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Fluid Journal : Fluid Journal 2008-2009
10 Fluid Journal WINTER 2008 Research has shown that tillage and rotation were more important than residue removal for changing carbon sequestration in the soil profile. Further studies have shown that SOM is derived from the root system more than the above-ground biomass. A consequence of reducing the SOM levels is to decrease the water-holding capacity of the soil. Water availability in different soils is directly affected by organic matter content and field capacity value of the soil increases rapidly as organic matter increases. The impact of modifying soil water availability cannot be dismissed in the current climate regime where rainfall events have and will become more variable. A number of studies have found that the amount of residue required to maintain SOM levels ranged from 900 to 15,000 lbs/A/yr. These summaries reveal that there are complex interactions among residue production, residue removal, and the impact on subsequent crop production and SOM content. These Figure 2. Seasonal water patterns for corn grown on a Clarion and Webster soil in central Iowa during 2000 using fall and spring-applied N management practices. water evaporation rate. Studies have shown that corn residue reduces daily water evaporation by 0.016 in/day compared to a bare soil surface. They also found that the properties of the residue change during the over-wintering period so that the residue acts more as a water-absorbing material in the spring because the waxy cuticle is no longer present. This changes the dynamics of the residue layer on the soil microclimate. The presence of fresh crop residue has had more of an impact on soil temperatures in the fall than in the spring for several reasons. In the fall the residue has a different reflectance and absorbs less energy, while in the spring the residue has decayed and often has become denser because of the degradation process and also settling over the winter due to the presence of snow on the residue. Soil temperatures are affected in the upper four inches of the soil profile. The effect is to decrease the maximum temperature extremes inthelayerbyasmuchas10to15o F and the minimum temperatures by 5 to 10o F. Moderation of the soil temperature extremes and the increased soil water content in this layer promotes a more active soil microbial and megafauna system (earthworms, etc.). This increase in biological activity helps to incorporate the residue material into the soil layer and maintain the SOM content. There is a linkage between the decline of microbial activity, SOM, and the decrease in crop productivity. One of the aspects of crop production is water use by the crop and the concept of water- use efficiency (WUE) that relates yield or total biomass production changes have potentially large environmental impacts and the effects of residue removal need to be thoroughly understood before implementing residue removal programs. On soil water and microclimate effects. Crop residue has a major impact on the surface energy balance. Energy balance of the soil surface determines the rate at which water is evaporated from the soil as well as the temperature extremes of the upper soil profile. The presence of crop residue acts as a barrier to the soil surface and solar radiation is absorbed at the upper boundary of the residue layer. Because the crop residue is typically a fairly porous material with a large amount of air space, the rate at which heat or water vapor is transferred through the residue layer is quite slow. Evidence of this is seen in how long it takes the soil surface to dry when there is a layer of residue on the soil. Crop residue provides a layer on the soil surface and reduces the soil Corn Water Use 2000 Day of Year Water Use (mm) 100 120 140 160 180 2002 20 2402 60 280 0 100 200 300 400 500 600 Clarion Spring N (100 kg/ha) Webster Spring N (100 kg/ha) Clarion Fall N (200 kg/ha) Webster Fall N (200 kg/ha)
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