Challenging the claims on the potential of biochar to mitigate climate change.

Thesis (PhD) - Wageningen University, Wageningen. Promotors: T. W. Kuyper, E. Hoffland, B. J. M. Arts, Wageningen University. Co-promotor: Etelvino Henrique Novotny, CNPS

Rapid decomposition of traditionally produced biochar in an Oxisol under savannah in Northeastern Brazil.

Soil amendment with biochar has been claimed as an option for carbon (C) sequestration in agricultural soils. Most studies on biochar/soil organic carbon (SOC) interactions were executed under laboratory conditions. Here we tested the stability of biochar produced in a traditional kiln and its effects on the stocks of native SOC under field conditions

A review and meta-analysis of mitigation measures for nitrous oxide emissions from crop residues

Crop residues are of crucial importance to maintain or even increase soil carbon stocks and fertility, and thereby to address the global challenge of climate change mitigation. However, crop residues can also potentially stimulate emissions of the greenhouse gas nitrous oxide (N$_{2}$O) from soils. A better understanding of how to mitigate N$_{2}$O emissions due to crop residue management while promoting positive effects on soil carbon is needed to reconcile the opposing effects of crop residues on the greenhouse gas balance of agroecosystems. Here, we combine a literature review and a meta-analysis to identify and assess measures for mitigating N$_{2}$O emissions due to crop residue application to agricultural fields. Our study shows that crop residue removal, shallow incorporation, incorporation of residues with C:N ratio > 30 and avoiding incorporation of residues from crops terminated at an immature physiological stage, are measures leading to significantly lower N$_{2}$O emissions. Other practices such as incorporation timing and interactions with fertilisers are less conclusive. Several of the evaluated N$_{2}$O mitigation measures implied negative side-effects on yield, soil organic carbon storage, nitrate leaching and/or ammonia volatilization. We identified additional strategies with potential to reduce crop residue N$_{2}$O emissions without strong negative side-effects, which require further research. These are: a) treatment of crop residues before field application, e.g., conversion of residues into biochar or anaerobic digestate, b) co-application with nitrification inhibitors or N-immobilizing materials such as compost with a high C:N ratio, paper waste or sawdust, and c) use of residues obtained from crop mixtures. Our study provides a scientific basis to be developed over the coming years on how to increase the sustainability of agroecosystems though adequate crop residue management

High application rates of biochar to mitigate N2O emissions from a N-fertilized tropical soil under warming conditions.

Biochar application has been suggested as a strategy to decrease nitrous oxide emissions from agricultural soils while increasing soil C stocks, especially in tropical regions. Climate change, specifically increasing temperatures, will affect soil environmental conditions and thereby directly influence soil N2O fluxes

Predicting field N2_{2}O emissions from crop residues based on their biochemical composition: A meta-analytical approach

Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N$_{2}$O). Previous meta-analyses have linked various biochemical properties of crop residues to N$_{2}$O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N$_{2}$O emissions from specific residues. Here we combine comprehensive databases for N$_{2}$O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N$_{2}$O emissions. On average, crop residue incorporation increased soil N$_{2}$O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N$_{2}$O emissions. Crop residue effects on N$_{2}$O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N$_{2}$O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N$_{2}$O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N$_{2}$O emissions, whereas mature residues with opposite characteristics had marginal effects on N$_{2}$O. The most important crop types belonging to the immature residue group – cover crops, grasslands and vegetables – are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N$_{2}$O emissions

Soil pH-increase strongly mitigated N2O emissions following ploughing of grass and clover swards in autumn: A winter field study

Emissions from crop residues contribute largely to the total estimated N2O emissions from agriculture. Since low soil pH increases N2O production by impairing the last denitrification step, liming has been suggested as a mitigation strat- egy; however, it may also increase N2O emissions by enhancing mineralization and nitrification. To gain field-based empirical knowledge, we measured N2O fluxes with an autonomous field-flux robot in limed and control plots before and after autumn ploughing of 3-year-old grass, clover grass or red clover swards under different N fertilization re- gimes. Dolomite applied before establishing the swards raised soil pHCaCl2 from ~4.8 to ~5.8 in limed plots. Higher pH halved emissions from ploughed leys despite higher soil mineral N contents. It also reduced emissions be- fore ploughing. We observed substantial N2O fluxes after ploughing, with peaks during a relatively warm wet period after freezing and higher peaks during diurnal snowmelt over frozen soil. Average N2Ofluxes were strongly positively correlated with high herbage yields in the preceding growing seasons rather than with the presence of clover. The yield-scaled average N2O fluxes were strongest in low pH soils at all yield levels; this was a true effect of soil pH on N2O, as herbage yields were not increased by liming. Here, yield-scaled flux was defined as the average N2O flux after ploughing divided by the dry matter. Fluxes in red clover plots were similar to those in grass plots, despite the lower C/N ratio and higher total amount of N in clover residues. However, clover in mixtures with grass increased yields and N2O emissions. This suggests that higher ley production enhanced microbial activity, including nitrifiers and denitrifiers, and that the pH effect on facilitating complete denitrification to N2 overrode any effect on minerali- zation and nitrification, thus resulting in N2Omitigation

