Phosphorus and water flux dynamics in runoff and plant uptake in forested headwaters

Forest stands are known to be affected by shortage in phosphorus, an important nutrient for plants. The main source of phosphorus in forested ecosystems is either recycling of litter by microbial activity in the shallow soil layers (recycling system) or weathering of the parent material in deeper depth of the soil profile (acquiring system). The project investigates the role of subsurface runoff processes that impact the processes of nutrient supply by export of phosphorus from forest stands. Results from the first project phase indicate, that large fraction of total plant available P is lost from the forest stand by deep percolation. To better quantify this loss and to study the recovery of phosphorus supply at various soil depth after a rainstorm, hydrometric and hydrochemical monitoring at three experimental hillslopes in Germany is planned. The instrumental setup consists of a 3 m deep trench, soil moisture measurements at 5 different depth, a > 15 m deep groundwater well and spring- and streamflow measurements at the bottom of the hillslope and will be extended by a horizontal drainage that allows sampling of percolation in deeper soil depth below the trench. This experimental setup will allow quantifying the vertical re-distribution of water and nutrients during controlled sprinkling experiments and the following period of recovery of phosphorus. In addition, sap flow of trees will be monitored and sampled to better understand and distinguish the sources and coupling of water and nutrient supply of plants in recycling and acquiring ecosystems. The project aims at a better characterization of differences in susceptibility of recycling and acquiring ecosystems to an anticipated increase in nutrient flushing frequency under changed climatic conditions. The findings are planned to be generalized, integrated and tested in a model that allows to simulate ecosystem nutrition strategies in forest stands.