Antropogenic input of nitrogen and riverine export from a Mediterranean catchment. The Celone, a temporary river case study

In a catchmnent, nitrogen (N) export from terrestrial ecosystems to rivers is controlled by the hydrological processes and N balance. In the present paper, the soil system N budget and riverine export were quantified in a Mediterranean watershed, the Celone (South-East, Italy). The study area (72 km2) consists of agricultural land (70%) and deciduous forests with three small residential areas. Major N inputs derived from fertilizers and animal manure, corresponding for the whole watershed area to 68 and 12 kg N ha−1 yr−1, respectively. N input from point sources was 1% of total input and atmospheric depositions measured in a gauging station near the study area was quantified in ∼6 kg N ha−1 yr−1. Crop N uptake was the main N output from agricultural land; it was estimated in ∼37 kg N ha−1 yr−1 by using data on crop yields provided by local farmers. Total flux of N in surface water was quantified for a year at the outlet of the study area by using continuous measures of flow and discrete measures of N concentrations carried out with a different frequency during the normal and low flow and when floods occurred. The hydrological regime of the stream, which is a temporary river, plays an important role in N transport. Water quality was found to vary considerably through the year in terms of both nutrient concentrations and loads. Riverine N export was quantified in 41% of total N input, and it was mainly transported during flood events, ∼60% of the annual load was delivered during floods occurred in 38 days. Organic nitrogen and nitrate were the main N forms in surface water, and the contribution per hectare was about 24 and 14 kg N, respectively. On a yearly basis, the difference between N inputs and outputs including riverine export was estimated in about 4 kg N ha−1 yr−1 for the whole watershed area. This amount partly accumulates in soils in different N forms and the remaining part, mainly in form of nitrate, percolates through unsaturated soil towards groundwater. This study reports an important analysis of N pollution in a Mediterranean watershed with a temporary river system and limited data availability. Data acquisition and handling have proved to be an important challenge to overcome in N balance quantification. The results and the methodology of the present work can be useful for understanding nitrogen loss dynamics and for functional water management and land use planning.