• Energy Research

AbstractAmong strategies suggested to decrease agricultural soil N2O losses, the use of nitrification inhibitors such as DMPP (3,4-dimethylpyrazole phosphate) has been proposed. However, the efficiency of DMPP might be affected by soil amendments, such as biochar, which has been shown to reduce N2O emissions. This study evaluated the synergic effect of a woody biochar applied with DMPP on soil N2O emissions. A incubation study was conducted with a silt loam soil and a biochar obtained from Pinus taeda at 500 °C. Two biochar rates (0 and 2% (w/w)) and three different nitrogen treatments (unfertilized, fertilized and fertilized + DMPP) were assayed under two contrasting soil water content levels (40% and 80% of water filled pore space (WFPS)) over a 163 day incubation period. Results showed that DMPP reduced N2O emissions by reducing ammonia-oxidizing bacteria (AOB) populations and promoting the last step of denitrification (measured by the ratio nosZI + nosZII/nirS + nirK genes). Biochar mitigated N2O emissions only at 40% WFPS due to a reduction in AOB population. However, when DMPP was applied to the biochar amended soil, a counteracting effect was observed, since the N2O mitigation induced by DMPP was lower than in control soil, demonstrating that this biochar diminishes the efficiency of the DMPP both at low and high soil water contents.

In the Mediterranean arid region such as Southeast (SE) Spain, a considerable part of the fluvial network runs permanently dry. Here, many dry watercourses are embedded in catchments where agriculture has brought changes in carbon (C) and nitrogen (N) availability due to native riparian vegetation removal and the establishment of intensive agriculture. Despite their increasing scientific recognition and vulnerability, our knowledge about dry riverbeds biogeochemistry and environmental drivers is still limited, moreover for developing proper management plans at the whole catchment scale. We examined CO2 and N2O emissions in five riverbeds in SE Spain of variable agricultural impact under dry and simulated rewetted conditions. Sediment denitrifying capacity upon rewetting was also assessed. We found that, regardless of agricultural impact, all riverbeds can emit CO2 under dry and wet conditions. Emissions of N2O were only observed in our study when a long-term rewetting driving saturated sediments was conducted. Besides, most biogeochemical capabilities were enhanced in summer, reflecting the sensitiveness of microbial activity to temperature. Biogeochemical processing variation across rivers appeared to be more controlled by availability of sediment organic C, rather than by agriculturally derived nitrate. We found that the studied dry riverbeds, agriculturally affected or not, may be active sources of CO2 and contribute to transitory N2O emissions during rewetting phenomena, potentially through denitrification. We propose that management plans aiming to support ecosystem biogeochemistry through organic C from native vegetation rather than agricultural exudates would help to reduce anthropogenic greenhouse gases emissions and excess of nutrients in the watershed and to control the nitrate inputs to coastal ecosystems.

Olive mill wastes represent an important environmental problem in Mediterranean areas where they are generated in huge quantities in short periods of time. Their high phenol, lipid and organic acid concentrations turn them into phytotoxic materials, but these wastes also contain valuable resources such as a large proportion of organic matter and a wide range of nutrients that could be recycled. In this article, recent research studies for the valorisation of olive mill wastes performed by several authors were reviewed: second oil extraction, combustion, gasification, anaerobic digestion, composting and solid fermentation are some of the methods proposed. Special attention was paid to the new solid waste generated during the extraction of olive oil by the two-phase system. The peculiar physicochemical properties of the new solid waste, called two-phase olive mill waste, caused specific management problems in the olive mills that have led to the adaptation and transformation of the traditional valorisation strategies. The selection of the most suitable or appropriate valorisation strategy will depend on the social, agricultural or industrial environment of the olive mill. Although some methods are strongly consolidated in this sector, other options, more respectful with the environment, should also be considered.

ABSTRACTThe interaction of biochar with mineral fertilization has attracted attention as a strategy to reduce N losses and enhance nitrogen use efficiency. In this study, we investigated the coapplication of biochar with two optimized fertilization strategies based on split urea and a microbial inoculant (Azospirillum brasilense) in a commercial pointed white cabbage crop. Additionally, we evaluated a third optimized N fertilization alternative, a biochar‐based fertilizer (BBF) enriched in plant‐available N, which was developed from the same biochar. We assessed environmental impacts such as greenhouse gasses (GHG) and NH3 emissions, yield‐scaled N2O emissions, and global warming potential (GWP). Additionally, we evaluated agronomical outcomes such as crop yield, plant N, and chlorophyll concentration. Moreover, we examined the N‐fixing gene's total and relative abundance (nifH and nifH/16S). Biochar and BBF exhibited similar crop yield, GHG, and NH3 emissions compared to split applications of the synthetic fertilizer. The main difference was associated with the higher soil C sequestration in biochar and BBF treatments that reduced the associated GWP of these fertilization strategies. Finally, biochar favored the activity of the N‐fixing bacteria spread, compared to the sole application of bacteria and BBF demonstrated a promoting effect in the soil's total abundance of natural N‐fixing bacteria.

Soil organic matter comprises all dead plant and animal residues, from the most recent inputs to the most intensively humified. We have found that traces of fresh substrates at microg g(-1) soil concentrations (termed 'trigger molecules') activate the biomass to expend more energy than is contained in the original 'trigger molecules'. In contrast, we suggest that the rate limiting step in soil organic matter mineralisation is independent of microbial activity, but is governed by abiological processes (which we term the Regulatory Gate theory). These two findings have important implications for our understanding of carbon mineralisation in soil, a fundamental process in the sequestration of soil organic matter.

More than two thirds of global nitrous oxide (N2O) emissions originate from soil, mainly associated with the extensive use of nitrogen (N) fertilizers in agriculture. Although the interaction of black carbon with the N cycle has been long recognized, the impact of biochar on N2O emissions has only recently been studied. Herein we reflect on proposed hypotheses to explain N2O decrease with biochar, linking them to specific mechanisms for N2O formation and consumption in soil. Moreover, to assist in elucidating key mechanisms in which biochar may act in mitigating emissions of N2O, we undertook a meta-analysis using published literature from 2007 to 2013. This quantitative analysis used 30 studies with 261 experimental treatments. Overall, we found that biochar reduced soil N2O emissions by 54% in laboratory and field studies. The biochar feedstock, pyrolysis conditions and C/N ratio were shown to be key factors influencing emissions of N2O while a direct correlation was found between the biochar application rate and N2O emission reductions. Interactions between soil texture and biochar and the chemical form of N fertilizer applied with biochar were also found to have a major influence on soil N2O emissions. While there is clear evidence that, in many cases, emissions of N2O are reduced, there is still a significant lack in understanding of the key mechanisms which result in these changed emissions. As such, we have guided readers with suggestions to address specific research gaps, which we anticipate will enhance our knowledge and understanding of biochar's N2O emission mitigation potential.

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.