Soil microorganisms require a range of essential elements for their optimal functioning and store several elements in the microbial biomass (MB), such as carbon (C), nitrogen (N), phosphorus (P) and sulphur (S), as well as other secondary and trace elements. The C, N and P content of the microbial biomass has been quantified in many studies for many years, whereas S has been the focus only in a few studies, despite the availability of methods and the relevance of MBS for the S turnover in soils. To illustrate the relevance of MBS, this review aims at summarizing the current state of knowledge on the quantities of MBS in different soils, influencing environmental and agricultural management factors, methodological shortcomings, and prospects for soil microbial biomass research. Median MBS contents were 6.0 µg g⁻¹ soil in arable, 7.6 µg g⁻¹ soil in grassland, and 5.7 µg g⁻¹ soil in forest soils. All extractants used led to similar MBS contents in soils with similar soil organic (SO) C contents. MBC and soil pH positively explained MBS, using multiple linear regression analysis. Median MB-C/S ratios increased in the order arable (55), grassland (85), and forest (135) soils. As the overall quantity of MBS data is still small, future studies are required to verify these observations. Moreover, future research needs to more strongly consider stoichiometric relationships of elements in the soil and the soil microbial ionome. The role of S and its complex relationship with the availability of other elements in soils for the soil microbial biomass and its functions remains to be elucidated.

Crop-specific cultivation practices including crop rotation, cover cropping, and fertilisation are key measures for sustainable farming, for which soil microorganisms are important components. This study aims at identifying links between agronomic practices, potato yield and quality as well as soil microorganisms. We analysed the roles of cover crops and of the soil prokaryotic, fungal, and protistan communities in a long-term trial, differing in crop rotation, i.e. winter wheat or silage maize as pre-crop, presence and positioning of oil radish within the rotation, and fertilisation, i.e. mineral fertiliser, straw, manure, or slurry. Up to 16% higher yields were observed when oil radish grew directly before potatoes. Losses of potato quality due to infection with Rhizoctonia solani-induced diseases and common scab was 43–63% lower when wheat + oil radish was pre-crop under manure or straw + slurry fertilisation than for maize as pre-crop. This contrast was also reflected by 42% higher fungal abundance and differences in β-diversity of prokaryotes, fungi, and protists. Those amplicon sequence variants, which were found in the treatments with highest potato qualities and differed in their abundances from other treatments, belonged to Firmicutes (2.4% of the sequences) and Mortierellaceae (28%), which both comprise potential antagonists of phytopathogens. Among protists, Lobosa, especially Copromyxa, was 62% more abundant in the high potato quality plots compared to all others, suggesting that specific higher trophic organisms can improve crop performance. Our findings suggest that successful potato cultivation is related (1) to planting of oil radish before potatoes for increasing yield and (2) to fertilisation with manure or straw + slurry for enriching the microbiome with crop-beneficial taxa.

Secondary and trace elements may be limiting soil microbial functioning, albeit microbial demand and content remain largely unknown and methods for their in situ detection are limited. Thus, the objective of the present study was to take the first step towards the method development for the assessment of the soil microbial ionome, that is, the elemental composition of soil microbial communities. Chloroform (CHCl ₃ ) fumigation extraction was used for the detection of microbial CHCl ₃ ‐labile secondary and trace element concentrations in soils. The suitability of two extractants (NH ₄ NO ₃ , CaCl ₂ ) for the quantification of CHCl ₃ ‐labile concentrations of phosphorus, sulphur, potassium, sodium, and magnesium, as well as selenium, iron, zinc, manganese, copper, cobalt, nickel, molybdenum, vanadium, boron, silicon, barium, arsenic, and cadmium, were tested in six agricultural soils. Additionally, three soil to extractant ratios (1:5, 1:10, and 1:20) and two extraction durations, 1 or 2 h, were tested in a subset of two soils. Out of the two extractants tested, 0.01 M CaCl ₂ was found to be the best‐suited extractant. For CaCl ₂ , a soil‐to‐extractant ratio of 1:20 with an extraction time of 1 h was best for the majority of elements in the two soils tested. In a limited number of agricultural soils, we were able to show that CHCl ₃ fumigation extraction can successfully be applied to the elements phosphorus, sulphur, potassium, sodium, magnesium, zinc, manganese, copper, nickel, vanadium, boron, silicon, and barium to yield a CHCl ₃ ‐labile element fraction. Conversion values to microbial biomass, accounting for elements contained in the cell envelope components, which are mostly not extractable, and to account for adsorption to soil colloids during extraction are yet to be determined in a larger variety of soils. To overcome some of the limitations of the fumigation extraction approach for secondary and trace elements, a pre‐extraction step may provide a suitable solution.