Soil microbes are key to nutrient cycling and soil formation, yet the impact of soil properties on microbe biomass remains unclear. Using 240 soil cores of 0–15 cm depth, taken at random points across six cattle-grazed pastures on an undulating landscape, we evaluated the biomass of microbes in soil as affected by naturally occurring variation in soil organic carbon (SOC), clay content, and local topography. The study pastures varied in historic land-use for crops or forage seeding. SOC was found to be greater in topographically low areas. In contrast, clay content was not related to topography, and clay deposition possibly varies with glaciation legacy. Microbial biomass carbon (MBC) was correlated positively with SOC, increasing from 700 mg kg⁻¹ MBC at 25 g kg⁻¹ SOC to 2240 mg kg⁻¹ MBC at 90 g kg⁻¹ SOC. Most likely, SOC promotes MBC through the release of water-soluble organic carbon. However, the response of MBC to clay content was negative, decreasing from 1340 mg kg⁻¹ MBC at 5% clay to 880 mg kg⁻¹ MBC at 30% clay. Small voids in association with clay particles likely restrict the access of microbes to SOC. The relationship between SOC and MBC illustrates the important role of SOC for soil function, in terms of nutrient availability and development of soil structure via the contribution of microbes. Lastly, there was considerable spatial variability in MBC across the 65 ha site, highlighting the importance of land-use histories and gradients in environmental variables, to determine the biomass of microbes in soil.

Primarily using cropped systems, previous studies have reported a positive linear relationship between microbial biomass carbon (MBC) and soil organic carbon (SOC). We conducted a meta-analysis to explore this relationship separately for grasslands and croplands using available literature. Studies were limited to those using fumigation–extraction for MBC for field samples. Trials were noted separately where records were distinct in space or time. Grasslands were naturally occurring, restored, or seeded. Cropping systems were typical of the temperate zone. MBC had a positive linear response to increasing SOC that was significant in both grasslands (p < 0.001; r² = 0.76) and croplands (p < 0.001; r² = 0.48). However, MBC increased 2.5-fold more steeply per unit of increasing SOC for grassland soils, as compared to the corresponding response in cropland soils. Expressing MBC as a proportion of SOC across the regression overall, slopes corresponded to 2.7% for grasslands and 1.1% for croplands. The slope of the linear relationship for grasslands was significantly (p = 0.0013) steeper than for croplands. The difference between the two systems is possibly caused by a greater proportion of SOC in grasslands being active rather than passive, relative to that in croplands, with that active fraction promoting the formation of MBC.

Early-season growth of flax (Linum usitatissimum L.) was reduced in the field following canola (Brassica napus L.) compared to rotation with wheat (Triticum aestivum L.). This effect was associated with reduced formation of arbuscular mycorrhizae in flax caused by rotation with canola. Greater uptake of copper (Cu), phosphorus (P), and zinc (Zn) was seen for flax after wheat. Mycorrhizal stimulation of uptake of these elements likely relates to their immobility in soil. The ratio of biomass of flax after wheat to flax after canola was 1.6. Corresponding ratios of nutrient uptake were 1.5 to 1.8 for Cu, P, and Zn but were between 1.2 and 1.4 for macronutrients other than P. These data illustrate the importance of consideration of a range of nutrients for concentration, uptake, and ratios of uptake among treatments to evaluate the impact of arbuscular mycorrhizae. Rotation of flax after canola should be avoided.