Contribution of belowground biomass carbon to the stable soil organic matter pool

General introduction

This now completed doctoral research study (ILVO and Ghent University) investigated the relative contribution of belowground versus aboveground biomass to the buildup of stable soil organic carbon (SOC) pool. 

The result is clear: relatively speaking, the root biomass of corn contributes twice to three times more to long-term carbon accumulation in the soil, than the above-ground crop residues (corn straw). However, the total root biomass in corn is smaller than the amount of corn straw. These findings are relevant when trying to predict soil carbon content through modeling, or when weighing the extent to which corn straw can be disposed of, as a feedstock in the bioeconomy. 

Research approach

The data in this study were obtained by following up well-documented field experiments and by resampling and analyzing field plots that were also sampled in the 1990s. Physical fractionation and 13C isotope analysis were used to determine the extent to which carbon derived from maize roots is physically protected in the soil. It was hypothesized that encapsulated organic matter, derived from dead plant material, in soil microaggregates led to some degree of physical protection against microbial degradation. This mechanism could be responsible for the longer residence time of C derived from underground biomass compared to above-ground crop residues. Underground crop residues have a more direct contact with soil aggregates. The objective was to know to what extent aggregation in different textures actually explains the relative stability of C derived from belowground biomass. The measurements show that this phenomenon does not explain the higher stability of root biomass. A second experiment tested the effect of soil type and maize variety on underground biomass. The root biomass of different maize varieties and soil textures were compared. The amount of root biomass in maize was found to be mainly determined by the soil. Root biomass hardly differed among the 8 maize varieties tested by soil type. Another interesting conclusion is that above-ground crop yield for maize shows no correlation with root biomass in the tillers. Thus, when modeling soil carbon content, one should not simply assume a positive relationship between aboveground yield and root biomass. Soil type is relevant in modeling because soil type strongly influences the relationship between root biomass and aboveground biomass.     

Relevance/Valorisation

This doctoral research has clarified the precise role of underground biomass (crop roots) in the build-up of stable carbon (C) in soil. Figures now exist for both grain and silage corn, both important for Flanders. Aboveground crop residues return organic matter and nutrients (in addition to the roots) to the agricultural soil, but they are also a potentially interesting source of biomass outside the field, for other applications in the circular economy. If the biomass from crop residues is to be used sustainably, one needed to know their precise contribution to the build-up of C in the soil.