Wednesday, October 3, 2012

Plant Carbon sequestration capacity in a high CO2 world overestimated

With climate change stimulating plant growth due to elevated CO2 concentrations climate models calculated this would also stimulate uptake and sequestration of carbon. But a new long term (13 year) study looking at plant biomass growth in a high CO2 environment with Nitrogen nutrients indicates that plants also require enriched soil nitrogen to escalate uptake and sequestration of carbon from elevated CO2. Soil nitrogen just cannot be increased on a widespread basis.

Related: Rising atmospheric Carbon Dioxide levels threaten crop yields and food security

The world's oceans absorb about 30 per cent of CO2 being emitted resulting in ocean acidification, with another 30 per cent being absorbed on land through plant biomass.

The research was done by ecologists Peter Reich and Sarah Hobbie at the University of Minnesota in St Paul. They suggest that estimates of how much CO2 land plants can absorb are far too optimistic in the present climate models. To achieve a higher rate of absorption plants need soil nutrients such as nitrogen and phosphorus. Few long term studies have tested the level of biomass CO2 uptake in conjunction with looking at soil nutrients.

The scientific paper published in Nature Climate Change - Decade-long soil nitrogen constraint on the CO2 fertilization of plant biomass (Full Paper) - reported:
As nitrogen limitation to productivity is widespread, persistent nitrogen constraints on terrestrial responses to rising CO2 are probably pervasive. Further incorporation of such interactions into Earth system models is recommended to better predict future CO2 fertilization effects and impacts on the global carbon cycle.

“This work addresses a question that’s been out there for decades,” said Bruce Hungate, an ecosystem scientist at Northern Arizona University in Flagstaff. "It's a hard question to answer, because it takes a long time to see how ecosystem carbon and nitrogen cycles change."

The study involved establishing 296 field plots with different numbers and combinations of perennial grassland species under ambient and elevated atmospheric CO2 with either ambient or enriched soil nitogen supply. In the first three years of the study plant biomass increased by 11 per cent under ambient and enriched soil nitrogen supply. But in the last ten years elevated CO2 stimulated plant biomass by only half as much under ambient than enriched soil Nitrogen supply. "This difference in response across time was similar if the first two years were compared with the subsequent eleven." says the paper.

Caption: The relative effect of elevated CO2 on mean plant biomass (g m−2; above-ground plus 0–20 cm depth fine-root biomass measured twice per year) at ambient and enriched N levels for two periods, early in the experiment (1998–2000) or during the following 10 years (2001–2010). The relative effect was calculated as (annual mean biomass at elevated CO2–annual mean biomass at ambient CO2) at each N treatment level. One standard error of the among-year means is shown.

Adrien Finzi, a biogeochemist at Boston University in Massachusetts told Nature Climate News “The real strength in this study is that now we have this 13-year record of a single ecosystem. It provides a really strong case for the claim that soil resources and nitrogen limitation in particular can impose a major constraint on carbon storage in terrestrial ecosystems.”

My conclusions from this study? Climate models will need to adjust the carbon sink capacity of plant biomass to take into account the limited soil nitrogen fertilization opportunities. And we humans need to try doubly hard in reducing our carbon emissions.

Sarah Hobbie is a professor in the ecology, evolution and behavior department in the College of Biological Sciences at the University of Minnesota.

Professor Peter B Reich is Regents Professor and Distinguished McKnight University Professor in the University of Minnesota's Department of Forest Resources. He won the 2009 Frontiers of Knowledge Award in Ecology and Conservation Biology. For this award his work as a plant ecologist was assessed as "radically improves our understanding of and ability to predict terrestrial ecosystem compositional and functional responses to global environmental change, including climate change and biodiversity loss."


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