Spent coffee grounds by-products and their influence on soil C-N dynamics.

SCG are a bio-waste generated in great amount worldwide which are attractive as soil amendment for their high content of organic matter and nutritive elements. Nevertheless, several studies have shown that soil application of untreated SCG has detrimental agronomic and environmental effects due to their high degradability and content of noxious compounds (phenols, caffeine, and tannins). However, SCG can be valorised, in the frame of circular economy, by extraction of energy and valuable products (carbohydrates, proteins, bio-oil, bio-diesel) and generation of solid by products (biochar, hydrochar, compost) that can be utilized as soil fertilizers and amendments. Therefore, the aim of this work was the characterization of different solid SCG by-products (as second-generation products) and their assessment as effective organic amendments. The novelty of this study is that for the first time 8 different by-products derived from the same SCG were characterized and comparatively evaluated for their impact on the C and N cycles of soil. SCG was collected and treated to generate 8 different SCG by-products (biochars produced at 270 and 400 °C, hydrochars produced at 160 and 200 °C, vermicompost, defatted SCG and biochars produced from defatted SCG at 270 and 400 °C). SCG and derived by-products were characterized for SEM micromorphology, pH and EC values, and C, N, H, O, volatile matter, fixed C, LOI, carbonates, water soluble C and N, NO3- and NH4+ content. SCG and SCG by-products assessment as organic amendments was performed with an incubation experiment. The residues were added (2.5%) to a moist Mediterranean agricultural soil and the amended soil samples were placed in mesocosms and incubated at 20 °C for 30 days. During incubation, CO2 and N2O emissions were measured every 6 h by means of a gas chromatography automated system for GHG sampling and measurement. The percentage of added C remaining (CR) in the soil was calculated by fitting the cumulative respiration of amended soil to a two-pool model. After 2, 7 and 30 days of incubation, the control and amended soils were sampled and analyzed for their content of extractable organic C, N, NO3- and NH4+ and microbial biomass C and N. Results showed that SCG by-products presented a great variability in their properties. SCG and hydrochars presented higher contents in volatile matter and water soluble C and N, and low content of fixed C, while biochars showed an opposite behaviour. SEM images confirmed the different characteristics of the SCG by-products: the biochar presented a porous structure, honeycomb-like form, due to the loss of the more soluble compounds, while the SCG and hydrochars' pores were filled with amorphous carbonaceous materials. Consequently, soil addition of SCG by-products showed a distinct impact on C and N cycle and microbial biomass content. Addition of SCG and hydrochars generated the highest cumulative CO2-C emissions (2103-2300 µg g-1), the lower amount of CR (86.8-88.6%), increased the soil extractable organic C and microbial biomass C and N and caused N immobilization. On the other hand, the addition of biochars generated lower CO2-C emissions (542-1060 µg g-1), higher amounts of CR (96. 3-99.9%) and lower amounts of extractable compounds and microbial biomass C and N, generating also N immobilization, but to a lesser extent. The addition of vermicompost generated 723 µg g-1 of CO2-C and 98% of CR remaining. However, this by-product did not generate N immobilization being able to act as N fertilizer. None of the residues generated N2O emissions. The different properties of the SCG by-products and their impact on C and N cycle indicated that they can be effectively applied to soil to exert different agronomical and environmental functions.

Biochar, compost and biochar-compost blend as options to recover nutrients and sequester carbon.

This work assessed the potential environmental impact of recycling organic materials in agriculture via pyrolysis (biochar) and composting (compost), as well its combination (biochar-compost blend) versus business-as-usual represented by mineral fertiliser. Life cycle assessment methodology was applied using data sourced from experiments (FP7 project Fertiplus) in three countries (Spain, Italy and Belgium), and considering three environmental impact categories, (i) global warming; (ii) acidification and (iii) eutrophication. The novelty of this analysis is the inclusion of the biochar-compost blend with a focus on multiple European countries, and the inclusion of the acidification and eutrophication impact categories. Biochar, compost and biochar-compost blend all resulted in lower environmental impacts than mineral fertiliser from a systems perspective. Regional differences were found between biochar, compost and biochar-compost blend. The biochar-compost blend offered benefits related to available nutrients and sequestered C. It also produced yields of similar magnitude to mineral fertiliser, which makes its acceptance by farmers more likely whilst reducing environmental impacts. However, careful consideration of feedstock is required.

