The impact of biogas digestate typology on nutrient recovery for plant growth: Accessibility indicators for first fertilization prediction.

In recent years, anaerobic digestion of organic waste (OW) is rapidly appearing as a winning waste management strategy by producing energy and anaerobic digestates that can be used as fertilizers in agricultural soils. In this context, the management of the OW treatment process to maximize agro-system sustainability satisfying the crop nutrient demands represents the main goal. To investigate these traits, two protocols to assess the plant availability of digestate nitrogen (N) and phosphorus (P) were evaluated. With this aim, the N and P availability was determined on 8 digestates and 2 types of digestate-based compost from different OW via sequential chemical extractions (SCE). In addition, the digestates were tested in soil incubations and in plant pot tests with Italian ryegrass and compared with chemical fertilizer and a non-amended control soil. The N extracted from digestates via SCE was related to soil N mineralization and plant N recovery. The C: N ratio had negative impact on mineralized N and its recovery in shoots (ShootsN = -0.0085.(C/N)+0.172, r2 = 0.67), whereas water extractable mineral N was positevely related to the root N apparent recovery fraction (N-ARF) with (RootsN = 5E-5.Nsolublemin+0.0138, r2 = 0.53). The shoot P-ARF was positively correlated with the inorganic water extractable fraction of P (ShootsP =0.1153.H2O-Pi-0.2777.H2O-Po+0.0249, r2 = 0.71) whereas the root P-ARF was positively correlated with the less accessible fractions (RootsP = (b)   0.0955.NaHCO3-Po+0.0955.NaOH-Po-0.0584NaHCO3-Pi+0.0128, r2 = 0.8641). Feedstock digestate typology impacted the N and P recovery results leading to a better description of the typology properties and a first nutrients ARF prediction.

Fertilizing potential and CO2 emissions following the utilization of fresh and composted food-waste anaerobic digestates.

Wet and dry-batch anaerobic digestion, and composting are common technologies in food waste (FW) management, resulting in different outputs. However, the effects of composting on carbon dioxide (CO2) emissions, nitrogen (N) and phosphorus (P) fertilizing capacity in view of closing nutrient cycle are still poorly investigated. In this work, two FW anaerobic digestates from the wet (D1) and dry-batch process (D2), and their respective composts (C1 and C2) were tested in a soil incubation (84 days at 25 °C) to assess CO2 emissions in comparison with a mixed (animal slurry/energy crop) digestate (BD) and a reference municipal solid waste compost (MSWC). The same products were also tested for the relative P efficiency (RPE) in soil, in comparison with a chemical-P source (30 mg P kg-1). Lastly, the apparent recovery fraction of N (N-ARF) from the five organic products was determined in a pot test with ryegrass (84 days; 300 kg available N ha-1), compared to a chemical fertilizer (NPK). Composting strongly reduced net-CO2 emissions compared to the two digestates (625 vs. 2850 mg CO2 kg-1 soil). Oppositely, composting very modestly influenced RPE that ranged around 100-90% in D1 and C1, and ≈30% in D2 and C2. Moreover, composting did not significantly reduce N-ARF that ranked in descending order as follows: NPK (77.5%) > D1 = BD (17.7%) ≥ C1 (14.7%) > MSWC (3.6%) > D2 (1.2%) > C2 (-3.1%). Composting was shown a reliable strategy for FW digestate management, as it reduces potential CO2 emission without affecting these products' N- and P-fertilizing capacity.

Organic wastes as alternative sources of phosphorus for plant nutrition in a calcareous soil.

Recycled organic wastes (OW) can be a valuable P source; however, their P-fertilising capacity is still poorly known. In this study, we selected three anaerobic digestates [wastewater sludge (D1), winery sludge (D2), and bovine-slurry/energy crops (BD)] and two animal effluents [bovine slurry (BS) and swine slurry (SS)] to test their P-release and P-fertilising capacities via sequential chemical extraction (SCE), X-ray diffraction (XRD), and 31P-nuclear magnetic resonance (31P NMR). Subsequently, the three digestates (30 mg P kg-1 of soil) were compared for the release of Olsen-P during a soil incubation and for plant-P apparent recovery (ARF) in a pot experiment using ryegrass (112 days) in a soil with poorly available-P (Olsen-P < 5 mg kg-1), under a non-limiting N environment. The amount of labile-P (H2O + NaHCO3), as determined from SCE, related well to the Olsen-P following OW addition to the soil. It was shown via 31P NMR spectroscopy that orthophosphate was the leading P-form in highly P-releasing OW. The amount of labile-P, however, was affected by soil adsorption, thereby reducing plant-P uptake. The plant-P ARF (%) showed that the recycled P-sources were clustered in highly (BD and SS: ≈20%), intermediately (D1 and BS: ≈15%), and poorly performing OWs (D2: ≈10%) vs. chemical P-source (P-chem: 20%). Therefore, only BD and SS were effective alternatives to P-chem; however, the other OW can be efficient P-sources in soils with higher Olsen-P. Thus, crop fertilisation can be tailored on a P-basis by SCE as a function of soil adsorption capacity and on an N-basis according to the demand.

