Positive and negative priming effects induced by freshly added mineral-associated oxalic acid in a Mollisol

Abstract

Freshly added root exudates carbon (C) can accelerate or delay the mineralization of soil organic carbon (SOC) pool. Such short-term change is a phenomenon known as the ‘priming effect’. However, it is still unclear whether the minerals associated with fresh C will still be able to cause the priming effect given that in the mineral associated fresh carbon complex, minerals protect fresh C from microbial decomposition. The hypothesis of this study is that, although oxalic acid is bound to minerals, the addition of mineral-associated oxalic acid to soils will accelerate the decomposition of SOC. To clarify this, a study was performed using a series of experiments including adsorption experiment, where different concentrations of oxalic acid (OA) were mixed with clay minerals to obtain the mineral-associated oxalic acid (MAOA) including vermiculite-associated oxalic acid (VmAOA) and montmorillonite-associated oxalic acid (MtAOA) at different concentrations. The X-ray diffraction (XRD) analysis was performed before and after the adsorption of oxalic acid by clay minerals. The VmAOA and MtAOA were added onto a Mollisol, incubated for 21 days and thereafter, the priming effects were determined in different treatments. Nuclear Magnetic Resonance (NMR) spectroscopy was performed before and after incubation. The results revealed that the adsorption of oxalic acid onto minerals affected the mineral structure through a decrease in mineral peaks. The OA mineralization increased with incubation time, while SOC mineralization showed the opposite trend to OA after day 7. In all the treatments, the strongest positive priming effect was induced on day 1 and was considered an apparent priming effect. On day 4 of the high treatment, and on day 7 of the low treatment with oxalic acid, the positive priming effect was observed and suggested as a real priming effect. After day 7 of the high treatment and day 11of the low treatment with oxalic acid, the positive priming effect shifted to the negative priming effect and the amount of SOC mineralized decreased till the end of incubation time. The percentage of alkyl groups increased in VmOA while the percentage of carboxyl groups and ketones groups increased in MtOA, indicating that alkyl groups are well protected by vermiculite while carboxyl groups and ketones groups are well protected by montmorillonite. This finding reveals that freshly added carbon, although bound to the mineral, can still accelerate SOC decomposition, and once fresh carbon bound to the mineral becomes available, the preference for SOC decomposition shifts to that of fresh carbon, resulting in a shift from a positive priming effect to a negative effect. This relevant finding will improve the understanding of the key role played by root exudates (oxalic acid) in the C cycle.