Biochar amendment before or after composting affects compost quality and N losses, but not P plant uptake.

We investigated the use of biochar (10% on a dry weight basis) to improve the composting process and/or the compost quality by adding it to either the feedstock mixture or the mature compost. The addition of biochar to the feedstocks was essayed in a full scale trial using a mixture of green waste and the organic fraction of municipal solid waste. Addition of biochar to mature compost was performed in a medium scale experiment. The use of biochar, even in small amounts, changed the composting process and the properties of the end products. However these effects depended on the time of application. We observed a faster decomposition in the bio-oxidative phase and lower greenhouse gas emissions when biochar was added at the beginning of the composting process, and a reduction in readily available P when biochar was applied during compost storage. Biochar as a means to increase the C content of the compost was only effective during compost storage. The P fertilizer replacement value of the compost with and without biochar was tested in a plant trial with annual ryegrass. While there was a clear effect on readily available P concentrations in the compost, adding biochar to the feedstock or the compost did not affect the P fertilizer replacement value.

Long-term effects of gasification biochar application on soil functions in a Mediterranean agroecosystem: Higher addition rates sequester more carbon but pose a risk to soil faunal communities.

Biochar applications can have important implications for many of the soil functions upon which agroecosystems rely, particularly regarding organic carbon storage. This study evaluated the impacts of adding a highly aromatic gasification biochar at different rates (0, 12 and 50 t ha-1) to a barley crop on the provision of crucial soil functions (carbon sequestration, water content, greenhouse gas emissions, nutrient cycling, soil food web functioning, and food production). After natural ageing in the field for six years, a wide range of soil properties representative of the studied soil functions were measured and integrated into a soil quality index. Results showed that C sequestration increased with biochar rate (23 and 68% higher than in the control for the 12 and 50 t biochar ha-1 treatments, respectively). Water content was enhanced at the 50 t ha-1 treatment depending on the sampling date. Despite biochar additions neither abating nor increasing CO2 equivalent emissions (carbon dioxide plus nitrous oxide and methane), the system shifted from being a methane sink (-0.017 ± 0.01 mg CH4-C m-2 h-1 at the 12 t ha-1 treatment), to a net source (0.025 ± 0.02 mg CH4-C m-2 h-1 at the 50 t ha-1 treatment). In addition, biochar ageing provoked a loss of nitrate mitigation potential, and indeed ammonium production was stimulated at the 50 t ha-1 rate. The 50 t ha-1 treatment also adversely affected nematode and collembolan functional diversity. Lastly, biochar did not affect barley yield. The results of the soil quality index indicated that no biochar treatment provided more benefits to our agricultural soil, and, although the 50 t ha-1 treatment increased C sequestration, this was potentially offset by its harmful effects on soil faunal communities. Therefore, application of this biochar at high rates should be avoided to prevent risks to soil biological communities.

Fresh biochar application provokes a reduction of nitrate which is unexplained by conventional mechanisms.

Soil-applied biochar has been reported to possess the potential to mitigate nitrate leaching and thus, exert beneficial effects beyond carbon sequestration. The main objective of the present study is to confirm if a pine gasification biochar that has proven able to decrease soil-soluble nitrate in previous research can indeed exert such an effect and to determine by which mechanism. For this purpose, lysimeters containing soil-biochar mixtures at 0, 12 and 50 t biochar ha-1 were investigated in two different scenarios: a fresh biochar scenario consisting of fresh biochar and a fallow-managed soil, and an aged biochar scenario with a 6-yr naturally aged biochar in a crop-managed soil. Soil columns were assessed under a mimicked Mediterranean ambient within a greenhouse setting during an 8-mo period which included a barley crop cycle. A set of parameters related to nitrogen cycling, and particularly to mechanisms that could directly or indirectly explain nitrate content reduction (i.e., sorption, leaching, microbially-mediated processes, volatilisation, plant uptake, and ecotoxicological effects), were assessed. Specific measurements included soil solution and leachate ionic composition, microbial biomass and activity, greenhouse gas (GHG) emissions, N and O isotopic composition of nitrate, crop yield and quality, and ecotoxicological endpoints, among others. Nitrate content reduction in soil solution was verified for the fresh biochar scenario in both 12 and 50 t ha-1 treatments and was coupled to a significant reduction of chloride, sodium, calcium and magnesium. This effect was noticed only after eight months of biochar application thus suggesting a time-dependent process. All other mechanisms tested being discarded, the formation of an organo-mineral coating emerges as a plausible explanation for the ionic content decrease.

