Decomposition of Organic Compounds in Soil As Affected by Nutrient Addition.

Plant roots release a number of low molecular weight organic compounds (LMWOCs) which are involved in many rhizospheric processes. The rate of flux of these LMWOCs in rhizosphere depends on nutritional status, microbial biomass and soil particles. We examined the effects of nutrient addition on decomposition of LMWOCs in soil in an incubation experiment. Three organic acids (acetic, malic and citric acid) were added at 10 mg C g^-1 soil to a clay loam soil from Wirrabara, South Australia (Xerult- USDA) and incubated at 22⁰C for seven days in the presence of increasing levels of N (N1:100, N2:250, N3:500 and N4:750 mg N kg^-1 soil) and P (P1:20, P2:50, P3:80 and P4:120 mg P kg^-1 soil). CO₂ produced was trapped by 0.5 M NaOH and was quantified by a titration method using 0.2 M HCl. The decomposition rate, as measured by CO₂ evolution was modelled using a double first-order exponential decay equation. The rate of decomposition varied between the organic acids and followed the order: malic > citric > acetic acid.  Whilst the rate of decomposition of acetic acid increased with increasing addition of nutrients, it decreased with highest level of nutrients in malic and citric acids. The difference in the effect of nutrients on the rate of decomposition between organic acids may be attributed to the difference in the nutrient-induced changes in pH and ionic strength as measured by EC.  The results also indicated that N is more limiting for microbes than P in this soil.