Energy storage and stability of soil organic matter during the natural restoration of abandoned cropland

Approximately 220 million hectares of former croplands are abandoned worldwide, with a quarter of this area located in Russia. In these areas, natural zonal vegetation is developing, and the soils, including soil carbon (C) stocks, are recovering. Accumulated organic C serves as a structural and energetic source for soil microorganisms. Postagricultural soil restoration affects organic matter stability, microbial organic carbon decomposition, and C cycling. We studied a chronosequence of abandoned croplands in the Eastern Siberian forest steppe zone (Haplic Luvisol) to assess the effects of cropland natural restoration on the stability of soil organic matter (SOM) and energy stocks. The energy content and thermal stability of C in the bulk soil, in free and occluded particulate organic matter (fPOM and oPOM), and in mineral-associated organic matter (MAOM) were analysed by thermogravimetry and differential scanning calorimetry as proxies for С available for microbial decomposition. The soils sequestered 0.85 Mg C ha−1 y−1 (0–30 cm) during the first 25 years after abandonment. In abandoned soils, thermally labile C with an activation energy of 69±0.6 kJ·mol⁻¹ accumulated 2.8 times faster than did stable C with a higher energy barrier to combustion (80±0.5 kJ·mol⁻¹). This increased energy availability and microbial activity led to faster C and nutrient cycling in the abandoned soils than in soils of current croplands. MAOM stored 1.7 times more energy per unit of C (42 kJ·g⁻¹ C) than did the free and occluded POM fractions (25 kJ·g⁻¹ C). fPOM was the dominant energy pool for labile C in the restored soil and increased microbial activity. Due to the preferential accumulation of C with lower thermal stability and with greater susceptibility to microbial decomposition in the restored soils, greater CO2 release could occur if these lands are tilled again. The content of thermally stable C also increased in abandoned soils; therefore, we must account for the ecological balance between C sequestration and release after the natural restoration of croplands.