Timescales of C turnover in soils with mixed crystalline mineralogies, Kruger National Park, South Africa
Organic matter-mineral associations stabilize much of the carbon stored globally in soils. Metastable short-range-order (SRO) minerals such as allophane and ferrihydrite provide one mechanism for long-term stabilization of organic matter in soil. However, ancient and highly weathered soils that cover a large fraction of land area lack SRO minerals. Here we evaluate the role of different minerals on the amount and turnover time (TT) of carbon in a field setting designed to minimize the role of
... imize the role of SRO by taking advantage of multiple lithologies in Kruger National Park, South Africa. Density separation demonstrated that most of the C was associated with minerals, even in surface soils. A parallel separation of clay-sized material demonstrated that 9–47 % of the organic C in these soils was stabilized by clays. Organic C associated with clay-sized material had average TT of 1020 ± 460 years in surface soils. The mean TT of this clay-associated C increased with depth and with fraction of total clay that was smectite. Because the C associated with smectite clay was so old, the amount of smectite (2 : 1 clays) controlled the age of bulk soil C across Kruger landscapes. The TT of the majority of soil C – not stabilized by clays – was much shorter, 190 ± 190 years in surface horizons. We suggest that this faster component reflects timescales of weaker C stabilization by crystalline Fe and Al oxyhydr)oxides and kaolinite (1 : 1) clays, as well as LF fractions not associated with minerals. Thus, bulk or HF carbon integrates C stabilized by mechanisms with inherently different TT, something that is often inferred from radiocarbon measurements. While SRO mineral concentrations were very low in these soils, the soils with most SRO had very high C content but also very young C. In other environments, SRO can be very stable and sorb C on very long timescales. We hypothesize that the seasonal wetting and drying in the KNP may reduce the age of SRO minerals as well as the C associated with them. Across the varying lithologies and a precipitation gradient found in the KNP, we found mineralogy to be the most important explanatory factor for C content (related to crystalline Fe) and turnover time (related to the amount of smectite).