CARBON AND NITROGEN IN THE ROOT-ZONE OF BARLEY (HORDEUM-VULGARE L) SUPPLIED WITH NITROGEN-FERTILIZER AT 2 RATES

Below-ground carbon (C) production and nitrogen (N) flows in the root- zone of barley supplied with high or low amounts of N-fertilizer were investigated. Interest was focused on the effect of the level of N-fer tilizer on the production of root-derived C and on gross immobilizatio n (i) and gross mineralization (m) rates. The plants were grown for 46 days in a sandy loam soil. Principles of pool dilution and changes in N-15 pool abundances were used in conjunction with mathematical model ling to calculate the flows of N. N was applied at a high or a low rat e, as ((NH4)-N-15)2SO4 solution (17.11 atom% N-15 excess), before sowi ng. Nitrification was inhibited by using nitrapyrin (N-Serve). Pots we re sampled four or five times during the experimental period, i.e. 0, 22, 30, 38 and 46 days after germination. On the three last sampling o ccasions, samples were also collected from pots in a growth chamber wi th C-14-labelled atmosphere. The release of C-14, measured as the prop ortion of the total C-14 translocated below ground, was higher in the high-N treatment, but the differences between treatments were small. O ur results were not conclusive in demonstrating that high-N levels sti mulate the decomposition and microbial utilization of root-released ma terials. However, the internal circulation of soil-N, calculated N flu xes (m), which were in accordance with C mineralization rates and amou nts of unlabelled N found in the plants (PU), suggested that the decom position of native soil organic matter was hampered in the high-N trea tment. Apparently, towards the end of the experimental period, microor ganisms in the low-N treatment used C from soil organic matter to a gr eater extent than C they used from root released material, presumably because lower amounts of mineral N were available to microorganisms in the low-N treatment. Immobilization of N appeared to be soil driven ( organisms decomposing soil organic matter account for the N demand) at low-N and root-driven (organisms decomposing roots and root-derived C account for the N demand) at high-N.