Pore scale processes in dry soils

Abstract:

Almost all soils experience regular drought and rewetting events. Yet most of our understanding of soil processes focuses on the moist periods, when plants are growing and nutrients are actively cycling. Yet, as soils dry, processes continue, yet change. Microbes shift their metabolic pathways from growth to survival, producing extracellular polymeric substances (EPS), sporulating, and going dormant. Under dry conditions, biotic processes are constrained but abiotic, chemical processes continue potentially altering soil aggregation and structure; in clayey California annual grassland & woodland soils pools of bioavailable water extractable organic carbon (WEOC) increase as does microbial biomass. Finally at rewetting, the pulse of water mobilizes resources, stimulates microbial activity and produces a flush of respiration and nutrient mineralization that can mobilize resources that had been previously inaccessible. One question that has driven much research has been where the organic matter comes from that drives these processes. We had hypothesized that the source of C for the dry-season increases was from the previous winter’s dead roots, but field experiments where we maintained plots plant-free for two years showed no decline in the production of WEOC, nor in the early-season respiration pulses following rewetting. In this presentation, we will discuss recent work integrating measurements on aggregation (driven both by biotic and abiotic processes), EPS production, and the dynamics of WEOC and microbial biomass and how they function differently under dry and moist conditions.