Effect of long-term fertilization on organic carbon mineralization of cinnamon soil

Objectives Mineralization of soil organic carbon is an essential process of biochemistry in soils, which is closely related to release of soil nutrients, maintenance of soil quality, and formation of greenhouse gases. This paper is to study the effects of different fertilizing treatments on dynamic changes of soil organic carbon mineralization rate and cumulative decomposition in Cinnamon soils for better managements of soil fertility, increasing carbon sinks and decreasing greenhouse gas emissions.

Methods Since 1988, a field experiment was conducted in 8 different fertilization treatments: no fertilizer (CK), inorganic N (N, 120 kg/hm²), inorganic N and P (NP, P₂O₅ 75 kg/hm²), low amount of organic manure (M₁, 22500 kg/hm²), low amount of organic manure plus inorganic N (M₁N), low amount organic manure plus inorganic N and P (M₁NP), high amount organic manure plus inorganic N (M₂N) and high amount organic manure plus inorganic N and P (M₂NP). Soil samples were collected at 0–20 cm depth before sowing in 2013. Carbon dioxide cumulative emission and mineralization rate were determined by incubation method. First-order kinetic model was used to calculate potential mineralization and turnover rates.

Results All the long-term fertilization treatments increased total organic carbon in the 0–20 cm depth of soil. The effects of manure combined with chemical fertilizer application, especially M₂N and M₂NP were more obvious than those in CK total organic C in M₂N and M₂NP was increased by 121.1% and 166.8%, respectively. The incubation results show that mineralization rates of soil organic carbon in all the treatments are the highest in the first day and then decreased rapidly. After 5 days incubation, the mineralization rate decrease slowdown. The changes of mineralization rates of soil organic carbon of all the treatment were in agreement with the logarithmic function relationship. Long-time fertilization enhanced the mineralization rates of soil organic carbon in the Cinnamon soils in the following order: manure with chemical fertilizer > manure > chemical fertilizer > CK. The cumulative CO₂ during 57 days incubation period was in the range of 555.0-980.3 mg/kg. The M₂NP and M₁N had the highest amounts which were 1.77 and 1.73 fold higher than in CK, respectively. The percentage of cumulative CO₂-C in TOC during the 57 days incubation period in all the fertilization treatments were lower than in comparing to CK. The M₂NP treatment decreased the most with 6.3% lower than that in CK. The potential CO₂-C evolved from the fertilization treatments were all higher than that from the CK, especially the M₁N, and M₂NP that produced 923.7 mg/kg and 926.4 mg/kg, which were 74.0% and 74.5% higher than CK. The fertilization treatment increased the turnover rate of soil organic carbon significantly and reduced the turnover time. Among the fertilization treatments, M₁NP and M₂NP showed strong impact on these.

Conclusions Long-term application of chemical fertilizer, manure, and manure with chemical fertilizer can effectively increase the accumulation of organic carbon in Castano-cinnamon soils and increase organic carbon mineralization rate and turnover rate. However, the fertilization treatments reduce the cumulative mineralization rate (the ratio of accumulated mineralization amount to total organic carbon), and enhance the soil carbon sequestration capacity. Among these treatments, application of 45000 kg/hm² manure, 120 kg/hm² and P₂O₅ 75 kg/hm² shows the most significant impact.