Fertilizer replacement value of digestate, processed animal manure, compost, biochar and struvite as renewable P sources

Thijs Vanden Nest, Lydia Fryda, Christophe Bogaerts, Jennifer Bilbao, Bart Vandecasteele

    Onderzoeksoutput: Hoofdstuk in Boek/Rapport/CongresprocedureC3: Congres abstract


    At the end of the processing cascade, biomass processing in the bioeconomy results in new products with potential as alternative P fertilizer. 90% of the biomass fed to biogas plants for energy production leaves the biogas reactor in form of digestate. Digestate contains large amounts of nutrients and organic matter, and has therefore a high potential as renewable nutrient source or soil conditioner. Digestate processing is widely adopted by the biogas producers. On large scale pig and cattle farms, manure processing is also increasingly introduced. This results in a large range of organic products from NK or P rich products that could replace mineral fertilizers, to soil conditioners which release little nutrients, but contain large amounts of stable organic matter. Further processing of digestate, manure and other waste streams may result in production of biochar and struvite. All these organic fertilizers/soil conditioners contain P, and can also be regarded as a renewable P source. Traditionally, these products are only characterized by total NPK content, dry matter (DM) and organic matter (OM) content and the potential N mineralization. However, little is known about the bioavailability of P from theses organic fertilizers. We aimed at forecasting the P fertilizer replacement value (FRV) based on the chemical composition of these renewable P sources. In 4 greenhouse pot trials we assessed the FRV of P for 19 processed digestate products, 10 types of processed animal manure, 2 types of compost, 2 types of struvite, 5 types of biochars and 18 blends of digestates and/or compost and/or animal manure, either as liquid, solid or pelletized material. The DM and OM content ranged from 4.2 to 93.3% and 1.4 to 83.9%/DM, respectively. The P content had a range of 0.26 to 12.1%/DM and C/P and N/P ratios ranged from <1 to 163.7 and <1 to 8.6, respectively. The soil used for the pot trial was a sandy loam soil with C% 1.63, pH-KCl 5.4, N% 0.152 and an ammonium lactate extractable amount of 4 mg P/100g dry soil. This soil was chosen as the P content was low, which makes differences in bioavailability of P detectable in plant growth. The different products were mixed through the soil at an amount equivalent to 90 kg P2O5/ha. Extra mineral N (NH4NO3 27%N) and K (KCl 60%K2O) were added to reach a minimum dose of 180 kg mineral N/ha and 180 kg K2O/ha. Tests with other N and K doses proved that these doses already exceeded the crop demands as no further increase in crop yield was detected by exceeding these NK doses. Westerwoldic ryegrass (Lolium multiflorum L. subsp. Westerwoldicum) was sown in the pots and the DM matter crop yield and the P export of the first 4 grass cuttings was measured to determine the P fertilizer replacement value (FRV) as an estimator of the P bioavailability (FRV(%)==[(Xorg, fertilizer – XP0)/(XP90-XP0)]*100, with X = DM yield or P export). The references for this calculation were P0, were no P fertilizer was added and P90 were 90 kg P2O5/ha was added as triple superphosphate (45% P2O5). The grass cuttings were also analyzed for N, K, Mg, Ca and Na content to check whether none of the other macro-elements were limiting for plant growth. The FRV based on the DM yield of the grass cuttings ranged from 46 to 170%, in comparison with triple superphosphate. This suggests that some organic fertilizers have a slow release of the P, or the P is present in unavailable forms (FRV<100%). On the contrary, some organic fertilizers result in more bioavailable P than triple superphosphate (FRV>100%). Organic acids released by the organic fertilizers can compete with P adsorption on Ca, Fe or Al binding site of the mineral soil particles and eventually stimulate P desorption or prevent P desorption. Although the last pot trials are still ongoing, we already observed a significant negative correlation (R² = 0.37 and p<0.001) between the bioavailability of P and the Ca:P molar ratio. Results on the link between P bioavailability and the composition of the tested organic fertilizers will be presented.
    TitelPhosphorus 2020 - Challenges for synthesis, agriculture, and ecosystems : 8th International Phosphorus Workshop - IPW8
    StatusGepubliceerd - 12-sep-2016
    Evenement8th International Phosphorus Workshop - University of Rostock, Rostock, Duitsland
    Duur: 12-sep-201616-sep-2016

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