How the Birch effect differs in mechanisms and magnitudes due to soil texture

Global climate change is predicted to intensify temperature and precipitation extremes, exposing soils to frequent and/or intense alternate wetting and drying regimes. Rewetting a dry soil causes a burst of CO₂ known as the “Birch effect”. This spike in respiration can be attributed to: (i) release of cellular solutes (metabolites) accumulated during drought due to rapid increase in water potential upon re-wetting; (ii) sudden death of certain microbes serving as carbon (C) sources for surviving microbes, leading to microbial community shifts; and (iii) release of physically protected C due to aggregate breakdown upon repeated drying-wetting. The relative importance of these mechanisms may change in different soil textures, but very few studies examine all three processes. In this study, we evaluated the effects of repeated drying and wetting cycles (transient state moisture conditions) on the Birch effect and elucidated the mechanisms that contributed to this effect in different textured soils. Soils of three distinct textures (sandy, loamy, and clayey) were incubated for 140 days under five alternate cycles of drying (10% WHC) and wetting (100% WHC) conditions. Control soils were held at 55% WHC for the same duration. Microbial biomass C (MBC), extractable organic C (EOC), metabolites, microbial community structure, and changes in aggregate associated C were determined at each time point. Sandy soil had the lowest respiration rate followed by loamy and clayey soils, and cumulative CO₂ loss was higher under transient moisture state compared to steady state. In sandy and clayey soils, changes in bacterial abundance controlled the Birch effect, while in loamy soil, the release of aggregate protected C majorly controlled the Birch effect.