Prolonged drought moderates flood effects on soil nutrient pools across a rainfall gradient

Climate change is manifesting through increased intensity and frequency of extreme events such as droughts and floods. Drought causes significant stress by limiting water availability, resulting in reduced plant growth and belowground carbon (C) inputs as well as plant nutrient uptake and microbial activity, thus impacting C and nutrient cycling. Severe floods can increase leaching of nutrients, which affect nutrient availability for plants and microorganisms potentially compounding drought-induced effects on C and nutrient cycles. However, few studies have assessed the interactions between drought and floods, constraining our ability to predict ecosystem-level effects of projected climate shifts toward fewer but larger rainfall events with longer dry spells between events. We conducted a study to assess how long-term drought moderates the effect of flooding on soil C, nitrogen (N) and phosphorus (P) pools across a rainfall gradient in south-eastern Australia. We collected soil monoliths from field rainfall manipulation facilities with a 5–7 years history of simulated reduced rainfall at three sites representing a rainfall gradient (∼200–800 mm) and exposed them to a 1-in-100-year flood event under laboratory conditions. We assessed nutrients and C in soil and microbial pools before and after the flood and in flood-induced leachates. The results showed that flooding caused significantly greater losses of N in leachates, with greater effect sizes in the drought treatments compared to ambient rainfall. This response was particularly pronounced for nitrate (152 % increase on average), especially at the site with the highest mean annual precipitation (>25-fold increase). This suggest that drought may significantly reduce the capacity of ecosystems to retain N following flood events, particularly in mesic ecosystems where C and nutrient pools are greater.