Acidification and alkalinization pretreatments of biowastes and their effect on P solubility and dynamics when placed in soil

BACKGROUND

Sustainability concerns as well as recent increases in fertilizer prices exacerbates the need to optimise the use of biowastes as fertilizers. For this reason, we investigated how different pretreatments affect the P dynamics when biofertilizers are placed in the soil.

METHODS

Sewage sludge (SS), sewage sludge ash (SS-ash), meat and bone meal (MBM), and the solid fraction of biogas digestate (BGF) were pretreated with H₂SO₄, NaOH, and Ca(OH)₂ and incubated for 2 and 12 days, respectively, in a one-dimensional reaction system for detailed studies of the interactions in the biomaterial-soil interface and the soil adjacent to the placement zone.

RESULTS

Our results showed that acidification and treatment with NaOH increased the P solubility of the biomaterials. The P loss from the biomaterial layer to the soil was correlated with water-extractable P in the biomaterials (0.659) and water-extractable P in the soil (0.809). Acidification significantly increased the total amount of P depleted from the biomaterial to the soil whereas NaOH pre-treatment did not. However, for NaOH-treated SS and SS-ash, the apparent recoveries were significantly higher compared to the acidification due to a decrease in soil P sorption capacity as the soil pH increased due to residual alkalinity in the biomaterials.

CONCLUSIONS

Acidification showed promising results by increasing the P solubility of all the biomaterials, and the alkalinization of SS and SS-ash with NaOH by increasing the apparent recovery in the soil. However, further studies are needed to assess the effects of these treatments on plant growth and P uptake.

Potential ammonia volatilization from 39 different novel biobased fertilizers on the European market - A laboratory study using 5 European soils

Current political focus on promoting circular economy in the European Union drives great interest in developing and using more biobased fertilizers (BBFs, most often waste or residue-derived). Many studies have been published on environmental emissions, including ammonia (NH₃) volatilization from manures, but there have only been a few such studies on BBFs. Ammonia volatilization from agriculture poses a risk to the environment and human health, causing pollution in natural ecosystems when deposited and formation of fine particulate matter (PM_x). Furthermore, NH₃ volatilization results in removal of plant-available N from agricultural systems, constituting an economic loss for farmers. The aim of this laboratory study was to determine the potential NH₃ volatilization from 39 different BBFs commercially available on the European market. In addition, this study aimed to investigate the effect of incorporation, application rate, soil type, and soil moisture content on potential NH₃ volatilization in order to derive suggestions for the optimal field application conditions. Results showed a great variation between BBFs in potential NH₃ volatilization, both in terms of their temporal pattern of volatilization and amount of NH₃ volatilized. The potential NH₃ volatilization varied from 0% of applied total N (olive oil compost) to 64% of applied total N (manure and crop digestate) during a 27- or 44-day incubation period. Characteristics of BBFs (pH, NH₄⁺-N, NO₃^--N, DM, C:N) and their interaction with time could explain 89% of the variation in accumulated potential NH₃ volatilization. Incorporation of BBFs into an acidic sandy soil effectively reduced potential NH₃ volatilization by 37%-96% compared to surface application of BBFs. Potential NH₃ volatilization was not significantly affected by differences in application rate or soil moisture content, but varied between five different soils (with different clay and organic matter content), with the highest NH₃ volatilization potential from the acidic sandy soil.

Impacts of empty fruit bunch applications on soil organic carbon in an industrial oil palm plantation

Extensive oil palm plantations have often resulted in the decrease of soil organic carbon (SOC). Several options exist to counter this, such as recycling empty fruit bunches (EFB) as a soil amendment. However, the extent to which EFB increase SOC has been disputed. Since EFB could also be used as a climate change mitigation tool, it is necessary to truly understand their impact on SOC. The investigation of the impact of nine EFB treatments (differing in frequency and application rates) on a 27-year-old large-scale experiment (Lampung, Sumatra, Indonesia) revealed that, while EFB impacts are heterogeneous throughout the plantation, they can positively affect total SOC and permanganate oxidisable carbon (POX-C) both at shallow and deep depths (measured up to 100 cm). POX-C was closely correlated to SOC, but showed significant increases compared to the untreated control in all treatments, while total SOC was only increased in a few treatments with small and frequent rates of EFB application. Overall, between 12 (±16) and 56 (±12) t ha^-1 of carbon were sequestered under the harvesting path after 21 years. Focussing on the mineral nutrition value of the EFB, oil palm companies apply a rate of 60 t of EFB every second year for their commercial production, and the analysis of three commercial plots showed that the commercial rate only increased POX-C while it had no effect on the total SOC and SOC stocks. It seems obvious that a change of paradigm is necessary to consider EFB recycling as a new management perspective, where the potential for carbon sequestration becomes an important variable for climate change mitigation besides the initial objective of integrating EFB application into the fertiliser management plan of a plantation.

