How Does the Management-Induced Change in Soil Organic Matter Control Carbon Dynamics and Organo-Mineral Associations in a Volcanic-Ash Soil in Central Japan?.

Volcanic-ash soils hold significantly greater amounts of organic matter (OM) than non-volcanic soils largely because inorganic constituents unique to such soil (e.g., poorly-crystalline minerals) have high capacity to stabilize OM. Better understanding of stabilization and destabilization processes is critical to effectively manage volcanic-ash soils for C sequestration and fertility. Here we focused on an allophonic Andisol from a long-term experimental field (Tsukuba, Japan) and compared three surface soil samples (0-20 cm) of contrasting OM contents that resulted from agricultural practices. We examined how C mineralization rate, its temperature sensitivity, and OM distribution across different organo-mineral assemblages change along the gradient of soil OM contents. One soil was from the plot under no-tillage plus leaf-manure addition for two decades and had 10% total C (NT soil), presumably representing a maxima. Another soil with no fertilizer/manure treatment had 4% OC (NF soil), representing a minimum OM content for surface soils in the area. The last soil was from conventional tillage with no manure and had 5% C (CT soil). One-month lab incubation showed that the amount of respired C as well as the fraction of total C respired increased in the following order: NF < CT < NT 5-20cm < NT 0-5cm, suggesting that NF soil had the lowest amount and quality of C. Temperature sensitivity (Q10) was higher for NF. The rest of samples, however, had similar Q10 despite 3-fold difference in total C and 9-fold difference in respired C among them, indicating non-linear response of Q10 to soil C pool size. We will also present our initial results on C structure based on 13C-NMR as well as the distribution of OM and extractable Fe, Al, Si (by oxalate and pyrophosphate) among density fractions.