Spatial heterogeneity and distribution patterns of dissolved carbons and total ammonia nitrogen in the upper Changjiang River under 2022 summer extreme drought conditions

The biogeochemical cycling of carbon in aquatic systems is profoundly regulated by extreme hydrological events, particularly through their impacts on dissolved carbon species (DCs) and total ammonia nitrogen (TAN). Despite growing recognition of these interactions, the spatial correlations and environmental linkages between DCs and TAN during meteorological extremes remain poorly constrained in large river systems. To address this critical uncertainty, we conducted a field campaign during the unprecedented summer drought (June-September 2022) in the Changjiang River Basin, collecting 24 water samples across three lateral positions along the upper Changjiang River mainstem. Our analyses revealed three key findings: First, dissolved inorganic carbon (DIC) constituted the predominant DC component (> 75%), while dissolved organic carbon (DOC) exhibited marked spatial variability (coefficient of variation > 35%). Second, bank-specific correlations emerged between carbon fractions and TAN, with DCs-TAN relationships showing strong correlations along river banks but no significant association in the river center. Third, spatial autocorrelation analyses using univariate and bivariate Moran's I indices quantified these heterogeneities, particularly revealing a striking positive association between DOC and TAN in the right bank (Moran's I = 0.64). This spatial variability suggests synergistic controls by drought-induced hydrological forcing, land-use derived inputs, and water quality parameters. Our findings establish a mechanistic framework linking extreme drought conditions to lateral carbon-nutrient coupling patterns, providing critical baseline data for modeling climate-driven biogeochemical shifts in monsoon-regulated river systems.