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Quantifying nitrogen leaching from diffuse agricultural and forest sources in a large heterogeneous catchment

Kopáček, Jiří, Hejzlar, Josef, Posch, Maximilian
Biogeochemistry 2013 v.115 no.1-3 pp. 149-165
agricultural watersheds, centrally planned economy, drainage, forested watersheds, forests, hydrologic models, land use change, leaching, market economy, markets, mineralization, nitrate nitrogen, nitrogen, organic soils, pastures, pollution load, soil organic matter, surface water, watershed hydrology, Czech Republic
Using mass budget and hydrological models, we quantified the contribution of major diffuse nitrogen (N) sources to surface water loading in a large heterogeneous catchment (upper Vltava river, Czech Republic, about 13,000 km²) over the last 52 years. The catchment reflects the typical development in central and eastern European countries, which witnessed socio-economic shifts from a market to a planned economy in the 1950s and back to a market economy in the 1990s. The former shift was accompanied by increasing N inputs to agricultural and forest areas with ranges for the 1950–1980s of 60–160 and 14–30 kg ha⁻¹year⁻¹, respectively, and with intensive draining of waterlogged farmland. The shift in the 1990s resulted in ~40 and ~50 % reduction of N inputs to agricultural areas and forests, respectively, and farmland draining ceased. The N exports from agricultural land (E AL) and from forests (E FO) varied within 3–45 and 1.6–7.1 kg ha⁻¹year⁻¹, respectively (with maxima in the 1980s). The E AL and E FO fluxes exhibited several similar patterns, being dominated by NO₃-N, increasing with N inputs, and having similar inter-annual variability related to hydrology. The N losses from forests were stable (19 % of N input on average), while those from agricultural land increased from ~10 % in the 1960s up to 32 % in the 2000s, due probably to the previous extensive drainage and tillage of waterlogged fields and pastures. These land use changes reduced the water residence time in agricultural land and induced mineralization of soil organic matter. Continuing mineralization of soil organic N pools thus was the most probable reason for the remaining high E AL fluxes despite a ~40 % reduction in N inputs to agricultural land, while the E FO fluxes decreased proportionally to the decreasing N deposition during 1990–2010.