Respiration After Rewetting of Air-Dried Soil Has Site-Specific Proportionality to Soil Organic Carbon

Microbe activity in soil can be estimated whereby a soil is air-dried, rewetted to a level favorable to microbes, and determination made of the CO2 subsequently released. Previous studies indicate this so-called burst respiration has a positive linear response to soil organic carbon (SOC). Investigation was made here of the consistency of this relationship, both regionally for pastures, and among crops fields on a single farm. The three pasture sites were separated from each other by about 100 km across Manitoba, Canada. Sample depths to 100 cm were used to provide a range of values of SOC within a pasture site. Samples were collected from 72 ha at an experimental pasture, as well as from commercial pastures of 3 ha and 16 ha. For all pastures, burst respiration was greater for soils collected at shallow depth. Burst respiration across depths had a linear positive relationship to SOC over the range investigated from 5–80 g C kg-1 at the experimental pasture. Corresponding relationships were found at both commercial pastures, but the regressions were distinct at each pasture. Background variability in SOC at 0–5 cm depth was evaluated among three cropped fields over two years at a single farm near Portage la Prairie, Manitoba. Each field had a different phase of the rotation soybean-corn- canola. Once again, significant but unique positive regressions between burst respiration and SOC were found for each field, despite differences in crop-year combinations among soil samples for a given field, and with SOC across years and fields ranging from 35–60 g C kg-1. Burst respiration was not related (P=0.65) to natural variation in clay from 14–38% across the crop fields, indicating that the quality of organic matter from historic management likely dictates the nature of the relationship between burst respiration and SOC at a site locally, and by extension, regionally. The consistency and linearity of the relationship between burst respiration and SOC at a location with consistent management history suggests that SOC can be taken as an index of soil health for a given field. Yet, to compare among sites, or even among fields on the same farm, measurement of burst respiration retains greater utility to estimate soil health.