Abstract

HELFRICH, M., FLESSA, H., MIKUTTA, R., DREVES, A. and LUDWIG, B. (2007):

European Journal of Soil Science 58, 1316-1329.

Stable soil organic matter (SOM) is important for long-term sequestration of soil organic carbon (SOC), but the usefulness of different fractionation methods to isolate stable SOM is open to question. We assessed the suitability of five chemical fractionation methods (stepwise hydrolysis, treatment with _H2O2, _Na2S2O8, NaOCl, and demineralization of the NaOCl-resistant fraction (NaOCl + HF)) to isolate stable SOM from soil samples of a loamy sand and a silty loam under different land use regimes (grassland, forest and arable crops). The apparent C turnover time and mean age of SOC before and after fractionation was determined by ^13Cand ^14Canalysis. Particulate organic matter was removed by density fractionation before soils were exposed to chemical fractionation. All chemical treatments induced large SOC losses of 62-95% of the mineral-associated SOC fraction. The amounts of _H2O2- and _{Na2S2O8-resistant} SOC were independent from land use, while those of NaOCl- (NaOCl + HF)- and hydrolysis-resistant SOC were not. All chemical treatments caused a preferential removal of young, maize-derived SOC, with _Na2S2O8 and _H2O2 being most efficient. The mean ^14Cage of SOC was 1000-10000 years greater after chemical fractionation than that of the initial, mineral-associated SOC and mean ^14Cages increased in the order: NaOCl < NaOCl + HF ≤ stepwise hydrolysis _H2O2 ≈ _Na2S2O8. None of the methods appeared generally suitable for the determination of the inert organic matter pool of the Rothamsted Carbon Model. Nonetheless, our results indicate that all methods are able to isolate an older, more stable SOC fraction, but treatments with _H2O2 and _Na2S2O8 were the most efficient ones in isolating stable SOM.