Effect of soil organic carbon on soil water retention

Reports about the relationship between soil water retention and organic carbon content are contradictory. We hypothesized that this relationship is affected by both proportions of textural components and amount of organic carbon. To test the hypothesis, we used the U.S. National Soil Characterization database and the database from pilot studies on soil quality as affected by long-term management. Regression trees and group method of data handling (GMDH) revealed a complex joint effect of texture and taxonomic order on water retention at −33 kPa. Adding information on taxonomic order and on taxonomic order and organic carbon content to the textural class brought 10% and 20% improvement in water retention estimation, respectively, as compared with estimation from the textural class alone. Using total clay, sand and silt along with organic carbon content and taxonomic order resulted in 25% improvement in accuracy over using textural classes. Similar but lower trends in accuracy were found for water retention at −1500 kPa and the slope of the water retention curve. At low organic carbon contents, the sensitivity of the water retention to changes in organic matter content was highest in sandy soils. Increase in organic matter content led to increase of water retention in sandy soils, and to a decrease in fine-textured soils. At high organic carbon values, all soils showed an increase in water retention. The largest increase was in sandy and silty soils. Results are expressed as equations that can be used to evaluate effect of the carbon sequestration and management practices on soil hydraulic properties.