Mineralogical controls on soil black carbon preservation

Abstract

Black carbon (BC) has long been considered a chemically resistant component of soil organic carbon (SOC). However, there is substantial evidence that the chemistry of most C compounds is less important for long‐term storage than is physical protection (e.g., mineral sorption). We explored BC retention in grasslands that lie along a climate gradient that produces strong differences in short range order (SRO) minerals known to drive landscape‐scale retention of SOC. We measured soil BC content using 13C nuclear magnetic resonance (NMR) spectroscopy, and used radiocarbon dating on a subset of samples to relate BC content to long‐term soil C storage. Black C concentrations in soil ranged from 0.2 to 2.9%, representing 10–30% of SOC and spanning levels found in temperate grasslands around the world. Black C concentrations were significantly correlated with SRO minerals, but the strongest single predictor of BC content was simply SOC. The ratio of BC/OC was fairly insensitive to SRO minerals, suggesting that BC responds similarly to reactive minerals as does OC. Direct links between SOC and BC retention warrant further study. We found no evidence that BC is preferentially retained relative to OC, with soil radiocarbon ages apparently driven primarily by inputs of new OC. These results indicate that physical protection may play a strong role in BC retention, and that BC cycling in soils may be more similar to OC cycling than is generally thought.