Search

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

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