María Luz Cayuela

Biochar and denitrification in soils: when, how much and why does biochar reduce N2O emissions?

Agricultural soils represent the main source of anthropogenic N2O emissions. Recently, interactions of black carbon with the nitrogen cycle have been recognized and the use of biochar is being investigated as a means to reduce N2O emissions. However, the mechanisms of reduction remain unclear. Here we demonstrate the significant impact of biochar on denitrification, with a consistent decrease in N2O emissions by 10–90% in 14 different agricultural soils. Using the 15N gas-flux method we observed a consistent reduction of the N2O/(N2 + N2O) ratio, which demonstrates that biochar facilitates the last step of denitrification. Biochar acid buffer capacity was identified as an important aspect for mitigation that was not primarily caused by a pH shift in soil. We propose the function of biochar as an “electron shuttle” that facilitates the transfer of electrons to soil denitrifying microorganisms, which together with its liming effect would promote the reduction of N2O to N2.

Bioenergy by-products as soil amendments? Implications for carbon sequestration and greenhouse gas emissions

An important but little understood aspect of bioenergy production is its overall impact on soil carbon (C) and nitrogen (N) cycling. Increased energy production from biomass will inevitably lead to higher input of its by‐products to the soil as amendments or fertilizers. However, it is still unclear how these by‐products will influence microbial transformation processes in soil, and thereby its greenhouse gas (GHG) balance and organic C stocks. In this study, we assess C and N dynamics and GHG emissions following application of different bioenergy by‐products to soil. Ten by‐products were selected from different bioenergy chains: anaerobic digestion (manure digestates), first generation biofuel by‐products (rapeseed meal, distilled dried grains with solubles), second‐generation biofuel by‐products (nonfermentables from hydrolysis of different lignocellulosic materials) and pyrolysis (biochars). These by‐products were added at a constant N rate (150 kg N ha−1) to a sandy soil and incubated at 20 °C. After 60 days, >80% of applied C had been emitted as CO2 in the first‐generation biofuel residue treatments. For second‐generation biofuel residues this was approximately 60%, and for digestates 40%. Biochars were the most stable residues with the lowest CO2 loss (between 0.5% and 5.8% of total added C). Regarding N2O emissions, addition of first‐generation biofuel residues led to the highest total N2O emissions (between 2.5% and 6.0% of applied N). Second‐generation biofuel residues emitted between 1.0% and 2.0% of applied N, with the original feedstock material resulting in similar N2O emissions and higher C mineralization rates. Anaerobic digestates resulted in emissions <1% of applied N. The two biochars used in this study decreased N2O emissions below background values. We conclude that GHG dynamics of by‐products after soil amendment cannot be ignored and should be part of the lifecycle analysis of the various bioenergy production chains.

Biochar accelerates organic matter degradation and enhances N mineralisation during composting of poultry manure without a relevant impact on gas emissions

A composting study was performed to assess the impact of biochar addition to a mixture of poultry manure and barley straw. Two treatments: control (78% poultry manure + 22% barley straw, dry weight) and the same mixture amended with biochar (3% dry weight), were composted in duplicated windrows during 19 weeks. Typical monitoring parameters and gaseous emissions (CO2, CO, CH4, N2O and H2S) were evaluated during the process as well as the agronomical quality of the end-products. Biochar accelerated organic matter degradation and ammonium formation during the thermophilic phase and enhanced nitrification during the maturation phase. Our results suggest that biochar, as composting additive, improved the physical properties of the mixture by preventing the formation of clumps larger than 70 mm. It favoured microbiological activity without a relevant impact on N losses and gaseous emissions. It was estimated that biochar addition at 3% could reduce the composting time by 20%.

