Very low uptake of organic N from dual-labelled (13C and 15N) green manure by wheat

Summary

Experimental data is still lacking for determining the significance of plant uptake of dissolved organic nitrogen (DON) in agricultural soils amended with organic fertilizers. Pulse-injection studies with dual-labelled amino acids have confirmed that non-mycorrhizal crops possess the capacity to take up DON but failed to quantify the uptake relative to total N uptake. In this study, dual-labelled green manure was added to soils with wheat plants. An advantage of using dual-labelled green manure as opposed to pulse injection of dual-labelled amino acids is that the amino acids in the fertilizer are released gradually, thus more naturally, and are therefore less susceptible to immediate microbial immobilization. The plants were harvested on days 27, 56, and 84 after sowing and analyzed for 13C and 15N. The initial 50% loss of fertilizer C in soil at day 27 indicates that the microbial decomposition decoupled substantial amounts of 13C/15N labelled compounds early in plant development, thus giving the microorganisms a pre-emptive competitive advantage in the acquisition of easily available 13C/15N labelled substrates. The δ13C signatures in roots were significantly higher in labelled than unlabelled fertilized treatments at day 27 with a difference of 1.02x10-3 atom% 13C. This difference diminished with time and was not significant on the last two sampling occasions. Assuming that the uptake was in specific amino acid forms, the calculated values of amino acid N uptake of total N are 1.6%, 0.8%, and 0.5% for glycine, glutamate, and lysine, respectively, on day 27. The use of dual-labelled green manure was thus suitable to detect even a low uptake of DON. These results suggest that crop uptake of DON is only of minor importance in agricultural soil with organic fertilizer amendments.