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Different activation and N-doping treatments were used to produce biochar-based fertilizers (BBFs) with increased N concentration and slow N release. Pristine biochars were produced by pyrolysis of olive tree pruning feedstock at low and high temperatures (400 and 800 °C). These biochars were activated either by ultrasonication, or oxidation with hydrogen peroxide (H2O2) or nitric acid (HNO3) to increase their N retention potential. Subsequently biochars were enriched with N with either urea or ammonium sulfate. The activation of low-temperature biochars with HNO3 was the most effective treatment leading to new surface carboxylic groups that facilitated the later enrichment with N. When treated with urea, BBFs reached 7.0 N%, whereas the H2O2 activation only allowed an increase up to 2.0 N%. The use of urea as the external N source was the most efficient for incorporating N. Urea treated biochars had a water-soluble fraction that represented up to 14.5 % of the total N. The hydrolyzable N fraction, composed by amides and simple N heterocycles originated by the N-doping treatments, and nitro groups generated from HNO3 activation, represented up to 60 % of the total N. This study relates the N chemical forms in the new BBFs to potential N availability in soil. The presence of water-soluble, hydrolyzable and non-hydrolyzable N implied that these BBFs may supply N that would be progressively available for plants, acting as slow-release fertilizers

Biochar is considered a potential technology to enhance chemical fertilizer use efficiency through intensification of the interactions between nutrients and the functional groups on biochar surfaces. However, little is known about how the application of activated biochars mixed with urea influences nitrogen (N) mineralization and crop performance in paddy fields. Here, a sawdust-derived fresh biochar (FBC) (ca. 400 °C) was activated chemically with 15% hydrogen peroxide and biologically with a nutrient solution mixed with a soil inoculum to obtain a chemically activated biochar (CBC) and a biologically activated biochar (BBC), respectively. The chemical and surface properties of FBC, CBC, and BBC were evaluated using spectroscopic methods, i.e., Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance, and potentiometric charge determination. The N retention capacity of biochars and their interaction with urea hydrolysis were examined in a laboratory incubation experiment. Additionally, a field experiment was carried out in a paddy field with the biochars unmixed or mixed with urea at a 1:1 ratio. Our results showed that negative surface functional groups and negative charges were increased on both activated biochars, especially CBC. Both activated biochars contributed to a significant reduction in urea-biochar suspension pH and increased N retention in the incubation experiment. Despite the enhanced surface properties of the activated biochars, no similar increases in rice biomass and grain yield were observed for these biochars in the field experiment. However, rice biomass, grain yield, apparent N use efficiency, and agronomic N use efficiency were significantly higher with the application of the three biochars compared to no-biochar application. Altogether, the results indicate that the application of urea mixed with biochar could enhance crop performance, especially in the case of activated biochar, which would enhance N retention in the soil

Novel fertilization strategies, such as the use of biochar-based fertilizers (BBFs) and the co-application of biochar with mineral fertilizers, have shown promising results for mitigating nitrous oxide (N2O) emissions and reducing N losses in agroecosystems. Two greenhouse experiments were performed with radish to evaluate: (1) the mitigation of yield-scaled N2O emissions using BBFs, produced at either 400 or 800 °C and enriched with urea, compared to the co-application of raw biochars with urea; and (2) the N2O mitigation potential of low rates of raw biochars, equivalent to those used with BBFs fertilization, co-applied with low and high N rates (90 and 180 kg N ha−1). BBF produced at 800 °C reduced yield-scaled N2O emissions by 32% as compared to the urea treatment, and by 60%, as compared to the combination of raw biochar with urea. This reduction was attributed to the slow rate of N release in BBF. On the contrary, the co-application of low rates of biochar with urea increased yield-scaled N2O emissions as compared to the fertilization with urea alone. Low rates of biochar (1.4–3.1 t ha−1) reduced yield-scaled N2O emissions only with a high rate of N fertilization. High-pyrolysis-temperature biochar, co-applied with synthetic fertilizer, or used to produce BBFs, demonstrated lower yield-scaled N2O emissions than biochar produced at a lower pyrolysis temperature. This study showed that BBFs are a promising fertilization strategy as compared to the co-application of biochar with synthetic fertilizers

