BIOCHEMICAL-CHARACTERIZATION OF SOIL ORGANIC-COMPOUNDS EXTRACTED BY 0.5-M K2SO4 BEFORE AND AFTER CHLOROFORM FUMIGATION

The organic C (E(C)), total N (E(N)), anthrone-, phenol- and orcinol-reactive C (ARC, PRC and ORC, respectively), ninhydrin-reactive N (NRN), Folin-Ciocalteu's reagent-reactive C and N compounds (FRC and FRN) and deoxyribose containing compounds (diphenylamine-reactive compounds, DRC) extracted by 0.5 M K2SO4 before (non-fumigated extracts) and after (fumigated extracts) CHCl3 fumigation were evaluated in 8 very different soils. All quantities were significantly linearly correlated between them and with total organic C (TOC) and total N (TN) as well. The frequency distribution of the significance levels passing from non-fumigated to fumigated extracts and gamma-values (fumigated minus non-fumigated values for the same class of compounds) showed a shift towards the highest significances; this may indirectly confirm the action of chloroform in lysing soil microbial cells. The significance levels (P) of TOC correlated vs fumigated extracts were higher than vs non-fumigated extracts and gamma-values, thus indicating a release of non-biomass C organic compounds after CHCl3 fumigation. Since TN vs fumigated extracts showed significance levels higher than non-fumigated "tracts but equal to gamma-values, any release of non-biomass N probably did not occur. These considerations were confirmed by multiple regression analysis when E(C) and E(N) (dependent variables) were reconstituted taking into account TOC and TN as additional independent variables, respectively. However, released non-biomass C organic compounds (likely sugars) compared to TOC were very low and could be considered to be negligible. A low release of ribose-containing compounds (ORC) occurred, thus indicating that the amount of RNA material released after CHCl3 exposure was negligible. Deoxyribose-containing compounds rendered extractable after CHCl3 fumigation were probably derived from bacterial plasmids rather than from bacterial and eukaryotic chromosomes. The possibility of characterizing soil microbial biomass C and N compounds by adopting suitable colorimetric methods, as demonstrated in this work, is a further advantage of the fumigation-extraction method when compared with the fumigation-incubation method.