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Title: SUBSTRATE INDUCED RESPIRATION IN SOILS USING C-14 (CARBOXYL) BENZOATE

Author
item Morris, Dolen
item SNYDER, G

Submitted to: World Congress of Soil Science
Publication Type: Proceedings
Publication Acceptance Date: 12/15/2001
Publication Date: 8/14/2002
Citation: Morris, D.R., and G.H. Snyder, 2002. Substrate induced respiration in soils using c-14 (carboxyl) benzoate. Transactions of the 17th World Congress of Soil Science. CD Rom paper 784:1-8.

Interpretive Summary: Soil in the Everglades Agricultural Area is subsiding at around 1.4 cm per year. Currently there is only about 30 to 150 cm of soil remaining on top of hard limestone bedrock. Most of the soil loss is due to microbial activity. Management methods to stop the microbial activity are needed. However, there are few practical methods that can be used under replicated field conditions to measure those losses. One of the more promising methods was one developed by Dr. Tate in 1979, which involved adding a 14C labelled compound to the soil and measuring CO2 evolved under a 2 hr incubation period. Dr. Tate did not use enzyme saturating condition, which is in contrast to classical enzymologist that use enzyme saturating conditions. An experiment was set up to determine how enzyme saturating conditions correspond to non saturating conditions using 14C benzoate. There was a positive correlation between enzyme saturating and non saturated levels. But, enzyme activity in one soil declined after reaching the maximum activity, which indicates possible toxicity of substrate or product and could result in errors when interpreting the data. Using the Tate method is better than the classical enzyme method for determining soil loss potential in the Everglades Agricultural Area.

Technical Abstract: Studies involved in measuring microbial induced respiration in soils using a 14C labelled substrate have been used to provide estimates of soil organic matter oxidation potential. Two laboratory experiments were conducted to determine how enzyme saturating conditions correspond to non saturating conditions. Labelled 14C carboxy benzoate was appled to Florida Histosols and sandy soils at rates ranging from 3.2 to 800 uM/g soil and incubated for 2 hrs. Soil enzyme responses followed Michaelis Menton equations. In the both experiments, the Km of the Histosols were significantly different from the sandy soils indicating different enzyme efficiencies and affinities. Vmax was different for each soil due to different quantities of enzyme present. Also, in both experiments, one of the sandy soils had activity that declined when Vmax was reached, which indicates that for some soils, applying substrate at enzyme saturating conditions and comparing microbial activities could lead to erroneous results. A correlation of the enzyme activities was performed on the samples in the second experiment. There was a significant relationship between the activity at the lowest substrate rate and the higher rates as indicated by significant correlation coefficients. Using less than enzyme saturated conditions should be adequate for estimating soil organic matter oxidation potential.