Distinguishing fertilization effects on soil microbial indices from unintentional variations during a 60-week incubation at 5 and 25 °C

The core aim of the current study was to distinguish true fertilization and temperature effects on soil microbial indices from unintentional variations of data obtained by well-known methods, sampled repeatedly during a 60-week period at 5 and 25 °C. Two floodplain soils were analyzed by chloroform fumigation-extraction (CFE), multi-substrate induced respiration (MSIR), and fungal ergosterol. The two soils were similar in texture and pH but differed in management history. The organic farming (OF) soil was fertilized with farmyard manure. The conventional farming (CF) soil regularly received inorganic fertilizers and straw, which increased the ergosterol content. The 60-week storage at 5 °C had generally a minor impact on all properties analyzed in the two soils. However, significant differences on microbial indices were repeatedly observed even at this temperature, which cannot be explained by growth and death processes. The reasons for these unintentional variations are discussed in detail. In the slightly alkaline to neutral range of the two soils, nitrification caused a small but consistent decline in soil pH. This was the most significant factor affecting functional microbial diversity, i.e., the response of soil microorganisms to glucose and other low-molecular weight substances of the MSIR approach. Microbial biomass C (MBC) by SIR as well as MBC and MBN by CFE declined by 30 to 40 % at 25 °C in comparison with 5 °C. The decline in ergosterol content was even stronger. The CF-soil exhibited a stronger loss in soil organic matter accompanied by a higher NO₃N mineralization rate, particularly, at 5 °C and a decline in K₂SO₄ extractable C at 25 °C. Also the microbial biomass and functional diversity of the OF-soil were more resilient to temperature-induced starvation than those of the CF-soil.