Assessment of the nitrogen fertilising value and carbon sequestration potential of digestates from different feedstocks

The drumbeat to reach net-zero emissions by 2050 and foster a European circular economy has arguably thrust anaerobic digestion front and centre due to its ability to produce renewable energy and fertilisers from recycled nutrients in the form of digestate. In this context, it is of paramount importance to better understand the fate of the nitrogen (N) and carbon (C) from digestate when applied to soil. In this study, the C and N mineralisation rates of five digestates from various feedstocks, undigested pig slurry and compost were studied in back-to-back incubation experiments in which predetermined amounts of these products were applied to soil. The treated soil mixtures were placed in PVC tubes, packed to a bulk density of 1.4 g cm-3, and their water content adjusted to 50% water-filled pore space. For soil incubations aimed at assessing N mineralisation patterns, destructive sampling was carried out every 20 days (over 4 months), during which ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) were measured. For C soil incubations aimed at monitoring organic carbon (Corg) mineralisation, each PVC tube was placed inside an airtight glass jar with an NaOH 0.5 M vial to trap CO2 from the treated soils. The treatments were incubated over 149 days, during which time the NaOH traps were periodically removed (17 sampling moments) and back titrated with 0.5 M HCl to measure the evolved CO2. Table 1 summarises the main results of the C and N incubations. Net N mineralisation (Nmin) was determined by subtracting the mineral N content of each treatment on day 0 from the mineral N at all subsequent measurements and expressed as percentage of applied organic N (Norg). As shown in Table 1, a positive Nmin (%Norg added) was observed for all products at the end of the experiment (day 127). Regarding C incubations, the amount of mineralised C (Cmin) from the treatments was calculated by subtracting the cumulative CO2 evolved from the untreated soil and was expressed as a percentage of added Corg from each product. A second-order kinetic model was fitted to the Cmin pattern of each of the tested products (Sleutel et al.,2005), from which the humification coefficient (Hc) of each treatment, defined as the amount of C remaining after one year, could be extrapolated. Results show that the Hc ranged from 50 to 81% for raw digestates (Table 1). The initial TC:TN ratio of the products was significantly negatively correlated (p < 0.01) to the amount of Cmin on the last day of C incubations. Also on the final day of N incubations, Nmin (p < 0.05) was significantly positively correlated with the initial NH4+-N:TN ratio of the products and negatively correlated with TC:TN (p < 0.05), as highlighted previously (Tambone & Adani, 2017). Thus, the TC:TN ratio of products proved a reliable predictor of C and N mineralisation rates in the incubated soils. Moreover, the Nmin rates (%Norg) of the studied digestates on the last day of the experiment ranged from 20 to 39% (Table 1). In other words, this suggests that anywhere between 60 to 80% of Norg was still present in the fertilised soil treatments after 127 days. Therefore, more attention might be given to the Norg fraction that still remains after application. In this respect, field trials would be warranted to assess the long-term fate of mineralised N when digestates are applied to a plant-soil system. Under this scenario, the synchrony, or possible asynchrony, between the mineralisation rates of the remaining Norg pools from the digestates and plant N uptake could be better monitored. Also of note, while digestates all exhibited an initial N immobilisation phase (not shown), the dynamics were very different from one product to the other, with some showcasing a comparatively faster remineralisation (from day 20 onwards) than others (from day 40 onwards). Thus, an understanding of the immobilisation/remineralisation pattern of a specific digestate could constitute a useful tool, among others, to reach an informed decision in the field regarding appropriate timing of N fertilisation, thereby reducing potentially counterproductive effects on crop growth. The amounts of Hc after one year, from the C incubation experiment, ranged from 50 to 81% for the digestates (Table 1). This result suggests that, without reaching thelevels of compost (99%), these products had an equally interesting C sequestration potential in the incubated soil, aside from their fertilising value.