<<One implication of this is that biogenic emissions from burning biofuels and biomass are increasingly less likely to be absorbed by future plant growth. Same goes for fossil fuel emissions>>
1. Commentary on new report in Nature Geoscience:
Plants need the right mix of nutrients to grow. Two of the most important nutrients are nitrogen and phosphorus. But there isn't an endless supply in soils for plants to use, lead author Dr Will Wieder, from the National Centre for Atmospheric Research in Colorado, tells Carbon Brief:
"Many ecosystems appear to be co-limited, meaning that both nitrogen and phosphorus are important for plant growth. There are places where one element or the other may be slightly more limiting, but at the end of the day plants need both to build roots, leaves and wood. This is why many fertilizers used in gardens and farms come with both nitrogen and phosphorus."
While nitrogen is abundant in the air we breathe, most plants can only take it up from the soil. Nitrogen gets into the soil by being 'fixed' from the air by microbes and certain plants, such as soy, Wieder says. Phosphorus primarily originates from rocks, and reaches the soil when they are worn down by the weather.
Nutrients can come from a little further afield as well, Weider adds:
"Both nitrogen and phosphorus can be moved around and transported through the atmosphere as dust or air pollution. The subsequent deposition of nitrogen and phosphorus also can contribute new nutrients to an ecosystem."
Limits to growth
Most climate models used for the latest Intergovernmental Panel on Climate Change (IPCC) report assume that enough additional nitrogen and phosphorus would be available for extra plant growth. But this might not actually be the case, Weider says:
"This 'new' nitrogen and phosphorus would have to come from somewhere, and we found it is unlikely to be supplied from outside the ecosystem, meaning that the increases in plant growth would have to be met through accelerated recycling of nutrients within ecosystems."
When Wieder and his colleagues included realistic amounts of nitrogen and phosphorus in their models, they found it limited the boost to plant growth from the extra carbon dioxide.
In the graph below, the black line shows the increase in plant growth the IPCC models project under a high-emissions scenario. Without limiting nitrogen and phosphorus, plant growth increases by 63% by 2100.
When the team included future limits to nitrogen (red line), they found extra plant growth dropped to 29%. With limits to both nutrients (blue line), the boost to growth dropped even further to 20%.
2. Abstract of the report in Nature Geoscience
The size of the terrestrial sink remains uncertain. This uncertainty presents a challenge for projecting future climate–carbon cycle feedbacks1, 2, 3, 4. Terrestrial carbon storage is dependent on the availability of nitrogen for plant growth5, 6, 7, 8, and nitrogen limitation is increasingly included in global models9, 10, 11. Widespread phosphorus limitation in terrestrial ecosystems12 may also strongly regulate the global carbon cycle13, 14, 15, but explicit considerations of phosphorus limitation in global models are uncommon16. Here we use global state-of-the-art coupled carbon–climate model projections of terrestrial net primary productivity and carbon storage from 1860–2100; estimates of annual new nutrient inputs from deposition, nitrogen fixation, and weathering; and estimates of carbon allocation and stoichiometry to evaluate how simulated CO2 fertilization effects could be constrained by nutrient availability. We find that the nutrients required for the projected increases in net primary productivity greatly exceed estimated nutrient supply rates, suggesting that projected productivity increases may be unrealistically high. Accounting for nitrogen and nitrogen–phosphorus limitation lowers projected end-of-century estimates of net primary productivity by 19% and 25%, respectively, and turns the land surface into a net source of CO2 by 2100. We conclude that potential effects of nutrient limitation must be considered in estimates of the terrestrial carbon sink strength through the twenty-first century.