Effects of elevated CO2 and mineral nitrogen deposition on growth, allocation and decomposition of Sphagnum fallax, Polytrichum strictum and Eriophorum vaginatum in a Sphagnum bog.

Atmospheric CO2-C sequestration is directly related to the formation of peat which in turn is dependent on the production and decomposition of plant material. We studied the various aspects of qualitative and quantitative growth, litter production and decomposition of three characteristic plants found on a Sphagnum bog, produced and decomposed under treatment conditions. Particular emphasis was placed on distinguishing between the effects of litter quality and environmental conditions on litter decomposition. We endeavoured to do this over three different experiments undertaken during the 1997 growing season. Two separated treatments were applied: 1) atmospheric CO2 experiment: CO2= elevated, 560 ppm CO2; AIR= ambient, 360 ppm CO2 2) mineral nitrogen fertilisation experiment: N+=30; N0=0 [kg NH4NO3/ha/a]. Growth of mosses – Both treatments had a negative effect on the two mosses’ height growth and may cause Polytrichum strictum to outcompete Sphagnum fallax in a long term. The water table effect on growth was significant and accounted for almost half the total variance on the ANOVA. NH4 ++ NO3 - concentrations in the bog’s surface water of the CO2/AIR experimental sub-site were correlated to height growth of Sphagnum fallax. Growth of Eriophorum vaginatum – Elevated CO2 had no effect on the plant’s biomass or the root/shoot ratio, but did produce significant cross effects (treatment x time) on leaf dynamics, such as on the vitality index or the production of young leaves per tiller. N fertilisation did not significantly enhance plant biomass, in spite of a clear positive trend for all plant parts. At final harvest, N fertilisation had enhanced the number of tillers by as much as 38%. The outcome was an increased green leaf pool, which lead a 32% reduction in leaf-turnover. Surprisingly, soluble phenols and lignin were affected in the opposite way, where we observed a differential N fertilisation effect on CBSC content. These disparities could have various origins. Phosphorus was generally reduced under both treatments, and may play an important role in the production of CBSC. Soluble phenols concentrations of living parts may have triggered allelochemical inhibitions. As anticipated, the BP may play an important role in the storage of nutrients and photosynthates. Decomposition – P. strictum grown under elevated CO2 pre-treatment decomposed less than those grown in the control, whereas elevated CO2 treatment did not affect the decomposition of any species’ litter. Eriophorum plants grown under N fertilisation decomposed less than those of the control. This could partly be due to enhanced lignin, soluble phenols initial litter concentrations. Furthermore, S. fallax litter decomposition was enhanced under N fertilisation conditions (treatment). Next to litter qualitative changes, the decomposition “efficiencies” of chemical compounds were also affected by the N fertilisation independently from their initial concentrations. N fertilisation pre-treatment increased the loss of soluble phenols and decreased the bacterial activity (ATP). A significant positive relationship has been found between the soluble phenols and bacterial ATP. The N fertilisation treatment reduced the N immobilisation in Eriophorum and decreased the bacterial ATP among litterbags. We concluded that treatment-induced changes in the decomposition process could occur both in the presence or in the absence of changes in the initial litter quality.