What the number rests on now
In the reads, the archaea made up % of the DNA against % for the bacterial oxidizers. That is a real and unusually lopsided ratio, and everything true about it is true of the sequences. It is not a count of cells or a rate of work, though: a marker counts gene fragments, and a number can lose its meaning at either step, from fragments to organisms and from organisms to activity. Read shares are a genetic potential, and the site says so wherever the ratio appears.
Three specific gaps sit between that potential and a claim about the soil, and none of them can be closed by re-reading the sequences. Each can be closed at the bench.
Three things the marker cannot see
- The pH under each core was never measured. The soil was described as from regional models, with a consensus surface pH of about - but that is a map value, not a reading from these thirteen holes. Because pH and nitrogen move together in soil, the archaeal pattern that looks like a nitrogen story could as easily be a pH story, and no amount of sequence can tell them apart.
- The nitrogen itself was never measured either. Grazing animals deliver nitrogen in dung and urine, dropped in patches that were not mapped - and it was never recorded whether stock reach the weedy patch these cores came from at all. The legumes' fixed nitrogen reaching the soil was inferred from who was growing rather than measured in the ground. The whole chain from clover to ammonia to archaea is argued, not weighed.
- A third oxidizer is invisible to this marker. Some Nitrospira oxidize ammonia all the way to nitrate on their own - comammox - and they do best in exactly the low-ammonia soil this ratio implies. The 16S gene cannot separate them from ordinary nitrite oxidizers, so the % this study recorded for Nitrospira is an upper bound on a competitor, not a measurement of one. The two-guild ratio may be missing a third player entirely.
The first measurement: count the genes directly
Ammonia oxidation runs on one gene, amoA, and the archaea, the bacteria and the comammox Nitrospira each carry their own version of it. Quantitative PCR of the three amoA variants on retained DNA does two things at once. It replaces a ratio of read shares with a count of gene copies per gram - a quantity, not a proportion - and it brings the invisible third oxidizer onto the same footing as the other two, because comammox amoA is a target a primer can hit even though the 16S gene cannot resolve it.
The reason to expect the qPCR to corroborate rather than overturn the ratio is that the archaea are known to win at low ammonia supply, which is the condition an unfertilized pasture is in - and fertilizer is what usually tips the balance back to the bacteria, which this ground has not had. Expecting the qPCR to agree is not the same as having run it, though. Nobody has run it.
The second measurement: read the soil, not the map
The other measurement is older and cheaper: pH and inorganic nitrogen - ammonium and nitrate - on the soil from each core. It is ordinary bench chemistry, and it settles the confound the study currently can only concede. With a pH and an ammonium reading beside each core's archaeal share, the pH story and the nitrogen story stop being indistinguishable: the archaeal share can be regressed on each with the other held down, and the data get to say which one it follows.
This measurement also meets an argument the study already made from the sequences alone. The archaea here were the near-neutral lineage, not the obligate acidophiles - the neutrophiles outweighed the acidophiles by to one in the reads - which is a reason to doubt that sour ground explains their dominance. A measured pH per core would confirm or break that inference directly, rather than leaving it resting on which archaea are present.
What it would take, and what it would be worth
Everything above turns on one question that is not about science at all: whether frozen soil or extracted DNA from these cores was kept. If it was, both measurements are modest - a qPCR run and a chemistry panel on thirteen samples, no new field season, no new sequencing. If it was not, the same measurements need the cores taken again, which moves them from a weekend's bench work to a return to the field.
Either way, what it buys is the same. The archaeal result does not depend on this soil being unusual, and these two measurements would turn it from a striking correlation into a measured, three-guild account of who oxidizes ammonia in an unfertilized pasture - the pH confound closed and the hidden competitor counted. That would stand as a paper on its own, and all it needs is bench time. The reanalysis on the gradient page is what can be published without it; this is what would let the study claim more.