Disentangling the effects of mineral fertiliser N, P and K on microbial biomass, necromass and ionome in soil from the Askov long-term field experiment

Given the increasingly recognised importance of microbial biomass (MB) in soil organic carbon (SOC) sequestration, knowledge of the microbial ionome beyond carbon (C), nitrogen (N) and phosphorus (P) becomes crucial. The microbial ionome could indicate nutritional restrictions related to MB growth and microbial necromass C (MN-C) accumulation. In this study, soils receiving different combinations of mineral N, P and potassium (K) were sampled in the Askov long-term field experiment, Denmark, and analysed for MB elemental composition including N, P, K, magnesium (Mg), manganese (Mn) and zinc (Zn) using fumigation-extraction and ICP-OES. Furthermore, bacterial, archaeal and fungal gene abundance was determined by qPCR as microbial community shifts may relate to microbial ionome shifts. MN-C was determined by amino sugar analysis. MB-C was unaffected by fertiliser treatments and not correlated with MN-C. N fertilisation increased MN-C. N and K additions increased plant-derived SOC, indicating the importance of N for microbial and plant-derived SOC accumulation. Availability of P and K increased MB-P and MB-K, respectively, and reduced the MB-C:P ratio but not MB-C:K. N fertilisation reduced the pH and increased Mn availability which increased MB-Mn and reduced MB-C:Mn. ITS1 gene copies responded positively to P availability. A reduced MB-C:Mn ratio was associated with a relative increase in fungal gene copy abundance. This was linked to an increase in SOC, indicating a positive link between Mn availability, fungal abundance and SOC level, yet this was not reflected in MN-C accumulation, but by the accumulation of plant-derived SOC, possibly due to reduced plant C turnover.