Establishment and verification of the relationship between soil organic carbon storage and exogenous carbon input

Objectives Soil organic carbon (SOC) plays a crucial role in soil fertility, and exogenous carbon (C) input is an important source to regulate its balance. The establishment of their internal relationship will provide an effective tool for quantitative soil fertility improvement in agricultural ecosystem.

Methods Two localized fertilization experiments were used in this research: one was 25-year long and the other was 8-year long. The data were collected from the treatments of no fertilizer (CK), chemical N and P (NP), chemical NPK fertilizers (NPK), NPK with manure (NPKM₁), 1.5 times of NPKM₁ (1.5NPKM₁) and manure (M₂) alone, and the data were used to establish the relationship between SOC and exogenous C input in the 25-year experiment. The data were also collected from the four treatments in the 8-year experiment, including chemical NPK fertilizers (T₀) and NPK combined with 15 t/hm² (T₁), 30 t/hm² (T₂) and 45 t/hm² manure (T₃). These data were used to verify the accuracy of the established relationships.

Results Compared with CK in the 25-year experiment, the application of chemical fertilizer (NP and NPK) significantly increased the annual C input of crop residues by 0.45–0.75 t/hm². The application of manure (M₂) and manure combined with chemical fertilizer (NPKM₁ and 1.5NPKM₁) showed an annual C input of crop residues with 1.59–9.36 t/hm², which was significantly higher than that in CK, NP and NPK treatments (P < 0.05). However, the annual C input of crop residues had no significant difference among T₀, T₁, T₂ and T₃ treatments in the 8-year experiment. SOC storage under long-term fertilizations could reach a steady value, namely SOC storage did not change with the increase of experimental years, with 24.01, 25.16, 48.44, 48.46 and 49.83 t/hm² under NP, NPK, NPKM₁, 1.5NPKM₁ and M₂ treatments, respectively. The mineralized amount of SOC storage was 4.69 t/hm² based on long-term different fertilizations. To maintain the initial SOC storage, 8.52 t/hm² of exogenous C was inputted. There was a 17% error in predicting SOC storage through the relationship between the change of SOC storage and exogenous C input. Considering the difference of initial SOC storage, only 3% error existed in the prediction of SOC storage through the relationship between SOC storage and exogenous C input. According to the relationship between SOC storage and exogenous C input, the amount of exogenous C needed to maintain initial SOC_a storage and increase SOC storage to SOC_b was 54.35 × 34.62 / (48.71 − SOC_a) − 1 and 1881.60 × (SOC_b − SOC_a) / (48.71 − SOC_b) × (48.71 − SOC_a), respectively.

Conclusions The relationship between SOC storage and exogenous C input based on the initial soil fertility status, could be used to accurately quantify the exogenous C input to improve soil fertility.