Rapid estimation of microbial biomass in acid red soils with and without substrate incorporation

Purpose

A rapid and alternative measurement of microbial biomass in acid red soils with and without substrate incorporation is proposed for soil quality evaluation.

Materials and methods

Soil microbial biomass C (SMBC) and N (SMBN) in 24 typical red soil samples developed from two parent materials (granite and arenaceous shale) were measured using fumigation-extraction followed by dry combustion method in comparison with ultraviolet (UV) spectrophotometry (increase in absorbance at 280 nm, ΔUV₂₈₀). The reliability of microbial biomass estimation by UV spectrophotometry was verified using six representative red soils amended with biochar (0, 1, 3 and 5%) and glucose (0, 100, 500 and 1000 mg kg^?1) separately.

Results and discussion

ΔUV₂₈₀ was strongly correlated with SMBC and SMBN measured by dry combustion, regardless of biochar/glucose incorporation. Validated conversion equations from unamended soil data were dependent on confounding effects of organic C and particle size and can be described as follows: SMBC?=?27.08?×?ΔUV₂₈₀ (R²?=?0.67, n?=?24) and SMBN?=?3.62?×?ΔUV₂₈₀ (R²?=?0.69, n?=?24). Regression models for rapid estimation of microbial biomass in red soils from different parent materials had to be calibrated separately in case of amendments. In most cases, SMBC (R² of 0.75–0.76 and root mean square error (RMSE) of 22.2–29.3 mg kg^?1) and SMBN (R² of 0.74–0.80 and RMSE of 2.60–14.2 mg kg^?1) can be predicted from ΔUV₂₈₀ in biochar/glucose-amended soils using these equations. The slope of the regression of SMBC against ΔUV₂₈₀ shifted in biochar-amended granite soils, mainly due to uncoordinated changes of SMBC in response to the difference in parent material-induced nutrient availability, while shifts of SMBC (or SMBN) against ΔUV₂₈₀ in glucose-amended arenaceous shale soils were attributed to particle size distribution.

Conclusions

Soil microbial biomass (SMBC and SMBN) in red soils can be rapidly predicted by fumigation-extraction with UV spectrophotometry detection and corresponding correction of calibration curves, depending on soil nutrient availability, particle size distribution and organic C levels.