Nutrient conditions in drained peatlands along a north-south climatic gradient in Sweden

Abstract

Soil nutrient conditions were investigated in a series of five peatland afforestation trials (established 1971) along a north-south gradient in Sweden. All areas were drained low-sedge mires and planted with Scots pine ( Pinus sylvestris (L.)). Soil samples from the topsoil (0–20 cm) were collected 22 years after drainage with two ditch spacings (7.5 and 60 m) and PK-fertilization with three fertilization regimes. Total and available nutrient concentrations were determined. Results indicated that pH and total concentrations of N, P and Ca in the topsoil of extensively drained areas increased with decreasing temperature sum in similar site types. Concentrations of K and Mg were not correlated to temperature sum. Tree height and volume were positively correlated to concentrations of P, K and Ca, but not to N, and only for temperature sums ≥ 950 d.d. (day degrees). Hence, it appeared that P and/or K directly or through their effects on N mineralization, limited tree growth in the south, whereas another growth factor was limiting in the north. We examined the available/total concentration ratios for the soil nutrients, with the intention to reveal possible climate-dependent trends in those ratios. The available/total ratios of P and Ca were found to increase with increasing temperature sum. For Ca in southern areas the ratios were higher in intensively drained and PK-fertilized plots than in the extensively drained and unfertilized treatment, possibly indicating that peat mineralization had been more intensive. The Ca-ratios for the two northernmost areas, which showed little or no tree growth reaction, did not increase even in the intensively drained and fertilized treatments. We hypothesise that the treatment-induced changes in available/total Ca ratio along the climate gradient also reflect post-drainage intensity of N-mineralization. Hence, the absence of increased Ca-ratios at temperature sums below 950 d.d. may indirectly indicate that tree growth is limited by the microbial release of organically bound N.