Solving the riddle of the Amazon's nitrogen cycle

Sand vs. clay content (averaged 0.1° latitude/longitude) for the global soil 15N data set (so far). Red points are the Amazon. The one high blue point is from Panama.

When examining all the soil nitrogen isotope data from around the world, the Amazon rainforests always stood out as being too enriched based on everything else we knew about it. The Amazon is hotter and wetter than most other places, but it's del15N of soil organic matter was still 3‰ too high.

Something was different about the Amazon rain forests. 

There was either more gaseous N loss coming out of there than expected or more microbial processing that was enriching the soils. 

But what was it?

I looked at soil pH. Nothing unique there.

Soil carbon and nitrogen. Still couldn't explain the Amazon effect away.

One "outlier" for the Amazon has always been the white sands forests. Their 15N was never as enriched and those forests were always considered "nutrient poor".

Here's what Quesada et al. 2010 recently wrote on the topic: 

"That nitrogen is in excess for most tropical ecosystems (except forests growing on white sands) is, of course, already widely accepted, but from our pedogenic viewpoint, however, we also argue that as these losses continue to occur, the soil C:N ratio of the soils should also gradually increase (Fig. 12c) until a critical threshold around 30 mg kg−1 is reached. Beyond this, the system once again becomes more closed with respect to the nitrogen cycle, and thus enrichment ceases and the soil _15N tends back towards the isotopic composition of the input precipitation (ca. 0‰). The white sands forests of Amazonia have long been known to have relatively depleted _15N (Martinelli et al., 1999), and a large fraction of the sites on the “downside” of the curve are indeed sandy soils. But importantly, there are also other soils types with similar low P levels on the low side of the “breakpoint” (Fig. 11). This suggests that rather than just being a soil type effect, extremely low soil P concentrations may itself be the cause of these soils having strongly depleted _15N, with the phosphorus shortage itself leading to a lower rate of nitrogen return to soil. That is to say (in simplistic terms) at very low P availability, the forests actually become nitrogen limited."

When I look at the data, the one thing that stands out about the Amazon is the high clay content.

There just are few places in the world with such high clay concentrations. 

The white sands of the eastern Amazon might be the outlier for the Amazon, but it's the "black clays" of the western Amazon that are the outlier globally.

Quesada's argument about C:N and P might have some validity (I can work through that later), but the once you take into account clay concentrations, the Amazon effect almost disappears (<1 average="average" enrichment="enrichment">

1‰ is tolerable.

So how clay impacts soil 15N will be important. Does it control P availability and decomposition patterns that impact gaseous N loss? 

Or is it by impacting retention of enriched substrates and microbial processing?

Answering this question will get at whether Amazonian rainforests have a unique N cycle or not, but we at least now a proximal driver.