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United States Department of Agriculture

Agricultural Research Service

Research Project: DEVELOPMENT OF MODELS AND CONSERVATION PRACTICES FOR WATER QUALITY MANAGEMENT AND RESOURCE ASSESSMENTS Title: Soil organic C:N vs. water-extractable organic C:N

Authors
item Haney, Richard
item Franzluebbers, Alan
item Jin, Virginia
item Johnson, Mari-Vaughn -
item Haney, Elizabeth -
item White, Michael
item Harmel, Daren

Submitted to: Open Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2012
Publication Date: September 15, 2012
Repository URL: http://handle.nal.usda.gov/10113/59390
Citation: Haney, R.L., Franzluebbers, A.J., Jin, V.L., Johnson, M.V., Haney, E.B., White, M.J., Harmel, R.D. 2012. Soil organic C:N vs. water-extractable organic C:N. Open Journal of Soil Science. 2(3):269-274.

Interpretive Summary: Soil testing labs use a variety of methods to determine % organic matter in soil. This is accomplished by measuring soil organic C and multiplying by 1.74. With new developments in instrumentation, we can now measure organic C and N from soil by extracting soil with water. The water extractable C and N is highly related to the soil C and N pool but is better correlated with soil microbial activity, therefore reflecting the pool of organic C and N that soil microbes readily act upon. The water extractable organic C and N pool is much smaller than the soil organic C and N pool (roughly 40 times smaller) and, therefore, a much more sensitive measurement of the active C and N involved in nutrient cycling. This will allow us to better predict the availability of nitrogen in soil to a growing plant.

Technical Abstract: Traditionally, soil-testing laboratories have used a variety of methods to determine soil organic matter, yet they lack a practical method to predict potential N mineralization/immobilization from soil organic matter. Soils with high microbial activity may experience N immobilization (or reduced net N mineralization), and this issue remains unresolved in how to predict these conditions of net mineralization or net immobilization. Prediction may become possible with the use of a more sensitive method to determine soil C: N ratios stemming from the water-extractable C and N pools that can be readily adapted by both commercial and university soil testing labs. Soil microbial activity is highly related to soil organic C and N, as well as to water-extractable organic C (WEOC) and water-extractable organic N (WEON). The relationship between soil respiration and WEOC and WEON is stronger than between respiration and soil organic C (SOC) and total organic N (TON). We explored the relationship between soil organic C: N and water-extractable organic C: N, as well as their relationship to soil microbial activity as measured by the flush of CO2 following rewetting of dried soil. In 50 different soils, the relationship between soil microbial activity and water-extractable organic C: N was much stronger than for soil organic C: N. We concluded that the water-extractable organic C: N was a more sensitive measurement of the soil substrate which drives soil microbial activity. We also suggest that a water-extractable organic C: N level > 20 be used as a practical threshold to separate those soils that may have immobilized N with high microbial activity.

Last Modified: 10/21/2014
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