Winter cover crops have the potential to increase soil organic C in the corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation in the upper Midwest. Management effects on soil C, however, are often difficult to measure because of the spatial variation of soil C across the landscape. The objective of this study was to determine the effect of oat (Avena sativa L.), rye (Secale cereale L.), and a mixture of oat and rye used as winter cover crops following soybean on soil C levels over 3 yr and both phases of a corn-soybean rotation using terrain attributes as covariates to account for the spatial variability in soil C. A field experiment was initiated in 1996 with cover crop treatments, both phases of a corn-soybean rotation, and a controlled-traffic notill system. Oat, rye, and oat-rye mixture cover crop treatments were overseeded into the soybean phase of the rotation in late August each year. Cover crop treatments were not planted into or after the corn phase of the rotation. Soil C concentration was measured on 450 samples taken across both rotation phases in a 7.62-m grid pattern in the late spring of 2000, 2001, and 2002. Slope, relative elevation, and wetness index (WI) were used as covariates in the analysis of variance to remove 77% of the variation of soil C caused by landscape driven patterns of soil C. Soil C concentrations were 0.0023 g C g soil 21 higher in 2001 and 0.0016 g C g soil 21 higher in 2002 than in 2000. The main effects of cover crops were not significant, but the interaction of cover crops and rotation phase was significant. The rye cover crop treatment had 0.0010 g C g soil 21 higher soil C concentration than the no-covercrop control in the soybean phase of the rotation, which included cover crops, but had 0.0016 g C g soil 21 lower C concentrations than the control in the corn phase of the rotation, which did not have cover crops. Using terrain covariates allowed us to remove most of the spatial variability of soil C, but oat and rye cover crops planted every other year after soybean did not increase soil C concentrations averaged over years and rotation phases.

As a part of establishing suitability classification for forage production, use of the national soil and climate database was attempted for Italian ryegrass (Lolium multiflorum Lam., IRG) in Gangwon Province. The soil data base were from Heugtoram of the National Academy of Agricultural Science, and the climate data base were from the National Center for Agro-Meteorology, respectively. Soil physical properties including soil texture, drainage, slope available depth and surface rock contents, and soil chemical properties including soil acidity and salinity, organic matter content were selected as soil factors. The crieria and weighting factors of these elements were scored. Climate factors including average daily minimum temperature, average temperature from March to May, the number of days of which average temperature was higher than 5 from September to December, the number of days of precipitation and its amount from October to May of the following year were selected, and criteria and weighting factors were scored. The electronic maps were developed with these scores using the national data base of soil and climate. Based on soil scores, the area of Goseong, Sogcho, Gangreung, and Samcheog in east coastal region with gentle slope were classified as the possible and/or the proper area for IRG cultivation in Gangwon Province. The lands with gentle or moderate slope of Cheolwon, Yanggu, Chuncheon, Hweongseong, Pyungchang and Jeongsun in west side slope of Taebaeg mountains were classified as the possible and/or proper area as well. Based on climate score, the east coastal area of Goseong, Sogcho, Yangyang, Gangreung and Samcheog could be classified as the possible or proper area. Most area located on west side of the Taebaeg mountains were classified as not suitable for IRG production. In scattered area in Chuncheon and Weonju, where the scores exceeded 60, the IRG cultivation should be carefully managed for good production. For better application of electronic maps.

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The quantification of evapotranspiration and soil evaporation is crucial for agricultural water management. The FAO-56 Penman-Monteith and E-DiGOR models were used to compute reference evapotranspiration (Eto) and bare soil evaporation, respectively, at 17 meteorological stations of South Korea, from 1980 to 2009. The same soil parameters were assumed for all stations in order to compare actual soil evaporation (Ea) rates jointly dominated by atmospheric evaporative demand and soil water availability, as well as the size of rainfall events. The sensitivity of Penman-Monteith type equations to the major climatic variables was determined based on 1-year dataset. The long-term mean annual precipitation and Eto calculated at selected stations over the country were 1339.7 mm and 1087.1 mm, respectively. Precipitation showed noticeable interyear fluctuations, and the annual Eto increased gradually during the study period. A strong correlation between pan evaporation (Epan) and Eto was observed (R = 0.808, P < 0.001), based on daily data of 30 years. Similarly, a significant correlation between Epan and potential soil evaporation (Ep) was existent (R = 0.622, P < 0.01). The Ep rates were lower than the Eto rates (Ep = 0.8 × Eto). The magnitude of Ea, as calculated with the model, reached a level of 63% of Ep. On the other hand, Ea accounted for 29.4% to 50.3% of the total precipitation over South Korea. Potential soil evaporation was more sensitive to net radiation, while reference evapotranspiration was mostly affected by the relative humidity. Wind speed was the less effective variable. The contribution of soil heat flux was negligible. The sensitivity of both Ep and Eto to the same climatic variables showed significant differences among seasons and locations. The aridity index ranged from 0.85 to 2.13, and all the study sites could be classified as humid areas. An aridity index of less than 1 appeared about once every 6 to 7 years, based on the station averages.

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