Soil mass and volume affect soil‐test biological activity estimates

Abstract

AbstractSoil‐test biological activity is a key indicator for soil health assessment. Methodological details may affect accuracy and precision of this indicator. Accuracy and precision of soil‐test biological activity estimates were determined for 10 replications of 10 mass and volume treatments under standard laboratory conditions using an alkali trap technique with acid titration. Five soils varying in texture and organic C and N concentrations were used to assess a gradient of soil mass and volume conditions on C mineralization following rewetting of dried soil during 0–3, 3–10, and 10–24 d of incubation at 50% water‐filled pore space and 25°C. Soil type explained ≥90% of variation in C mineralization, but soil mass and volume treatments also had significant effects on estimates (p < .001). In sets of treatments with the same surface area exposed, increasing soil volume led to lower C mineralization across soil types. Entrapment of CO2 in soil pores may have been the main reason for lower C mineralization, since the effect diminished over time. Treatments with 50 to 100 g of soil were nearest the median estimates of C mineralization for each soil. Precision of C mineralization estimates was greatest when soil mass was ≥50 g of soil. A consistent surface area‐to‐volume of ∼0.2 cm−1 is suggested with sufficient soil mass to account for potentially enhanced random variation of C‐enriched surface soils.