Considering inorganic P binding in bio-based products improves prediction of their P fertiliser value

Prediction of the relative phosphorus (P) fertiliser value of bio-based fertiliser products is agronomically important, but previous attempts to develop prediction models have often failed due to the high chemical complexity of bio-based fertilisers and the limited number of products included in analyses. In this study, regression models for prediction were developed using independently produced data from 10 different studies on crop growth responses to P applied with bio-based fertiliser products, resulting in a dataset with 69 products. The 69 fertiliser products were organised into four sub-groups, based on the inorganic P compounds most likely to be present in each product. Within each product group, multiple regression was conducted using mineral fertiliser equivalents (MFE) as response variable and three potential explanatory variables derived from chemical analysis, all reflecting inorganic P binding in the fertiliser products: i) NaHCO₃-soluble P, ii) molar ratio of calcium (Ca):P and iii) molar ratio of aluminium + iron (Al + Fe):P. The best regression model fit was achieved for sewage sludges with Al-/Fe-bound P (n = 20; R² = 79.2%), followed by sewage sludges with Ca-bound P (n = 11; R² = 71.1%); fertiliser products with Ca-bound P (n = 29; R² = 58.2%); and thermally treated sewage sludge products (n = 9; R² = 44.9%). Even though external factors influencing P fertiliser values (e.g. fertiliser shape, application form, soil characteristics) differed between the underlying studies and were not considered, the suggested prediction models provide potential for more efficient P recycling in practice.

Contrasting effects of biochar on phosphorus dynamics and bioavailability in different soil types

We investigated how two different biochars (wood biochar - WBC and straw biochar - SBC) affected P dynamics and bioavailability in five different soils differing in pH, C%, texture, Fe, Al, Ca, and Mg giving a range of soils with low (S1 and S2), intermediate (S4), and high (S3 and S5) P sorption capacities. Langmuir and Freundlich equations were fitted to the sorption data of soil and soil/biochar mixtures. P fertilizer applied to all treatments was fractioned into strongly sorbed P (qS), easily available sorbed P (qA) and solution P (c) by determining the anion exchange resin (AER)-extractable P in samples from the sorption experiment. A pot experiment was conducted to measure P uptake by maize grown in S1, S2 and S3 amended with WBC or SBC at two P fertilizer levels (0 or 70mgPkg^-1). Only WBC could sorb P from solution partly due to a high content of calcite. SBC did not have any effect on P sorption isotherms, whereas WBC increased the P sorption in S1, S2, and S4, yet decreased P sorption in acidic soil S5. qS increased in S1, S2, and S4, and decreased in S5 in WBC treatments, whereas, qS decreased in SBC treatments in soils S2, S4, and S5. Accordingly, there was a significant interaction between soil type and biochar on maize growth and P uptake. Biochar had no effect in an alkaline soil (S3), whereas, WBC and SBC had positive effects on maize growth in slightly acidic soils S1 and S2, depending on the soil P status, however, the P uptake was lower in WBC compared to SBC treatments. Biochar and soil properties and the P status of the soil affect P bioavailability. The study provides useful information for optimizing the use of biochar in agricultural P management.

Changes imposed by pyrolysis, thermal gasification and incineration on composition and phosphorus fertilizer quality of municipal sewage sludge

Fertilizer quality of ash and char from incineration, gasification and pyrolysis of a single municipal sewage sludge sample were investigated by comparing composition and phosphorus (P) plant availability. A process for post oxidation of gasification ash and pyrolysis char was developed and the oxidized materials were investigated as well. Sequential extraction with full elemental balances of the extracted pools as well as scanning electron microscopy with energy dispersive X-ray spectroscopy were used to investigate the mechanisms driving the observed differences in composition and P plant availability in a short-term soil incubation study. The compositional changes related mainly to differences in the proximate composition as well as to the release of especially nitrogen, sulfur, cadmium and to some extent, phosphorus (P). The cadmium load per unit of P was reduced with 75-85% in gasification processes and 10-15% in pyrolysis whereas no reduction was observed in incineration processes. The influence on other heavy metals was less pronounced. The plant availability of P in the substrates varied from almost zero to almost 100% of the plant availability of P in the untreated sludge. Post-oxidized slow pyrolysis char was found to be the substrate with the highest P fertilizer value while ash from commercial fluid bed sludge incineration had the lowest P fertilizer quality. The high P fertilizer value in the best substrate is suggested to be a function of several different mechanisms including structural surface changes and improvements in the association of P to especially magnesium, calcium and aluminum.

Low temperature circulating fluidized bed gasification and co-gasification of municipal sewage sludge. Part 2: Evaluation of ash materials as phosphorus fertilizer

The study is part 2 of 2 in an investigation of gasification and co-gasification of municipal sewage sludge in low temperature gasifiers. In this work, solid residuals from thermal gasification and co-gasification of municipal sewage sludge were investigated for their potential use as fertilizer. Ashes from five different low temperature circulating fluidized bed (LT-CFB) gasification campaigns including two mono-sludge campaigns, two sludge/straw mixed fuels campaigns and a straw reference campaign were compared. Experiments were conducted on two different LT-CFBs with thermal capacities of 100kW and 6MW, respectively. The assessment included: (i) Elemental composition and recovery of key elements and heavy metals; (ii) content of total carbon (C) and total nitrogen (N); (iii) pH; (iv) water extractability of phosphorus after incubation in soil; and (v) plant phosphorus response measured in a pot experiment with the most promising ash material. Co-gasification of straw and sludge in LT-CFB gasifiers produced ashes with a high content of recalcitrant C, phosphorus (P) and potassium (K), a low content of heavy metals (especially cadmium) and an improved plant P availability compared to the mono-sludge ashes, thereby showing the best fertilizer qualities among all assessed materials. It was also found that bottom ashes from the char reactor contained even less heavy metals than cyclone ashes. It is concluded that LT-CFB gasification and co-gasification is a highly effective way to purify and sanitize sewage sludge for subsequent use in agricultural systems.