Residues of bioenergy production chains as soil amendments: Immediate and temporal phytotoxicity

The current shift towards bioenergy production increases streams of bioenergy rest-products (RPs), which are likely to end-up as soil amendments. However, their impact on soil remains unclear. In this study we evaluated crop phytotoxicity of 15 RPs from common bioenergy chains (biogas, biodiesel, bioethanol and pyrolysis). The RPs were mixed into a sandy soil and the seedling root and shoot elongation of lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), and wheat (Triticum aestivum L.) were measured. Immediate phytotoxic effects were observed with biodiesel and bioethanol RPs (root elongation reduced to 14-60% for the three crops; P<0.05). However, phytotoxicity was no longer significant after seven days. Digestates had no phytotoxic effect whereas biochars ranged from beneficial to detrimental depending on the original feedstock and temperature of pyrolysis. Biochar amendment alleviated phytotoxicity of bioethanol by-products for wheat and radish. Phytotoxicity assessment is critical for successful soil amendment with bioenergy RPs.

Potential of olive mill waste and compost as biobased pesticides against weeds, fungi, and nematodes

The phytotoxic and antimicrobial properties of olive mill wastes have been widely investigated and demonstrated over the past decade. However, their potential utilization as biodegradable pesticides against plant pathogens is still poorly understood. In this study, a series of laboratory bioassays was designed to test the inhibitory effects of sterile water extracts of two-phase olive mill waste (TPOMW) and TPOMW composts with different degrees of stabilization on several different plant pathogens. Fungicidal properties of TPOMW extracts, assayed in a microwell assay format, showed that the growth of Phytophthora capsici was consistently and strongly inhibited by all TPOMW extracts diluted 1:10 (w:v). In contrast, suppression of Pythium ultimum and Botrytis cinerea by the extracts was not as strong and depended on the specific TPOMW sample. Mature compost inhibited P. capsici and B. cinerea at dilutions as great as 1:50, w:v. Neither TPOMW nor TPOMW compost extracts were able to inhibit the growth of the basidiomycete root rot agent Rhizoctonia solani. In addition, studies were conducted on the allelopathic effects of TPOMW extracts on seed germination of four highly invasive and globally distributed weeds (Amaranthus retroflexus, Solanum nigrum, Chenopodium album and Sorghum halepense). Both the TPOMW and immature TPOMW compost extracts substantially inhibited germination of A. retroflexus and S. nigrum, whereas mature composts extracts only partially reduced the germination of S. nigrum. Finally, TPOMW extracts strongly inhibited egg hatch and second-stage juvenile (J2) motility of the root-knot nematode Meloidogyne incognita. However, only higher concentrations of stage-one and stage-two TPOMW compost extracts exerted a suppressive effect on both J2 motility and on egg hatch. The study shows the high potential of naturally occurring chemicals present in TPOMW and TPOMW composts that should be further investigated as bio-pesticides for their use in sustainable agricultural systems.

Biochar increases soil N2O emissions produced by nitrification-mediated pathways

In spite of the numerous studies reporting a decrease in soil nitrous oxide (N2O) emissions after biochar amendment, there is still a lack of understanding of the processes involved. Hence the subject remains controversial, with a number of studies showing no changes or even an increase in N2O emissions after biochar soil application. Unraveling the exact causes of these changes, and in which circumstances biochar decreases or increases emissions, is vital to developing and applying successful mitigation strategies. With this objective, we studied two soils (Haplic Phaeozem (HP) and Haplic Calcisol (HC)), which showed opposed responses to biochar amendment. Under the same experimental conditions, the addition of biochar to soil HP decreased N2O emissions by 76%; whereas it increased emissions by 54% in soil HC. We combined microcosm experiments adding different nitrogen fertilizers, stable isotope techniques and the use of a nitrification inhibitor (dicyciandiamide) with the aim of improving our understanding of the mechanisms involved in the formation of N2O in these two soils. Evidence suggests that denitrification is the main pathway leading to N2O emissions in soil HP, and ammonia oxidation and nitrifier-denitrification being the major processes generating N2O in soil HC. Biochar systematically stimulated nitrification in soil HC, which was probably the cause of the increased N2O emissions. Here we demonstrate that the effectiveness of using biochar for reducing N2O emissions from a particular soil is linked to its dominant N2O formation pathway.

