Characterization of soil organic matter and black carbon in dry tropical forests of Costa Rica

Abstract

Soil organic matter (SOM) in dry tropical forests (DTFs) of Costa Rica is less studied compared to wet tropical forests. Specifically, black carbon (BC) from biomass burning during the dry season but also prescribed burning during any prior pastoral land use contributes to SOM in DTF. Data on BC are needed for predicting the response of the soil organic carbon (SOC) pool to the projected climate change as BC is among the most persistent SOC fractions. Therefore, SOM was characterized and BC contents estimated selectively as soot (soot-BC) for 0–10 cm depth of a > 60 yr. old live oak (Quercus oleoides) forest (Qu) and a > 400 yr. old-growth forest (Of) at Sector Santa Rosa of Área de Conservación Guanacaste in northwestern Costa Rica. We expected that SOM chemistry at Qu is more closely related to the previous pastoral land use than at Of, and that a higher frequency of fire at Qu contributes to higher soot-BC concentrations than at Of. SOM was characterized by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, and soot-BC estimated by both NMR after chemical oxidation with sodium hypochlorite (NaOCl), and by C analysis and mass balance after thermal oxidation in air at 375 °C. SOM at Qu was more aliphatic but less aromatic than at Of as the duration in which woody litter input was dominant was shorter in the younger compared to the older forest. Alkyl C at Of was relatively enriched after chemical oxidation, and aryl C and lignin relatively depleted. In contrast, only small changes in these spectral regions were observed at Qu after oxidation may be due to stabilization by Fe oxides. Thus, higher inputs along with higher proportions of stabilized soot-BC fractions in the mineral phase may have contributed to higher soot-BC concentrations at Qu compared to Of determined by NMR after chemical oxidation (31.3 vs. 14.8% of SOC in the form of soot-BC). However, soot-BC values after thermal oxidation were lower (3.9 and 3.7% for Of and Qu, respectively), and not related to values obtained by NMR which highlights the need for the development of a standardized quantification method for soil BC. This method must be applied to study BC in the entire soil profile on a larger number of plots, especially in landscape depressions that receive run-on during the rainy season, to elucidate the fate of BC in the DTF which may be prone to frequent wildfires in the future.