Current and residual phosphorous availability from compost in a ryegrass pot test.

Compost can provide nitrogen (N) and especially phosphorous (P) available for plant growth, thus representing a potential alternative to chemical P-fertilizers a non-renewable resource. However, little is known about their residual capacity to provide plant-available P. In this study four compost: a green waste compost (GWC), one from anaerobically-digested bio-waste (DC), one from sewage sludge (SSC), and one from bio-waste (BWC), were compared (10 and 20 Mg VS ha-1) in a ryegrass pot test (112 days), for their N- and P-relative mineral fertilizer equivalence (MFE; %) vs. a chemical fertilizer (NPK). After the test period, the exploited treatments were tested for their MFE during an additional ryegrass growth cycle (112 days) in an N-rich environment (N+). After 112 days, the pot test showed that DC and SSC produced dry biomass in the same range as did NPK, attaining the best N-MFE (80-100%) and P-MFE (100-125%), whereas GWC and BWC performed poorly (60-80 and 80-90%; N-MFE and P-MFE). At the end of the first growth cycle, DC and SSC still showed relevant Olsen-P (20-30 mg kg-1). This was reflected in the best ryegrass P-MFE in DC and SSC at the end of the second growth cycle (N+), after 224 days (100-110%), whereas BWC and GWC poorly performed (90-95%). DC and SSC may therefore represent valuable sources of N available for plant nutrition in the short term, and also represent medium-term valuable P sources, alternative to rock phosphate P fertilizers. This promising approach need further field-scale investigation to confirm the medium-long term capacity of composts to be alternative to rock phosphate P fertilizers.

Effect of iron sulphate on the phosphorus speciation from agro-industrial sludge based and sewage sludge based compost.

Composting is considered a suitable process for organic waste management, providing stable products that can be safely utilized as fertilizers, but little is still known about the variation of phosphorous (P) extractability during the stabilization process. In this work, sequential chemical extraction (SCE) with increasing strength extractants (H2O; 0.5M NaHCO3 pH 8.5; 0.1M NaOH, 1M HCl) was applied for P speciation over 56days of composting of either agro-industrial or urban wastewater sludge with green waste treated (AICFe+; SSCFe+) or not (AICFe-; SSCFe-) with FeSO4 (2%v/v). Composting strongly reduced the H2O-P, promoting the organic-P (Po) mineralization from the labile fraction (H2O+NaHCO3 40%), in addition to the increases of NaHCO3- and HCl-extractable inorganic-P (Pi) in both AICFe- and SSCFe- (+20% on average). The FeSO4 treatment did not negatively affect the process, reducing the Po mineralization during composting by increasing the NaOH-P, also protecting this fraction from fixation in the sparingly soluble fraction. The final P fractionation (%) was in AICFe-: NaOH (41)=NaHCO3 (38)>HCl (18)>H2O (3); in AICFe+: NaOH (53)>NaHCO3 (24)=HCl (22)>H2O (2); in SSCFe-: NaOH (46)>NaHCO3 (29)>HCl (21)>H2O (4) and in SSCFe+: NaOH (66)>NaHCO3 (13)>HCl (20)>H2O (1). Composting reduced the more easily leachable fraction (labile-Po), reducing the risk of P loss by increasing the long-term available P fraction (NaOH-P). This was enhanced by the FeSO4 addition. Further investigation into soil behaviour and plant availability of P from this source is needed.

Phosphorous extractability and ryegrass availability from bio-waste composts in a calcareous soil.

In this work four stable bio-waste composts (C1, C2, C3, C4) were selected on the basis of their increasing water soluble P (H2O-P). The P speciation was assessed via sequential chemical extraction (SCE) on the same products. Moreover, the plant-available P was assessed via apparent recovery fraction approach (ARF) in a pot test on ryegrass over 21 weeks at 15 mg P kg-1 of soil. An inorganic P source (P-chem) was added as a reference at the same P rate in addition to a non-fertilized control (Control). SCE showed that the sparingly soluble P (HCl-P) was the most important fraction in all composts: C1 (HCl 65% > NaHCO3 17% = NaOH 17% > H2O 1%); C2: (HCl 51% > NaOH 23% > NaHCO3 18% > H2O 7%); C3: (HCl 58% > NaOH 21% > NaHCO3 12% > H2O 9%); C4: (HCl 39% > NaOH 23% > NaHCO3 22% > H2O 16%). The plant test showed that the different treatments had a different ARF (%) at the first harvest: P-chem (14.7)> C4 (14.4)> C3 (14.1)> C2 (3.4)>C1 (3.1), compared to the cumulated ARF (%) of the six harvests: C4 (50.1)> C3 (35.0)> C1 (21.1)> C2 (18.3)> P-chem (17.4). Data showed a good correlation of H2O-P vs. plant ARF at the first harvest and a good correlation of labile P (H2O-P + NaHCO3-P) vs. total plant ARF over 21 weeks. The free and labile P forms from SCE can be a valuable tool in the assessment of fast and middle term plant-available P from stable bio-waste composts in calcareous soils.

