Nutrient dynamics in water and soil under conventional rice cultivation in the Vietnamese Mekong Delta [version 2; peer review: 1 approved, 2 approved with reservations]

Background The evaluation of nutrient variability plays a crucial role in accessing soil potentials and practical intervention responses in rice production systems. Synthetic fertilizer applications and cultivation practices are considered key factors affecting nutrient dynamics and availability. Here, we assessed the nutrient dynamics in surface, subsurface water and soil under local water management and conventional rice cultivation practices in the Vietnamese Mekong Delta. Methods We implemented a field experiment (200 m 2) in the 2018 wet season and the 2019 dry season in a triple rice-cropping field. Surface water, subsurface water (30–45 cm), and topsoil (0–20 cm) were collected eight samples during the rice-growing seasons to clarify its nutrient dynamic. Results The results showed that N-NH 4 +, P-PO 4 3- and total P peaks were achieved after fertilizing. Irrespective of seasons, the nutrient content in surface water was always greater than that of subsurface water ( P<0.001), with the exception of N-NO 3 -, no significant difference was disclosed ( P>0.05). When comparing the wet and dry seasons, nutrient concentrations exhibited minor differences ( P>0.05). Under conventional rice cultivation, the effects of synthetic fertilizer topdressing on the total N, soil organic matter (SOM), and total P were negligible in the soil. Higher rates of N fertilizer application did not significantly increase soil N-NH 4 +, total N, yet larger P fertilizer amounts substantially enhanced soil total P ( P<0.001). Conclusions Under conventional rice cultivation, the low concentration of N-NH 4 +, P-PO 4 3- and total P in the subsurface water indicated that nutrient losses mainly occur through runoff rather than leaching. Notably, nutrient content in soil was fairly high, whilst SOM was varied from low to medium between seasons. Future work should consider the nutrient balance and nutrient dynamic simulation on surface and subsurface.