Insights gained from two decades of intensive monitoring: hydrology and nitrate export in a tile-drained agricultural catchment

Nitrate (NO3
−) export from agricultural land poses an ongoing threat to both
inland and coastal waters. Experimental studies investigating the hydrologyNO3
−-export mechanisms require long-term data to identify reliable causal
relationships. In this study, utilizing a 23-year continuous dataset with a high
temporal resolution (daily to twice a week), we aim to identify potential drivers
for NO3-losses and assess the impact of nitrogen (N) soil surface budgets
on NO3-export. A drainage plot (4.2  ha) and a ditch catchment (179  ha) were
fully equipped to register hydrological parameters, including water sample
collection. Mean annual NO3
−-N concentrations (loads) for the drainage plot and
the ditch catchment were 9.4  mg  l−1
(20.6  kg  ha−1) and 6.0  mg  L−1 (20.9  kg  ha−1), respectively. Annual discharge was closely positively correlated with annual NO3-losses, highlighting the significant influence of prevailing weather and, consequently, hydrologic conditions on NO3-export rates. The majority of the annual NO3 load was exported during winter (56% at the drainage plot, 51%
at the ditch catchment), while the rest was exported during spring (28, 29%),
summer (9, 9%) and fall (7, 11%).
We could not find any direct relationships
between N soil surface budgets and NO3-losses. Putting all results together, it
can be concluded that agricultural activities for many decades resulted in high
soil N stocks, which determined the general high NO3-N concentration levels.
Nevertheless, temporal NO3-export dynamics during the last two decades were
clearly driven by hydro-meteorological conditions, nearly independently of land
management and N soil surface budgets on the fields