Carbon and nitrogen in the root-zone of barley (Hordeum vulgare L.) supplied with nitrogen fertilizer at two 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-fertilizer on the production of root-derived C and on gross immobilization (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 ¹⁵N pool abundances were used in conjunction with mathematical modelling to calculate the flows of N. N was applied at a high or a low rate, as (¹⁵NH₄)₂SO₄ solution (17.11 atom% ¹⁵N excess), before sowing. Nitrification was inhibited by using nitrapyrin (N-Serve). Pots were 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 occasions, samples were also collected from pots in a growth chamber with ¹⁴C-labelled atmosphere.The release of ¹⁴C, measured as the proportion of the total ¹⁴C translocated below ground, was higher in the high-N treatment, but the differences between treatments were small. Our results were not conclusive in demonstrating that high-N levels stimulate the decomposition and microbial utilization of root-released materials. However, the internal circulation of soil-N, calculated N fluxes (m), which were in accordance with C mineralization rates and amounts of unlabelled N found in the plants (PU), suggested that the decomposition of native soil organic matter was hampered in the high-N treatment. Apparently, towards the end of the experimental period, microorganisms in the low-N treatment used C from soil organic matter to a greater 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.Abbreviations AU Ammonium N-unlabelled - AL Ammonium N-labelled - AT Ammonium N-labelled and unlabelled (total) - NU Nitrate N-unlabelled - OU Organic N-unlabelled - OL Organic N-labelled - OT Organic N-total - PU Plant N-unlabelled (shoots and roots) - PL Plant N-labelled (shoots and roots) - PT Plant N-total (shoots and roots) - SL Sink or source of N-labelled - S Source or sink of N, mainly to and from the outer part of the cylinder - SU Sink or source of N-unlabelled - m Mineralization rate - i Immobilization rate - ua Uptake of ammonium - un Uptake of nitrate - la Loss of ammonium.