Abstract
Abstract. Soil organic carbon (SOC) constitutes the largest terrestrial C stock, particularly in the Andosols of volcanic areas. Quantitative information on distribution of SOC stocks is needed to construct a baseline for studying temporal changes in SOC. The spatial variation of soil short-range-order minerals such as allophane usually explains the variability of topsoil SOC contents, but SOC data for deeper soil layers are needed. We found that within a 1 km2 Costa Rican basin covered by coffee agroforestry, SOC stocks in the upper 200 cm of soil were highly variable (24 to 72 kg C m−2). Topsoil SOC stocks were not correlated with SOC stocks present in deeper layers. Diffuse-reflectance mid-infrared (MIR) spectroscopy made possible the analysis of a large number of samples (69 soil profiles, i.e. 598 soil samples) for ammonium-oxalate and sodium-pyrophosphate-extractable forms of Al, Fe, and Si, as well as SOC content and bulk density. Using the MIR spectra, we identified two different soil materials, which were identified as allophanic and halloysitic soil material. Allophanic soil occurred on top of the halloysitic soil. The thickness of the allophanic soil material, rich in SRO minerals and related to a young andic A horizon, explained the variability of SOC. This study illustrates that knowledge of topography and pedogenesis is needed to understand and extrapolate the distribution of SOC stocks at landscape scales.
Highlights
Soil organic carbon (SOC) contributes importantly to soil fertility and productivity, but is a larger pool of C than the world’s vegetation and atmosphere combined (Lal, 2004)
The SOC stocks of the studied 1 km2 watershed were much larger than the SOC stocks stored in vegetation biomass
Knowing surface SOC stocks provided little information about deep SOC stocks, whereas SOC stocks within the 0–200 cm depth were positively correlated with the thickness of an allophanic soil material layer, likely a young andic A horizon
Summary
Soil organic carbon (SOC) contributes importantly to soil fertility and productivity, but is a larger pool of C than the world’s vegetation and atmosphere combined (Lal, 2004). Those facts suggest that SOC is a potential sink for atmospheric CO2, especially in soils whose formerly high levels of SOC have become depleted through land use. High spatial variations in SOC content can occur even at small scales (metres) (Gessler et al, 2000; Chevallier et al, 2000), and such variations increase the uncertainty of comparisons among SOC stocks under different land-management practices Development of accurate, low-cost techniques is needed for quantifying SOC contents at the necessary spatial, horizontal, and vertical scales
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