Estimation of nitrogen budgets for contrasting catchments at the landscape scale

A comprehensive assessment of nitrogen (N) flows
at the landscape scale is fundamental to understand spatial
interactions in the N cascade and to inform the development
of locally optimised N management strategies. To explore
these interactions, complete N budgets were estimated
for two contrasting hydrological catchments (dominated by
agricultural grassland vs. semi-natural peat-dominated moorland),
forming part of an intensively studied landscape in
southern Scotland. Local scale atmospheric dispersion modelling
and detailed farm and field inventories provided high
resolution estimations of input fluxes. Direct agricultural inputs
(i.e. grazing excreta, N2 fixation, organic and synthetic
fertiliser) accounted for most of the catchment N inputs, representing
82% in the grassland and 62% in the moorland
catchment, while atmospheric deposition made a significant
contribution, particularly in the moorland catchment, contributing
38% of the N inputs. The estimated catchment N
budgets highlighted areas of key uncertainty, particularly N2
exchange and stream N export. The resulting N balances suggest
that the study catchments have a limited capacity to store
N within soils, vegetation and groundwater. The “catchment
N retention”, i.e. the amount of N which is either stored
within the catchment or lost through atmospheric emissions,
was estimated to be 13% of the net anthropogenic input in
the moorland and 61% in the grassland catchment. These
values contrast with regional scale estimates: Catchment retentions
of net anthropogenic input estimated within Europe
at the regional scale range from 50% to 90 %, with an average
of 82% (Billen et al., 2011). This study emphasises the
need for detailed budget analyses to identify the N status of
European landscapes.