One field, one season, one ledger

The Nitrogen the Harvest Takes Away

A legume can fix a great deal of nitrogen and still leave little or none for the next crop, because fixation is an input to the whole plant while protein-rich grain is an export from the field.

The arithmetic below is deliberately narrow. It asks how much atmospheric nitrogen entered a plant and how much plant nitrogen left in the harvested product. It does not measure the change in total soil nitrogen.

01 / Put the harvest on the scale

Partial nitrogen balance −5.7 kg N/ha
retain allremove all
fixed N − exported N
174.7 − (180.4 × 1.00) = −5.7 kg N/ha

The soybean preset uses published study means. Grain contained 180.4 kg N/ha. The partial balance crosses zero only when more than 96.8% of that grain-N pool is removed.

At full grain harvest, exported N is 5.7 kg N/ha greater than whole-plant fixation.

81.8%reported whole-plant Ndfa, mean ± 1.0% SE

02 / Where did plant nitrogen come from?

The answer depends on the reference

Fixation was not seen directly. It was inferred from isotope mixing. The published whole-plant result also includes an estimated root contribution: root dry mass was set to 20% of shoot dry mass, not measured per area. Each preset holds an illustrative plant signal while you move its reference end member. The estimated atmospheric fraction moves with that choice.

81.8% estimated plant N derived from atmosphere
(reference − plant) / (reference − atmosphere)
(1.1000 − 0.5000) / (1.1000 − 0.3663) = 81.8%

The study paired long-term enriched soil with a near-isogenic non-nodulating soybean. These controls use illustrative isotope signals, not the unavailable raw study observations.

03 / Three answers that are not interchangeable

What, exactly, is being credited?

Nitrogen fixed

174.7 ± 8.9

kg N/ha, whole plant, mean ± SE, including the study's modeled root contribution. This is atmospheric nitrogen entering the sampled crop.

Partial balance after grain

−5.7 ± 2.0

kg N/ha, mean ± SE. This is fixed N minus seed N removed, not measured change in soil total N.

Following-crop fertilizer credit

not measured

A fertilizer credit is an empirical response of a later crop. This experiment did not estimate one.

A large input did not predict the balance

Across the harvested mesocosms, whole-plant fixation and partial balance had R² = 0.02 and p = 0.84. In this study, knowing fixation alone told almost nothing about whether harvest export overtook it.

Schematic of a relationship with R squared 0.02 Twenty-two deterministic illustrative points form a nearly flat cloud. This is not a reconstruction of the study observations. whole-plant fixed N →partial balance → reported R² = 0.02; p = 0.84
The line and 22-point cloud are a schematic keyed to the published sample count and statistics. Raw observations were not available from the cited Dryad record on 2026-08-25, so this is not presented as the study scatterplot.

04 / The check

A balance with teeth

The page calculation, an independent tissue route, and the paper's published anchor meet at the same rounded result. Then a deliberately poisoned input is required to fail.

Uncertainties and free choices
  • The anchor is one season at one Iowa site. Only 22 of 36 mesocosms were harvested.
  • The whole-plant estimate includes a modeled root contribution. Root biomass per area was not measured; root chemistry came from 14 of 22 harvested mesocosms, and the authors set root dry mass to 20% of shoot dry mass. This is a study-specific assumption, not a universal soybean root correction.
  • The reported ±2.0 kg N/ha is retained. The page does not combine the marginal 8.9 and 9.3 SE values because paired fixed and seed N covary.
  • Partial balance omits gaseous loss, leaching, deposition, mineralization, rhizodeposition, and longer-term residue turnover.
  • The normalized cover-crop ledger chooses 100 kg N/ha removable biomass and 70 kg N/ha fixed solely to expose the arithmetic. It is not a crop recommendation or published average.
  • All isotope signals in the counterfactual panel are illustrative free choices. Only the paper's reported 81.8 ± 1.0% whole-plant Ndfa is an anchor result.