Long-term effects of experimental fertilization and soil warming on dissolved organic matter leaching from a spruce forest in Northern Sweden

Abstract Nitrogen deposition and increasing temperature are two of the major large-scale changes projected for coming decades and the effect of this change on dissolved organic matter is largely unknown. We have utilized a long-term fertilization and soil warming experiment in Northern Sweden to study the effects of increased nutrient levels and increased temperature on DOC transport under the O horizon. The site is N limited and mean annual temperature 2 °C. Experimental fertilization with ammonium nitrate and a physiological mixture of other macro- and micro-nutrients has been going on for 22 years and soil warming, 5 °C above ambient soil temperature for 14 years, prior to the study. Experimental plots have been irrigated to avoid drying and we also studied the effect of this long-term irrigation on DOC by establishing control plots receiving no irrigation. DOC concentrations and fluxes under the O horizon were approximately 50% higher in fertilized plots than in non-fertilized control plots. We did not find any statistically significant effect of soil warming. There was a statistically significant effect of long-term irrigation on DOC with higher DOC concentration and fluxes in irrigated plots than in plots without irrigation. There were no major effects on DOC quality measured by specific UV absorbance. Fertilization approximately doubled soil organic matter stocks in the O horizon, whereas there were no such effects of warming or irrigation on soil organic matter amounts. There was no statistically significant treatment effect on DOC collected from the B horizon. We hypothesize that the positive effect of fertilization on DOC is related to increased soil C stocks.