Use efficiency of nitrogen from biodigested plant material by ryegrass

Apparent nitrogen-use efficiency of the applied mineral N (NUE(min)) in effluent from biodigested plant material (BE; C : N(org) ratio 14:1; mineral N-to-total N ratio 0.5:1) and a nitrate-based inorganic fertilizer (IF), both applied at two rates was investigated in a six-month pot experiment with Italian ryegrass (Lolium multiflorum Lam.). Dry-matter (DM) production was 7% lower and total amount of N in aboveground biomass was 8% lower in BE than in IF at 40 d after sowing (DAS), equal at 81 DAS, and higher in BE than in IF at 136 and 172 DAS. NUE(min) calculated on the basis of accumulated N in aboveground biomass of ryegrass in fertilized treatments compared to a control without N application was significantly lower in BE than in IF up to the third cut (136 DAS). Total NUE(min), total N recovery, and amount of foliage DM were similar for both fertilizers at the end of the experiment. Root biomass, total DM produced including roots and stubble, the fraction of root N to total plant N, and soil mineral N at 172 DAS were higher for BE than for IF. Mineral N applied with biogas-reactor effluent was almost as effective as the nitrate-based mineral fertilizer used for comparison. Within the six-month experimental period net N mineralization, estimated at 12% of organic N in effluent, was not substantial. Hence, the organic compounds in the effluent were relatively recalcitrant.