Ninth International Conference on Permafrost ... - IARC Research

Soil Structural Change Effects on Greenhouse Gas Production and Carbon Loss inThawing SoilsG.A. LehrschUSDA-ARS Northwest Irrigation & Soils Research Laboratory, Kimberly, ID, USAR.S. DunganUSDA-ARS Northwest Irrigation & Soils Research Laboratory, Kimberly, ID, USAIntroductionFreezing alters soil structure, affects microbial populationsand activities, increases the emission of greenhouse gases(GHG) such as CO 2and N 2O, and redistributes the soilsolution and its constituents within soil profiles. None of theseprocesses are well characterized, being affected in variousways by the interacting effects of numerous factors, includinginitial soil water content, freezing rate, and number of freezethawcycles (FTC) experienced (Lehrsch 1998, Lehrsch etal. 1991). As a soil freezes, its soil water is redistributed, attimes and in places strengthening aggregates and elsewherefracturing them. Increases in aggregate stability are thoughtto be due to clay accumulation or to the precipitation, atparticle contact points within aggregates, of soil solutionconstituents, such as Ca 2+ or soluble silica (Lehrsch et al.1991). Experimental evidence for such processes, however,is lacking. Moreover, aggregate breakdown could releasesoluble organic C (SOC). Increased SOC concentrationsin the soil solution, upon thawing, may spur localizedmicrobial activity, increasing GHG emissions. The extentand speed with which soil water is redistributed determineswhether these processes will occur and, if so, their extentand significance. The exposing of new fracture surfaces frombroken aggregates may increase N 2O flux, since there maythen be more nitrogen in the soil solution and more substrateavailable to support microbial denitrification (Sehy et al.2004). Since wintertime losses of N 2O from agriculturalsoils can be 2 to 4 times as great as summertime losses, thestudy of physicochemical and microbial processes affectingwintertime N 2O flux are critically needed to assess globalN 2O budgets (van Bochove et al. 2000). As active layer orseasonally frozen soils thaw, greenhouse gases are emittedbut little is known about the effects of freezing-inducedsoil structural changes upon in situ microbial production ofCO 2and N 2O. Thus, in the laboratory, we characterized theeffects of organic carbon, soil water content, and FTC onaggregate stability and the emissions of CO 2and N 2O fromthawing soils.Methods and MaterialsWe studied three lots of Portneuf silt loam (DurinodicXeric Haplocalcid) collected from the 0 to 0.15 m depthof a field site (42°31′N, 114°22′W) located about 2.1 kmsouthwest of Kimberly, Idaho, USA, on 3 October 2007.One lot (hereafter referred to as soil with aged manure) hadreceived about 35 Mg ha -1 (dry weight) of dairy manureeach spring for 2 y prior to sampling, one lot 35 Mg ha -1 offresh manure at study initiation, and one lot no manure. ThePortneuf’s Ap horizon contained about 560 g silt kg -1 , 220 gclay kg -1 , and where no manure had been added, about 9.3g kg -1 of organic carbon (C). When collected, the Portneuf’saggregate stability was about 88%. Field-moist soil (watercontent of 0.07 kg kg -1 ) passing an 8 mm sieve was packedto a dry bulk density of 1.15 Mg m -3 into 97 mm diameter,0.13 m long plastic cylinders then, at a temperature of +2° C,slowly wetted to saturation and thereafter drained by tensionto a matric potential of -5 or -10 kPa (approximate watercontents of 0.48 and 0.37 m 3 m -3 , respectively). The packedcylinders, insulated using extruded foam to ensure freezingdownward from the surface, in an environmental chamberwere then subjected to 0, 1, 2, or 4 FTC, each of whichconsisted of slow freezing at -7°C for 72 h, then thawingat +2°C for 72 h. The 0, 1, 2, and 4 FTC were chosen forstudy because (1) the greatest microbial effects occur withthe first few cycles, and (2) important structural changesoccur with the first few cycles (Lehrsch 1998). The unfrozencontrols (0 FTC) were held at +2° C for the entire period ofall FTC. A calibrated, infrared photoacoustic detector (FieldGas-Monitor Model 1412, Innova AirTech Instruments,Ballerup, Denmark) was used to simultaneously measureCO 2and N 2O concentrations, after correction for watervapor, in gas samples collected every 8 to 12 h from the 560ml headspace of intact but thawing cores in sealed containersheld at 2, 10, 20, or 30°C for 72 h in an environmentalchamber. After each sample had been frozen for the last timebut not yet thawed, it was removed from the cylinder andsectioned into 20 to 25 mm thick layers. After each layer wassplit longitudinally, water content and aggregate stabilitywere measured on one half, and CO 2and N 2O emitted fromthe other half using the photoacoustic detector as the soilthawed in a sealed container at 2, 10, 20, or 30°C for 72h. Before measuring aggregate stability, the frozen soil wasthawed at +2°C for 72 h. Thereafter, aggregate stability wasmeasured by wet sieving 1 to 4 mm aggregates in deionizedwater for 180 s.Results and DiscussionSoil water and soil structurePreliminary data revealed that relatively slow freezing at-7°C redistributed the water in the relatively wet soil core(Fig. 1A). Due primarily to thermal potential gradients, waterflowed from the 33 mm depth to the 10 mm depth. Comparedto the water content at 33 mm, the water content at 10 mmincreased about 19%, to a water content of about 0.44 m 3 m -3175