Interactions among phosphorus, pH and Eh in reforested mangroves, Vietnam: a three-dimensional spatial analysis

Sediment reduction is frequently proposed to increase available phosphorus (P) but several studies found also decreases. Another important factor for P liberation is the pH. We investigated the relative importance of Eh and pH on P cycling in reforested mangroves. Sediment P compounds, pH and Eh were analysed over depth along five transects of two areas in the Saigon River Delta and compared with leaf P levels. A three-dimensional spatial approach was used to investigate pH and Eh effects on P compound distribution at different sediment depth and locate layers of predominant P uptake. Along an inundation gradient, submergence durations of 254 to 2 days per year caused a large Eh gradient within the top 20 cm, whereas Eh response was small within 20-45 cm depth. At individual layers, a correlation between Eh and Al/Fe-P was only found in the upper depth interval (0-20 cm). No significant effect of Eh or Al/Fe-P on Morgan-P (available P) was detected. Minor effects on P composition changes by the Eh were caused through generally strongly reduced sediment at deeper layers (> 20 cm). In contrast, pH variations produced extreme differences in both, Ca-P and available P content at layers of apparently predominant P uptake (> 20 cm). As available P was correlated with Ca-P (p < 0.001), leaf P (p < 0.001) and pH (non linear correlation) it is likely that the pH sensitive Ca-P fraction is a more effective source for mangrove tree growth than Al/Fe-P. The predominant pH effect on P uptake within these reforested mangroves differs from a proposed reduction-governed P cycling in wetlands.