Multitemporal Relationships Between the Hydroclimate and Exports of Carbon, Nitrogen, and Phosphorus in a Small Agricultural Watershed

Agriculture affects the biogeochemical cycles of carbon, nitrogen, and phosphorus, leading to a deterioration of surface water quality. The increasing magnitude of climate change raises questions about potential additional or mitigating effects of climate change on this deterioration. One way to understand these potential effects is to cross‐analyze the dynamics of nutrient concentrations and hydroclimatic variables at multiple time scales. Here, we used a 16‐year data set, from a 5 km2 agricultural watershed in France with a temperate oceanic climate, that contains a daily record of nutrient concentrations and hydroclimatic variables from 2002–2017. We calculated Mann‐Kendall and Theil‐Sen tests, Fourier transforms, and daily hydroclimatic distributions associated with extreme stream concentrations to investigate long‐term trends, seasonal dynamics and their interannual variations, and the daily time scale, respectively. Dynamics of dissolved organic carbon (DOC) and nitrate (NO3) concentrations displayed opposite patterns at the three temporal scales, while soluble reactive phosphorus concentrations showed decoupled dynamics, related more to extreme hydrological events. Climate and past agricultural changes seem to have a synergetic effect that leads to long‐term NO3 decrease and DOC increase. Precipitation and, to a greater extent, watershed wetness controlled seasonal and event‐driven dynamics.