Nitrogen‐Related Constraints of Carbon Uptake by Large‐Scale Forest Expansion: Simulation Study for Climate Change and Management Scenarios

Abstract

Increase of forest areas has the potential to increase the terrestrial carbon (C) sink. However, the efficiency for C sequestration depends on the availability of nutrients such as nitrogen (N), which is affected by climatic conditions and management practices. In this study, I analyze how N limitation affects C sequestration of afforestation and how it is influenced by individual climate variables, increased harvest, and fertilizer application. To this end, JSBACH, the land component of the Earth system model of the Max Planck Institute for Meteorology is applied in idealized simulation experiments. In those simulations, large‐scale afforestation increases the terrestrial C sink in the 21st century by around 100 Pg C compared to a business as usual land‐use scenario. N limitation reduces C sequestration roughly by the same amount. The relevance of compensating effects of uptake and release of carbon dioxide by plant productivity and soil decomposition, respectively, gets obvious from the simulations. N limitation of both fluxes compensates particularly in the tropics. Increased mineralization under global warming triggers forest expansion, which otherwise is restricted by N availability. Due to compensating higher plant productivity and soil respiration, the global net effect of warming for C sequestration is however rather small. Fertilizer application and increased harvest enhance C sequestration as well as boreal expansion. The additional C sequestration achieved by fertilizer application is offset to a large part by additional emissions of nitrous oxide.

Quantifying and Comparing Effects of Climate Engineering Methods on the Earth System

“…This CDR potential of reforestation is higher than previous estimates (e.g., House et al, ) and is due to increased forest cover in combination with enhanced terrestrial carbon uptake in a warm and high‐CO 2 climate, suggesting that the CDR potential of reforestation depends on the background state of the Earth system (Sonntag et al, ). The CDR potential may be lower in a model including nitrogen limitation, as demonstrated recently by Kracher (). Yet, the study by Kracher () used a different baseline climate (RCP4.5 instead of RCP8.5) and found a large effect mostly on boreal forest expansion, which makes a quantitative estimate of the effect of nitrogen in our scenario difficult.…”

“…The CDR potential may be lower in a model including nitrogen limitation, as demonstrated recently by Kracher (). Yet, the study by Kracher () used a different baseline climate (RCP4.5 instead of RCP8.5) and found a large effect mostly on boreal forest expansion, which makes a quantitative estimate of the effect of nitrogen in our scenario difficult. Terrestrial carbon is very similar in experiments rcp45 and CE‐land (Figure e), because of two compensating effects: a decrease in soil carbon due to weaker soil respiration because of lower temperatures and an increase in vegetation carbon in CE‐land compared to rcp45 (Figure b).…”

“…For example, the model does not include permafrost, which would lead to an additional warming in warmer (e.g., RCP8.5) compared to colder (e.g., RCP4.5) scenarios (e.g., Schaefer et al, ) and thus could lead to an underestimation of the cooling effects of the CE methods. Our model also lacks a nitrogen cycle, which could affect the carbon response in the reference and in the CE experiments (e.g., Kracher, ). However, potentially overestimated terrestrial productivity and carbon content partly cancel out when looking at differences between model experiments.…”

Impacts of Enhanced Weathering on biomass production for negative emission technologies and soil hydrology

“…The additional AR P demand, obtained for the 21 st century for an N-unlimited and N-limited AR scenario (Kracher, 2017) was approximated by stoichiometric P:N ratios for mean and range (5 th and 95 th percentiles), which is a similar approach done by Sun et al (2017). The ratios were derived from databases of hard-and softwood (Pardo et al, 2005) and foliar biome-specific 105 nutrient content (Vergutz et al, 2012).…”

“…Sources for biomass-based energy production are crop and forestry residues (Smith, 2012;Smith et al, 2012;Tokimatsu et al, 2017), dedicated bio-energy grass (BG) plantations (Smith, 2012;Smith et al, 2012) or short rotation woody biomass from forestry (Cornelissen et al, 2012;Smeets and Faaij, 2007). Large-scale AR, as well as bio-energy plantations, require extensive 40 landscape modifications for growing forests or natural regrowth of trees in deforested areas to increase terrestrial CDR (Kracher, 2017;Boysen et al, 2017a;Popp et al, 2017;Humpenöder et al, 2014), and huge quantities of irrigation water (Boysen et al, 2017b;Bonsch et al, 2016). The biomass yields of AR and agricultural bio-energy crops directly correlate with fertilizer application, which in turn could reduce CDR efficiency due to related emissions of N2O (Creutzig, 2016;Popp et al, 2011) and initiate unwanted side-effects like acidification of soils (Rockström et al, 2009;Vitousek et al, 1997), streams/rivers, andGarrels, 1983).…”