Integrating periphyton dynamics into an agroecosystem model for rice paddy N footprint evaluation

Periphyton at the soil-water interface regulates nitrogen (N) cycling in rice paddies, yet its role in multi-pathway N losses is overlooked in agroecosystem models, leaving a critical gap in assessing the integrated environmental N footprint (NF) of rice systems. The core challenge lies in capturing the seasonal sink-source transition of periphyton under coupled environmental controls and upscaling its heterogeneous N accumulation to regional scales. Here we incorporated a process-based periphyton module into the rice version of the soil Water Heat Carbon Nitrogen Simulator (WHCNS_Rice) and upscaled site-scale observations to watershed-scale application via a soil organic carbon-based parameter transfer function (μ₀ = 6.05 + 5.77 × SOC, r = 0.79). The improved model, constrained by temporal measurements, published N-loss data, and field surveys from 14 paddy sites, reproduced the seasonal sink-source shift (r = 0.95) and the spatial heterogeneity of periphyton biomass N (PBN; mean 13.9 kg N ha^-1, CV = 30.1%) at watershed scale. Periphyton decreased NH₃ volatilization by 17.8%, while increasing denitrification, N leaching, and runoff N losses by 28.6%, 2.7%, and 7.0%, respectively, with crop N uptake remained nearly unchanged (+0.4%). As the reduction in NH₃ volatilization dominated the integrated response, the yield-scaled NF declined by 5%, from 9.3 to 8.8 kg N-eq t^-1 on average. The NF reduction scaled positively with PBN (r = 0.55, p < 0.001), with each 1 kg N ha^-1 increase in PBN corresponding to a 0.02 kg N-eq t^-1 decrease in NF, indicating greater mitigation potential in areas with larger PBN. These findings demonstrate that representing periphyton is essential for unbiased regional N footprint assessment, particularly in paddy fields with higher periphyton N storage, and provide a modelling framework to support region-specific evaluations.