Effects of biochar on mitigating nitrous oxide emission from an intensive vegetable field and crop yields

Objectives The study evaluated the long-term effect of biochar amendment on both greenhouse gas (GHG) emissions and crop yields, which are crucial for the efficient field application of biochar.

Methods A field experiment was conducted on an intensive vegetable field in 2016 in Nanjing, Jiangsu. Four treatments included no nitrogen fertilizer control (CK), urea in each crop (N), urea in each crop and fresh biochar (NC_F) once in 2016, and urea in each crop and field-aged biochar which was amended into the field for four years in 2012 (NC_A), with biochar amendment at 40 t/hm². Vegetables of pakchoy, water spinach, edible amaranth and spinach were planted in sequence during the observed consecutive year from November 2016 to November 2017. Urea (N 240 kg/hm²) was applied once as basal fertilization for each crop, except for water spinach which received an extra topdressing N 240 kg/hm² after its first harvest with 3 harvests in total. The static chamber-gas chromatograph method was used to measure dynamically nitrous oxide (N₂O) emissions.

Results The N₂O was mainly emitted during the second and third crop seasons, with yield-scaled N₂O emissions of 0.038–0.131 and 0.107–0.482 kg/t, respectively, with being 0.033–0.209 and 0.007–0.070 kg/t for the first and fourth crop seasons, respectively. Neither soil temperature nor inorganic nitrogen was correlated significantly with N₂O emission in all treatments. Significant correlation (P < 0.01) was detected between soil moisture and N₂O fluxes with WFPS (water filled pore space) ranging from 37%–93% during the experimental period. Relative to the N treatment, the NC_F treatment significantly reduced the annual N₂O cumulative emissions and the annual N₂O emission factors by 35.6% and 46.2%, respectively. Relative to the N treatment, the NC_A treatment significantly reduced the annual N₂O cumulative emissions and the annual N₂O emission factors by 38.8% and 49.9%, respectively. Compared with N treatment, the NC_F and NC_A treatments improved crop yield by 4.6% and 17.9%, respectively, being significant for the NC_A treatment. Moreover, relative to the N treatment, the NC_F and NC_A treatments significantly reduced yield-scaled N₂O emission by 49.8% and 41.3%, respectively.

Conclusions Since field-aged biochar showed obvious effects on N₂O mitigaion and crop yield after 4 years, biochar incorporation showed long-term effect on GHGs mitigation and crop improvement in the intensive vegetable ecosystem.