Direct effect of fertilization on microbial carbon transformation in grassland soils in dependence on the substrate quality

Abstract

AbstractResponse of microbial metabolism (growth, substrate utilization, energetic metabolism) to fertilization by N and P and resulting changes in soil‐organic‐matter (SOM) decomposition (priming effect) were studied in grassland soils with relatively high organic‐matter content. Treatments with and without glucose addition were studied to simulate difference between rhizosphere and bulk soil. Our expectation was that fertilization would decrease soil respiration in both treatments due to an increased efficiency of microbial metabolism. At first, fertilization activated microbial metabolism in both treatments. In glucose‐nonamended soils, this was connected with a short‐term apparent priming effect but if glucose was available, the higher energetic demand was covered by its mineralization in preference against SOM, causing significant SOM savings as compared to unfertilized soils. After a relatively short period of 1–3 d, however, the phase of deprived microbial metabolism occurred in both treatments, which was characterized by lower soil respiration in fertilized than in unfertilized soils. Fertilization further decreased net microbial growth following glucose addition, shortened turnover time of microbial biomass and changed the partitioning of assimilated glucose within microbial biomass (decreased accumulation of storage compounds and increased the proportion of mineralized glucose). As a result, fertilization reduced soil respiration mainly due to a deprivation of microbial metabolism. The rate and range of microbial response to fertilization and also the amount of saved soil C were larger in the soil with higher SOM content, likely driven by the higher content of microbial biomass.