EFFECTS OF CARBOHYDRATE AMENDMENTS ON NUTRIENT PARTITIONING, PLANT AND MICROBIAL PERFORMANCE OF A GRASSLAND-SHRUB ECOSYSTEM

Nutrient partitioning, microbial and plant nutrient assimilation and b iomass production were investigated after addition of carbon of low an d high decomposability (sawdust and sugar, respectively) to the soil o f a non-agricultural mixed grassland-shrub ecosystem at intervals duri ng one growing season. The aim was to investigate how the energy sourc e available for soil microorganisms, and the quality of the source, in teracted with ecosystem nutrient circulation and plant performance. By the end of the season, sugar, but not sawdust amendment, had reduced the pools of soil inorganic N and P strongly whereas the pools of N an d P in the microbial biomass had increased. Nutrient uptake and biomas s production of herbs, but not of shrubs, declined to about one third of that in non-sugar amended soils, and tissue N and P concentrations declined. Total P in circulation between the soil inorganic, the micro bial and the plant biomass pools was unchanged, whereas N declined as a result of a comparatively larger decrease in plant biomass N and soi l inorganic N than the simultaneous gain in microbial biomass N. T The results show, firstly, that microbial nutrient immobilization and com petition with plants for inorganic soil nutrients can affect plant nut rient uptake and reduce biomass production strongly if microbial activ ity is stimulated by an increase of the available carbon in the soil. Secondly, the lack of increase of P or, for N, the decline of the amou nts in circulation between the biological pools and the soil inorganic pool indicates that ecosystem gross mineralization was not stimulated when extra labile carbon became available. Hence, the increased micro bial demand for nutrients without any stimulation of nutrient minerali zation depleted the soil inorganic nutrient pool, led to reduced plant nutrient uptake and biomass production, and to a higher proportion of ecosystem nutrients allocated to the microbial biomass.