Soil microbial biomass as an indicator for soil health

Soil health is defined by productivity, such as forage growth and weight gains for grazing animals. An indicator for soil health, if available, would comprise of measurements on soil that would allow advanced prediction. Microbial biomass carbon (MBC) is a measurement of the quantity of microbes in soil, estimated by fumigation of soil by chloroform and extraction of the soluble carbon released. Given the microbes contribute to the success of plant growth, MBC was evaluated as a possible indicator for soil health under two grazing systems, continuous and planned, in a replicated and randomized field experiment. From theory, planned grazing is expected to generate greater soil health, because of a longer forage rest period combined with low selectivity of grazing. Values of MBC corresponding to 2.1% of total soil organic carbon (SOC) for the trial were consistent with values for grasslands reported across the North America and Europe. Over two years following establishment of the grazing trial, no difference in MBC could be discerned between grazing treatments. This outcome means that either MBC fails as an indicator of soil health, or else that more time is needed for differences in soil health, if there are any, to emerge between the two treatments. The second interpretation seems likely, because no differences in forage or animal production for the experimental site could be found in available data from related studies. Soil clay content varied from 5-30% across the experiment and increasing clay in soil was associated with a linear reduction in MBC and microbial biomass nitrogen (MBN). Most likely, this outcome relates to shielding by clays, which separates microbes from the organic matter they utilize, leading to a reduced quantity of microbes. Where possible, bulking of soil samples to average across textural variability should be employed to maximize sensitivity of MBC in attempts to discern any expected differences in soil health.