Effect of Residue Application Methods and Long-Term No-Tillage Management on Residue Decomposition and Priming Effect in Japanese Andosol: Evidence from a Field Incubation Experiment

 

Recent studies observed that long-term no-tillage (NT) management enhanced soil carbon stock in Japanese Andosols. However, limited quantitative data about residue decomposition characteristics under NT hinder a full understanding of its mechanisms. To address this, we conducted a field incubation study to examine how different residue application methods, related to tillage management, affect residue decomposition and priming effect in Japanese Andosol. Using PVC cylinders (0-15 cm), we collected undisturbed soils from NT plots, while we also collected disturbed soils from conventional tillage (CT) plots at a long-term experiment site (Ibaraki, Japan). We applied ¹³C-labelled residue (Ryegrass, C:N=25.8, ¹³C=7 atm%) at a rate of 2 kg C m^-2 under three treatments: (1) surface application on NT soil (NTSA), (2) mixing residue with CT soil (CTMIX), and (3) surface application on disturbed CT soil (CTSA), with two controls (NTCK and CTCK) (n=4). We monitored the flux of soil- and residue-derived CO₂ for three month incubation period, including the priming effect, and measured the microbial biomass carbon (MBC). The total C content at the 0–5 cm depth was significantly higher in NT soil (64 g kg^-1) compared to CT soil (45 g kg^-1), while the bulk density was slightly greater in CT soil (0.78 g cm^-³) than in NT soil (0.72 g cm^-³) at the same depth. Cumulative residue C mineralization in surface residue treatments, such as NTSA (92 g C m^-²) and CTSA (88 g C m^-²), were clearly larger than in CTMIX (44 g C m^-²) after three months. This suggests different physical protection and accessibility of surface residue particles under NT or CT, as mixing residues into the soil enhances adsorption to mineral surfaces under CT, reducing microbial decomposition, and probably promoting aggregation, which decreases the residue decomposition physically. Cumulative soil C mineralization was also enhanced in soils treated with labelled residue compared to control soils, reflecting positive priming effect. The priming effect was greatest in CTSA (120 g C m^-²), followed by NTSA (83 g C m^-²), with the lowest priming effect observed in CTMIX (32 g C m^-²) after three month. The results of one month of incubation showed that relative changes in MBC with respect to each control soil was negative in NTSA (-13%), while it was positive in CTSA (33%) at 0–2.5 cm depth. This implies that MBC in NTSA was more stable to residue application compared with CTSA contributing to its lower priming effect. Less changes in MBC observed in CTMIX (9%) than CTSA may be due to dilution of carbon inputs by the mixing effect, which also contributed to lower priming effects. In summary, surface residue application enhanced the residue mineralisation with larger priming effect, compared to mixing residue application, for three month in Japanese Andosols.