Former land use and tree age affects nitrate leaching from European forest soils

Forest ecosystems are typically associated with good water quality; an ecosystem service often claimed as a benefit of afforestation schemes. However, legacy effects of historical land use, plus decades of elevated nitrogen deposition inputs from traffic pollution, and agricultural and industrial activities in combination with higher nitrogen scavenging by forests, have led to elevated nitrate leaching from forested lands across Europe. Elevated nitrate leaching threatens the quality of surface and groundwater. It is also related to soil acidification and depletion of base cations, compromising the nutritional status of the soil and, subsequently, current and future trees generations. Several variables that affect the response of nitrate leaching to elevated deposition inputs have previously been identified in long-term forest monitoring datasets. Here we collated a European-scale dataset from published literature of throughfall nitrate concentrations and nitrate leaching, and variables affecting this relationship, e.g. soil type, surrounding land use and climate, broadening the evidence beyond these long-term monitoring datasets.

We identified a variation in response to elevated deposition between coniferous and broadleaved forests. This could be partly attributed to the former land uses typically associated with the different tree species. Broadleaf forests planted on former arable land exhibited a different response to elevated deposition than afforested heathlands/grasslands and conifers planted on arable land. An age effect was also observed, with nitrate leaching from forest soils increasing with tree age until 80 years old for conifers and 50 years old for broadleaves, then declining as trees aged further. This research provides evidence to assess the timescale over which afforestation schemes can deliver expected benefits to water quality. It also highlights that considering former land use is important to identify locations in forested landscapes where groundwater nitrate concentrations may be elevated.