Optimal application rate and frequency of apple-pruning biochar for crop productivity, soil properties, and nitrous oxide mitigation of sweet potato uplands

Purpose: This study evaluated the effects of the application rate and frequency of apple pruning-derived biochar on sweet potato productivity, soil properties, and nitrous oxide (N 2 O) emissions, with the aim of providing optiaml biochar application strategy that balances agronomic productivity and environmental sustainability.Methods: A field experiment was conducted from 2023 to 2025 in a sandy loam upland field in Iksan, Jeonbuk State, Republic of Korea.Apple pruning-derived biochar produced by pyrolysis at 400°C for 4 h was applied at rates of 100, 200, 400, and 800 kg 10a -1 under annual, biennial, and triennial application regimes.Sweet potato growth, plant nutrient concentrations, and marketable yield, as well as soil physical and chemical properties, and nitrous oxide (N 2 O) emissions were investigated.Results: Biochar application significantly affected soil physical and chemical properties depending on application rate and frequency.Soil bulk density decreased and porosity increased in most treatments, whereas soil organic carbon (SOC) and cation exchange capacity generally increased following biochar application.Most aboveground growth parameters showed marginal responses, although stem diameter (P = 0.0018), SPAD value (P < 0.001), and internode length (P = 0.0213), shoot fresh weight (P = 0.0371) differed among treatments.The highest marketable yield (5,578 kg 10a -1 ) was observed under the annual application of 400 kg 10a -1 , while relatively high yields were also observed under the biennial and triennial applications of 400 kg 10a -1 .Lower cumulative N 2 O emission were observed by biochar applications compared to the control (7.42 kg N 2 O ha -1 ), with the lowest value (6.25 kg N 2 O ha -1 ) recorded under the annual application at 400 kg 10a -1 , representing a 15.8% lower value than the control.Conclusions: This study shows that the annual application of biochar at 400 kg 10a -1 was is an optimal biochar application strategy to achieve multiple benefits of biochar application including marketable yield, soil physical properties, SOC sequestration, and N 2 O mitigation.These findings provide the biochar application strategy for climate-smart cultivation of sweet potato using apple pruning biomass.