The demand for biostimulants has been growing at an annual rate of 10 and 12.4% in Europe and Northern America, respectively. The beneficial effects of humic substances (HS) as biostimulants of plant growth have been well-known since the 1980s, and they can be supportive to a circular economy if they are extracted from different renewable resources of organic matter including harvest residues, wastewater, sewage sludge, and manure. This paper presents an overview of the scientific outputs on application methods of HS in different conditions. Firstly, the functionality of HS in the primary and secondary metabolism under stressed and non-stressed cropping conditions is discussed along with crop protection against pathogens. Secondly, the advantages and limitations of five different types of HS application under open-fields and greenhouse conditions are described. Key factors, such as the chemical structure of HS, application method, optimal rate, and field circumstances, play a crucial role in enhancing plant growth by HS treatment as a biostimulant. If we can get a better grip on these factors, HS has the potential to become a part of circular agriculture.

The addition of alkaline and high-cation exchange capacity (CEC) biochars is a suitable strategy to increase the CEC of weathered soils. The aim of this study was to evaluate the effect of biochar from different feedstocks and pyrolysis temperatures on the CEC of two contrasting Oxisols. Biochars produced from chicken manure (CM), eucalyptus sawdust (ES), coffee husk (CH) and sugarcane bagasse (SB),plus a control (without biochar), at 350, 450, and 750 °C were mixed with the soils at 2; 5; 10 and 20% (w/w) and incubated for 9 months. Feedstock, pyrolysis temperature and addition rate of biochar were key factors controlling the alteration of soil CEC. The CH biochar pyrolyzed at 350 °C was the most effective matrix at increasing soil CEC. In a rate-dependent way, ES and SB biochars increased C contents of both soils without improving soil CEC. The efficiency of high-ash biochars in enhancing soil CEC in both Oxisols was limited by the alkalization caused by high rates of CH and CM biochars. The increase in CEC is soil-dependent and modulated by high-ash biochar CEC and application rate, as well as by the original soil CEC

Biochar has been found to interact with N transformations in soil but the mechanisms remain largely unknown. In this study we investigated the priming effect of combined biochar and urea inputs on soil inorganic N pools through an isotope tracer approach. Biochar was applied in combination with urea in two complementary laboratory experiments: (i) in the first one, three 15N-labeled organic amendments (wheat straw (WS)), its biochars produced at 350 °C (B350) and at 550 °C (B550) were added to soil in combination with unlabeled urea; (ii) in the second experiment the three same, but unlabeled, amendments were added to soil in combination with 15N labeled urea. This system allowed partitioning between three N sources: native soil N, biochar-derived N and urea-derived N. In addition, CO2 fluxes were measured to follow total C mineralization in soil and N2O emissions were monitored. The proportion of N that mineralized from biochar was always below 0.5% of the added N. The co-addition of urea increased the concentration of NH4+ derived from B350, but not from B550, demonstrating the lower mineralization of N in biochars produced at 550 °C. Whereas the addition of WS led to a rapid immobilization of N, we found that despite their high C:N, none of the biochars, applied at a rate of 1.5%, immobilized inorganic N in soil. On the contrary, significantly higher NH4+concentrations derived from native soil organic N (SON) and urea were found throughout the incubation when B550 was added. This effect can be attributed to an apparent priming effect since a net decrease in CO2 fluxes was recorded when biochar was added to the soil. The addition of glucose (a low molecular weight carbon source) stimulated an increase in CO2 fluxes in all treatments along with a net N immobilization in soil. However, both biochars significantly reduced C readily available to microbes, as proved by lower soil CO2 fluxes, and limited the immobilization of NH4+ induced by glucose addition. Our result