Effects of pyrogenic carbon feedstock and pyrolysis temperature on the oxidation kinetic and benzene polycarboxilic acids formation.

Pyrogenic carbon (PyC) has a high potential to soil carbon sequestration. Among the methods used to quantify PyC, the method of benzene polycarboxilic acids (BPCA) is well-established. The oxidation step of this method is essential for a reliable PyC quantification. Up to now there are no studies on the influence of feedstock and pyrolysis temperature in the oxidation kinetic of PyC and BPCA formation; and then these are the purposes of this study. For this, different PyC were oxidised at 170 ºC during different times. The data showed similar kinetic curves for all PyC, but different BPCA production depending upon feedstock and pyrolysis temperature. All PyC showed maximum production of BPCA at 6-8 h under nitric acid oxidation at 170 ºC. We concluded that 8 h of nitric acid oxidation at 170 ºC produces a reliable data to PyC quantification, thus representing a robust method

A review and meta-analysis ofmitigation measures for nitrous oxide emissions from crop residues

Crop residues are of crucial importance to maintain or even increase soil carbon stocks and fertility, and thereby to ad- dress the global challenge of climate change mitigation. However, crop residues can also potentially stimulate emis- sions of the greenhouse gas nitrous oxide (N2O) from soils. A better understanding of how to mitigate N2O emissions due to crop residue management while promoting positive effects on soil carbon is needed to reconcile the opposing effects of crop residues on the greenhouse gas balance of agroecosystems. Here, we combine a literature review and a meta-analysis to identify and assess measures for mitigating N2O emissions due to crop residue applica- tion to agricultural fields. Our study shows that crop residue removal, shallow incorporation, incorporation ofresidues with C:N ratio > 30 and avoiding incorporation of residues from crops terminated at an immature physiological stage, are measures leading to significantly lower N2O emissions. Other practices such as incorporation timing and interac- tions with fertilisers are less conclusive. Several of the evaluated N2O mitigation measures implied negative side- effects on yield, soil organic carbon storage, nitrate leaching and/or ammonia volatilization. We identified additional strategies with potential to reduce crop residue N2O emissions without strong negative side-effects, which require fur- ther research. These are: a) treatment ofcrop residues before field application, e.g., conversion ofresidues into biochar or anaerobic digestate, b) co-application with nitrification inhibitors or N-immobilizing materials such as compost with a high C:N ratio, paper waste or sawdust, and c) use ofresidues obtained from crop mixtures. Our study provides a scientific basis to be developed over the coming years on how to increase the sustainability of agroecosystems though adequate crop residue management

Secondary data of N2O emissions associated with the return of crop residues from field studies

The value of this dataset is: ● This dataset contains a summary of data from 75 field studies on N2O emissions associated with the return of crop residues. It includes the biochemical quality of the residues returned to the soil. The data is grouped according to the type of crop, type of residue generated and residue maturity. All these categories were explained in the papers [papers below] and highlighted the importance of biochemical quality and maturity of the residue to predict N2O emissions. ● This dataset can be used by researchers and stakeholders interested in exploring the drivers of soil N2O emissions from a given residue type. ● These data can be used as a starting point for future meta-analyses and review studies. This dataset is used in the following review papers: • D. Abalos, T. Rittl, S. Recous, P. Thiébeau, et al. 2022 Predicting field N2O emissions from crop residues based on their biochemical composition: a meta-analytical approach. STOTEN, 812, 152532 • D. Abalos, S. Recous, K. Butterbach-Bah, C. De Notarisl et al. A review and meta-analysis of mitigation measures for nitrous oxide emissions from crop residues(under revision in Stoten