Browsing by Subject "N use efficiency"

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Digestate composition affecting nitrogen fertilizer value, soil carbon mineralization, and nitrous oxide emissions
(2025) Häfner, Franziska; Möller, Kurt
The variety of organic feedstocks that can be used for anaerobic digestion leads to digestates with different physico-chemical compositions. Different feedstocks yield distinct digestate compositions, hence different fertilizer potential, which consequently may affect carbon (C) and nitrogen (N) cycling in soils. This present thesis aimed to explore the relationship between feedstocks used for anaerobic digestion, digestate composition and fertilizer properties, and their consequences for agricultural and environmental sustainability. It was investigated, how digestate properties can be related to its N fertilizer value, C storage potential, or greenhouse gas (GHG) emission potential. Two experiments were conducted to assess (i) differences in the N fertilizer value of seven digestates from different feedstocks in a practice-oriented 2-year field experiment with spring wheat on a silty loam, with two different application techniques and (ii) the degradability of organic matter (OM) in the digestates within an aerobic incubation experiment for 56 days. In addition, digestates were labeled with the stable isotope 15N, either in both NH4+-N and organic N, or only in NH4+-N. Those labeled digestates were examined in a 2-year field experiment to assess short-term nitrous oxide (N2O) emissions after application from the digestate and native soil-N pool. The labeled digestates were also examined in a pot study with a crop rotation of ryegrass and maize in a sandy and loamy soil. The 15N-labeling approach was applied to distinguish the N contribution in total N offtakes from the digestate, its solid and liquid fraction, and from the soil. In a first step, seven digestates from different feedstocks were collected from existing biogas plants either from farms or municipalities in Baden-Württemberg, Germany. The digestates showed differences in dry matter (DM), C/N ratio, ammonium (NH4+) concentration, macro nutrients, and salt content. The range was according to the literature, and no limits for heavy metals were exceeded. The food waste digestate stands out among the other digestates with lowest C/N ratio and raw fiber content, highest ammonium concentration and highest table salt (NaCl) content. In the field study, the mineral fertilizer equivalents (MFE) ranged from 18% to 60% in the 1st year and from 39% to 83% in the 2nd experimental year. Highest MFE in both years was exhibited with food waste (49-70%), while most other digestates from agricultural feedstocks showed MFE´s in the range of 28-49%. The fertilizer properties (C/N, lignin, acid detergent fiber (ADF), NH4+/N and organic C/organic N (Corg/Norg)) were found to predict 58.9-74.2% of the variance in N offtakes among the digestates. Incubation of the different digestates showed that 61% of the organic C added by food waste digestate had been mineralized after 56 days. In contrast, only 16-22% of the supplied organic C was decomposed for the other plant and manure-based digestates. The composition of the digestates could explain up to 90.4% of the CO2 evolution, with C/N being the predominant factor (35.5%), followed by ADF (26.5%), raw fiber content (9.2%), lignin (8.7%) Corg/Norg (6.7%) and Carbonate-C (1.2%). The pot study showed comparable N fertilizer replacement values (NFRV) as MFEs in the field, with 33.5-76.0% and 40.6-76.3%, in sand and loam, respectively. Highest NFRV was measured from sugar-beet and food-waste-based digestates, with on average 70% across the two soils. N offtakes and yield were affected by fertilizer, soil, and their interaction, meaning digestate performance among soils varied to different extents. Digestate`s N fertilizer value could be related to three main compositional parameters (C/N, Corg/Norg, NH4+/N), which accounted for 44.5% of the variance of total N offtakes. Digestate-15N recovery in aboveground biomass after five applications showed that between 28-78% of N was derived from the digestates. Total 15N-fertilizer recovery in biomass was 18-43% among digestates and did not correspond to higher total N offtakes or N use efficiency (NUE). The solid fraction within the raw digestate contributed only 4.0-8.3% to digestate-N offtakes, compared to 32.5-45.3% from the liquid fraction. N mineralization of the solids was estimated at 6-13% after five applications. Cumulative N2O emissions after application of the different digestates were higher in the 1st year (312–1580 g N2O-N ha⁻¹) compared to the emissions (133–690 g N2O-N ha⁻¹) in the 2nd year. Differences among digestates were only found in the 1st year, where sugar beet leaves had higher cumulative N2O than digestates from cattle slurry or food waste. Emission factors determined in the two years’ field study (0.21–0.75%) were all within the range of the IPCC default value. Based on 15N-labeling, it could be observed, that only 16–38% of N2O emissions originated from digestate N. From the results of field, pot, and incubation study, it can be concluded that differences in fertilizer value and OM stability are related to variations in digestate composition. Differences in C mineralization depend on the overall degradability of the digestates, as shown by C/N and fiber fractions. However, the relevance for yield, N recovery and C storage potential is in most cases rather low, except for digestates with very distinct properties, such as food waste or sugar beet digestates. The 15N labeling of digestates shows that the N taken up by the majority of digestate-treated crops derived more than 50% of N from the soil pool, underlining the significance of the soil for N supply via mineralization. After several crop cycles and applications of digestate, it appears that the impact of soil on digestate-derived 15N efficiency decreases. With repeated application and mineralization of Norg from the solids, the initially 2.4-4.6% lower N availability of the applied 15N from the solid fraction increased to a similar range as the NUE of the liquid fraction (~31-24%). The eight labeled digestates showed differences in NUE and 15N recovery rates, which indicate different impacts of the digestates on N immobilization and mineralization in the soil. Field application of the 15N-digestates shows that more than 60% of N2O emissions originate from the native soil N pool. The first peak following fertilization contained mainly soil N, presumably due to denitrification of soil nitrate, likely induced by the applied OM. Whereas later N2O peaks after rainfall events contained more denitrified digestate-N, as indicated by 15N signals in N2O, signifying that digestate derived N becomes the primary N source. It can be concluded that for N2O emissions that after digestate application, the main N source for emissions is the soil N pool, probably triggered by the supplied OM. Furthermore, the influence of environmental factors such as water-filled pore-space and soil N pool, on overall N2O emissions are of main relevance, and less the digestate type. In summary, while digestate composition does have some influence on its performance as fertilizer, the broader context of environmental conditions and the role of native soil N pools are of higher significance than the feedstock. Understanding these factors will be crucial for optimizing the use of digestates in agricultural and environmental management practice.