Plant versus microbial controls on soil aggregate stability in a seasonally dry ecosystem

• Microbial activity formed the most macroaggregates under intermediate moisture.
• Abiotic mechanisms required wet conditions to form macroaggregates.
• Microbial-produced aggregates were spherical while abiotic aggregates were platy.

The formation of water-stable macroaggregates in soil is beneficial for many reasons, including carbon (C) sequestration, nutrient retention, and erosion control. A mix of biotic (e.g., plant C input, microbial activity) and abiotic factors (e.g., water, mineral interactions) contribute to form macroaggregates. However, in order to better model and manage soil macroaggregates, we need to know more about the relative contributions of these mechanisms. Previous experiments to separate microbial and abiotic mechanisms have been hampered by the need to add sterilant dissolved in water, thus limiting our ability to draw conclusions about the role of soil moisture in controlling aggregation and preventing conclusions about dry soil. Our first goal was to quantify the contribution of plant growth (and fresh plant C inputs) by continuously removing plants for 2 years in a seasonally dry grassland. Our second goal was to quantify microbial vs. abiotic contributions to macroaggregate formation under a range of soil moisture conditions by using chloroform vapor to sterilize soil without adding water or destroying soil structure. In the field, regardless of dry season length, removing plants reduced the average size of soil aggregates by 22–33%, which was primarily driven by a shift from large macroaggregates (2–9 mm diameter) to small macroaggregates (0.25–2 mm). In the laboratory, in sterile soils macroaggregate production increased with the moisture content. The resulting physicogenic aggregates appeared planar and angular at both macro- and micro-scales. In contrast, biogenic aggregates were formed most at intermediate moisture levels and were spherical. Our results suggest that—even in dry climates—soil macroaggregates are preserved by the presence of even dead plant roots, but are engineered by live microbes.