Nitrous oxide (N2O) is a potent greenhouse gas that is produced during microbial nitrogen transformation processes such as nitrification and denitrification. Soils represent the largest sources of N2O emissions with nitrogen fertilizer application being the main driver of rising atmospheric N2O concentrations. Soil biochar amendment has been proposed as a promising tool to mitigate N2O emissions from soils. However, the underlying processes that cause N2O emission suppression in biochar-amended soils are still poorly understood. We set up microcosm experiments with fertilized, wet soil in which we used 15N tracing techniques and quantitative polymerase chain reaction (qPCR) to investigate the impact of biochar on mineral and gaseous nitrogen dynamics and denitrification-specific functional marker gene abundance and expression. In accordance with previous studies our results showed that biochar addition can lead to a significant decrease in N2O emissions. Furthermore, we determined significantly higher quantities of soil-entrapped N2O and N2 in biochar microcosms and a biochar-induced increase in typical and atypical nosZ transcript copy numbers. Our findings suggest that biochar-induced N2O emission mitigation is based on the entrapment of N2O in water-saturated pores of the soil matrix and concurrent stimulation of microbial N2O reduction resulting in an overall decrease of the N2O/(N2O + N2) ratio.

The application of biochar as a soil amendment to improve soil fertility has been suggested as a tool to reduce soil-borne CO 2 and non-CO 2 greenhouse gas emissions, especially nitrous oxide (N 2 O). Both laboratory and field trials have demonstrated N 2 O emission reduction by biochar amendment, but the long-term effect (>1 year) has been questioned. Here, we present results of a combined microcosm and field study using a powdered beech wood biochar from slow pyrolysis. The field experiment showed that both CO 2 and N 2 O emissions were still effectively reduced by biochar in the third year after application. However, biochar did not influence the biomass yield of sunflower for biogas production (Helianthus annuus L.). Biochar reduced bulk density and increased soil aeration and thus reduced the water-filled pore space (WFPS) in the field, but was also able to suppress N 2 O emission in the microcosms experiment conducted at constant WFPS. For both experiments, biochar had limited impact on soil mineral nitrogen speciation, but it reduced the accumulation of nitrite in the microcosms. Extraction of soil DNA and quantification of functional marker genes by quantitative polymerase chain reaction showed that biochar did not alter the abundance of nitrogen-transforming bacteria and archaea in both field and microcosm experiments. In contradiction to previous experiments, this study demonstrates the long-term N 2 O emission suppression potential of a wood biochar and thus highlights its overall climate change mitigation potential. While a detailed understanding of the underlying mechanisms requires further research, we provide evidence for a range of biochar-induced changes to the soil environment and their change with time that might explain the often observed N 2 O emission suppression.

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Core Ideas The main aim was to reduce N2O emission through crop residue removal or N immobilization through straw addition in fall. Annual N2O emission was reduced by 74% through removal of vegetable crop residues in fall. N2O emission in the removal treatment was as low as in the unfertilized control. Straw addition in autumn failed to reduce N2O emission (p = 0.057). Vegetable production, such as cauliflower (Brassica oleracea var. botrytis L.) or broccoli (Brassica oleracea var. italica P.), is often associated with high N surpluses, posing the risk for substantial N losses. Straw addition in autumn to immobilize surplus N over winter or removal of vegetable crop residues were shown to reduce nitrate leaching efficiently. However, the effect of these management measures on the release of nitrous oxide (N2O) is still unclear. We determined N2O fluxes from a vegetable field with a silty texture in southern Germany over 2 yr in the following treatments: no N fertilization (−N), conventional N fertilization without (CON), and N fertilization with crop residue removal (−CR) or straw addition (+S). Marketable fresh matter yields and N uptake showed only minor differences among all N‐fertilized treatments. Enhanced N2O fluxes occurred over a period of nearly 6 mo in the first year in autumn and winter after crop residue incorporation. Positive correlations between N2O fluxes and driving soil variables suggested denitrification as the major N2O source. Cumulative N2O emission ranged between 5.2 (−CR) and 37.2 kg N2O–N ha−1 yr−1 (CON). Crop residue removal reduced N2O emission in CON very efficiently by 74%. Straw addition reduced N2O emission in only 1 yr. The N2O emission factors were 4.3, 1.9, and 3.1 for CON, −CR, and +S, respectively. The high N2O reduction by crop residue removal seems to be promising in terms of mitigation, but long‐term effects, such as missing humus reproduction, should be considered in future studies.