Soil Organic C:N vs. Water-Extractable Organic C:N

Richard L. Haney, Alan. J. Franzluebbers, Virginia. L. Jin, Mari-Vaughn. Johnson, Elizabeth. B. Haney, Mike. J. White, Robert. D. Harmel

Texas AgriLife Research, Blackland Research & Extension Center, Temple, TX.
USDA-Agricultural Research Service (ARS), Grassland, Soil & Water Research Laboratory, Temple, TX, USA.
USDA-ARS, Agroecosystem Management Research Unit, Lincoln, NE; 4USDA-NRCS, Resources Inventory and Assessment Division, Temple, TX.
USDA-ARS, J. Phil Campbell, Sr. Natural Resource Conservation Center, Watkinsville, GA.
USDA-NRCS, Resources Inventory and Assessment Division, Temple, TX.

DOI: 10.4236/ojss.2012.23032                

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 micro-bial 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.

Keywords

Soil Microbial Activity; C:N Ratios; Soil Organic C; N Mineralization; N Immobilization; Soil Testing

Conflicts of Interest

The authors declare no conflicts of interest.

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