Eucalypt Leaf Litter and a Drying-Rewetting Cycle Shape Wetland Soil-Water Nutrient Dynamics: A Laboratory Microcosm Study

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This study used laboratory column microcosms with River Murray (Australia) wetland soils and water to examine how drying-rewetting (DRW) and river red gum (Eucalyptus camaldulensis) litter inputs interactively affect nitrogen (N), phosphorus (P), and carbon dynamics across soil, pore water, and overlying water. We quantified N and P species and dissolved organic carbon (DOC) under two water regimes (constant submergence versus DRW) with and without litter addition to resolve cross-compartment responses at the aquatic-terrestrial interface. DRW enhanced net inorganic-N mobilization, resulting in pronounced post-rewetting pulses in pore-water and overlying-water N (especially NH4+) compared to constant submergence, with concentrations gradually converging during prolonged inundation. Conversely, total P and inorganic P (Pi) were higher under constant submergence, consistent with sustained reducing conditions promoting P release in solution. Although litter inputs did not substantially alter total nutrient levels in overlying water, they shifted pore-water chemistry by increasing the proportion of inorganic P and strongly elevating DOC. Following rewetting, DOC increased sharply in litter-amended treatments, ~ 2.9-fold in overlying water and ~ 16.8-fold in pore water, compared with no-litter treatments. Litter addition also reduced nitrate availability in soil and pore water during resubmergence, consistent with stronger microbial N retention and/or NO3− consumption under rapidly re-established reducing microsites after rewetting. Overall, DRW and litter inputs interact to amplify short-lived C and N pulses while modifying P speciation, with implications for predicting nutrient availability and water-quality risk in wetlands experiencing increasingly variable hydrological regimes.