Appendix 1: Experimental site
The three experimental plots established at the Sapporo and Memuro sites (CO, SM and SR), measure 10 m × 15 m and 10 m × 10 m, respectively. No field replications were made of the experimental plots due to the lack of available space. All plots were kept bare during the two-year observation period (November 2008 to October 2010) to enable precise observations of the atmosphere-snow-soil interactions. Air temperature and precipitation were measured at each site, and snow cover height, albedo of the ground or snow cover surface, and soil temperatures at depths of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.7 m were measured in each plot.Measurements were conducted as follows: snow cover height was measured using a ruler2 to 3 times weekly;the albedo was determined as the ratio of individually-measured daily mean reflection to incoming short wave radiation using an albedo meter (PCR-02,Kipp and Zonen B.V.); soil temperature was measured using T-type thermocouples inserted to soil depths as specified above; and soil frost depth was recorded as the 0˚C fringe estimated by the interpolation.
During the two-year observation period the annual mean air temperature was lower at the Memuro site (6.0˚C and 6.2˚C for the first and second study years, respectively) than at the Sapporo site (7.9˚C and 8.0˚C, respectively). Similarly, the mean early-to-midwinter air temperature (December–February) was lower at the Memuro site (-6.3˚C and -7.9˚C for the first and second study years, respectively) than at the Sapporo site (-2.5˚C and -3.7˚C, respectively). Annual precipitation was roughly 10% higher at the Memuro site (1003 mmand 1183 mm for the first and second study years, respectively) than at the Sapporo site (910 mm and 1043 mm, respectively), whereas cumulative early-to-midwinter precipitation was lower at the Memuro site (152 mm and 132 mm for the first and second study year, respectively) than at the Sapporo site (269 mm and 190 mm, respectively).
The maximum snow cover height at the CO and SM plots was 0.86 and 0.79 m at the Sapporo site, and 0.72 and 0.69 m at the Memuro site in the first and second study years, respectively (Fig. 2). The application of charcoal onto the snow cover as an ASA in the SM plots caused a decline in the albedo of the snow cover surface from 0.7-1.0 to 0.2-0.4, and enhanced the process of snowmelt, particularly at the Memuro site. Snowmelt was completed 14 and 10 days earlier in the SM plot (in comparison with the respective CO plot) at the Memuro site in the first and second study year, respectively, but 6 and 3 days earlier at the Sapporo site in the respective years. Except for the Memuro site in the first study year, several snowfall events occurred after the ASA application and negated the decrease in albedo (data not shown). Despite theunexpected snowfall, the albedo decreased again immediately after the snow covering the ASA had melted, and finally snowmelt was stimulated to a maximum extent. In the SR plots snow cover height was occasionally below 0.2 m, and thus reduced successfully by the operations (Fig. 2). At both sites, the annual maximum soil frost depth at the CO and the SM plots ranged from 0.0 m to 0.15 m. However, the results were not the same for the SR plot where depths exceeded 0.2 m: at the Memoru sitedepths of 0.42 m and 0.53 m were recorded in the first and second study year, respectively; and at the Sapporo site depths of 0.21 m and 0.27 m were recorded. The maximum soil frost depth was observed in February at both sites.
Appendix 2: Measurement of CO2 and N2O fluxes of soil
A circular stainless steel collar (275 mm OD)with awater groove (20 mmin height) was embedded at the centerof the subplot (2 m× 2m), and potassium nitrate was supplied therein at a rate of 50 kg N ha-1during late October 2008 and 2009 to mimic the excess residual mineral N level in local farmlands after harvest. Chambers made ofcolored polyvinylchloride measuring 0.25-m in diameter, 0.40 m in height, and 19.6-L in volume were used to observe the net N2O and CO2 emission rates. Air samples drawn from the chamberheadspace at 1, 11, 21 and 31 minafter the onset of measurements, were individually injected into a pre-evacuated 20-mL vial capped with a butyl rubber septum or a pre-evacuated 150-mL Tedlar® bag. The N2O concentration was determined using a gaschromatograph (Shimadzu 14B, Shimadzu Co. Ltd, Japan) equippedwith an electron capture detector, whereas the CO2 concentration was determined using an infrared analyzer, (ZFP9, Fuji Electric), or gas chromatograph (GC-14B, Shimadzu) equipped with a thermal conductivity detector. Emission rates (μgNm-2 h-1 and mg C m-2 h-1) werederived as the slope of a linear regression computed for measuredN2O and CO2 concentration data in the chamber versus time (ppm h-1).To keep the area between the chamber and the groundsurface airtight, the groove on the collarwas filled with liquidwateras a seal. However, during the period when the collar was under the snow cover, thechamber was placed directly onto the snowpack surface to minimizeany disturbance of the snowcover height. At that time, the qualityof the air-tightness between the chamber and the snowpacksurface was uncertain; therefore, the slope values were statisticallytested as shown in Yanai et al. (2011). Any slope that did not differsignificantly from 0 (Student’s t-test, α = 0.20, thus p ≥0.10) wasregarded as 0 ppm h-1, i.e., no net gas emissions occurred. The gas sampling was carried out at two-to three-day intervals during the snowmelt period (March–early April) and weekly for the remainder of the time. Cumulative emissions over the monitoring period (kg Nha-1 and Mg C ha-1) were calculated by linear interpolation of the observedemission rates.
In addition, the CH4 flux was not measured because of its negligibly small contribution to the total greenhouse gas balance in an arable upland field (Koga et al. 2004; Koga et al. 2006), and it was not known whether the CH4 flux responded significantly to the seasonal freeze-thaw conditions of soil (see Flessa et al. 1995; Maljanen et al. 2007).
ESM Table 1 Local snow cover manipulation techniques conducted in agricultural fields in Hokkaido, northern Japan.
ESM Table 2 Physico-chemical properties of wood-based waste charcoal† used as an agricultural snowmelt agent.
Values shown are on an oven-dry basis
†: dark 4 mm-granular, pyrolysis at 800°C
‡: Specific surface area, N2 as sorbent
ESM Fig. 1 Schematic diagram of N2O flux in early spring affected by seasonal soil-freezing and insulation of soil from cold air during the snow-melting period. Modified from Yanai et al. (2012)
References
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