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High δ15N of tree foliage in forests subject to high N supply has been attributed to 15N enrichment of plant available soil N pools after losses of N through processes involving N isotope fractionation (ammonia volatilization, nitrification followed by leaching and denitrification, and denitrification in itself). However, in a long-term experiment with high annual additions of NH4NO3, we found no change in the weighted average δ15N of the soil, but attributed the high δ15N of trees to loss of ectomycorrhizal fungi and their function in tree N uptake, which involves redistribution of N isotopes in the ecosystem (Hogberg et al. New Phytol 189:515–525, 2011), rather than a loss of isotopically light N. Here, we compare the effects of additions of urea and NH4NO3 on the δ15N of trees and the soil profile, because we have previously found higher δ15N in tree foliage in trees in the urea plots. Doing this, we found no differences between the NH4NO3 and urea treatments in the concentration of N in the foliage, or the amounts of N in the organic mor-layer of the soil. However, the foliage of trees receiving the highest N loads in the urea treatment were more enriched in 15N than the corresponding NH4NO3 plots, and, importantly, the weighted average δ15N of the soil showed that N losses had been associated with fractionation against 15N in the urea plots. Thus, our results in combination with those of Hogberg et al. (New Phytol 189:515–525, 2011) show that high δ15N of the vegetation after high N load may be caused by both an internal redistribution of the N isotopes (as a result of change of the function of ectomycorrhiza) and by losses of isotopically light N through processes fractionating against 15N (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).

Dissolved organic carbon (DOC) is an important component in the terrestrial carbon cycle. Yet, the relative importance of different inputs of DOC to the soil solution remains uncertain. Here, we used a large-scale forest girdling experiment to examine how the supply of recent photosynthate to tree roots and their mycorrhizal fungi affects DOC, in particular low-molecular weight organic acids (LMWOA). We also studied effects of tree girdling on non-structural carbohydrates in microorganism, and examined the effects of freezing of soil and the presence of roots in the soil samples on soil solution DOC and LMWOA in this experiment. The concentration of DOC was reduced by 40%, while citrate was reduced by up to 90% in the soil solution by the girdling treatment. Other LMWOA such as oxalate, succinate, formate and propionate were unaffected by the girdling. We also found that girdling reduced the concentrations of trehalose (by 50%), a typical fungal sugar, and of monosaccharides (by 40%) in microorganisms in root-free soil. The effect of freezing on DOC concentrations was marked in samples from control plots, but insignificant in samples from girdled plots. Release of DOC from cell lysis after freezing was attributed equally to roots and to microorganisms. Our observations suggest a direct link from tree photosynthesis through roots and their mycorrhizal fungi to soil solution chemistry. This direct link should impact solute transport and speciation, mineral weathering and C dynamics in the soil compartment. Importantly, our finding of a substantial photosynthate driven production of DOC challenges the paradigm that DOC is mainly the result of decomposition of organic matter.