Changes in leaching patterns of nitrogen and phosphorus after artificial drainage of a boreal forest — a paired catchment study in Lappajärvi, western Finland

The effect of ditching of boreal (Finnish) peat-rich forest land on leaching patterns of phosphorus and nitrogen was assessed using a paired catchment approach (response stream + control stream) for a 4 + 1 year study period. After the ditching operations, the leaching of the organic nitrogen (TON) decreases, while that of the inorganic nitrogen (ammonium and nitrate) and phosphorus (total reactive P and total non-reactive P) increases. The TON does not decrease as much as TOC does, indicating that the pool of organic material being leached after the ditching is enriched in nitrogen. The increased leaching of ammonium and nitrate after ditching is related most likely to ammonification and nitrification processes in peat biomass after the release of excess surface water. The behaviour of P is complex. The data indicate that the major controls of the determined P fractions are the Fe redox chemistry and the behaviour of Fe hydroxides.