Active carbon pool-size is enhanced by long-term manure application

We studied the dynamics of soil organic carbon (SOC)-pool mineralisation in agricultural soil. A laboratory incubation experiment was conducted using the soil from a long-term experiment involving the following fertilisation regimes: no fertilisation (CK); mineral (NPK); organic (M), and combined organic-inorganic fertilisers (MNPK). SOC mineralisation rate decreased as follows: MNPK > M > NPK > CK. Cumulative SOC mineralisation (Cm) ranged between 730.15 and 3 022.09 mg/kg in CK and MNPK, respectively; 8.81% (CK) to 20.45% (MNPK) of initial SOC was mineralised after a 360-day incubation. Soil Cm values were significantly higher under NPK, M, and MNPK compared to those under the CK treatment. Dynamic variation in Cm with incubation time fitted a double exponential model. Active carbon pools accounted for 2.06-6.51% of total SOC and the average mean resistant time (MRT1) was 28.76 days, whereas slow carbon pools accounted for 93.49-97.94% of SOC, with an average MRT2 of 8.53 years. The active carbon pool in fertilised soils was larger than in CK; furthermore, it was larger in M- and MNPK- than under NPK-treated plots. SOC decomposed more easily in long-term fertilised plots than in non-fertilised plots.