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.
Citation
ID:
216403
Ref Key:
braakhekke2017earthnitrogen