Volatilized
If you don't know where you are: The ratio of the nitrogen isotopes tells ecologists whether a forest belongs to the tropics or to the temperate latitudes. Curiously, though, rain seems to change this characteristic. Researchers have now pieced together an explanation, one clue after the other.
Somehow it didn't fit. Normally, tropical rainforests are among those habitats in which the nitrogen isotope 15N is enriched. What is meant by this is that compared to our forests in temperate latitudes, for example, there is a little more 15N relative to the contingent of 14N: the isotope -Ratio 15N to 14N is higher in the tropics than here. However, researchers found in their measurements that in tropical areas with higher rainfall, the relative proportion of 14N in the soil as well as in the plants increases - without being able to really explain the pattern. Apparently, the climate and nutrient cycle were linked here. But how?
The input is to blame, some suspect: The biological nitrogen fixation or differences in the load that clouds and rain bring should cause the changes in the system. Others, on the other hand, attribute it to the output: the lighter 14N isotope tends to be washed out and thus causes a relative enrichment of the heavier partner. Benjamin Houlton and his colleagues at Princeton University chose field work over speculation and got to the bottom of things in Hawaiian forests.
For years, six pieces of forest have been meticulously examined here, which, apart from the amount of rain, hardly differ in other environmental conditions - the best terrain to elucidate the fate of nitrogen. Like a true circumstantial process, the researchers collected data from rainwater, soil, soil water, streams, leaves and Co. And first of all refuted the theses discussed so far.
Thesis 1 - changed input - clearly failed: The scientists could not find any connection between the occurring isotope ratios and the amount of precipitation. With the exception of the two wettest forests, Houlton and his team also found relatively higher 15N values in the soils than 14N values than in the entries – so the forests had to lose the lighter isotope somehow.
Thesis 2 - selective washing out - did not work either. There were indeed differences according to the amount of precipitation: in the drier forests, nitrate dominated in the streams, while in the wetter variants dissolved organic nitrogen, a collective parameter that includes several nitrogen fractions. The nitrate is largely missing. However, the isotopic ratio of 14N and 15N in soils and streams was very similar - which is curious: would 14 N easier to travel with, there should be more of it in the water in the wetter regions and less in the soil. In both cases, the researchers tended to observe the opposite. Conclusion: This explanation is not correct either.
Houlton and his team still don't find themselves in need of explanation. From their point of view, the results confirm one thing above all: a third thesis that they championed. As a result, the culprit you are looking for is not at the entrance or exit, but in the system itself - in the form of specialized bacteria. Or to put it in a nutshell: denitrification, the conversion of nitrate to atmospheric nitrogen, confuses the number games.
Receipts required? Among other things, the researchers also found a telltale pattern for oxygen. Thus, its isotope ratios of 18O to 16O in nitrate were with those of 15N to 14N closely correlated - an indication of an intensive denitrification, the actors of which are apparently less willing to process the heavier candidates and therefore increase their respective share in the milieu relatively equally.
And what does the rain have to do with the whole story? A bit of imagination is required here. In the drier forests, the scientists conclude, the conversion of nitrate does not take place completely or across the board. Therefore, 15N-rich nitrate can escape from the local denitrification zone and still announce what is happening in distant regions such as the draining streams. "We are still working on the basic understanding of the nitrogen cycle"
(Lars Hedin) But in the wetter areas, where conditions that are far more low in oxygen and thus promote denitrification occur, nothing remains of the telltale nitrate, which is why its typical isotope signature is also missing in the streams and soils. The image at the exit now resembles the image at the entrance again, without the slightest hint of the bacterial restructuring that is taking place in between. "As these results show, we are still working on the fundamental understanding of the nitrogen cycle," commented study leader Lars Hedin on the new insights.
And that these should by no means be underestimated is shown by the scientists' calculations of how much nitrogen the system loses in this way: Houlton and his colleagues determined that about one evaporates in this way in dry forests Quarter of the nutrient, in the humid forests even more than half. And the rain covers the tracks.