Relationships between stability, maturity, water-extractable organic matter of municipal sewage sludge composts and soil functionality.

Compost capability of restoring or enhancing soil quality depends on several parameters, such as soil characteristics, compost carbon, nitrogen and other nutrient content, heavy metal occurrence, stability and maturity. This study investigated the possibility of relating compost stability and maturity to water-extractable organic matter (WEOM) properties and amendment effect on soil quality. Three composts from municipal sewage sludge and rice husk (AN, from anaerobic wastewater treatment plants; AE, from aerobic ones; MIX, from both anaerobic and aerobic ones) have been analysed and compared to a traditional green waste compost (GM, from green manure, solid waste and urban sewage sludge). To this aim, WEOMs were characterized through chemical analysis; furthermore, compost stability was evaluated through oxygen uptake rate calculation and maturity was estimated through germination index determination, whereas compost impact on soil fertility was studied, in a lab-scale experiment, through indicators as inorganic nitrogen release, soil microbial biomass carbon, basal respiration rate and fluorescein di-acetate hydrolysis. The obtained results indicated that WEOM characterization could be useful to investigate compost stability (which is related to protein and phenol concentrations) and maturity (related to nitrate/ammonium ratio and degree of aromaticity) and then compost impact on soil functionality. Indeed, compost stability resulted inversely related to soil microbial biomass, basal respiration rate and fluorescein di-acetate hydrolysis when the products were applied to the soil.

Effects of hydrothermal pre-treatments on Giant reed (Arundo donax) methane yield.

Twelve hydrothermal pre-treatment combinations of temperature (150 and 180 °C), time (10 and 20 min) and acid catalyst (no catalyst; H2SO4 at 2% w/w immediately before steam cooking or in 24-h pre-soaking) were tested to assess their effects on methane yield of Giant reed biomass vs. untreated control. A batch anaerobic digestion was conducted with 4 g VS l(-1) at 53 °C for 39 days. Untreated biomass exhibited a potential CH4 yield of 273 ml g(-1) VS; the four pre-treatments without acid catalyst achieved a 10%, 7%, 23% and 4% yield gain in the respective temperature/time combinations 150 °C/10 min, 150 °C/20 min, 180 °C/10 min and 180 °C/20 min. Conversely, the eight pre-treatments with H2SO4 catalyst incurred a methanogenic inhibition in association with high SO4(2-) concentration in the hydrolysate, known to enhance sulphate reducing bacteria. Furfurals were also detected in the hydrolysate of five strong pre-treatments with H2SO4 catalyst.

The evaluation of stability during the composting of different starting materials: comparison of chemical and biological parameters.

Three blends formed by: (i) food processing waste (CP(FP)), (ii) waste water sewage sludge (CP(WW)), and (iii) their mixture (CP(FP+WW)), blended with tree pruning as bulking agent, were composted over 3 months. During composting the blends were monitored for the main physical-chemical characteristics: temperature, oxygen saturation level (O(2)%), pH, total and volatile solids, total organic carbon, and organic nitrogen (N(org)). In addition to the main parameters, the dissolved organic carbon (DOC), the inorganic nitrogen and the Oxygen Uptake Rate (OUR) were monitored. All the mixtures easily reached a peak temperature around 70°C, related to the lowest O(2)%. After 90 d, CP(FP), CP(FP+WW), and CP(WW) showed an organic matter mineralization of 43%, 35% and 33%, respectively; CP(FP) fitted an exponential model while both CP(FP+WW), and CP(WW) fitted a logistic model. During composting an OUR reduction of 79%, 78% and 73% was registered in CP(FP), CP(FP+WW), and CP(WW), respectively; the OUR successfully fitted the adopted exponential model and well reflected the stabilization process in time. The N(org) recovery at the end of the process was positive only in CP(WW) (11.6%). The DOC significantly decreased during the composting process but did not successfully fit any model. The mineral nitrogen did not follow the typical pattern with NH(4)(+) disappearance and NO(3)(-) accumulation. Strong NO(3)(-) losses were evident in all blends, while NH(4)(+) accumulations were detectable only in CP(FP), and CP(FP+WW). The NH(4)(+)/NO(3)(-) ratio did not satisfactorily reflect the composting process over time. The comparison of the first order (exponential) and logistic (sigmoidal) models applied to the OUR and OM course highlights the role of mineral nitrogen as limiting factor during composting of the more stabilized sludge.

