Spatial and seasonal dynamics of nitrification pathways in sandy sediments along the Guangdong Coast, China

Nitrification is a key process in the nitrogen cycle, yet its pathways and controls in permeable sandy sediments remain poorly understood. Here, we examined sandy sediments along the Guangdong coast of China in summer and winter, using potential nitrification rates (PARs) assays and qPCR quantification of comammox, AOA, and AOB genes. Results showed clear seasonal shifts: PAR_total (0.09-0.85 μg N g^-1 d^-1), PAR_aoa (0.03-0.39 μg N g^-1 d^-1), PAR_cmx (0.03-0.24 μg N g^-1 d^-1), and PAR_aob (0.01-0.31 μg N g^-1 d^-1) were generally higher in summer than in winter (PAR_total: 0.05-0.58; PAR_aoa: 0.01-0.42; PAR_cmx: 0.01-0.15; PAR_aob: 0.00-0.25 μg N g^-1 d^-1). In terms of relative contribution, while AOA (36.41%) remained the dominant driver in winter, the contribution of comammox notably increased to 34.88% (compared to 25.54% in summer), indicating niche differentiation and functional redundancy. Stepwise regression identified TOC, temperature, NO₂^-, Fe²⁺:Fe³⁺, and sulfide as key drivers of pathway variation (p < 0.05). Although the PAR of sandy sediments was lower than that of muddy sediments, we used extrapolation to estimate the nitrification potential of Chinese coastal sandy sediments (0-5 cm) were estimated at approximately 1122.13 ± 19.03 t N yr^-1. These findings revealed that sandy sediments, despite low nutrient retention and organic matter, acted as complementary and buffering components of the nitrogen cycle under dynamic coastal conditions, with important implications for nitrogen flux regulation and eutrophication management.