Evaluation of extracted organic carbon and microbial biomass as stability parameters in ligno-cellulosic waste composts.

Extracted organic C and microbial biomass were evaluated as stability parameters in 3 different ligno-cellulosic waste composts. Organic C was extracted by both water and alkali and further separated in humic-like carbon (HLC) and nonhumic carbon (NHC). Conventional humification parameters, such as humification index and degree of humification were calculated from NHC and HLC. Microbial biomass carbon (B(C)) was determined as an indicator of the degree of biochemical transformation, whereas ninhydrin reactive N (B(NIN)) was measured to obtain the stability parameter B(NIN)/N(TOT) (N(TOT), total N). The water-extracted organic C did not provide reliable information on the transformations underwent by the ligno-cellulosic wastes during composting, since its content remained almost unaltered during the whole process. In contrast, parameters based on the alkali-extracted organic C and microbial biomass clearly reflected organic matter (OM) changes during the process. There was an increase in the net amount of HLC in the alkali extracts throughout composting, especially in the first 7 to 12 wk of the process, as well as a relative enrichment of HLC with respect to NHC. Values of humification index and degree of humification in end products were consistent with an adequate level of compost stability. The stability parameter B(NIN)/N(TOT) showed to be a reliable indicator of stability in ligno-cellulosic wastes. Parameters based on the alkali-extracted C and microbial biomass clearly reflected the transformation of the OM during composting and can be used as stability parameters in ligno-cellulosic waste composts.

An integrated chemical, thermal, and microbiological approach to compost stability evaluation.

The evaluation of compost stability is of the utmost importance for the reliability of composting as a recycling strategy. To date there is no single parameter that can give a sure indication of the stability of composts from different starting materials. This paper investigates different methods of evaluating the dynamics of transformation of materials and the stability level of the end products in a composting process. The following parameters were determined on compost samples of different ages from cotton (Gossypium herbaceum L.) cardings and yard wastes: humification index (HI), degree of humification (DH), thermogravimetry (TG) microbial biomass C (B(C)), and ninhydrin-reactive N (B(NIN)). Finally, from TG, derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) thermal stability parameters were deduced. Humification parameters in the end products (0.2 and 81% for HI and DH, respectively) showed the effective stability reached by the organic matter (OM). Thermal analysis evidenced the presence of two main organic pools with different thermal stability. During composting a relative increase in the more stable organic pool was indicated by the variation of the thermostability index R1 from 0.41 to 0.74. The parameter R1 was significantly correlated with both HI (r = -0.94; P < 0.05) and DH (r = 0.97; P < 0.05). Microbial biomass content dynamics reflected the availability of readily decomposable substrates. The ratio between B(NIN) and total N in the end product was 0.96%, indicating a good stability level. The simultaneous application of different approaches, considering different properties of composting materials, provides a more complete description of the stability and quality reached by the organic materials.

Biochemical changes and GHG emissions during composting of lignocellulosic residues with different N-rich by-products.

Nitrogen availability plays a critical role in the biodegradation of organic matter during composting. Although the optimal initial C/N is known to be around 25-30, the chemical form in which N is present influences microbial activity and therefore degradation rate and gaseous losses. This study was conducted to evaluate the influence of N availability on the composting of a mixture of lignocellulosic materials. Three composting piles were made of a mixture of wheat straw and cotton waste, each pile containing different N-rich animal by-products. The evolution of the main physico-chemical parameters was monitored (temperature, pH, electrical conductivity, C/N, NH(4)(+), NO(3)(-), water soluble C and N) as well as the enzymatic activity related to the cycle of the main nutrients (ß-glucosidase, protease, alkaline phosphatase and fluorescein diacetate hydrolysis). Additionally, fluxes of CO(2), CH(4) and N(2)O emitted from the composting piles were measured by the closed-chamber technique. Cumulative CO(2) emissions were fitted to five different kinetic models with biological significance to C mineralization data. The application of the different N-rich residues had a significant effect on the C and N dynamics during composting. However, most enzymatic activities followed similar patterns in the three piles. The major CO(2) fluxes were recorded during the thermophilic phase, showing a direct relationship with temperature peaks. No CH(4) fluxes were detected for any of the composting piles during the whole trial, whereas low N(2)O emissions were found at the early beginning and during the maturation stage.

