Alteration of quality and stability of organic matter in grassland soils of Southern Brazil highlands after ceasing biannual burning

The impact of ceasing biannual burning on the composition and the amount of soil organic matter (SOM) in grassland soils from Southern Brazil was studied by means of solid-state C-13 and N-15 NMR spectroscopy using soil duplicates of fields without 1, 2, 5 and 22 years of burning. The examination of the duplicates demonstrated good reproducibility. Demineralization of the soil with hydrofluoric acid (HF) indicated that the fire frequency had no impact on the amount of HF-extractable SOM. Exclusion of fire resulted in a considerable decrease of the organic C stocks, whereas N and S stocks were only slightly affected. The content of pyrogenic organic C (PyOC) in the soils was determined via a combination of chemical oxidation with acid dichromate and solid-state C-13 NMR spectroscopy of the oxidation resistant fraction. This analysis revealed that this decline was caused by the reduced input of decaying plant roots remaining in the soil after combustion of the aboveground vegetation during a fire. In spite of frequent burning for centuries, the PyOC concentrations were unexpectedly low in the top 5 cm of the studied soils but increased clearly with soil depth, indicating that some charcoal constituents can be translocated with the soil solution. With respect to the stability of charcoal in the grassland soils, our study revealed that comparable to SOM, pyrogenic organic matter (PyOM) is composed of fractions with different stability against microbial degradation. Whereas the labile PyOM fraction showed C losses similar to that of O-alkyl C, the more stable PyOM was selectively preserved together with an alkyl C fraction. Based on the results of the present study, one can conclude that frequent burning of grassland can lead to an increased C-sequestration potential of a soil. Because the input of the additional C is mainly caused by a higher biomass production after the fire, the additional SOM contributes to the labile pool with short turn-over times rather than to the recalcitrant carbon fraction. After ceasing burning, the reduced litter input cannot compensate the fast consumption of this pool and a quick decrease of the C stocks is expected. (C) 2012 Elsevier B.V. All rights reserved.