OOS 31-10
Do plants and microbes engineer dry soil?
The biotic and abiotic formation of water-stable macroaggregates (WSM) in soil is beneficial for many reasons, including carbon (C) sequestration, nutrient retention, aeration, erosion control, microenvironments for soil biodiversity, and improved seed bed for plants. Some mix of biotic (e.g., C input, microbial activity) and abiotic factors (e.g., water) is needed to form WSM. However, in order to better model and manage WSM, we need to know more about the relative contributions of these mechanisms. Previous experiments to separate biotic and abiotic WSM have been hampered by the need to add sterilant dissolved in water, thus preventing conclusions about dry soil. Our first goal was to quantify the contribution of fresh plant C inputs on WSM by hand-weeding plants for 2 years in a seasonally dry grassland. Our second goal was to quantify the microbial contribution to WSM formation as a function of soil moisture. In the laboratory, we ground soil to <250 μm and used chloroform vapor to sterilize soil without adding water. For 6 months, soils were exposed to all combinations of three treatments: microbial biomass (live vs. sterile); fresh C input (with vs. without plants); and moisture (5%, 10%, 15%, and 20% of water-holding capacity).
Results/Conclusions
In the field, removing plants reduced the average size of water-stable soil aggregates by 22% in both years, which was primarily driven by a shift from large WSM (2—9 mm diameter) to small WSM (0.25—2 mm). This effect occurred whether there was an 8-month dry season or no dry season (i.e., irrigated). In the laboratory, effects did not depend on plant removal. Sterile soils formed more WSM with increasing moisture. Abiotic WSM—likely cemented by polyvalent cation bridging—appeared planar and angular at both macro- and micro-scales. In contrast, live soils appeared spherical and formed more WSM at intermediate moisture. Microbes are known to secrete extracellular polymeric substances (EPS) that glue together WSM. We hypothesize that under the driest conditions, microbes cannot access soil C to make EPS. Given a small amount of water, C substrates can diffuse to microbes which have incentive to connect the dry soil matrix to access more resources. Under wetter conditions, microbes surprisingly reduced the formation of WSM, possibly by interfering with cation bridging. Thus, biotic and abiotic processes can interact. Furthermore, our results suggest that—even in dry climates—plants preserve WSM and microbes engineer WSM.