Change in the Content of Total and Easily Degradable Organic Matter in Soddy–Podzolic Soil Associated with a Long-Term Straw Incorporation

Abstract

Plant residues and by-products of cereal and leguminous crops are the most important resource for the preservation and regeneration of soil organic matter (SOM). To assess the state of SOM and its role for agriculture, it is important to have information not only about total organic carbon (C_org), but also about the content and ratio of labile, easily mineralized and biologically accessible components that are especially essential for soil nutrient status and maintaining microbial activity and effective soil fertility in general. However, it the scientific literature, there are almost no data on changes in the content and dynamics of various SOM pools and fractions associated with a long-term straw incorporation. Thus, the influence of prolonged and repeated incorporation of cereal and leguminous straw on the content of C_org and easily degradable SOM fractions, i.e., water-soluble (cold- and hot-water extractable) carbon and mortmass, as well as their relation with the crop yields have been studied in a long-term field experiment on soddy–podzolic sandy loam soils in Meshchera Lowlands at the end of the fourth stage of 5-year grain–row crop rotation (winter wheat, lupine, potatoes, barley, and annual grasses). The repeated incorporation of winter wheat, lupine, and barley straw in the amount of 3 t/ha (in total, 36 t/ha during 4 years of crop rotations) together with annual application of mineral fertilizers (on average N54P51K57 per year) have increased the content of C_org by 13% (compared to the initial content), mortmass reserves by 2.42 times, carbon stocks in mortmass (C_mm) by 2.36 times, cold-water extractable carbon (C_w) by 11%, and hot-water extractable carbon (C_hw) by 31% relative to the option without fertilizers and thus, improved the arable layer of soddy–podzolic soil. In the variants with the straw application, the proportions of C_hw and C_mm fractions in SOM are higher, which indicates an increase in the availability of labile easily degradable carbon in soil. The correlation between yields of cultivated crops and the content of SOM components under study have showed to be positive with varying degree of coherence. Thus, the regular straw incorporation in soil is an effective method of SOM preserving/regenerating and increasing the availability of labile carbon.