INFLUENCE OF LIGHT-INTENSITY ON THE DISTRIBUTION OF CARBON AND CONSEQUENT EFFECTS ON MINERALIZATION OF SOIL-NITROGEN IN A BARLEY (HORDEUM-VULGARE L)-SOIL SYSTEM

A pot experiment was conducted in a C-14-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with N-15. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of N-15-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated C-14 was lower in the low-I treatment than in the high-I treatment. The lower rates of C-14 and N-15 incorporation into the soil biomass were associated with less root-derived C-14. During the chamber period ((CO2)-C-14-atmosphere), mineralized amounts of N-15 (measured as plant uptake of N-15) were small and represented about 6.8 to 7.8% of the initial amount of organic N-15 in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil N-15 which was stabilized. The amounts of N-15 mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.