C-CYCLING AND N-CYCLING DURING DECOMPOSITION OF ROOT MUCILAGE, ROOTS AND GLUCOSE IN SOIL

C and N dynamics were followed during decomposition of root mucilage, roots and glucose in soil, incubated for 6 months at 25-degrees-C. Eac h of the substrates, derived from maize plants, was added at two rates , which ranged from ca 100 to 450 mg C kg-1 soil. Carbon mineralizatio n from each substrate was determined from C-13 variations at natural a bundance. Nitrogen fluxes were calculated by artificial N-15 tracing o f soil nitrate-N. The kinetics of C and N fluxes, expressed per unit o f added C, were almost independent of the C rate for each of the three substrates. The calculated true mineralization rate of root mucilage- C was comparable to glucose mineralization; mineralization of roots-C was slower. A positive 'priming effect' was found and was proportional to the amount of substrate-C added. The priming increased throughout the incubation and was more pronounced for glucose than for mucilage o r roots. In contrast, there was no priming effect for N during glucose decomposition. Gross immobilization of labelled nitrate-N reached a m aximum of 72, 19 and 61 mg N g-1 added C for mucilage, roots and gluco se, respectively. This immobilization was obtained when the true miner alization rates of carbon were 35, 38 and 40%. Gross N immobilization and true C mineralization were highly correlated during root decomposi tion. Total N assimilation, i.e. gross immobilization of NO3-N plus mi crobial assimilation as NH4-N or organic N, was estimated by applying the isotope dilution method to biomass-N. It reached 88, 66 and 61 mg N g-1 added C for mucilage, roots and glucose, respectively. The highe r N assimilation for mucilage was probably due to exclusive bacterial decomposition, whereas fungi were involved for the two other substrate s. The remineralization of N and the decline in biomass-N during the s econd part of the incubation were markedly faster and more complete fo r mucilage than for roots and glucose.