Soil organic carbon stocks and forest productivity in volcanic ash soils of different age (1835-30,500Â years B.P.) in Mexico

Volcanic soils have a large potential for organic carbon storage and for forest growth, in particular under moist temperate climate. Documenting both potentials may offer incentives for forest conservation and exploitation, and for the allocation of bonds for environmental services. We studied sites on lava flows of different ages (1835, 4200, 6200, 8000, 10,000 and 30,500 years B.P.) covered by volcanic ashes within the Sierra del Chichinautzin Volcanic Field, under pine or fir forest. We analyzed the accumulation of organic carbon in the soils and its relation with the evolution of site quality, forest productivity indicators and C storage in tree biomass. We also studied the relation between soil C accumulation and different soil properties as soil pH, clay contents and pyrophosphate extractable Al and oxalate extractable Al and Si contents. Results show that carbon accumulation in the soil increases linearly with age within 1835 and 10,000 years old sites with a mean accumulation rate of 4.33 kg m− 2 each 1000 years. However, between 10,000 and 30,500 years the amount of C stored decreases again by about 30% of the maximum soil C accumulation. During the first 8000 years of the chronosequence site quality improves with soil development. Tree growth indexes approach stabilization at sites of ages between 10,000 and 30,500 years. Adult pine trees reach an accumulation of 27 kg C per tree each year. During the first 10,000 years carbon accumulation in mineral surface horizons is associated with the formation of Al-bearing organo-mineral complexes (characterized by pyrophosphate extractions), which seem to compete with the neoformation of short range order clay minerals. As clay and Al-oxide formation progresses these soil variables contribute also to stabilize soil C, although they are by far not as efficient as the organo-mineral complexes.