The effect of new leonardite-based soil amendments on soil organic carbon and carbon dioxide emissions from black soil

The current declining soil fertility along with the need for intensification of crop production and greenhouse gas (GHG) emissions reduction is a great challenge, which may be mitigated by sound soil organic carbon (SOC) management. Organic soil amendments have long been known to increase the organic matter content of the soil. Locally available organic soil amendments are an essential source of carbon and nutrients. The significance of the production of new soil amendments has become even clearer considering the escalating prices of chemical fertilizers. Leonardite (oxidized lignite) has a high content of carbon (48.3%) and can be an efficient raw material for soil amendment production. To increase the degree of humification of leonardite and to improve its fertilization value mechano-activation with different materials was used. Iron as a variable-valent metal played the role of a catalyst for the synthesis of the humic substance while bentonite was used as matrice to their synthesis. Molasses was used as the binder during the granulation of soil amendments. For the production of organo-mineral amendment to organic granules obtained a mineral fertilizer layer was added to achieve an N:P:K ratio of 1:0.86:0.95. Correct dose and method of soil amendment application play an important role in its efficient use. We have tested two methods of application of new soil amendments (band incorporation and broadcasting) at two rates of total nitrogen fertilization (30 and 60 kg N ha^-1) in a field trial for two years. The sum of positive temperatures on the experimental field is 2400-2900 ^○С. The average annual precipitation is 465-680 mm. The soil - Chernozem Podzolic with organic carbon content by the Tyurin method is 2.37 %. Application of organo-mineral amendment (N₆₀) led to an increase of total organic carbon content in the topsoil by 10% compared to the no treatment and mineral fertilization. The integral indicator of the complex dielectric permeability revealed a decrease of polarity of the humic supramolecular structure due to organo-mineral amendment application. This change led to the formation of a more complex, polydisperse, and heterogeneous molecular aggregate organization with high hydrophobicity and stability. The data obtained during the growing period of maize showed that the intensity and the dynamics of carbon dioxide (CO₂) emissions from soil depend on the hydrothermal conditions and the type of soil amendments. Cumulative CO₂ flux increased with an increasing application rate of soil amendments. The highest mean value of CO₂ emissions (0.77 kg CO₂/ha/hour) during the growing season was observed under broadcasting of organic soil amendment (N₆₀). Further long-term measurements of soil organic carbon dynamics on different soil types across a variety of technological operations of new soil amendments application are important to reliably quantify rates of SOC accumulation and GHG emissions.