Microrespiration: A field method for measuring microbial activity in arable soils

Microrespiration: A field method for measuring microbial activity in arable soils

Franziska Weinrich¹, Katharina Keiblinger¹, Christoph Rosinger^1,2 Gernot Bodner²

¹Institute of Soil Research, Department of Forest- and Soil Sciences, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria (franziska.weinrich@students.boku.ac.at)

²Institute of Agronomy, Department of Crop Sciences, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria

The debate on soil health is more topical than ever. Large-scale soil monitoring is therefore an important tool for assessing the state of soil health. In this context, relatively simple yet reliable methods are needed to accompany lab-based analytics of different soil health indicators, such as microbial activity. Microbial activity is a useful parameter because most soil processes are mediated by soil microorganisms. The aim of this study is to develop a simple and affordable field test for broad applicability of measuring microbial activity in arable soils. The method was tested for sensitivity to different arable management systems and effects of land use as well as the influence of soil texture.

The principle of the field method is based on the colour change of a pH-indicator due to the acidic reaction with CO₂ that is released during soil respiration. For this purpose, 13 agricultural sites with varying soil texture, three management systems as well as two types of land use (arable and natural vegetation) were examined. Pioneer management is characterized by management aiming for an increase of soil health by applying conservational or regenerative practices, Standard management involves state of common knowledge practices and the Reference, representing a different land use, is provided by a semi-natural vegetation strip.

The method was developed for different amounts of soil, moisture conditions, incubation periods and substrates (glucose powder, milled straw, milled alfalfa). For the validation of the field method, the colour change of the indicator (evaluated by means of RGB data) is compared to respiration measurements with gas chromatography in the laboratory. Additionally, microbial biomass carbon and ergosterol concentrations as well as their ratio were determined to evaluate changes in the abundance of the microbial community and bacteria vs. fungi composition.

The results show that the continuous colour change of the indicator is highly correlated with the CO₂ concentrations measured in the laboratory (r² = 0.62; p < 0.001). Furthermore, the metabolic quotient and the colour change correlate well with each other (r² = 0.72; p < 0.001). The differentiation between agricultural management systems (Pioneer vs. Standard) is not so clear, land use however can clearly be distinguished with this method.  The influence of soil texture on the results of the field test is clearly visible. However, it is not possible to derive an indication on the microbial community composition with this method.

The performance of the field method leads to reasonable results and proves to be suitable for the simple determination of microbial activity in arable soils. After further improvement, this method provides a rather simple and affordable tool for soil health monitoring of arable soils.