizer had no effect. In freeze-dried low-P plant tissue, sampled after 1.5, 6, and 12 months, the conserving effect of adding fertilizer N was confirmed. The addition of P, in contrast, enhanced the rate of decomposition. After 6 months, about a third of the C remained, and after 12 months, about one-quarter. It is concluded that P, whether intrinsic or added, can increase the rate of decomposition of organic residues in soil, but there is a strong interaction with N, which has a predominant influence. The effects of N depend on the form it is in. Increased intrinsic tissue N can increase the rate of C loss, whereas added inorganic N can decrease the rate of C loss during decomposition.
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