Influence of Fertilizer Systems on Soil Organic Carbon Content and Crop Yield: Results of Long-Term Field Experiments at the Geographical Network of Research Stations in Russia

The results of long-term experiments (LTEs) performed at the Geographical Network of experimental stations encompassing major soil types in Russia (soddy-podzolic (Retisols, Luvisols, and Podzols); gray forest (Luvic Greyzemic Phaeozems); leached, typical, and southern chernozems (Chernozems); chestnut soils (Kastanozems); meadow brown soils (Albic Stagnic Luvisols); and meadow chernozem-like soils (Luvic Greyzemic Stagnic Phaeozems)) within the entire country (from the north to the south of the European part of Russia, Siberia, and the south of the Far East region (Amur oblast, Khabarovsk, and Primorskii regions)) are summarized. Most of the stationary LTEs were lasting 20-45 years; ten LTEs lasted from 50 to 81 years. It was found that the organomineral fertilizer system (OMFS) exceeded the mineral fertilizer system (MFS) in its effect on the yield by 1-40% in 18 experiments, while the MFS was more efficient than the OMFS by 2-3% in only three LTEs. For the same soil type, the differences in Corg content between unfertilized trials reach 0.56-1.55% for soddy-podzolic soils and 1.58-3.93% for chernozems. Differences in soil Corg content between the unfertilized and OMFS (manure + NPK) trials were less than 10% in four LTEs, 11-20% in six LTEs, and >20% in ten LTEs; the average difference reached 0.27% of Corg. The levels of minimum (C-min) and optimum (C-opt) organic carbon content have been determined with due account for the soil types, soil texture classes, and the applied fertilizer system. They can be used as reference values for the studied soils.