Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s t020Water content, m 3 m -30.35 0.40 0.45 0.50AAggregate stability, %0 30 35 40 45B800.0700.0Air temperature increasedfrom -7 to + 2 ° CCO 2 conc.40600.0Ambient CO 2 conc.Soil depth, mm6080100120Figure 1. Soil water contents and aggregate stability in a relativelywet core of Portneuf silt loam after freezing at -7°C.(Fig. 1A). There was little change in water content below 58mm. Lehrsch et al. (1991) found that water content of 0.34m 3 m -3 in Portneuf silt loam was sufficient to decrease itsaggregate stability when it was frozen, then thawed. In thisstudy, at water content nearly one-third greater, structuralchanges will occur.Indeed, soil structure near the surface reflected the oftobservedinverse relationship between water content andaggregate stability after freezing (Fig. 1B) (Lehrsch et al.1991). As water content decreased with depth, aggregatestability increased from 35% at the 10 mm depth to 40%at the 33 mm depth, a 1.14-fold increase. In the middle ofthe core, however, aggregate stability changed little. Therelatively low aggregate stability near the bottom of thecore may have been a consequence of difficulty experiencedwhen dissecting the core prior to analysis.CO 2and N 2O emissionsPreliminary data revealed that both CO 2and N 2Oconcentrations measured in the headspace above intact soilcores increased with time as once-frozen soil containingaged manure thawed at +2°C (Fig. 2). CO 2concentrationsincreased by about 200 ppmv (37% greater than ambient)in the first 30 h after air temperatures in the environmentalchamber were increased from -7 to +2°C. For subsequentFTC and thawing temperatures of +2°C, CO 2emissions fromintact cores may not be as great. The C pool that provides thesubstrate for microbial production of CO 2as repeatedly frozensoils thaw is apparently limited in size, since Herrmann andWitter (2002) found that most CO 2was produced in the first4 FTC. Data in Figure 2 also reveal that N 2O concentrationsafter one incidence of freezing increased by about 5.7 ppmv(more than eight times greater than ambient) in the first 30 hafter thawing began. The concentration of CO 2peaked about24 h, and N 2O peaked about 26 h after the temperature wasincreased (Fig. 2). Lysed microbial cells may have providedsubstrate for microbial denitrification (Sehy et al. 2004).Gas concentration, ppmv500.010.07.55.02.50.0Ambient N 2 O conc.0 5 20 25 30Elapsed time, hSummaryAvailable data indicate that FTC (1) caused soil water toredistribute and (2) interacted with water content to alterthe near-surface aggregate stability of wet soil. Moreover,compared to ambient conditions, emissions of CO 2increasedby a third, and N 2O increased by a factor of eight, eachpeaking 24 to 26 h after thawing began, from intact cores ofsoil frozen only once and thawed at only +2°C.ReferencesN 2 O conc.Figure 2. CO 2and N 2O emissions as a function of thawing timefrom intact cores frozen once.Herrmann, A. & Witter, E. 2002. Sources of C and Ncontributing to the flush in mineralization upon freezethawcycles in soils. Soil Biology & Biochemistry 34:1495-1505.Lehrsch, G.A. 1998. Freeze-thaw cycles increase nearsurfaceaggregate stability. Soil Science 163: 63-70.Lehrsch, G.A., Sojka, R.E., Carter, D.L. & Jolley, P.M. 1991.Freezing effects on aggregate stability affected bytexture, mineralogy, and organic matter. Soil ScienceSociety of America Journal 55: 1401-1406.Sehy, U., Dyckmans, J., Ruser, R. & Munch, J.C. 2004.Adding dissolved organic carbon to simulate freezethawrelated N 2O emissions from soil. Journal ofPlant Nutrition and Soil Science 167: 471-478.van Bochove, E., Jones, H.G., Bertrand, N. & Prevost, D.2000. Winter fluxes of greenhouse gases from snowcoveredagricultural soil: Intra-annual and interannualvariations. Global Biogeochemical Cycles 14:113-125.176