On-farm soil management distinctly influences the stabilization of dissolved organic carbon within soil aggregates

Dissolved organic carbon (DOC) is a highly active carbon pool which can easily be utilized by soil microbes and is thereby a crucial fraction of the soil carbon cycle. Occlusion within soil aggregates is one important stabilization mechanism of DOC in the soil and is affected by various factors including soil management and site-specific conditions. However, details about this mechanism and its regulating factors are still largely unclear. Thus, this study aims to investigate the stabilization mechanism of DOC in soil aggregates under different soil managements across varying soil textures (fine, medium, and coarse). In the study, the soil managements were categorized into three different systems depending on the degree of conservation measures implemented on the farms: state-of-the-art system (standard), conservation systems (pioneer) which were primarily characterized by intensive use of cover cropping together with crop diversification and rotation, and semi-natural grassland (reference). We employed a combination of ultrasonication and online UV-visible spectroscopy to examine the concentration of DOC released from soil aggregates decayed. The experimental setup followed the theory that the duration of low-amplitude ultrasonication energy correlates with higher aggregate stability (resistant to decay), which in return affects the stability of DOC. Influential factors were assessed from relationships with soil physico-chemical and biological characteristics. Based on the observed release pattern, the study deduced that DOC was stabilized within soil aggregates across three different stability levels: low, moderate, and high. 

The results revealed that soil management systems exerted a significant effect on DOC in moderately and highly stable aggregates. Standard systems exhibited the lowest DOC concentration, while reference systems demonstrated the highest concentration. A significant effect of soil texture was found only at the moderate aggregate stability level where coarse soil displayed the lowest DOC concentration. Contrastingly, neither soil management systems nor soil texture had a significant effect on DOC concentration at the low aggregate stability level. The stabilized DOC within moderately stable aggregates, as evidenced by DOC concentration at the moderate level of aggregate stability, exhibited more pronounced correlations with soil microbial variables (i.e., microbial biomass C and ergosterol) than with soil texture as identified through particle size distribution. Therefore, our results suggest that changes induced by soil management, particularly in microbiological attributes, have a more crucial role on the stabilization of DOC in highly stable aggregates than site-specific conditions such as soil texture. Furthermore, the application of low-energy ultrasonication in this study enables the differentiation of soil managements, even within arable systems, in an on-farm setting.