Effects of long-term soil managements on the nature of OC in organo-mineral associations in temperate arable soils: selective dissolution approach

Conservation agricultural management (CA) including organic fertilization and crop rotations has been adopted to enhance organic carbon (OC) storage in arable soils mainly via increasing OC input to soils. However, it remains unclear how CA can affect OC in mineral-associated OM (MAOM) which is important for long-term C storage. For example, CA would potentially influence organo-mineral associations in MAOM by changing soil pH and/or the input of base cations (e.g. Calcium (Ca²⁺)). In the present study, to estimate OC bound to metal cations and reactive mineral phases, we first assessed soil extraction with pyrophosphate-dithionite (PD, pH 7.5) by measuring PD extractable metals and co-dissolved OC in soil and comparing with conventional extraction techniques (pyrophosphate, acid oxalate and dithionite-citrate). We then examined the extent to which long-term (> 20 yrs) CA enhances OC in MAOM and OC bound to metal cations and reactive mineral phases compared to inorganic fertilization (control) in arable topsoils under temperate climate.

Soils were sampled from eight long-term experimental sites with different soil managements (CA vs control) and under contrasting soil mineralogy (i.e. Andisol group (n=3) and non-Andisol group (n=5)) located in Japan, Canada, and France. Density fractionation (cut-off density: 1.8 g cm^-3) with sonication (475 J mL^-1) was conducted to isolate particulate organic matter (POM) and MAOM fractions in these soils. The nature of OC in MAOM fraction was then assessed by PD and other chemical extraction techniques.

As for the assessment of selective dissolution techniques, PD at near-neutral pH appeared to dissolve aluminum (Al) and iron (Fe) extractable by the conventional extractions, which suggests that PD extraction is a practical method to approximate OC bound to metal cations and reactive mineral phases. As for the effects of soil managements, POM-C was effectively enhanced by CA managements for both soil groups, whereas CA effectively enhanced MAOM only for non-Andisoil group. Among C pools in MAOM PD-extractable OC, which contributed 20±7 SD % of MAOM-C, was not enhanced significantly by CA. In the presentation we also plan to discuss relative importance of extractable metals (Al, Fe, and Ca) in OC present in organo-mineral associations of the studied soils.