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Title: EFFECT OF SOIL ORGANIC CARBON ON SOIL WATER RETENTION

Author
item Rawls, Walter
item Ritchie, Jerry
item Pachepsky, Yakov
item SOBECKI, T - USDA-NRCS
item BLOODWORT, H - USDA-NRCS

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2002
Publication Date: 6/1/2003
Citation: RAWLS, W.J., RITCHIE, J.C., PACHEPSKY, Y.A., SOBECKI, T.M., BLOODWORT, HL, EFFECT OF SOIL ORGANIC CARBON ON SOIL WATER RETENTION. GEODERMA. 2003. V. 16. P. 61-76.

Interpretive Summary: The ability of soil to retain water under draining or drying conditions depends on soil texture and structure, i.e. on the proportion of particles of different sizes and their spatial arrangement to form pores to hold water. Organic compounds affect ability of soil particles to form aggregates and, thus, affect soil structure. The literature contains contradictory reports on the role of organic carbon in soil water retention. We hypothesized that this role is affected by a) proportions of textural components, b) amount of organic carbon, and c) quality of organic matter. To test the hypothesis, we used data on 12,000 samples from National Soil Characterization. We observed a complex joint effect of texture, organic carbon, and taxonomic order on water retention. Adding information on taxonomic order and on taxonomic order and organic carbon content to the textural class brought an improvement in water retention estimation in comparison with estimation from the texture alone. The highest sensitivity of the water retention to changes in organic matter content was sandy soils. An increase in organic matter content lead to an 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. We expressed our results as equations, and tested them with the data from NRCS pilot studies on soil quality as affected by long-term management. Results were satisfactory, and we propose to use those equations to project carbon sequestration and management practices effects on soil hydraulic properties.

Technical Abstract: Reports about the relationship between soil water retention and organic carbon content are contradictory. We hypothesized that this relationship is affected by a) proportions of textural components, b) amount of organic carbon, and c) quality of organic matter. To test the hypothesis, we used National Soil Characterization data base and the data base from pilot studies on soil quality as affected by long-term management. Regression trees and Group Method of Data Handling 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. 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 lead 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.