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.

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Field-cured onions cv. Hyskin ( Allium cepa L.) supplied with organic nitrogen fertilizer were studied. The fertilizer was applied by broadcasting and harrowing, broadcasting and rotary cultivation, or placement between rows. Nitrogen dynamics were monitored throughout the growing season by soil sampling. Variation in quercetin content in the onion scales was analyzed by HPLC. The organically fertilized onions were compared with inorganically fertilized onions grown in the same field. Inoculation with arbuscular mycorrhizal fungi (AMF) in the row at sowing or during commercial transplant production was tested but did not significantly affect mycorrhizal root colonization levels in the field. Onions that received no fertilizer at all or that had fertilizer placed between rows had better establishment, probably due to more favorable soil nitrogen concentrations for seedling emergence. Broadcast application led to higher nitrogen concentration in the root zone, resulting in fewer but larger individual onions. Quercetin levels were not significantly altered as a result of nitrogen fertilizer source (inorganic or organic), application method, or mycorrhizal inoculation. However, variation between years was significant, with quercetin levels in 2004 almost twice as high as those in 2005.

Soilless cultivation of vegetables with digestate fertilizer from biogas production as a nutrient source is a promising method for integrating food production and organic waste management. In this study, bok choy (Brassica rapa var. chinensis) was cultivated in a hydroponic nutrient film technique system with biogas digestate as the only fertilizer source. Nitrification in moving bed biofilm reactors (external and/or integrated into the hydroponic cultivation system) was employed to lower the high ammonium concentration in the digestate prior to use. Treatments with differing nitrification and digestate input strategies were compared with respect to pH dynamics, crop growth, shoot water content, and shoot mineral content. The results showed that < 20% longer cultivation time (< 1 week) gave similar yield in biogas digestate-based hydroponics as in conventional hydroponics based on synthetic fertilizers. Automatic pH control through addition of digestate resulted in similar shoot dry weight as in the mineral fertilizer reference system. It can be concluded that biogas digestate is a suitable plant nutrient source for hydroponic production of bok choy, considering productivity and circularity aspects. The combined impact of digestate ammonium content, digestate pH, and the nitrification step needs to be considered when designing a hydroponic system with biogas digestate as the fertilizer source.