KMnO4 determination of active carbon for laboratory routines: three long-term field experiments in Austria

Recent studies show that a labile soil carbon (C) fraction determined with potassium permanganate (KMnO4) reflects the type of soil management. The present study combines the method for determining the active C (AC) pool with an alternative titration of the 0.02m KMnO4 solution with sodium oxalate (Na2C2O4) for routine laboratory analyses. Three long-term field experiments investigated: (i) different cropping systems and C-14-labelled organic amendments, (ii) three different tillage systems, and (iii) the application of four different kinds of compost. The results showed the depletion of AC in the permanent bare-fallow system of the C-14-labelled field experiment. When calculating the ratio AC/total organic C (TOC), the depletion of the AC/TOC curve reflected a priming effect, in accord with previous work. We obtained significant positive correlations of AC with TOC, total nitrogen (N-t), humic acid-C and remaining C-14-labelled material. The AC in the tillage systems experiment was significantly (P<0.05) different between all three tillage treatments at 0-10cm depth, and the ratio AC/TOC also revealed a significant difference between minimum and conventional tillage treatments at 10-20cm. For the compost field experiment, significant differences occurred between plots fertilised solely with N and plots receiving organic amendments. The AC/TOC ratio of the sewage sludge amendment was significantly lower than in all other systems. Correlations of AC with TOC for all samples of the different long-term field experiments revealed different behaviours in different soil types. The correlations of AC with N-t showed higher coefficients than with TOC. The applied methodology has a potential for sensitive and reliable detections of differences in soil organic matter characteristics.