|Patterns of soil 15N and P availability in the Amazon. From Quesada et al. 2010.|
When we looked earlier, the degree of decomposition affects soil organic matter content and the isotopic ratio of the N in the soil.
For individual classes of organic matter, be in leaves, organic layers, mineral soils, or fractions of mineral soils, the more microbes process organic matter, the more C is lost and the more enriched the nitrogen becomes in 15N.
The "processing hypothesis" is a standard explanation for vertical profiles of organic matter in soils. Deep soils have lower C concentrations and higher del15N than shallow soils because the stuff at the bottom has been worked over by microbes**.
**Other mechanisms affect vertical profiles, too. Plants preferentially cycle light N up to the top. Illuviation can also transport C and N downwards.
What applies vertically, could also apply horizontally.
Yet geographic patterns have largely been explained with the fractionating loss hypothesis. Soils enriched in 15N are thought to be enriched because they have lost a larger proportion of their N to fractionating pathways compared to relatively depleted soils.
Two sets of observations come together to generate the main latitudinal patterns.
1) Tropical soils are enriched in 15N compared to temperate soils
2) Tropical soils have high rates of N2O flux.
Put together, the two reinforce one another to solidify a view of latitudinal gradients.
But why would that be?
Nitrification or denitrification are not thought to be temperature sensitive like nitrogen fixation.
Therefore, it's indirect controls.
One of the major hypotheses is the Climate-Nutrient hypothesis. Tropical systems are thought to be more P-limited, which increases the degree of N surplus. Greater N availability increases the likelihood of gaseous N loss.
Quesada's work (above) is a good example of data that calls this hypothesis into question.
Within the Amazon (which is all hot), across a gradient of P availability, 15N is lowest in low P soils, not high P soils. Low P soils are supposed to have the greatest excess N and the most gaseous N loss.
To maintain the Climate-Nutrient hypotheses, explanations get pretty complicated. Rates of N2-fixation by plants have to vary in ways that one wouldn't expect. Or losses have to become episodic and almost catastrophic.
A number of other questions come up. Recent work suggests that N losses via NO3- leaching or dissolved organic N loss to streams in tropical systems can be high, too. And N2O is just one of the gaseous fluxes of N. Denitrification also produces N2, which is nearly impossible to measure.
Are tropical systems losing a greater proportion of their N via gaseous pathways? That part has never been quantified directly.
There is enough evidence out there to at least question the traditional view of the fractionating loss hypothesis driving global patterns in soil 15N, if not our views of the N cycle in the hot, cold, wet, and dry.
The next question is whether the processing hypothesis can explain more variation with fewer mechanisms.
If so, global patterns of N cycling need to be reconsidered.