Wheat assimilation of carbon and nitrogen investigated by amendments of dual-labelled (13C and 15N) green manure

Summary

Experimental data is still lacking for determining whether plant uptake of organic nitrogen in agricultural soils contributes substantially to the total nitrogen (N) uptake. Pulse-injection studies with duallabelled amino acids have confirmed that non-mycorrhizal crops possess the capacity to take up organic N but failed to quantify the uptake relative to total N uptake. In this study wheat uptake of organic N was investigated in combination with monitoring N-dynamics by adding dual-labelled green manure to soil with wheat plants. An advantage of using dual-labelled green manure as opposed to pulse injection of uallabelled amino acids is that the amino acids in the fertilizer are released gradually and naturally in the same time that N dynamics and assimilation can be monitored. The wheat plants were harvested after 27, 56, and 84 days after sowing and analyzed for 13C and 15N. A small, but significantly higher 13C signature was measured in roots from the labelled treatments at 27 days, which indicates uptake of organic N. From this value, the uptake of organic N was estimated to be minor and constitute between 1.5 % and 4.4 % of total root N. Wheat N derived from the green manure was for all three harvest dates 83.3% (±0.9) and 88.6% (±0.7) for roots and shoots, respectively. This significant difference that is very large compared to natural abundance studies might be explained by a differential assimilation of N forms in the different plant compartments. From the soil-plant system there was 11-13 % loss of green manure N and 51 % loss of green manure C within the initial 27 days after which there was little or no losses. These initial losses suggest a pre-emptive decoupling of dual-labelled molecules. In conclusion, the study supports the hypothesis that organic N uptake does not contribute substantially to N acquisition in agricultural crops.