Mollisols are of major importance for food security worldwide but are increasingly degraded by soil erosion. Mollisols in Northeast China have been converted into agricultural use only since the nineteenth century, but gullies are widely distributed. Gully erosion history, rates and causes in this region remained unclear. We chose a study area with landforms and land-use history typical for the central Mollisol region of Northeast China to estimate the initiation years and rates of gully erosion from 1968 to 2018 by using aerial and satellite imageries. The outlet fan deposits of a large gully system were dated by Caesium-137 (137Cs) and artefacts. Local farmers were interviewed to verify the results. Gully volumes were measured by structure-from-motion technique using photos taken from an unmanned aerial vehicle. Our results showed that gully systems had already appeared on the steep slopes and along unpaved roads in 1968. They had become larger and more complex in 2018 by upslope retreat of the main gullies and side gully formation. Gully incision started in the 1950s and 1960s, when the original grassland and forest were completely converted into arable land. From 1968 to 2018, the gully density increased from 1.2 to 2.3 km km-2 and the gully heads retreated at speeds from 1.5 to 2.5 m yr-1. The soil loss from gully erosion ranged from 25.7 to 44.7 Mg yr-1 ha-1. These data demonstrate the severity of gully erosion in the Mollisol region of Northeast China and underline the importance of appropriate countermeasures.

Soil organic matter (SOM) is formed through partial decomposition and transformation of plant litter inputs. Thus, litter chemistry is generally regarded as the primary control over the formation efficiency of litter-derived SOM through selective preservation of recalcitrant litter fractions. Here we used model soils and showed that the SOM formation efficiency was, instead, controlled by discriminative protection of litter- and microbially-derived residues by different clay minerals. The SOM formation efficiency was higher for vermiculite than for kaolinite and illite because vermiculite protected more labile litter- and fungal residues through surface adsorption than did kaolinite or illite through pore entrapment. We developed a novel model to quantify mineral-protection strength following litter decomposition, and demonstrated that the mineral-protection strength explained well the variation in the SOM formation efficiency among the model soils, and could be predicted for a natural soil material from those of its compositional clay mineral types and their relative abundances in the soil. Our results provide solid evidence that soil clay mineralogy plays a critical role in SOM formation as known in long-term SOM stabilization, with important implications for model improvement to predict SOM dynamics for different soil types.