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.

Composting Olive Mill Waste and Sheep Manure For Orchard Use

An industrial-scale composting plant has been designed for producing organic fertilizers from olive mill waste using the windrow pile system. Materials to be composted, two phase olive mill waste (TPOMW) and sheep litter (SL), were characterized and made into three piles consisting of different proportions of each. Throughout the composting process, temperature (T), moisture (M), organic matter (OM), total organic carbon (Corg), total nitrogen (NT), germination index (GI), pH and electrical conductivity (EC) were monitored. The potential agronomic value of the final composts was ascertained by analyzing the bulk density, OM and Corg concentration, pH, EC, macro and micronutrient content (N, P, K, Ca, Mg, Fe, Cu, Mn, Zn, B), the concentration of humic and fulvic acids and inorganic nitrogen (NH4+,NO2−,NO3−). Each compost was applied to an area of one hectare within a six year-old olive plantation. Four months after application, the soils showed an increased OM concentration and cationic exchange capacity (CEC).

Two-phase olive mill waste composting: enhancement of the composting rate and compost quality by grape stalks addition

Two-phase olive mill waste (TPOMW) is a semisolid sludge generated by the olive oil industry. Its recycling as a soil amendment, either unprocessed or composted, is being promoted as a beneficial agricultural practice in the Mediterranean area. One of the major difficulties when composting TPOMW is the compaction of the material due to its dough-like texture, which leads to an inadequate aeration. For this reason, the addition of bulking agents is particularly important to attain a proper composting process. In this study we followed the evolution of two composting mixtures (A and B) prepared by mixing equal amounts of TPOMW and sheep litter (SL) (in a dry weight basis). In pile B grape stalks (GS) were added (10% dry weight) as bulking agent to study their effect on the development of the composting process and the final compost quality. The incorporation of grape stalks to the composting mixture changed the organic matter (OM) degradation dynamics and notably reduced the total amount of lixiviates. The evolution of several maturation indices (C/N, germination index, water soluble carbon, humification indices, C/N in the leachates) showed a faster and improved composting process when GS were added. Moreover, chemical (NH4+, NO3−, cation exchange capacity, macro and micronutrients, heavy metals) and physical properties (bulk and real densities, air content, total water holding capacity, porosity) of the final composts were analysed and confirmed the superior quality of the compost where GS were added.

Bioenergy from cattle manure? Implications of anaerobic digestion and subsequent pyrolysis for carbon and nitrogen dynamics in soil

Cattle manure can be processed to produce bioenergy, resulting in by‐products with different physicochemical characteristics. To evaluate whether application of such bioenergy by‐products to soils would be beneficial compared with their unprocessed counterpart, we quantified differences in greenhouse gas emissions and carbon (C) and nitrogen (N) dynamics in soil. Three by‐products (15N‐labeled cattle manure, from which anaerobic digestate was obtained, which was subsequently pyrolysed) were applied to a loess and a sandy soil in a laboratory incubation study. The highest losses of soil C from biological activity (CO2 respiration) were observed in manure treatments (39% and 32% for loess and sandy soil), followed by digestate (31% and and 18%), and biochar (15% and and 7%). Emissions of nitrous oxide (N2O) ranged from 0.6% of applied N from biochar to 4.0% from manure. Isotope labeling indicated that manure N was most readily mineralized, contributing 50% to soil inorganic N. The anaerobic digestate was the only by‐product increasing the mineral N pool, while reducing emissions of N2O compared with manure. In biochar treatments, less than 18.3% of soil mineral N derived from the biochar, while it did not constrain mineralization of native soil N. By‐products of anaerobic digestion and pyrolysis revealed soil fertility in addition to environmental benefits. However, the reported advantages lessen when the declining yields of C and N over the bioenergy chain are considered.