Hydrologic History Drives Divergent Soil Organic Matter Trajectories Across Hillslopes

Abstract

Water availability governs soil organic matter (SOM) decomposition, yet how fine‐scale hydrologic variability shapes seasonal SOM processing and persistence remains unclear. In seasonally dry forests, rewetting can trigger rapid mineralization, but antecedent moisture may leave biogeochemical legacies that modulate this response and its significance for carbon and nutrient cycling. We examined whether subsurface water storage and antecedent moisture regulate SOM processing and carbon trajectories. We leveraged a natural gradient in plant‐accessible water across two Douglas‐fir‐dominated hillslopes in Oregon's western Cascade Mountains (USA). These sites experience similar climate, vegetation, and soils, but differ in subsurface water‐storage capacity. We compared dry‐season (June) soils with wet‐season (December) soils, and paired continuous soil‐moisture profiles with high‐resolution mass spectrometry of water‐extractable (WEOM) and mineral‐associated organic matter (MAOM), bulk SOM pools, and microbial‐biomass. SOM chemistry was similar in June but diverged in hillslope‐specific ways by December. Soils from the high‐water‐storage hillslope accumulated OM in WEOM and MAOM and gained microbial biomass carbon, shifting toward more oxidized, relatively complex compounds. Soils from the low‐water‐storage hillslope lost OM across fractions and microbial biomass carbon, and retained more reduced, relatively simple compounds. High‐water‐storage profiles remained within water‐holding capacity longer—an optimum for microbial activity—whereas low‐water‐storage sites dried faster and experienced larger moisture extremes. Plant‐mediated hydraulic redistribution occurred on both hillslopes, but was not a key mechanism maintaining optimal soil water‐holding capacity. These insights support a hillslope storage framework: subsurface storage can imprint biogeochemical legacies of moisture conditions, shaping decomposer responses to cool‐season wetting in seasonally dry landscapes.