Impact of chemical fertilizer reduction and organic fertilizer combined application on soil organic carbon mineralization in apple orchard

【Objective】The current study was designed to reduce reliance on chemical fertilizers and to evaluate the combined application of organic and inorganic fertilizers. The aim of this research was to assess the impact of reduced chemical fertilizer inputs and the combined use of organic fertilizers on soil organic carbon (SOC) mineralization in apple orchards.【Methods】Long-term experiments were carried out on apple orchard soil in Xingcheng city, Liaoning province. Based on these long-term fertilization experiments, six treatments were established: CK (no fertilization), CF100 (100% chemical fertilizer), CM25CF75 (25% cow manure with 75% chemical fertilizer), CM50CF50 (50% cow manure with 50% chemical fertilizer), CM75CF25 (75% cow manure with 25% chemical fertilizer), and  CM100 (100% cow manure), respectively. Indoor mineralization culture experiments were conducted to analyze the effects of different application ratios on soil organic carbon mineralization in fertilized (F) and non-fertilized (NF) areas.【Results】The results showed that all treatments increased total nitrogen content in the fertilized area. Treatments involving 50%-100% chemical fertilizer reduction combined with organic fertilizer application significantly increased soil nitrate nitrogen content. In addition, all treatments increased the contents of available phosphorus and potassium in the apple orchard soil. In the non-fertilized area, the application of chemical fertilizer alone and 25% chemical fertilizer reduction combined with organic fertilizer decreased the total nitrogen content. Treatments with 50%- 100% chemical fertilizer reduction combined with organic fertilizer decreased soil nitrate nitrogen content. Available phosphorus and potassium were not affected by any treatment, except for the 50% chemical fertilizer reduction combined with organic fertilizer application, which influenced available phosphorus content in the non- fertilized area. The cumulative SOC mineralization in the fertilized and non- fertilized areas of each treatment increased with the increase in the duration of cultivation. Cumulative SOC mineralization increased rapidly at the beginning of the cultivation, but the rate of increase gradually slowed as incubation progressed. Moreover, the cumulative organic carbon mineralization in each treatment of the fertilized area was significantly higher than that in the non-fertilized area. The mineralization rate of soil organic carbon (SOC) was highest on the first day in both fertilized and non-fertilized areas, followed by a sharp decrease from the 2nd to the 4th day, and then stabilized after the 11th day. The SOC mineralization rate under chemical fertilizer reduction combined with cow manure (CM25CF75, CM50CF50, CM75CF25) and under cow manure alone (CM100) were higher than that under chemical fertilizer and the control treatment. The cumulative SOC mineralization increased with increasing duration of cultivation. In the fertilized area, all fertilization treatments increased the cumulative mineralization amount of SOC, with the CM100 treatment showing the highest value (1 332.14 mg·kg-1 ). In the non- fertilization area, the trend of mineralization rate among different treatments was basically consistent with that in the fertilization area. The mineralization rate was shown as followed: CM100＞ CM75CF25＞CM50CF50＞CM25CF75＞CF100＞CK. Compared to the CK treatment, the cumulative SOC mineralization in the CM100, CM75CF25, CM50CF50, CM25CF75, and CF100 treatments were increased by 107.45%, 81.78%, 70.01%, 55.18%, and 20.21%, respectively, after 35 days of cultivation. The SOC mineralization rate and cumulative SOC mineralization amount in the non-fertilized area were significantly lower than those in the fertilized area. A first-order kinetic model was applied to evaluate potential mineralizable carbon (C0), mineralization rate constant (K), and half-turnover period (T1/2). Fertilization treatments increased C0 and K and shortened T1/2. With increasing proportions of cow manure in the combined application, C0 and K increased, whereas T1/2 decreased. In the non- fertilization area, the CM50CF50, CM75CF25 and CM100 treatments significantly increased C0 and K, while significantly reduced T1/2 compared with the control treatment. The CM100 treatment exhibited the highest C0 and K and the shortest T1/2. In the fertilization area, there was a significant positive correlation between the total nitrogen, available phosphorus, C0, and K in the soil, while there was a significant negative correlation with T1/2. In the non- fertilization area, there was no obvious correlation among these indicators. 【Conclusion】It was concluded that long-term combined application of chemical and organic fertilizers modified the basic properties of apple orchard soil. This practice significantly improved soil fertility, as indicated by high total nitrogen, available phosphorus, and organic carbon contents. It also enhanced the mineralization rate of organic carbon and promoted the accumulation of soil organic carbon. Moreover, with increasing proportions of cow manure in the combined application, both the SOC mineralization rate and cumulative mineralization amount increased accordingly. The combined application of chemical fertilizer and cow manure increased potential mineralizable carbon (C0) and the mineralization rate constant (K), while shortening the half-turnover period (T1/2). In particular, the combined application of chemical fertilizer with cow manure was more conducive to soil organic carbon retention, especially under the CF25CM75 treatment.