Tropical Andosol organic carbon quality and degradability in relation to soil geochemistry as affected by land use

Land use is recognized to impact soil geochemistry on the centennial to millennial timescale, with implications for the distribution and stability of soil organic carbon (SOC). Young volcanic soils in tropical areas are subject to much faster pedogenesis, noticeable already on the centennial or even decadal timescale. As land use is a recognized factor for soil formation, it is thus conceivable that even relatively recent land use conversion in such areas would already bear a significant impact on the resulting formed soils., e.g., in terms of content of pedogenic oxides. Very scarce observational evidence exists, so such indirect implications of land use on SOC cycling are largely unknown. We here investigated SOC fractions, substrate-specific mineralization (SOC or added plant residue), and net priming of SOC as a function of forest or agricultural land use on Indonesian volcanic soils. The content of oxalate-extracted Al (Alo) correlated well with organic carbon (OC) associated with sand-sized aggregates, particularly in the subsoil. The proportion of SOC in sand-sized ultrasonication-resistant (400 J mL−1) aggregates was also higher in agricultural land use compared to pine forest land use, and a likewise contrast existed for Alo. These combined observations suggest that enhanced formation of Al (hydr)oxides promoted aggregation and physical occlusion of OC. This was, importantly, also consistent with a relatively lesser degradability of SOC in the agricultural sites, though we found no likewise difference in degradability of added 13C-labeled ryegrass or in native SOC priming between the pine forest and agricultural land uses. We expected that amorphous Al content under agricultural land use would mainly have promoted mineral association of SOC compared to under pine forest land use but found no indications for this. Improved small-scale aggregation of tropical Andosols caused by conversion to agriculture and high carbon input via organic fertilizer may thus partially counter the otherwise expectable decline of SOC stocks following cultivation. Such indirect land use effects on the SOC balance appeared relevant for correct interpretation and prediction of the long-term C balance of (agro)ecosystems with soil subject to intense development, like the here-studied tropical Andosols.