Application of nitrogen-rich sunflower husks biochar promotes methane oxidation and increases abundance of Methylobacter in nitrogen-poor soil

Abstract

The area of sunflower crops is steadily increasing. A beneficial way of managing sunflower waste biomass could be its use as a feedstock for biochar production. Biochar is currently being considered as an additive for improving soil parameters, including the ability to oxidise methane (CH₄) - one of the key greenhouse gases (GHG). Despite the high production of sunflower husk, there is still insufficient information on the impact of sunflower husk biochar on the soil environment, especially on the methanotrophy process. To fill this knowledge gap, an experiment was designed to evaluate the effects of addition of sunflower husk biochar (produced at 450-550 °C) at a wide range of doses (1-100 Mg ha^-1) to Haplic Luvisol. In the presented study, the CH₄ oxidation potential of soil with and without sunflower husk biochar was investigated at 60 and 100% water holding capacity (WHC), and with the addition of 1% CH₄ (v/v). The comprehensive study included GHG exchange (CH₄ and CO₂), physicochemical properties of soil (pH, soil organic carbon (SOC), dissolved organic carbon (DOC), nitrate nitrogen (NO₃^--N), WHC), and the structure of soil microbial communities. That study showed that even low biochar doses (5 and 10 Mg ha^-1) were sufficient to enhance pH, SOC, DOC and NO₃^--N content. Importantly, sunflower husk biochar was significant source of NO₃^--N, which soil concentration increased from 9.40 ± 0.09 mg NO₃^--N kg^-1 for the control to even 19.40 ± 0.26 mg NO₃^--N kg^-1 (for 100 Mg ha^-1). Significant improvement of WHC (by 11.0-12.4%) was observed after biochar addition at doses of 60 Mg ha^-1 and higher. At 60% WHC, application of biochar at a dose of 40 Mg ha^-1 brought significant improvements in CH₄ oxidation rate, which was 4.89 ± 0.37 mg CH₄-C kg^-1 d^-1. Higher biochar doses were correlated with further improvement of CH₄ oxidation rates, which at 100 Mg ha^-1 was seventeen-fold higher (8.36 ± 0.84 mg CH₄-C kg^-1 d^-1) than in the biochar-free control (0.48 ± 0.28 mg CH₄-C kg^-1 d^-1). CO₂ emissions were not proportional to biochar doses and only grew circa (ca.) twofold from 3.16 to 6.90 mg CO₂-C kg^-1 d^-1 at 100 Mg ha^-1. Above 60 Mg ha^-1, the diversity of methanotrophic communities increased, with Methylobacter becoming the most abundant genus, which was as high as 7.45%. This is the first, such advanced and multifaceted study of the wide range of sunflower husk biochar doses on Haplic Luvisol. The positive correlation between soil conditions, methanotroph abundance and CH₄ oxidation confirmed the multifaceted, positive effect of sunflower husk biochar on Haplic Luvisol. Sunflower husk biochar can be successfully used for Haplic Luvisol supplementation. This additive facilitates soil protection against degradation and has the potential to mitigate GHG emissions.

Keywords: Greenhouse gas; Luvisols; Methane uptake; Methanotrophy; Methylobacter; Sunflower husks biochar.