Methane emission from soil under long-term no-till cropping systems

Methane (CH₄) emission from agricultural soils increases dramatically as a result of deleterious effect of soil disturbance and nitrogen fertilization on methanotrophic organisms; however, few studies have attempted to evaluate the potential of long-term conservation management systems to mitigate CH₄ emissions in tropical and subtropical soils. This study aimed to evaluate the long-term effect (>19 years) of no-till grass- and legume-based cropping systems on annual soil CH₄ fluxes in a formerly degraded Acrisol in Southern Brazil. Air sampling was carried out using static chambers and CH₄ analysis by gas chromatography. Analysis of historical data set of the experiment evidenced a remarkable effect of high C- and N-input cropping systems on the improvement of biological, chemical, and physical characteristics of this no-tilled soil. Soil CH₄ fluxes, which represent a net balance between consumption (−) and production (+) of CH₄ in soil, varied from −40±2 to +62±78μg Cm⁻²h⁻¹. Mean weighted contents of ammonium (NH₄ +–N) and dissolved organic carbon (DOC) in soil had a positive relationship with accumulated soil CH₄ fluxes in the post-management period (r²=0.95, p=0.05), suggesting an additive effect of these nutrients in suppressing CH₄ oxidation and stimulating methanogenesis, respectively, in legume-based cropping systems with high biomass input. Annual CH₄ fluxes ranged from −50±610 to +994±105g Cha⁻¹, which were inversely related to annual biomass-C input (r²=0.99, p=0.003), with the exception of the cropping system containing pigeon pea, a summer legume that had the highest biologically fixed N input (>300kgha⁻¹yr⁻¹). Our results evidenced a small effect of conservation management systems on decreasing CH₄ emissions from soil, despite their significant effect restoring soil quality. We hypothesized that soil CH₄ uptake strength has been off-set by an injurious effect of biologically fixed N in legume-based cropping systems on soil methanotrophic microbiota, and by the methanogenesis increase as a result of the O₂ depletion in niches of high biological activity in the surface layer of the no-tillage soil.