Conclusions

There is strong experimental evidence that the supply of N exerts proximal control on forest growth in Fennoscandian boreal forests and that any positive effect of Ca is likely indirect and related to the long-term influence of soil pH on microbes turning over N in soils. The strong correlation between soil pH (and Ca supply) and forest growth occurs partly also because Ca and N are components of the same water fluxes in landscapes. The relative contribution of in situ N turnover processes versus, inflow of N to the greater N supply (and hence productivity) in groundwater discharge as compared to that in recharge areas is not known. A more thorough understanding of the biogeo-chemistry and controls on productivity in these forest ecosystems requires that important links between soil chemistry and soil biology are identified and explored. Such studies will probably reveal a discrepancy between short-term effects of chemical manipulations on biota and the relations between natural chemical variability and biota. Hence, there is a need for long-term experiments.

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