Effect of season and health status on nutrient dynamics in diseased Agathis australis forests

Kauri (Agathis australis), a long-lived conifer endemic to northern New Zealand, is threatened by kauri dieback disease caused by the soilborne oomycete Phytophthora agathidicida. The pathogen causes root damage and dieback, which may impair water and nutrient uptake and may alter soil processes. Field-based evidence on how nutrient availability varies with stand health status and season remains limited.

We investigated plant-available nutrient supply in kauri dominated forests across the Waitākere Ranges (New Zealand), using 6 plots (24 subplots) established across three sites (Cascades, Piha and Huia) including plots which show strong evidence of kauri dieback symptoms (symptomatic) and plots without strong dieback symptoms (asymptomatic). Plant-available nutrients (NO₃^--N, NH₄⁺-N, PO₄^3--P), base cations (Ca, Mg, K), micronutrients (Fe, Mn, Cu, Zn, B, S), and trace/toxic metals (Pb, Al) were measured using Plant Root Simulator^TM (PRS) probes, which provide an integrated, field-based measure of nutrient supply rates. Sampling was conducted from March 2024 to February 2025, with probes deployed and retrieved every three months.

Our findings show that health status had an effect on Cu and Pb supply with lower rates in symptomatic plots which may reflect a reduction in rhizosphere activity and organic matter cycling that decreases the exchangeable fraction of these strongly complexing metals. Season had an effect on most nutrients with lower supply rates during austral summer (Dec–Feb) likely reflecting drier-soil conditions limiting transport (reduced diffusion) combined with greater plant demand. NO₃^--N showed no seasonal or health status related effect, consistent with its high mobility and very low rates. In austral winter (Jun–Aug), NH₄⁺-N supply increased, likely due to reduced plant uptake and suppressed nitrification under cool, wet conditions. Overall, these results indicate that seasonal controls dominate plant-available nutrient supply across the Waitākere kauri forests, while health status is associated with more targeted shifts in specific elements and there is minimal evidence of health status × season interactions for most nutrients. This study underscores the importance of repeated, in-situ measurements for interpreting nutrient dynamics in disease-impacted ecosystems.