Organic matter dynamics in a forest soil as affected by climate change

Large amounts of carbon are stored in boreal soils as soil organic matter. Aim of the research presented in this thesis was to quantify the effects of climate change on decomposition soil organic matter in a boreal forest ecosystem by means of field and laboratory experiments. Field experiments were carried out within the framework of the Climate Change Experiment (CLIMEX). In this project in two covered catchments in southern Norway CO 2 concentration and/or temperature were increased. An increase in temperature resulted in a higher N availability caused by increased mineralization of N. Part of the extra N was taken up by the vegetation whereas export of N in runoff increased as well. Appearantly, at least initially, the higher N availability exceeded the N demand by the vegetation.Laboratory incubations of different soil layers showed that the effect of elevated temperature on decomposition decreased with increasing depth in the soil presumably due to a decrease in substrate quality. A 14C labeling experiment in growth chambers showed that, at elevated CO 2 , more C was fixed in a plant-soil system with heather. An increase in CO 2 concentration did not change allocation to shoot, root or soil. When N supplies increased, relatively more C was fixed in shoots at the expense of roots. On the short term (days) decomposition of labile, root-derived organic matter was stimulated both by elevated CO 2 and elevated N. On the longer term, (weeks) respiration from high-CO 2 soils was lower than that of low-CO 2 soils.Simulations with the NICCCE model showed that, under normal light conditions, the forest ecosystem acts as a sink for C in the next 100 years. Especially under low light conditions, a step increase in CO 2 and temperature causes the ecosystem to become a source for C. When CO 2 and temperature are increased gradually, the ecosystem becomes a small sink for C.</p

Nitrogen transformations in a forested catchment in southern Norway subjected to elevated temperature and CO2

Model predictions on the response of soil processes to global warming are mostly inferred from small-scale laboratory studies. In this study, a forested catchment in southern Norway was enclosed by a greenhouse and experimentally manipulated by increasing CO2 ( 200ll-1 above ambient) and temperature ( 3-5oC). This paper reports on the effects of the climate manipulation on N mineralization and nitrification. We measured net N mineralization and nitrification in a control and treated part of the greenhouse as well as in an uncovered reference catchment in plots dominated by Calluna vulgaris (L.) Hull or Vaccinium myrtillus L. Net N mineralization in the 0-10cm soil layer significantly increased, most likely as a result of increased temperature. The effect was largest in plots dominated by Calluna. Nitrification did not significantly increase. Soil moisture inside the incubated cores was not affected by the climate change treatment. Pre-treatment mineralization was similar inside and outside the enclosure whereas nitrification was higher inside the enclosure. The NH4 content was significantly lower inside the chamber due to removal of acidifying components from the precipitation and lower inputs of dry deposition. We found however no differences in pH, %C and %N of the LF and H layer and total C and N in the soil cores between the two catchments. Mineralization was generally higher under Vaccinium than under Calluna even though measured soil chemical and physical characteristics were similar. Nitrification was higher under Calluna than under Vaccinium

Nitrogen transformations in a forested catchment in southern Norway subjected to elevated temperature and CO2

Model predictions on the response of soil processes to global warming are mostly inferred from small-scale laboratory studies. In this study, a forested catchment in southern Norway was enclosed by a greenhouse and experimentally manipulated by increasing CO2 ( 200ll-1 above ambient) and temperature ( 3-5oC). This paper reports on the effects of the climate manipulation on N mineralization and nitrification. We measured net N mineralization and nitrification in a control and treated part of the greenhouse as well as in an uncovered reference catchment in plots dominated by Calluna vulgaris (L.) Hull or Vaccinium myrtillus L. Net N mineralization in the 0-10cm soil layer significantly increased, most likely as a result of increased temperature. The effect was largest in plots dominated by Calluna. Nitrification did not significantly increase. Soil moisture inside the incubated cores was not affected by the climate change treatment. Pre-treatment mineralization was similar inside and outside the enclosure whereas nitrification was higher inside the enclosure. The NH4 content was significantly lower inside the chamber due to removal of acidifying components from the precipitation and lower inputs of dry deposition. We found however no differences in pH, %C and %N of the LF and H layer and total C and N in the soil cores between the two catchments. Mineralization was generally higher under Vaccinium than under Calluna even though measured soil chemical and physical characteristics were similar. Nitrification was higher under Calluna than under Vaccinium

Carbon allocation and decomposition of root-derived organic matter in a plant-soil system of Calluna vulgaris as affected by elevated CO2.

The effect of elevated CO2 on C allocation in plant and soil was assessed using soil cores planted with 1-y-old heather (Calluna vulgaris (L.) Hull). Plants were pulse-labeled with 14CO2 at ambient and elevated CO2 and two nitrogen regimes (low and high). After harvesting the plants, the soil was incubated to monitor total respiration and decomposition of 14C-labeled rhizodeposits. Total and shoot biomass increased at high N but were not affected by CO2. Root biomass was not affected by either N or CO2 treatments. Total 14C uptake and shoot-14C increased upon adding N and elevating CO2 but the N effect was strongest. Total 14C uptake per unit shoot mass decreased with N, but increased with CO2. Root-14C content was not significantly affected by the N or CO2 treatment. Total soil-14C slightly increased at elevated CO2 whereas microbial 14C increased due to high N. C allocation to shoots increased at the expense of roots, soil and respiration at high N but was not affected by the CO2 treatment. Variation in 14C distribution within each treatment was small compared to variation in total 14C amounts in each plant-soil compartment. Initially, 14C respiration from rhizodeposits correlated well with root-14C, total soil-14C, soil solution-14C and microbial 14C, at harvest time and was increased by elevated CO2. By the end of the incubation, however, decomposition of labeled organic matter was not affected by the treatments whereas total (= 12C 14C) respiration was lowest for the elevated-CO2 soils. We speculate that initially, respiration is dominated by decomposition of fresh root exudates whereas in the longer term, respiration originates from decomposition of more recalcitrant root material that had been formed during the entire experiment. The increased net 14C uptake and unchanged distribution pattern, combined with an increased decomposition of easily-decomposable compounds and a decreased decomposition of more recalcitrant root-derived material indicated a small sink function of a Calluna plant-soil system under elevated CO2

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding 15 N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH4+, NO3−, 15NH4+ and 15 NO3−. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH4+ mineralization and NH4+ immobilization increased. In the H layer, however, both gross NH4+ mineralization and NH4+ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate 15N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability