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Low phosphorus levels limit carbon capture by Amazonian forests
A global study of forests 1 revealed that, between 1990 and 2007, they converted approximately 29% of human-generated carbon emissions into long-lived plant biomass and soil organic matter. This carbon sink prevents those past emissions from accumulating in the atmosphere and contributing to global climate change. Covering just 7% of Earth’s land surface, tropical forests are responsible for 33% of global biomass production and 50% of the global forest carbon sink 1 , 2 . The future of this carbon sink is uncertain, in part because plants sequester nutrients as well as carbon, and this could induce nutrient limitation that stalls plant growth and thereby reduces future carbon sequestration 3 . The nutrients most likely to limit growth when scarce are nitrogen (corresponding to approximately 1.5% of plant dry matter), phosphorus (0.2%), potassium (1.0%), calcium (0.5%) and magnesium (0.2%).
Read the paper: Direct evidence for phosphorus limitation on Amazon forest productivity
Writing in Nature , Cunha et al . 4 report results from the first experiment conducted in a tropical forest, or any other habitat, that tests the effect of adding nitrogen, phosphorus and major cations (potassium, calcium and magnesium added together) in all combinations. The authors’ experiment addresses societally important questions about the future of the tropical-forest carbon sink, and fundamental questions about which, if any, nutrient places limits on carbon capture in tropical forests and how the spectacular biomass productivity there is maintained despite impoverished soils.
This experiment addresses the long-standing theory that phosphorus and/or the major cations, but not nitrogen, limit the productivity of highly weathered tropical forest soils 5 . The theory arises from the idea that there is extensive leaching of all nutrients from highly weathered soil, whereas nitrogen alone is replenished through the ‘fixation’ and capture of atmospheric nitrogen by cyanobacteria and bacteria that prosper in the warm, wet climates characterizing tropical forests 5 , 6 . Previous experiments that tested the effect of nitrogen and phosphorus addition in tropical forests provide partial support for the theory, and indicate a similar strong positive response to the addition of both of these nutrients. However, many of the experiments failed to characterize the local soils, or were carried out in forests growing on relatively fertile soils 7 , 8 .
Cunha and colleagues conducted their experiment at a well-characterized site in the central Amazon rainforest where soils are highly weathered and soil phosphorus levels are among some of the lowest recorded for any tropical forest (Fig. 1). Thus, the authors predicted that there would be strong plant responses to the addition of phosphorus, and possibly to the addition of the major cations, but not to nitrogen supplementation.
Figure 1 Figure 1 Variations in the level of phosphorus in forest soil. Phosphorus is a key nutrient for plant growth. The graphs show total soil phosphorus (P) concentration in milligrams per kilogram of dry soil to a depth of 30 centimetres for: a , tropical humid lowland forests (located within 23.5° of the Equator and with a mean annual precipitation and temperature of more than 2,000 millimetres and 22 °C, respectively); and b , temperate and boreal forests (located poleward of 23.5° latitude). There is a strong regional difference. However, phosphorus concentrations vary 100-fold in both regions, with broad overlap in the levels. The sample sizes are 113 and 450 forests for a and b , respectively; the graphs show data compiled in ref. 11. Cunha et al . 4 conducted an experiment in which various nutrients were added to the soil of a tropical forest in Amazonia (the usual level of phosphorus found at the study site is indicated). The study revealed that low phosphorus is a limiting factor for plants, and thus hinders carbon capture, in this forest.
The pattern of allocation of plant resources, together with life-history considerations, complicate the authors’ prediction. Biomass production includes wood, roots and leaves. These tissues coordinate the acquisition of all essential resources, and addition of a limiting nutrient might alter biomass allocation between the tissues.
Specifically, nutrient supplementation might shift biomass allocation away from the fine roots that acquire the added nutrient and towards another tissue type to produce a second limiting resource, in addition to increasing the overall level of biomass production. Leaves capture atmospheric carbon through the process of photosynthesis, and leaf production should increase after the addition of a limiting nutrient if carbon is limiting, but remain unchanged if carbon is not limiting, or might even decrease if the added nutrient boosts per-leaf photosynthesis. Roots capture water as well as nutrients, and root production should increase after the addition of a limiting nutrient if water or a second nutrient is limiting, but decrease otherwise.
