Grasslands and Croplands Have Different Microbial Biomass Carbon Levels per Unit of Soil Organic Carbon

Terence McGonigle,William Turner

doi:10.3390/agriculture7070057

Abstract

Primarily using cropped systems, previous studies have reported a positive linear relationship between microbial biomass carbon (MBC) and soil organic carbon (SOC). We conducted a meta-analysis to explore this relationship separately for grasslands and croplands using available literature. Studies were limited to those using fumigation–extraction for MBC for field samples. Trials were noted separately where records were distinct in space or time. Grasslands were naturally occurring, restored, or seeded. Cropping systems were typical of the temperate zone. MBC had a positive linear response to increasing SOC that was significant in both grasslands (p < 0.001; r2 = 0.76) and croplands (p < 0.001; r2 = 0.48). However, MBC increased 2.5-fold more steeply per unit of increasing SOC for grassland soils, as compared to the corresponding response in cropland soils. Expressing MBC as a proportion of SOC across the regression overall, slopes corresponded to 2.7% for grasslands and 1.1% for croplands. The slope of the linear relationship for grasslands was significantly (p = 0.0013) steeper than for croplands. The difference between the two systems is possibly caused by a greater proportion of SOC in grasslands being active rather than passive, relative to that in croplands, with that active fraction promoting the formation of MBC.

Highlights

Microbial biomass carbon (MBC) comprises all microorganisms in soil and is typically measured as the carbon released in response to microbe death following chloroform fumigation [1]
substrate-induced respiration (SIR) gives different MBC values compared to fumigation extraction (FE), correlation is claimed for MBC values from these two methods [30]
Our meta-analysis of data from the literature shows a positive relationship between MBC and soil organic carbon (SOC)

Summary

Introduction

Microbial biomass carbon (MBC) comprises all microorganisms in soil and is typically measured as the carbon released in response to microbe death following chloroform fumigation [1]. MBC changes with greater agility than soil organic carbon (SOC) in response to residue management [2]. MBC acts as a pool of organic carbon that contributes cyclically to immobilization and release of minerals during formation and breakdown [5]. Soil health and soil quality are often considered as synonyms [6]. Soil health places more emphasis on biological activity in soil, such as the contribution of soil organisms to nutrient cycling [7]. A soil property employed effectively as an indicator of soil health should allow for prediction in advance of that production or service. SOC is considered a valuable indicator of soil health in agricultural soils [9]. SOC and Agriculture 2017, 7, 57; doi:10.3390/agriculture7070057 www.mdpi.com/journal/agriculture

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Conclusion