Is growth of soil microorganisms in boreal forests limited by carbon or nitrogen availability?

To study whether the biomass of soil microorganisms in a boreal Pinus sylvestris-Vaccinium vitis-idaea forest was limited by the availability of carbon or nitrogen, we applied sucrose from sugar cane, a C-4 plant, to the organic mor-layer of the C-3-C dominated soil. We can distinguish between microbial mineralization of the added sucrose and respiration of endogenous carbon (root and microbial) by using the C-4-sucrose as a tracer, exploiting the difference in natural abundance of C-13 between the added C-4-sucrose (delta(13)C -10.8parts per thousand) and the endogenous C-3-carbon (delta(13)C -26.6 parts per thousand). In addition to sucrose, NH4Cl (340 kg N ha(-1)) was added factorially to the mor-layer. We followed the microbial activity for nine days after the treatments, by in situ sampling of CO2 evolved from the soil and mass spectrometric analyses of delta(13)C in the CO2. We found that microbial biomass was limited by the availability of carbon, rather than nitrogen availability, since there was a 50% increase in soil respiration in situ between 1 h and 5 days after adding the sucrose. However, no further increase was observed unless nitrogen was also added. Analyses of the delta(13)C ratios of the evolved CO2 showed that increases in respiration observed between 1 h and 9 days after the additions could be accounted for by an increase in mineralization of the added C-4-C.