Un(der)explored links between plant diversity and particulate and mineral-associated organic matter in soil

Introduction

Recent studies have indicated that the amount of organic carbon (C) stored in soils can be sensitive to plant diversity^{1, 2, 3, 4–5}. Thus, widespread biodiversity losses^6,7 can be a threat to the diverse soil functions associated with soil organic C, and restoration or enhancement of plant diversity (e.g., diversified crop rotation and reforestation) could increase soil C storage and help mitigate climate change^{8, 9, 10–11}. The effectiveness of such management strategies, however, is hampered by incomplete knowledge of the processes by which plant diversity affects soil C storage. It remains especially unclear how plant diversity relates to the C that is stored in particulate organic matter (POM) and mineral-associated organic matter (MAOM), which tend to have different sources, formation pathways, residence times, and responses to land management^{12, 13, 14, 15, 16, 17–18}. We offer two explanations for the existence and persistence of this knowledge gap: (i) to the best of our knowledge, few of the hundreds of experimental studies of plant diversity gradients have measured POM and MAOM^{19, 20–21}, and (ii) although conceptual frameworks of POM and MAOM have been developed and revised, none of these prior efforts have focused on the role of plant diversity.

Here, we consider how plant diversity affects the formation of POM and MAOM via the quantity of inputs of plant-derived C to soil^{22, 23–24} and transformations of organic matter in soil, as mediated by the quality of organic matter inputs and the composition and metabolic activities of bacteria, fungi, and animals in soil^{25, 26, 27, 28, 29, 30, 31, 32, 33, 34–35}. For example, effects of plant diversity on plant litter quality³⁶ and soil microclimate³¹ could interact to determine conditions for microbial growth and utilization of the litter³⁷. This, in turn, could affect the decomposition or accumulation of POM and the formation of microbial residues in MAOM. Likewise, plant diversity-related changes in rhizodeposition²⁸ could affect the generation of dissolved organic matter and formation of MAOM via direct sorption. Here, we briefly review and synthesize how these and various other processes link plant diversity and the two key sub-pools of soil organic matter (SOM), highlight research gaps, and...