Improving climate models to account for plant behavior gives good news

Climatologists have not explained what plants do at night and it is a mistake. A new study from the Lawrence Berkeley National Laboratory of the Department of Energy (Berkeley Lab) found that the uptake of nutrients by plants in the absence of photosynthesis affects gas emissions greenhouse effect in the atmosphere.

In a study published today in Nature Climate changeThe lead author, William Riley, demonstrates how to improve climate models to better represent the biogeochemical dynamics of the land. With the help of a new global terrestrial model developed and integrated into DOE's Energy Exascale Earth System Model (E3SM), Riley and his team have found that plants can absorb more carbon dioxide and that soils lose less nitrous oxide than previously thought. Their global simulations imply terrestrial terrestrial ecosystem reactions with the atmosphere lower than those predicted by current models.

"This is good news for what is currently in climate models," said Riley, a scientist with the Berkeley Lab's Earth and Environmental Sciences. "But that's not good news in general, it will not solve the problem, and whatever the case may be, factories will not be tracking anthropogenic emissions of carbon dioxide – it's just that they could do better than what current models suggest. "

Humans have issued a record 34 gigatons of CO₂ per year, averaged over the last decade. About half of this remains in the atmosphere, the rest being absorbed by oceans and land (through photosynthesis); this latter quantity, called the terrestrial carbon sink, varies from year to year depending on factors such as fires, drought, land use and weather conditions.

Scientists are trying to understand the increasing impact of global carbon dioxide emissions on the terrestrial carbon sink, which currently ranges from 0 to 11 gigatonnes of CO.₂ per year, including land-use changes, with great interannual variability. Another complication is terrestrial nitrous oxide, a potent greenhouse gas naturally released from land and agricultural and industrial activities. In other words, to what extent can plants reduce anthropogenic emissions of carbon dioxide?

The new Berkeley Lab study found that by not properly accounting for what plants do at night and during the off season, climate models may underestimate the terrestrial carbon sink and overestimate the release of Nitrous oxide, the latter of 2.4 gigatonnes of CO.₂-equivalent per year. "This number is substantial compared to the current terrestrial carbon sink," said Riley, whose number varies between one quarter and more than 100%, depending on the year.

Plant-microbe competition for nutrients

The ability of plants to absorb carbon dioxide is limited by the availability of soil nutrients, particularly nitrogen and phosphorus. The more nutrients are abundant, the more plants can benefit from the increase in atmospheric carbon dioxide. Microbes in the soil also play an important role as they compete with plants for nutrients.

In fact, microbes play an important role in the carbon cycle, and interactions between plants, soil and microbes are complex and a challenge for climatologists. Most climate models assume that plants compete for nutrients in the soil only when they require it for photosynthesis, not at night or outside growth periods.

"What most climate models have ignored, is this fairly solid observational literature showing that plants acquire soil nitrogen even when they do not do photosynthesis" said Riley.

Berkeley Lab is focused on the theme of plant-soil-microbe interactions as part of its "Microbes to Biomes" initiative. This will be a central theme of the Center for Integrating Biological and Environmental Programs, or BioEPIC, a proposed facility that would house one of the experimental capabilities to advance the objectives of DOE's mission in energy and environmental sciences. One of the goals is to represent and study these processes on a large scale and in a controlled manner.

"This study demonstrates the progress made in the more mechanistic representation of terrestrial processes that are important for the climate and will also be for BioEPIC," said Riley.

Less nitrous oxide emissions

In this study, Berkeley Lab researcher, Qing Zhu, co-author of the paper, conducted a meta-analysis of 120 short-term nitrogen absorption experiments by plants to test their new terrestrial model called ELMv1. "We also compared observations of nutrient uptake at night versus day and during off-season," said Riley. "We are pretty confident that the basic mechanisms of the model are correct and that this meta-analysis and the observations of individual sites corroborate it."

They found that a significant portion of nutrient uptake occurs in the absence of photosynthesis, with plants and microbes competing for nutrients. "The quantities vary a lot depending on the latitude, but under high latitudes, such as the Arctic, about 20% of the annual nitrogen uptake by plants occurs outside of the growing season This can go up to 55% for nocturnal absorption in the tropics, "he added. I said. "This is a huge deal for plants and will facilitate the absorption of atmospheric carbon. It is currently completely ignored in most climate models.

"This type of model improvement will help us better understand the implications of future CO₂ emissions, "said Riley.

Jinyun Tang, a researcher at Berkeley Lab, also co-authored the paper titled "Weaker Interactions Between Land and Climate Resulting from Nutrient Absorption During Photosynthetic Inactivity". The study was funded by the DOE Science Bureau.

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More information:
W. J. Riley et al., Lower Feedback of Land and Climate of Nutrient Absorption During Photosynthetic Inactivity, Nature Climate change (2018). DOI: 10.1038 / s41558-018-0325-4