Articles | Volume 8, issue 4
Research article
12 Dec 2017
Research article |  | 12 Dec 2017

Nitrogen leaching from natural ecosystems under global change: a modelling study

Maarten C. Braakhekke, Karin T. Rebel, Stefan C. Dekker, Benjamin Smith, Arthur H. W. Beusen, and Martin J. Wassen

Abstract. To study global nitrogen (N) leaching from natural ecosystems under changing N deposition, climate, and atmospheric CO2, we performed a factorial model experiment for the period 1901–2006 with the N-enabled global terrestrial ecosystem model LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator). In eight global simulations, we used either the true transient time series of N deposition, climate, and atmospheric CO2 as input or kept combinations of these drivers constant at initial values. The results show that N deposition is globally the strongest driver of simulated N leaching, individually causing an increase of 88 % by 1997–2006 relative to pre-industrial conditions. Climate change led globally to a 31 % increase in N leaching, but the size and direction of change varied among global regions: leaching generally increased in regions with high soil organic carbon storage and high initial N status, and decreased in regions with a positive trend in vegetation productivity or decreasing precipitation. Rising atmospheric CO2 generally caused decreased N leaching (33 % globally), with strongest effects in regions with high productivity and N availability. All drivers combined resulted in a rise of N leaching by 73 % with strongest increases in Europe, eastern North America and South-East Asia, where N deposition rates are highest. Decreases in N leaching were predicted for the Amazon and northern India. We further found that N loss by fire regionally is a large term in the N budget, associated with lower N leaching, particularly in semi-arid biomes. Predicted global N leaching from natural lands rose from 13.6 Tg N yr−1 in 1901–1911 to 18.5 Tg N yr−1 in 1997–2006, accounting for reductions of natural land cover. Ecosystem N status (quantified as the reduction of vegetation productivity due to N limitation) shows a similar positive temporal trend but large spatial variability. Interestingly, this variability is more strongly related to vegetation type than N input. Similarly, the relationship between N status and (relative) N leaching is highly variable due to confounding factors such as soil water fluxes, fire occurrence, and growing season length. Nevertheless, our results suggest that regions with very high N deposition rates are approaching a state of N saturation.

Short summary
Nitrogen input in natural ecosystems usually has a positive effect on plant growth. However, too much N causes N leaching, which contributes to water pollution. Using a global model we estimated that N leaching from natural lands has increased by 73 % during the 20th century, mainly due to rising N deposition from the atmosphere caused by emissions from fossil fuels and agriculture. Climate change and increasing CO2 concentration had positive and negative effects (respectively) on N leaching.
Final-revised paper