Organic fertilizers have divergent effects on soil N2O emissions

A field experiment was conducted in a subtropical tea (Camellia sinensis (L.) O. Kuntze) plantation in Jiangsu Province, China, including the following treatments: no nitrogen (N) fertilizer (control), conventional mineral N fertilizer (urea) (CN), soybean cake fertilizer (SF), pig manure (PM), cattle manure (CaM), chicken manure (CM), and CM + biochar (CMB). Cumulative nitrous oxide (N2O) and nitric oxide (NO) emissions were 4.8 +/- 0.1 and 3.7 +/- 0.3 kg N ha(-1) year(-1) under CN, respectively, and increased to 5.4 +/- 0.2 and 4.6 +/- 0.3 kg N ha(-1) year(-1) under SF (P < 0.05), respectively. Treatments with livestock manures (PM, CaM, and CM) reduced N2O (41.4-49.6%) and NO (46.5-59.8%) emission in comparison to CN. Combined amendment of CM and biochar more effectively reduced N2O emissions than CM treatment alone. Based on a meta-analysis of 26 global paired measurements in acid soils, the threshold of C/N ratios of organic fertilizers between the positive and negative responses of N2O emissions to organic fertilizers was 8.6 with a range of 4.5-22.3 (95% confidence interval), indicating that reduced N2O emission under PM, CaM and CM was potentially due to their C/N ratios compared to the threshold. Organic fertilizer application did not influence tea yield, while combined application of CM and biochar increased tea yield and resulted in the least yield-scaled N2O emission. N2O and NO emission factors for N fertilizers applied under CN were 1.9 +/- 0.1% and 1.5 +/- 0.2%, respectively, and reduced to 0.08 +/- 0.04% and 0.12 +/- 0.04% under CMB, respectively. The results suggest that tea plantations in the subtropical region are hotspots for N2O and NO emissions. Combined application of chicken manure and biochar could mitigate N gas emissions and increase yield in the tea plantation systems.