Sizes and Ratios of Organic Carbon Pools in Gray Forest Soil under Long-Term Application of Mineral and Organic Fertilizers

Soil organic matter (SOM) is proposed to be subdivided into structural and process pools. The structural pools include particulate organic matter (CPOM) and mineral-associated organic matter (CMAOM) and the process pools comprise microbial biomass (Cmic) and potentially mineralizable organic matter (C0). The studies have been performed in a long-term microplot experiment on gray forest soil (Luvic Retic Greyzemic Phaeozem (Loamic)). Mineral (N, 90–360 kg/ha; P2O5, 75–300 kg/ha; and K2O, 100–400 kg/ha) and organic (25–100 t/ha of fresh cattle manure) fertilizers have been applied annually over nine years at increasing doses. The resulting increase in the soil organic carbon (Corg) stock from NPK and manure application has reached 5–10 and 38–83%, respectively. Extremely high doses of manure (700 to 900 t/ha over 9 years) have caused the soil saturation with Corg. The sizes of SOM pools decrease in the order CMAOM > CPOM > C0 > Cmic amounting to 7.91–12 g/kg (50–84% of Corg), 0.76–12 g/kg (8–50%), 0.32–1.71 g/kg (2.8–13.7%), and 0.09–0.56 g/kg (0.8–3.7%), respectively. The size of CPOM pool mainly depends on the dose of mineral and organic fertilizers, and the size of CMAOM pool, on the duration of fertilization. Both Cmic and C0 pools increase with the rise in the dose of manure and decrease with the rise in the NPK doses. The long-term manuring did not lead to a cumulative accumulation of C0 in the soil. It is emphasized that the separation of structural and process pools can be useful for monitoring of SOM quality and functions.