Nitrogen dynamics in soils fertilized with digestate and mineral fertilizers: A full field approach.

Highly stabilized digestate from sewage sludge and digestate-derived ammonium sulphate (RFs), were used in a comparison with synthetic mineral fertilizers (SF) to crop maize in a three-year plot trial in open fields. RFs and SF were dosed to ensure the same amount of mineral N (ammonia-N). In doing so, plots fertilized with digestate received much more N (+185 kg ha-1 of organic N) because digestate also contained organic N. The fate of nitrogen was studied by measuring mineral and organic N in soil at different depths, ammonia and N2O emissions, and N uptake in crops. Soil analyses indicated that at one-meter depth there was no significant difference in nitrate content between RF, SF and Unfertilized plots during crop season indicating that more N dosed with digestate did not lead to extra nitrate leaching. Ammonia emissions and N content in plants and grains measured were also similar for both RF and SF. Measuring denitrification activity by using gene makers resulted in a higher denitrification activity for RF than SF. Nevertheless, N2O measurements showed that SF emitted more N2O than RF (although it was not statistically different) (7.59 ± 3.2 kgN ha-1 for RF and 10.3 ± 6.8 kgN ha-1 for SF), suggesting that probably the addition of organic matter with digestate to RF, increased the denitrification efficiency so that N2 production was favoured. Soil analyses, although were not able detecting N differences between SF and Rf after three years of cropping, revealed a statistical increasing of total carbon, suggesting that dosing digestate lead to carbon (and maybe N) accumulation in soil. Data seem to suggest that N2O/N2 emission and organic N accumulation in soil can explain the fate of the extra N dosed (organic-N) in RF plots.

Using highly stabilized digestate and digestate-derived ammonium sulphate to replace synthetic fertilizers: The effects on soil, environment, and crop production.

Recovered fertilizers (a highly stabilized digestate and ammonium sulphate) obtained from anaerobic digestion of sewage sludge, were used on plot trials with a maize crop, in a comparison with synthetic fertilizers. After three consecutive cropping seasons, the soils fertilized with the recovered fertilizers (RF), compared to those fertilized with synthetic fertilizers (SF), did not show significant differences either in their chemical characteristics or in the accumulation of inorganic and organic pollutants (POPs). The RF ensured an ammonia N availability in the soil equal to that of the soil fertilized with SF, during the whole period of the experiment. Furthermore, no risks of N leaching were detected, and the use of RF did not result in a greater emission of ammonia or greenhouse gases than the use of SF. The agronomic results obtained using RF were equivalent to those obtained with SF (fertilizer use efficiency of 85.3 ± 10 and 93.6 ± 4.4% for RF and SF respectively). The data show that utilising a very stable digestate can be a good strategy to produce a bio-based fertilizer with similar performance to that of a synthetic fertilizer, without environmental risks.

Environmental Performance in the Production and Use of Recovered Fertilizers from Organic Wastes Treated by Anaerobic Digestion vs Synthetic Mineral Fertilizers.

Recovered fertilizers (RFs), in the form of digestate and digestate-derived ammonium sulfate, were produced from organic wastes by thermophilic anaerobic digestion (AD) at full scale. RFs were then used for crop production (maize), substituting synthetic mineral fertilizers (SFs). Environmental impacts due to both RF and SF production and use were studied by a life cycle assessment (LCA) approach using, as much as possible, data directly measured at full scale. The functional unit chosen was referred to as the fertilization of 1 ha of maize, as this paper intends to investigate the impacts of the use of RF (Scenario RF) for crop fertilization compared to that of SF (Scenario SF). Scenario RF showed better environmental performances than the system encompassing the production and use of urea and synthetic fertilizers (Scenario SF). In particular, for the Scenario RF, 11 of the 18 categories showed a lower impact than the Scenario SF, and 3 of the categories (ionizing radiation, fossil resource scarcity, and water consumption) showed net negative impacts in Scenario RF, getting the benefits from the credit for renewable energy production by AD. The LCA approach also allowed proposing precautions able to reduce further fertilizer impacts, resulting in total negative impacts in using RF for crop production. Anaerobic digestion represents the key to propose a sustainable approach in producing renewable fertilizers, thanks to both energy production and the modification that occurs to waste during a biological process, leaving a substrate (digestate) with high amending and fertilizing properties.

Measuring ammonia and odours emissions during full field digestate use in agriculture.

The use of digestate in agriculture represents an opportunity for reducing the use of synthetic fertilizers while promoting nutrient and organic matter recycling, i.e. contributing to a circular economy. However, some environmental impacts could result from digestate use, with particular reference to N emissions, which can contribute to particulate matter formation in the atmosphere. So, correct digestate spreading methods need to be tested to reduce ammonia emission and, possibly, also to avoid annoyance to the inhabitants. In this work a digestate from organic wastes was used as a fertilizer by its injection at 15 cm, in comparison with a synthetic one (urea) for three consecutive years in open fields, measuring ammonia and odours emission. On average, the ammonia emission from digestate was of 25.6 ± 9.4 kg N Ha-1 (11.6% ± 4 of Total Ammonia Nitrogen - TAN - dosed), while urea emitted 24.8 ± 8.3 kg N Ha-1 (13.4% ± 4.5 of TAN dosed). The injected digestate also emitted less odour than urea (601 ± 531 and 1767 ± 2221 OU m-2 h-1, respectively), being ammonia coming from urea hydrolysis responsible for odour productions. The different N fertilizers did not lead to differences in crop yields, i.e. 18.5 ± 2.9 Mg grain Ha-1 and 17.4 ± 1.2 Mg grain Ha-1 for digestate and urea respectively.

Microbial recycling cells (MRCs): A new platform of microbial electrochemical technologies based on biocompatible materials, aimed at cycling carbon and nutrients in agro-food systems.

This article reviews the mechanisms that drive nutrients and carbon sequestration from wastewaters by microbial electrochemical technologies (METs). In this framework, a new generation of METs is also presented (to be called microbial recycling cells, MRCs), based on 100%-recyclable materials (biomass-derived char coal, clay, terracotta, paper, ligno-cellulosic plant materials, etc.), which can act as bio-electrodes, separators and structural frames. In traditional METs architectures (based on technological materials such as carbon cloths, plastic panels, membranes, binders), inorganic salts precipitation and adsorption, as well as biofouling due to organic-matter deposition, are considered as main drawbacks that clog and hinder the systems over relatively short periods. In MRCs, these mechanisms should be maximized, instead of being avoided. In this perspective, both inorganic and organic forms of the main nutrients are sequestered from wastewater and deposited on METs modules. Once the systems become saturated, they can entirely be recycled as agricultural soil conditioners or as base for organic-mineral fertilizers.