Subsurface methane dynamics of a paddy field under long-term fertilization: 13C-evidence from in-situ belowground labeling

Rice paddies are a significant source and sink of methane (CH4). The field surface emission and laboratory turnover potential of CH4 have been extensively studied. However, there is a knowledge gap in underlying mechanisms linking in-situ belowground CH4 production and oxidation potentials affected by rice growth and fertilization. Here, we implemented an improved soil silicone tube approach and 13C-labeled CH4 to measure the in-situ production and oxidation of CH4 in the subsoil (20–30 cm) during rice growth under four long-term (7 years) fertilization treatments. Urea N (uN) fertilization decreased 3-folds CH4 production as compared to non-fertilized control (Control). In combination with uN, biochar application (uN + Biochar) resulted in similarly low [CH4], but manure fertilization (uN + Manure) stimulated CH4 production which increased with rice growth and reached 0.6 mmol L−1 of soil tube at mature stage relative to uN. During the rice growth, δ13C–CH4 values under all fertilizations progressively decreased as compared with the start of the experiment but remained in the range of acetoclastic methanogenesis (−75‰). Pronounced δ13C–CO2 enrichment under uN + Manure indicated the increasing contribution of CO2-reduction pathway. The CH4 oxidation was highest (20.1 μmol L−1) under uN + Manure at tillering stage, followed by uN (14.6 μmol L−1) at seedling stage. The lowest oxidation potential was under uN + Biochar along with the weakest CH4 concentration dynamics. Water-locked conditions, low measured O2 concentration in silicon tubes, as well as the positive relationship between CH4 oxidation and NO3− and DOC suggested a possibility of anaerobic oxidation of CH4 (AOM). In conclusion, the intensive rice growth during seedling and tillering stages controls subsoil CH4 concentrations via methanotrophic activity (anaerobic oxidation pathway). Manure fertilization offset the inhibiting effect of mineral N fertilization on CH4 production but stimulated CH4 oxidation. Among all fertilization treatments biochar strongly suppressed the CH4 turnover and can therefore be recommended for greenhouse gas mitigation strategy in cleaner rice production.