Land Use Effects on Organic Carbon in Andean Volcanic Ash Soils

ABSTRACT Background Both organic carbon (OC) stocks and labile OC (LOC) fractions are important indicators of soil health and are sensitive to land use change. Aims To study the effects of land use change on these indicators in montane volcanic ash soils, a soil transect was surveyed in northern Ecuador. Methods Samples were collected from 0–30, 30–60, and 60–90 cm soil depth at two agricultural sites with different time of cultivation and at three natural vegetation sites (tropical alpine grassland, páramo). LOC was determined as cold and hot water extractable OC (CWEOC and HWEOC). Molar absorptivity at 254 nm was determined in the extracts as a qualitative measure. Results Total OC stocks were high at the páramo sites (51.3–60.2 kg C m −3 ) and the younger agricultural site (50.8 kg C m −3 ; 20 years of cultivation), but significantly lower (30.1 kg C m −3 ) at the older agricultural site (at least 100 years of cultivation); CWEOC (0.1%–0.7%) and HWEOC (0.6%–4.1%) represented only a small part of the OC. Both LOC pools decreased with increasing cultivation time, with CWEOC reflecting short‐term and HWEOC long‐term effects. In contrast, the molar absorptivity was highest at the oldest site (198–307 L mol −1 cm −1 vs. 36–64 L mol −1 cm −1 at the other sites), indicating that easily degradable labile C was depleted leaving compounds with higher aromaticity. Conclusions The conversion of páramo into agricultural land negatively affects OC stocks and soil health, as indicated by reduced OC storage capacities and lower LOC contents.