Electrofocusing the compost organic matter obtained by coupling SEC-PAGE.

Humic acids (HA)-like extracted from compost at the beginning (t(0)) and after 130 days of composting (t(130)) were fractionated by coupling size exclusion chromatography to polyacrylamide gel electrophoresis (SEC-PAGE). HA-like fractions with the same molecular size (MS) and electrophoretic mobility were pooled and further characterised by analytical polyacrylamide gel electrofocusing (EF) and compared with HA separated from a Typic Chernozem soil. During the composting process all fractions were subjected to quantitative and qualitative modifications: the high MS fraction was degraded, the mid MS fractions were qualitatively changed, the content of low MS fractions increased and changed qualitatively. The main changes in EF pattern of the non fractionated HA-like t(130) were associated to low MS fractions. Such data seem to be reliable for explanation what mechanisms and monitoring of the evolution of the compost organic matter for their agricultural uses.

Evaluation of photochemical properties of compost humic-like materials.

Humic-like acids (HLA) were extracted from compost at the beginning and after 70, 130 and 730 days of maturation in order to be investigated for their ability to induce the transformation of 2,4,6-trimethylphenol under irradiation at 365 nm. The rate of 2,4,6-trimethylphenol phototransformation in the presence of HLA (25 mg l(-1)) varied within HLA 0<

Compost-based growing media: influence on growth and nutrient use of bedding plants.

The agronomic performance and the mineral composition and trace element content in Begonia semperflorens "Bellavista F1", Mimulus "Magic x hybridus", Salvia splendens "maestro", and Tagete patula xerecta "Zenith Lemon Yellow", were tested by growing the plants on substrates of white peat and 25-50-75-100% green waste and sewage sludge (80%+20%v/v) compost (CP). A commercial peat medium of black and white peat (2:1v/v) was used as control. At flowering, the agronomic parameters were compared by ANOVA and plant nutritional status was compared by vector analysis. Substrate-species interactions (P<0.001) were evident for all measured parameters. In the 25% CP medium all the species showed an increase or preservation of the studied agronomic parameters. Begonia grown in 25% CP, showed the highest dry weight (DW) and number of flowers. Other treatments were comparable to the control. Mimulus and Salvia showed the highest DW in the 25-50% CP. Mimulus, after a DW increase up to 50% CP, showed the steepest reduction as the CP increased further. Tagete showed no differences in DW up to 50% CP, or in flower number up to 25% CP, compared to the control. The additional increases of CP in the medium showed a DW decrease similar to that of Salvia. Vector analysis showed the use of compost mainly induced a decrease of P concentration in tissues, except for Begonia which remained unchanged. Plant tissues showed a general P reduction due to a dilution effect in the low compost mixtures (25-50%) and a deficiency in the higher CP mixtures. In contrast, an increase of Mg in the aboveground tissues of all species was detectable as compost usage increased, with the exception of Salvia which suffered a Mg deficiency. Vector analysis also highlighted a Ni and partial Fe deficiency in Tagete and Salvia.

A multivariate approach to the study of the composting process by means of analytical electrofocusing.

Three blends formed by: agro-industrial waste, wastewater sewage sludge, and their mixture, blended with tree pruning as bulking agent, were composted over a 3-month period. During the composting process the blends were monitored for the main physical and chemical characteristics. Electrofocusing (EF) was carried out on the extracted organic matter. The EF profiles were analyzed by principal component analysis (PCA) in order to assess the suitability of EF to evaluate the stabilisation level during the composting process. Throughout the process, the blends showed a general shifting of focused bands, from low to high pH, even though the compost origin affected the EF profiles. If the EF profile is analyzed by dividing it into pH regions, the interpretation of the results can be affected by the origin of compost. A good clustering of compost samples depending on the process time was obtained by analyzing the whole profile by PCA. Analysis of EF results with PCA represents a useful analytical technique to study the evolution and the stabilisation of composted organic matter.

Evolution of organic matter from sewage sludge and garden trimming during composting.

To use compost appropriately in agriculture it is extremely important to estimate the stabilization level of the organic matter. In this work, two different piles of compost were studied by means of (i) humification parameters (degree of humification--DH, humification rate--HR, humification index--HI) prior to and after enzymatic hydrolysis of the extracted organic carbon, (ii) water-soluble organic carbon (WSOC) and (iii) water-soluble nitrogen. A significant relationship between composting time, WSOC and humification parameters after enzymatic hydrolysis (DHenz; HRenz; HIenz) was found.