Soil organic carbon dynamics in long-term experiments with mineral and organic fertilizers in Russia

The “4 per 1000” initiatives encourages agricultural sector to apply practices aimed at soil organic carbon (SOC) management for greenhouse gases sequestration. We accessed the potential of Russian agricultural soils to store SOC under various managements. RothC model was used to simulate SOC stocks in seven Russian long-term experiments started in 1933–1980 with mineral and organic fertilization. Crop sequences included alternation of cereals, row crops and grasses, four experiments had crop rotations with bare fallow field. We used current weather data and yearly carbon input as input data. Carbon input was calculated using crop residues estimated from crop yield and aboveground biomass production. RothC satisfactorily simulated the observed changes in SOC on Podzols, Retisols and Chernozems, as evaluated through the root mean square error, coefficient of determination and the mean difference. However, in the absence of clear trend, RothC was less sensitive to the observed interannual SOC dynamics. To maintain initial C level annual input of 1.1–1.3 Mg C ha−1 yr−1 was required for sandy Podzols, while for loamy Retisols necessary rates were 1.4–2.0 Mg C ha −1 yr−1. Inputs of 2.6–2.9 Mg C ha−1 yr−1 were required to maintain soil C in Chernozem. If long-term C input was insufficient to maintain SOC, the stock of resistant plant material continuously decreased. The effect of agronomic practices on active C pools might lead only to a short-term C sequestration that was highly yield-dependent. Simulation of SOC dynamics for the plots that did not receive fertilizers and had the lowest SOC stock revealed that aboveground net primary production (NPP) input was sufficient for maintaining constant SOM stocks if these plots were converted to grassland for forage production and received farmyard manure. The average annual 7–17‰ increase in SOC was possible to reach for 30 years and more in all experiments on Podzols and Retisols in treatments with organic fertilization, while treatments with only mineral fertilizers were not sufficient to reach 4‰ level.