Below Central Amazonian forests

Below Central Amazonian forests: what’s hidden there and why is it important?

By Laynara Lugli, University of Exeter – UK

I could start this story by saying how exuberant and important Amazonian forests are, but I guess most of us already know it. The first idea that comes to my mind when trying to explain why we can find so many big trees, stocking a lot of carbon in Amazon is that the soils must be very fertile, right? Well, it’s not that simple! Actually, about 60% of Amazonian forests occur on very nutrient-poor soils and it is exactly over those poor soils that we find more biomass being stored aboveground.

In part this is explained by the very efficient cycling of nutrients in the soil, especially phosphorus (P). Phosphorus is an essential nutrient for plant metabolism, photosynthesis, etc. but it’s usually found in low concentrations in the majority of Amazonian soils, because these soils are very old and weathered. What we don’t know for sure is what exactly happens belowground to make these forests so efficient.

For this reason, during May, June and July 2016, we travelled from Exeter, UK to Manaus, Brazil for a big root and soil field campaign as part of the AFEX project (Amazon Fertilisation Experiment). In summary, AFEX is the first large-scale nutrient-addition experiment in Amazon forest and aims to determine the responses of all key components of the carbon cycle to nutrient addition. And because P might be a limiting nutrient in those forests, my PhD aims specifically to understand the mechanisms plants use, both above and belowground, to efficiently use and cycle this nutrient. Among these mechanisms, emphasis is given to nutrient concentration in green leaves and litter (and the difference between these, which gives nutrient resorption efficiency), nutrient concentration in soil (including the different fractions in which P occurs in soils), soil and root enzyme activities, root production, morphology and mycorrhizal colonisation.

Doing field work in the tropics is not an easy task. Even though we have good infrastructure at our field site (ZF-2 reserve), there are still so many details that we need to plan beforehand, especially when analysing fresh root and soil samples. In May we sampled 32 plots (40 x 40 m) and 5 root ingrowth cores (30 cm depth) in each, divided by 4 layers, meaning 640 root samples to collect in the field, clean, dry and weigh for biomass. Part of the samples were also used for phosphatase enzyme assays and for mycorrhizal colonisation analysis. It was a big team effort, working against the clock to keep the roots as fresh as we could and also praying for the rain to go away during our sampling hours.

In June we went back to the field with our soil augers and thanks to the very strong guys that were helping us, we were able to take some samples from this very (very!) clayey soil. We sampled all the 32 plots, being 4 soil cores in each plot (30 cm depth). This time, since we wanted to analyse our soils for ammonium, nitrate and phosphate concentrations (both total and also only in the soil microbial biomass), we had to bring the lab to the field to make sure our samples were fresh enough. Part of the team was in the field during the day sampling soils and the other part was at the base camp, testing the equipment and preparing the chemical solutions that would receive the soils later. Once the field day was over, the fun part started: sieving fresh clayey soils, weighing samples, agitating and centrifuging soils in solution and finally filtering them to have our final extracts.

In the end, after all the stressful planning and a lot of improvisation and help from our collaborators, all went well in our field campaign and we are now organising the data we have so far and planning for our next campaign at the same time. We’ll go back to Manaus in November, 2016 for more “fun with roots” and also to start fertilising our plots. In the following years we’ll repeat these measurements (and conduct many more!) and we’ll be able to compare how the forests function before and after nutrient addition. Is it P really limiting one or many of the forest compartments? Are nitrogen and cations maybe also limiting the way those forests work? Do we see changes below and aboveground in the same magnitude? Ultimately, do the trees will grow more and therefore assimilate more atmospheric CO2 because nutrient limitation is not constraining these forests anymore? Hopefully AFEX will help us elucidate many questions on how Central Amazonian forests work and will tell us more about nutrient limitation and the many mechanisms behind it!


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