Effects of long-term mineral fertilization on microbial biomass, microbial activity, and the presence of r- and K-strategists in soil

Long-term effects of mineral fertilization on microbial biomass C (MBC), basal respiration (RB), substrate-induced respiration (RS), β-glucosidase activity, and the r–K-growth strategy of soil microflora were investigated using a field trial on grassland established in 1969. The experimental plots were fertilized at three rates of mineral N (0, 80, and 160 kg ha−1 year−1) with 32 kg P ha−1 year−1 and 100 kg K ha−1 year−1. No fertilizer was applied on the control plots (C). The application of a mineral fertilizer led to lower values of the MBC and RB, probably as a result of fast mineralization of available substrate after an input of the mineral fertilizer. The application of mineral N decreased the content of C extracted by 0.5 M K2SO4 (Cex). A positive correlation was found between pH and the proportion of active microflora (RS/MBC). The specific growth rate (μ) of soil heterotrophs was higher in the fertilized than in unfertilized soils, suggesting the stimulation of r-strategists, probably as the result of the presence of available P and rhizodepositions. The cessation of fertilization with 320 kg N ha−1 year−1 (NF) in 1989 also stimulated r-strategists compared to C soil, probably as the result of the higher content of available P in the NF soil than in the C soil.