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

Abstract

Abstract.To study global nitrogen (N) leaching from natural ecosystems under changing N deposition, climate, and atmospheric CO 2 , we performed a factorial model experiment for the period 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 CO 2 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 CO 2 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.

Modelling the response of net primary productivity of the Zambezi teak forests to climate change along a rainfall gradient in Zambia

“…The projected NPP increase at Kabompo and Namwala is in the same direction as the results reported by other re-searchers (Alo and Wang, 2008;Mohammed et al, 2018;Pan et al, 2015) for some parts of Africa (Table 3). Some modelling studies on tropical forests (Braakhekke et al, 2017;Ciais et al, 2009;Doherty et al, 2010;Melillo et al, 1993;Midgley et al, 2005;Pan et al, 2015;Thuiller et al, 2006) also reported large positive effects of increased CO 2 concentration on forests' productivity. This positive effect is probably due to increased water-use efficiency (WUE, which is a measure of a plant's water use during photosynthesis in relation to the amount of water withdrawn; Grain Research and Development Cooperation, 2009) by the plants.…”

“…In LPJ-GUESS, crown area (m 2 per individual) is determined from stem diameter (see Eq. 6) and We used LPJ-GUESS version 3.0 and implemented a "cohort mode" for our study (Braakhekke et al, 2017;Smith et al, 2001). Though this model version accounts for nitrogen dynamics in soil and vegetation, we did not switch nitrogen on during our simulations.…”

“…The effects of these climatic changes vary with location, ecosystem types, and climate zones (Wu et al, 2011). While increased temperature stimulates plant productivity to its optimal temperature in some plants (Wu et al, 2011), it also exponentially stimulates autotrophic plant respiration (Burton et al, 2008;Wu et al, 2011). Such increasing temperature effects can either be enhanced or moderated, depending on whether water availability decreases or increases (Chen et al, 2013).…”

Exposure to Light Elicits a Spectrum of Chemical Changes in Soil

“…Mattsson et al (2003), and Williard et al (1997) and references therein. Higher values generally correspond to forests, with lower values (less than several hundred mg/m 2 /yr) in savannahs, tundra, shrublands, grasslands, and deserts (Braakhekke et al, 2017). All data are from unfertilized soil.…”

Controls of terrestrial ecosystem nitrogen loss on simulated productivity responses to elevated CO<sub>2</sub>

“…investigate the sensitivity of leaching to global change in a TBM (Braakhekke et al, 2017) of-the-art models along with available data (Tian et al, 2018). We contend that such efforts to consolidate the representation of ecosystem N loss processes could best be aided by field experiments that investigate N loss rates (and their partitioning between gaseous and leaching components) under perturbation in the regions that we identified as crucial with respect to modelled N loss uncertainty.…”