Phosphorus Transport in Subsurface Flow at Beech Forest Stands: Does Phosphorus Mobilization Keep up with Transport?

Phosphorus (P) is a limiting factor of primary productivity in most forest ecosystems but little is known about retention within and losses of P from forests. Subsurface flow (SSF) is one of the important pathways of P export but few attempts exist to quantify it. We present results of sprinkling experiments with ca. 150 mm, 2H labelled, total rainfall conducted at 200 m² plots on hillslopes with slopes between 14° and 28° at three beech forests in Germany in summer and spring. We aimed at quantifying vertical and lateral SSF and associated P transport in the forest floor, the mineral soil and the saprolite. The study sites differed regarding soil depth, skeleton content and soil P stocks (between 678 g/m² and 209 g/m², in the first 1 m soil depth). Vertical SSF in the mineral soil and in the saprolite was at least two orders of magnitude larger than lateral SSF in the same depth. Vertical and lateral SSF consisted mainly of pre-event water that was replaced by sprinkling water (piston flow mechanism). Short spikes of event water at the beginning of the experiment at two of the sites with high skeleton content indicate that preferential flow occurred in parallel to matrix flow. We observed a significant decrease in P concentrations in SSF with increasing soil depth suggesting effective retention of P by adsorption to soil particles in all three forest ecosystems. Higher P concentrations in SSF at the beginning of the experiments indicate nutrient flushing but P concentrations were nearly constant thereafter despite strong increase in SSF. P concentrations did also not change significantly with increasing share of event water in SSF. These chemostatic transport conditions suggests that P mobilization rates were similar to transport rates in both, P-rich and P-poor sites. The observed first flush effect implies that P export by SSF will increase as rainfall events with high transport capacity are predicted to occur more frequent under future climatic conditions. [ABSTRACT FROM AUTHOR]