Dynamics of Soil Organic Carbon Fractions Under Different Crop‐Growth Periods: A Comparative Study Between Long‐Term Fallow Land and Continuous Cultivated Land

ABSTRACT

Whether short‐term responses of soil organic carbon fractions to fertilisation differ between soils with long‐term fallow land versus continuous cultivation land remains unclear. Crop‐growth periods encompass seasonality, crop development and associated field‐management events; temporal variation is important for interpreting these short‐term fertilisation responses. A field experiment was conducted on Regosol sloping cropland with the same soil parent material but distinguished by two land‐use types: long‐term fallow and continuous cultivation. Five fertilisation treatments were implemented: no fertiliser (CK), chemical fertiliser only (CF), organic fertiliser only (OF) and two treatments with organic fertiliser replacing chemical fertiliser at a rate of 5% (OC1) and 30% (OC6) based on equal nitrogen input. On the fallow soil, the concentrations of dissolved organic carbon (DOC) and particulate organic carbon (POC) peaked in March, whereas mineral‐associated organic carbon (MAOC) concentration was lowest in July. The specific ultraviolet absorbance at 254 nm (SUVA ₂₅₄ ), indicating carbon aromaticity under OC1 was significantly higher than CK and CF by 23.4% and 31.3%, respectively. On the cultivated soil, DOC concentration peaked in April, POC concentration peaked in December, and MAOC concentration showed no significant differences. There were no significant differences among treatments for SUVA ₂₅₄ . The linear mixed‐effects model showed that all carbon fractions varied significantly among crop‐growth periods within each slope. Mantel analyses combined with partial least‐squares path modelling revealed that SUVA ₂₅₄ had a negative linkage with carbon fractions on fallow soil (path coefficient = −0.65, p  < 0.01), and a positive association on cultivated soil (path coefficient = +0.85, p  < 0.01). The findings indicate that land‐use legacy is an important context for interpreting short‐term soil organic carbon fraction responses under different fertilisation treatments.