Changes in C and N fractions with composted manure plus chemical fertilizers applied in apple orchard soil: an in‐situ field incubation study on the Loess Plateau, China

Abstract We investigated the effects of compost ( CM ), made from poultry and cattle manure with spent mushroom substrate, plus chemical fertilizers ( CF s) on soil organic carbon (C) and nitrogen (N) fractions in silty loam soil of the Loess Plateau. Eight fertilizer practices were applied in a 7‐year‐old ‘Red Fuji’ apple ( Malus domestica Borkh.) orchard for 360 days. Compared to CM alone, CM – CF s decreased slightly soil total organic C but increased total N by 4.3–11.6%. Notably, CM – CF s increased soil microbial biomass C ( MBC ) by 2.7–26.5% and microbial biomass N ( MBN ) by 7–13.7%. Soil water‐soluble carbon ( WSC ) was increased by 20.7% and 19.2% when 2% CM plus N and phosphorus (P) (2%M– NP ) and 4% CM plus N and P (4%M– NP ) were applied, respectively. Whereas 0.5% CM plus N and P (0.5%M– NP ) increased WSC by 9.3% on day 30 but decreased it by 7.2% from 30–360 days. Hot water‐soluble C increased by 13.1–14.6% from 0–180 day, but thereafter, the effect disappeared. Compared to CF s, CM – CF s increased MBN by 35.1–115.6%, and increased alkali‐hydrolyzable‐N by 3.5–55.8% over 180–360 days of incubation. Additionally, CM – CF s promoted N mineralization, increasing NH 4 ‐N and NO 3 ‐N contents. Based on the changes in C and N fractions and available nutrients, 2%M– NP (45 t/ha of CM plus 450 kg/ha of N and 157.5 kg/ha of P) may be the optimal fertilizer strategy for stimulating soil microbial growth and activity, and enhancing nutrient cycling for apple growth.