Litter quality and living roots affected the formation of new mineral-associated organic carbon but did not affect total mineral-associated organic carbon in a short-term incubation

Storing carbon (C) in soil organic matter (SOM) is critical for mitigating climate change as well as for providing food security under extreme weather events. Mineral-associated organic matter (MAOM), which is the organic matter attached to silt and clay, has a longer residence time than the light, sand-sized particulate organic matter (POM). Hence, increasing the C in this fraction is thought to be important for long-term C storage. However, the influence of litter quality and living plants on new MAOM formation and MAOM turnover is unclear. We hypothesized that lower C:N litter would form MAOM more efficiently and result in more MOAM than higher C:N litter because lower C:N litter favors biomass production of microbes, which are thought to be an important precursor of MAOM. In addition, we hypothesized that living plants would amplify the effect of litter quality on MAOM formation and turnover because roots growing with low C:N litter would be less N-stressed and rely less on MAOM turnover to access N than roots growing in high C:N litter. We conducted a greenhouse study where soil was mixed with four different plant litter treatments: a control without litter, soybean (Glycine max) with C:N = 21, buckwheat (Fagopyrum esculentum) with C:N = 35, and sudex (Sorghum bicolor [L.] Moench × S. sudanense [P.] Staph.) with C:N = 65. The four litter treatments were incubated with and without living oats (Avena sativa). The incubation was maintained for 40 days, and over this period soybean and buckwheat lost 89 % of their initial dry matter while sudex lost 69 % of its initial dry matter. Contrary to our expectation, the litter with the higher C:N (sudex) had the greatest proportion of degraded C transferred to MAOM (i.e., the litter MAOM-C transfer efficiency). However, the litter MAOM-C transfer efficiency was not associated with the total MAOM-C at the end of the incubation. The presence of living plants increased the amount of MAOM-C derived from soybean and sudex litter, but it did not increase the total MAOM-C. Thus, our results demonstrate that high C:N litter can efficiently form new MAOM-C, which can be enhanced by the presence of living plants, but the greater MAOM-C formation may be accompanied by a loss of pre-existing MAOM-C.