Let's Get Biogeochemical

In a previous post, I spoke about nine planetary boundaries, and how four of those boundaries have already been transgressed (Rockström et al., 2009). I'll link you again to Johan Rockström's TEDTalk about it here and refresh your memory with Figure 1.

Figure 1: The nine defined planetary boundaries. Biogeochemical flows is currently beyond the zone of uncertainty, indicating that it is at high risk. (Source: Stockholm Resilience Centre, accessed 2018)

As you can see, Biogeochemical flows are one of the nine boundaries transgressed beyond the zone of uncertainty. Human industry and agriculture have altered natural nitrogen and phosphorous flows, creating more reactive nitrogen than occurs naturally and mobilised significant amounts of phosphorous. Much of this ultimately ends up in terrestrial and aquatic ecosystems, polluting waters but fertilising soils (Stockholm Resilience Centre, accessed 2018). Similar occurrences have happened with increased CO2 output from fossil-fuel emissions. Tropical forests and large bodies of water are expected to take up a lot of this excess atmospheric carbon (Mahli et al., 2000; Sabine et al., 2004), although the terrestrial tropical response is hotly debated (Hickler et al., 2008). Since these are global occurrences, the Amazon has of course been affected too, especially considering the amount of forest converted to agricultural land (Hansen et al., 2013).

Increasing severity of droughts under El Nino and deforestation-induced droughts in the Amazon are likely to affect soil emissions of N₂O and CH₄ in the Amazon. A study into this by Nepstad et al. (2002) reduced rain throughfall in an area of the Amazon to replicate these drought effects, finding that the resultant reduction in productivity led to an increase in N₂O soil emissions and an increase in soil consumption of CH₄. Although aboveground productivity was also reduced, indicating a reduction in above ground carbon storage. This suggested that carbon and nitrogen emissions in Amazon forests are sensitive to small changes in rainfall. Ultimately, this means that increased drought under anthropogenic climate change and deforestation could significantly alter carbon and nitrogen cycling in the Amazon. A loss of nitrogen from Amazonian soils is concerning considering the deteriorating soil quality in the Amazon (Martinez and Zinck, 2004). 

Interestingly, Nepstad et al. suggested no decrease in leaf litter mineralisation, which is thought to be the main way nitrogen is fixed into soils in tropical forests (Körner, 2009). In contrast, Körner’s study suggested that tropical forests, including the Amazon, are likely to experience an increase in nitrogen fixation stimulated by elevated CO­₂ (which is set to increase in the future) and an increase in productivity and carbon storage as a result. However, the study also suggested that soil nitrogen is less of a limiting factor in growth than phosphorous is in tropical forests.

Tropical forest soil is typically of poor quality, so the previously mentioned mobilisation of phosphorous and increased nitrogen fixation (despite increased N₂O emissions) seems like a good thing. More phosphorous and nitrogen would increase soil quality and mitigate the phosphorous limitation, enabling increased biomass growth and above ground carbon storage, right? Taking carbon out of the atmosphere AND encouraging forest growth – that’s two for one! Well, it’s not that simple... 

While this does happen, we’re offsetting it with our physical destruction of the rainforest, by reducing Amazon forest extent and reducing biodiversity, which I’ve talked about in my previous posts. That hinders growth immensely. What’s more is the potential for Phosphorous- and Nitrogen-containing fertiliser used in agriculture to run off into the Amazon River and cause algal blooms that reduce light penetration and cause oxygen-starvation to Amazon River plumes as bacteria consume these algae (Lock et al., 2015; Stockholm Resilience Centre, accessed 2018). The fish can’t breathe! Let’s consider also that my previous post determined that dam building is already reducing fish diversity in the Amazon river basin! Concerning, I know.

The issue of biogeochemical cycling is a little less cut-and-dry than some of the other issues I’ve discussed in the Amazon. There appear to be benefits to the Amazonian terrestrial ecosystems; potential increases in productivity and soil fertility, but definite negatives to our effects on biogeochemical cycling, particularly in Amazonia’s aquatic ecosystems. At least we can be sure in the knowledge that we’re effecting change again. We do a lot of that, unfortunately.

See you next time, for the final, reflective post of the blog!

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