Effects of Carbohydrate Amendments on Nutrient Partitioning, Plant and Microbial Performance of a Grassland-Shrub Ecosystem

Abstract

Nutrient partitioning, microbial and plant nutrient assimilation and biomass production were investigated after addition of carbon of low and high decomposability (sawdust and sugar, respectively) to the soil of a non-agricultural mixed grassland-shrub ecosystem at intervals during one growing season. The aim was to investigate how the energy source available for soil microorganisms, and the quality of the source, interacted 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 and P in the microbial biomass had increased. Nutrient uptake and biomass 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 microbial and the plant biomass pools was unchanged, whereas N declined as a result of a comparatively larger decrease in plant biomass N and soil inorganic N than the simultaneous gain in microbial biomass N. The results show, firstly, that microbial nutrient immobilization and competition with plants for inorganic soil nutrients can affect plant nutrient uptake and reduce biomass production strongly if microbial activity 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 amounts 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 microbial demand for nutrients without any stimulation of nutrient mineralization 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.