Response to commentary on Tanga et al. (2025): Experimental design and interpretation of phosphorus concentration responses under Controlled Drainage

This letter responds to commentary on Tanga et al. (2025) regarding experimental design and interpretation of phosphorus concentration responses under controlled drainage. Our response clarifies that the spatial arrangement of controlled and free drainage plots was a deliberate design decision to minimize lateral seepage, supported by negligible seepage influence observed between adjacent plots. As well, the grouped plot comparisons are consistent with individual paired plot comparisons presented in the Supplementary Information. Based on literature, we also distinguish between Fe- and Al-driven phosphorus sorption mechanisms, noting that Al oxides are not redox-sensitive and may maintain phosphorus retention under elevated water table conditions - a plausible site-specific explanation for the observed dissolved reactive phosphorus reductions. Finally, we address the concentration comparison methodology, contending that non-flow periods under controlled drainage are a direct treatment effect and should not be excluded from analysis, consistent with established practice in the agricultural drainage management literature. We acknowledge the commentors’ points regarding replication, baseline soil phosphorus variability, and concentration comparison methods are valuable and will inform the design of future studies.