Controls on the export of C, N, P and Fe from undisturbed boreal catchments, Finland

Abstract

Associations between catchment characteristics (altitude, slope, proportion of peatland, site fertility, total stem volume, and its distribution among the main tree species), climatic drivers (temperature sum, latitude, precipitation and runoff) and atmospheric deposition and concentrations and export of C, N, P and Fe in 21 unmanaged boreal catchments (0.07 to 14 km2) were examined in data sets spanning 3 to 32 years. In the entire data base including catchments from southernmost Finland to the Arctic Circle, N and P concentrations were associated with climatic drivers and deposition, while a catchment characteristic, peatland percentage, reflecting topography and site fertility was the best predictor for TOC (total organic carbon) and total Fe concentrations and export. In a subgroup of 18 catchments located in similar climatic conditions and low N deposition in eastern Finland (<600 kg N km–2 yr–1), peatland percentage accounted for 52–75% of the export of TOC, TN (total nitrogen), TON (total organic nitrogen), NH4-N and total Fe, while slope was the best predictor of TP (total phosphorus) export (r2 = 0.56, p < 0.001). Furthermore, among these 18 catchments TOC export accounted for as much as 95% of the variance in TON export, and 61–73% of the variance in NH4-N, total Fe and TP export. Total stem volume of the tree stands and their distribution between tree species and vegetation types were not important predictors for concentrations/export. The peatland percentage of the 21 catchments ranged widely, from 0 to 88% , with an average value of 34% as in entire Finland. Consequently, among the 21 catchments there was also a large variability both in average TOC and pH, 1.2–30 mg l–1 and 4.2–7.1, respectively. The average annual TOC export was 6,200 kg km–2 yr–1 , significantly lower values were recorded for the export of TN, TP and total Fe (130, 5 and 280 kg km–2 yr–1, respectively). N and P export was mainly in an organic form. The TIN (total inorganic N) proportion averaged 13% and elevated NO3-N export occurred only in fertile, high pH sites in southernmost Finland, where stream C/N ratio was lowest. On average, 97% of the TOC, 94% of the TN and 79% of the TP was in a dissolved form.