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

Abstract

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.

The influence of landscape spatial configuration on nitrogen and phosphorus exports in agricultural catchments

“…Ca 2+ , NO3 -, conductivity). These results are consistent with current knowledge on transfer pathways of nutrients in catchments, with deeper flowpaths for N species than for P species (Strohmenger et al 2020), and explains why landscape features had less influence on NO3than P (Thomas and Abbott 2018). These landscape metrics, however, are relatively rigid, as topological influences (e.g.…”

Is a simple model based on two mixing reservoirs able to reproduce the intra-annual dynamics of DOC and NO3 stream concentrations in an agricultural headwater catchment?

“…During the 1970s, the intensified agricultural production lead to excessive N inputs (Cheverry, 1998) that have been reduced by environmental policies following the 1990s (Casal et al, 2019b). In this landscape, most DOC and NO3accumulate in wetland soils and groundwater, respectively (Aubert et al, 2013;Strohmenger et al, 2020). (Brittany, France).…”

“…The large pool of NO3in the groundwater originates from past and, to a lesser extent, ongoing agricultural activities in the catchment (Molénat et al, 2008;Basu et al, 2010;Aubert et al, 2013;Dupas et al, 2018;Strohmenger et al, 2020). The N legacy generated by the conversion of permanent grassland to arable land and from the massive addition of livestock slurry and manure since the intensification of agricultural in 1970-1976 (Casal et al, 2019b;Casal et al, 2019a) leached and accumulated in the vadose zone and groundwater (Cheverry, 1998;Casal et al, 2019b;Strohmenger et al, 2020). The lower NO3concentration in the riparian zone is explained by the heterotrophic denitrification that can occur there under anoxic conditions when the water table reaches the soil surface (Oehler et al, 2009;Montreuil et al, 2010;Bell et al, 2015;Casson et al, 2019).…”

“…This underestimation was likely due to the long-term trend in NO3time series. This multi-annual trend has been related to the gradual decrease in excess agricultural N that occurred mainly from 1998-2008 and induced a slow and delayed decrease in groundwater NO3concentration, and thus in stream concentrations (Dupas et al, 2018;Strohmenger et al, 2020). Since our model focused on intra-annual dynamics and assumed a constant groundwater concentration, it could not reproduce such a long-term trend.…”

“…Previous studies showed that seasonal variation in DOC and NO3are closely related to water table fluctuations in groundwater-fed catchments (Aubert et al, 2013;Thomas et al, 2016;Abbott et al, 2018;Strohmenger et al, 2020). In contrast, short-term variations in DOC and NO3have been connected to the activation of subsurface and surface flow paths during storm events and the subsequent hydrological connection of DOC-rich and NO3 --poor riparian soils to the stream, especially for near-surface soil layers (Bernal et al, 2002;Outram et al, 2014;Bowes et al, 2015;Fovet et al, 2018a;Strohmenger et al, 2020).…”