Effects of soil fertility on the carbon cycle of tropical forests: a novel soil fertility manipulation experiment in Amazonia (AFEX)


Why fertilise the Amazon forest?

Tropical forests growing on highly-weathered, infertile tropical soils are likely limited by soil phosphorus (P) availability. However, large-scale nutrient manipulation experiments necessary to confirm this assumption of nutrient limitation in Amazonian tropical forests are lacking. Therefore, the Amazon Fertilization Experiment (AFEX) is paramount in the understanding of tropical forest function in response to changing soil nutrient availability. This is important because Amazonian tropical forests account for up to 50% of the carbon uptake of tropical forests globally1.


Who are we?

The AFEX project is funded by Natural Environment Research Council (NERC, UK) from 2014–2019. Our network of investigators includes researchers in Brazil, Panama, Australia, and the United Kingdom. We currently have four master’s and four PhD Brazilian students based at the Instituto Nacional de Pesquisas da Amazônia (INPA) and the University of Exeter (funded by Ciência Sem Fronteiras, Brazil). The project is hosted by the Biogeochemistry Lab (Beto Quesada) and co-hosted by the Projeto Dinâmica Biológia de Fragmentos Forestais (PDBFF, José Luis Camargo) at INPA.


What is the experiment?

The AFEX project is a fully factorial experimental design replicated four times. Plots (50 x 50 m) were established on plateaus in a completely randomized block design with each of the eight nutrient treatments in one plot per block. Plots within each block are separated by 50 m and blocks are separated by 100–300 m. The treatments are: nitrogen (N) applied as 125 kg N/ha/yr, phosphorus (P) applied as 50 kg P/ha/yr, and cations applied as 50 kg KCl/ha/y and 160 kg dolomitic limestone/ha/yr, all combinations of these treatments and a control for a total of 8 treatments. Treatments are applied three to four times during the rainy season.


What do we measure?

We will monitor above and belowground carbon dynamics. This includes fine temporal scale responses such as soil microbial respiration to long-term trends such as tree growth using dendrobands. The first phase of AFEX will focus on process-based responses of the carbon cycle. We will examine root dynamics using various methods, mycorrhizal colonization and respiration, leaf litter production, and leaf physiology. In addition to measuring plant and microbe responses to nutrient addition, we will characterize the soil nutrients and tree species composition in all plots.


Where is the experiment?

Soils across Amazonia vary in soil nutrients and physical properties, depending on soil type and geologic age, due to weathering processes and loss of nutrients. Soils in western Amazonia at the base of the Andes mountains are relatively “fertile” compared to “infertile” soils in the east on the geologically old Brazilian and Guyana shields. Central Amazonian soils are characterized as Ferralsols, the most abundant soil type across the Amazon, with low pH and nutrients availability and highly aggregated clay particles. However, Central Amazonian soils near Manaus are derived from sediments from the Brazilian and Guyana shields. That is why AFEX is located in Central Amazonia at a site with soils that have experienced continuous weathering and perhaps represent some of the most nutrient limited soils occurring in Amazonia2. The experiment is located approximately 70 km north of Manaus, in Central Amazonia, in the Cueiras Reserve.


1. Pan Y, Birdsey RA, Fang J, Houghton R, Kauppi PE, Kurz WA, Phillips OL, Shvidenko A, Lewis SL, Canadell JG, et al. 2011. A Large and Persistent Carbon Sink in the World’s Forests. Science 333: 988–993.

2. Quesada CA, Lloyd J, Anderson LO, Fyllas NM, Schwarz M, Czimczik CI. 2011. Soils of Amazonia with particular reference to the RAINFOR sites. Biogeosciences 8: 1415–1440.

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