A simple automated system for measuring soil respiration by gas chromatography.

An automated dynamic closed chamber system for CO(2) sampling and analysis was developed for the measurement of soil respiration under laboratory conditions. The system is composed by a gas chromatograph linked to a fully computerised sampling system composed by 16 sample jars and 2 multiposition valves. Besides CO(2), the system can automatically and simultaneously measure CH(4), N(2)O and other gases of environmental interest. The detection limits of the system for CO(2), N(2)O and CH(4) were 2, 1 and 4ppmv, respectively. The accuracy of the system, expressed as percent bias, was -0.88, -0.94 and -3.17% for CO(2), N(2)O and CH(4), respectively, with relative standard deviation of 0.42, 0.68 and 0.61%. Measurement of CO(2) evolved following acidification of a known amount of reagent grade CaCO(3) showed a standard recovery of 96.8+/-2.5% reached within 30s after acidification. A linear response of CO(2) respiration was obtained for a wide range of operative conditions (5-60min accumulation time, 10-80g soil sample size, 10-60mLmin(-1) air flow rate, 15-25 degrees C temperature of incubation) demonstrating the flexibility of the system, which allows for the measurement of soil samples characterised by different rates of gas evolution. Moreover, the results obtained with soil samples showed that within the above conditions the proposed system is not affected by potential limitations of static closed chamber systems such as CO(2) dissolution in the soil solution, reduced rate of CO(2) diffusion from soil to headspace and CO(2) inhibition of microbial activity. The system was also capable to detect significant changes in N(2)O emissions from soil amended with different amounts of glutamic acid. The automatic and frequent measurements provided by the system make possible an accurate description of the dynamics of gas evolution from soil samples under laboratory conditions.

Plant and animal wastes composting: effects of the N source on process performance.

The aim of this work was to evaluate the impact of different N-rich animal wastes on the composting of ligno-cellulosic wastes by a range of classical and novel methods, with particular emphasis on microbial community composition. Two composting mixtures were prepared by adding to a mixture of cotton carding wastes and wheat straw: (i) meat and bone meal and (ii) blood meal and horn and hoof meal. Composts were analyzed using physico-chemical and biochemical properties, as well as nucleic acid microarrays. Results showed that physico-chemical and biochemical parameters differentiated composts depending on their degree of stability, while microarray hybridization discriminated compost samples according to the starting materials used in the compost production. Microarray analysis indicated not only the presence in the composts of bacteria involved in N(2) fixation and plant disease suppression, but also the presence of Acinetobacter calcoaceticus that is suspected to trigger an autoimmune response related to bovine spongiform encephalopathy. The present work highlights the importance of using parameters addressing different properties of the composting matrix for a proper evaluation of the process performance.

Biofiltration at composting facilities: effectiveness for bioaerosol control.

Biofiltration was evaluated as a method to control the airborne microorganisms released at composting facilities. Seven commercial composting plants were selected for this study because of their different operating conditions and biofilter designs. In all plants, the biofilters were originally designed for odor control. The concentrations of both Aspergillus fumigatus and mesophilic bacteria were measured in the air stream before and after passing through the biofilters and compared with the background concentrations in the surrounding area. Results showed that biofiltration achieved an average reduction greater than 90% and 39% in the concentrations of A. fumigatus and mesophilic bacteria, respectively. In all the plants, the airborne A. fumigatus concentration after the biofilter was lower than 1.2 x 10(3) cfu m(-3), independent of the inlet concentration, whereas the mesophilic bacteria concentration was dependent on the inlet concentration. The different behaviors of the two microorganism groups were thought to be due to the different aerodynamic characteristics of the particles that affected the capture by impact in the biofilter bed. The fungus, whose spores had a maximum of diameter size distribution between 2.1 and 3.3 microm, were more effectively captured in the biofilter than the bacteria, which had diameters mainly between 1.1 and 2.1 microm.