SOIL-NITROGEN DYNAMICS IN RESPONSE TO CARBON INCREASE IN A MEDITERRANEAN SHRUBLAND OF SW SPAIN

Most models predict that high atmospheric CO2 concentrations will lead to an increase in the C-to-N ratio of litter production in terrestria l ecosystems. The effect of an increase in the soil C-to-N ratio on th e nitrogen dynamics in a Mediterranean shrubland was simulated by mixi ng with the lifter layer wood shavings with a high C-to-N ratio. Sampl es of mineral soil, taken subsequently eight times during 404 d, were analyzed for total C, total N, total soil carbohydrates, potential net N mineralization, potential net nitrification and microbial biomass-N . We found significant increases in the concentration of total carbohy drates, C-to-N ratio and microbial biomass N in amended soils during t he experiment, while potential net N mineralization rate and net nitri fication rate significantly decreased; amounts of available nitrogen ( NH4+-N + NO3-N) were unaffected by the amendment treatment. However, b y the end of the experiment, no significant differences between amende d and control soil samples were found. The total carbohydrates-to-K2SO 4-extractable total-N ratio was the best predictor of both net mineral ization rate and microbial biomass N, showing that the available C-to- available-N ratio is a better indicator of N dynamics than the total C to total N ratio. Our results support the hypothesis that increasing C availability in soils leads to a decrease in N availability for plan ts through the immobilization of N in microbial biomass and to an incr ease in the temporal heterogeneity of soil properties in a Mediterrane an shrubland. (C) 1998 Elsevier Science Ltd. All rights reserved.