Effects of freezing-thawing on fluxes of CO2, DOC, DON and DIN from upland soils

Sammendrag

One presumed effect of climate change in the temperate region is increased frequency of freezing-thawing of the soil due to less permanent snow cover. Freeze-thaw episodes are often followed by an increased decomposition rate and microbial respiration rate and may be important for the winter C and N dynamic in the soil. In Norway roughly 70% of the land area is mountains, heathlands, and other unproductive, semi-natural systems. The systems are important contributors to the flux of C and N to air and water. Stable winters and snow cover has been common in these areas and the effects of increased temperatures may be more evident than in areas with more unstable winters. Consequently, investigating the effects of climate change and increased winter temperature in upland systems is important. In southern Norway, field experiments are ongoing to quantify the effect of climate change on fluxes of N and C from upland semi-natural terrestrial ecosystems. Effects of changes in winter soil temperature are examined in mini-catchments, which are untreated, insulated to simulate a stable snow cover, or subjected to snow removal. Due to natural variation between and within the catchments, we also designed an incubation experiment with controlled, but realistic conditions. The objectives of this lab experiment were to study 1) the effect of frequency and length of freezing episodes and 2) variations in effects between the different vegetation types represented in the mini-catchments. Intact soil cores were collected from the three main types of vegetation in the area: Heather (Calluna), grass (Molínia) and bog (Sphagnum). The soil cores were subjected to four different freezing/thawing regimes for four consecutive two weeks periods: 1) Slow cycling (SC) with one long freezing episode during each two weeks period, 2) fast cycling (FC) with four short freezing episodes during each two weeks period, 3) permanent frost (PF) and 4) permanent thaw (PT). The freezing temperature was ?5°C and the thawing temperature was 5°C. In the end of each two weeks period, and within the pre- and post incubation periods, CO2 emission was measured followed by water addition and soil water collection. There was no addition and withdrawal of water during the incubation period for the PF treatment. To get similar total amount of water from all treatments, the final water addition in the post incubation period was higher for PF. The soil water was analysed for DOC, DON, DIN, pH, conductivity and absorbance at different wavelengths. Preliminary results indicate that length of freezing periods seems to be more important for the concentrations and fluxes of DOC and DON than the frequency of freeze-thaw events. The cumulative DOC and DON fluxes showed the following order for the different treatments: PF >SC>PT>FC. The flux of NH4-N followed the similar pattern, whereas NO3-N showed an opposite trend. All the three different vegetation types responded fairly similarly to the different regimes. In general Calluna and Molínia gave higher fluxes than Sphagnum. The CO2 flux showed large variation, but the PF treatment seemed to give the highest flux following the PT treatment. And as for DOC and DON and DIN, Sphagnum showed lower respiration rate than Calluna and Molínia.