Accumulation of SOM fractions to croplands and plantations converted from cropland with black soil

Soil organic matter (SOM) decreases after the conversion of native grasslands to croplands and increases after the conversion of croplands to plantations. However, the reasons for loss and/or accumulation of SOM fractions remain unclear. Hence, we investigated SOM fraction quantity and quality of soils from cropland (CL), a plantation (PL) converted from cropland, and native grassland (NG) all converted from the black soil (Mollisols) region of China. Results showed that: (1) across all sites, coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM), and mineral-associated organic matter (mSOM) formed successively, and the mSOM carbon (C) content accounted for more than 70% of the bulk SOM C content. (2) The cPOM C contents in CL and NG were similar, but the fPOM and mSOM C contents in CL were only 11.35% and 52.95% of those in NG, respectively. The C contents of cPOM, fPOM, and mSOM in PL were 53.77%, 27.72%, and 72.80% of those in NG, respectively. (3) The accumulation of mSOM was affected by the quantity of fPOM, but had no obvious relationship with the quality of fPOM. However, the accumulation of fPOM was affected by both quantity and quality of cPOM. In general, promoting the accumulation of fPOM may increase the mSOM and SOM contents of CL, and increasing cPOM may be a feasible way to increase the SOM content of PL. The study provides a fundamental reference for promoting SOM accumulation of croplands and plantations in the black soil region.