The role of death and lysis of microbial and plant cells in the formation of soil organic matter

Processes occurring in soil after microbial and plant cell death and release, adsorption, or degradation of cell molecules and organelles after cell lysis are poorly understood despite their contribution to the formation of soil organic matter (SOM). In this review, we discuss the many knowledge gaps with respect to processes occurring after death and lysis of plant and microbial cells and propose directions for future research to fill these gaps. We specifically highlight that while model studies using surface-reactive particles, such as clay minerals, have studied adsorption and binding of important biological molecules, such as proteins (including enzymes) and DNA, these processes have not been studied for entire cell organelles. These can contribute to the formation of SOM via preservation of their contents by microbial processing and subsequent adsorption on surface-reactive soil particles or occlusion in soil aggregates as microbial necromass. Moreover, active enzymes can persist after cell death and contribute to the formation of necromass in a disorganized cell environment. However, the persistence of active enzymes in soil after cell death is poorly studied. We argue that high resolution imaging techniques, for instance nano-scale secondary ion mass spectrometry (NanoSIMS), can aid in closing the related research gaps by resolving the distribution of soil organic carbon at the micro and nanometer scales, at which the above processes occur. Finally, the quantification of microbial- and plant-derived SOM is still challenging because the most commonly used marker for microbial necromass (amino sugars) only represents a part of microbial cells. The risk of overestimation is high, and we propose combining amino sugar and extracellular DNA determination to confirm necromass origin and reduce this risk. Better knowledge of the processes occurring after death and lysis of plant and microbial cells and improved quantification of plant and microbial necromass will provide more detailed insights into the formation of SOM and allow for a more efficient management of soil organic C.