The early arrival of spring doesn’t boost annual tree growth
Wood production must support any increase in leaf biomass and might also increase if trees grow taller, which would intensify competition for sunlight. Rapid growth in tree height drives strong responses to nutrient supplementation in young forests recovering from disturbances, but whether this is the case in old-growth tropical forests is not known 8 . This uncertainty is compounded by the fact that long-lived trees that are adapted to infertile soils allocate substantial amounts of carbon and nutrients to storage and defence, and are thus characterized by both slow growth and low mortality rates. Added nutrients might be acquired and stored for many years before a productivity response becomes evident.
Cunha and colleagues report statistically significant responses to nutrient addition that are broadly consistent with these biogeochemical and life-history expectations. There was no significant response to nitrogen addition, whereas phosphorus addition increased the production of leaves and fine roots. The rise in leaf production was associated with increased leaf turnover rates, without an increase in leaf biomass. Thus, extra wood production was not required to support increased leaf biomass, and wood production was similar across all nutrient-addition treatments.
If phosphorus were the only limiting soil resource, fine-root production would be expected to decrease in response to phosphorus addition. The observed increase in fine-root production implicates a second, unidentified limiting soil resource as driving the rise in root production.
Previous work 9 indicates that addition of the three major cations causes notable changes in root-growth dynamics, root characteristics and root colonization by nutrient-supplying fungi called mycorrhizae. This suggests that cation levels also limit plant function and might limit biomass production over a longer timescale. Fundamental questions concerning how tropical forests maintain high levels of biomass productivity despite growing on impoverished soils remain to be explored through this rich ongoing experiment.
25 years of valuing ecosystems in decision-making
Cunha and colleagues’ results also have major societal implications. The phosphorus limitation of leaf and fine-root production will limit the acquisition of other resources, thereby lowering forest resilience as local climates change (for example, affecting water uptake during drought). Nonetheless, the phosphorus limitation of the forest carbon sink has not been proved fully. Leaves and fine roots turn over quickly and contribute to long-term carbon sequestration only indirectly, by enabling increases in the biomass of long-lived wood and decomposition-resistant soil organic material that persist on a timescale of decades to centuries 10 . Cunha and co-workers’ results are from the first two years of their experiment. Long-lived carbon pools will increase with phosphorus addition as the experiment continues, if decomposition-resistant organic compounds in leaves and roots are added to pools of soil organic matter, and if trees adapted to phosphorus-impoverished soils slowly increase wood production.
A previous compilation of soil phosphorus measurements provides a global perspective on the potential nutrient limitation of the forest carbon sink 11 . Highly weathered, low-phosphorus soils predominate across eastern and central Amazonia, and occur throughout the tropics. However, soil phosphorus concentrations vary 100-fold across humid, lowland tropical forests, and broadly overlap with those in temperate and boreal forests (Fig. 1).
This broad variation in soil phosphorus content, together with previous evidence for both nitrogen and phosphorus limitation in forests at all latitudes 7 , 8 , 12 , 13 , suggest that future projections of the forest carbon sink must incorporate nutrient cycles.
Eleven Earth-system models 14 , which simulate physical, chemical and biological processes to predict future climates, inform the sixth assessment of the Intergovernmental Panel on Climate Change 15 . Five of these models lack nutrient cycles, and only six include a nitrogen cycle (one of which includes both nitrogen and phosphorus cycles). Of these 11 models, the one with both nitrogen and phosphorus cycles predicts the smallest terrestrial carbon sink 14 . Amazonia comprises approximately 25% of the global forest carbon sink 1 , and an understanding of nutrient cycles in characteristic low-phosphorus Amazonian soils will improve predictions of the future global forest carbon sink. Cunha and colleagues’ experiment has provided, and should continue to provide, crucial information with which to parameterize and validate Amazonian nitrogen, phosphorus and cation cycles, and thereby improve the accuracy of such projections. .
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