Effects of land conversion to cropland on soil organic carbon in montane soils of Northeast China from 1985 to 2020

Soil organic carbon (SOC) plays important roles in soil ecological function, soil conservation, and the global carbon cycle. With the increase of population and food demand, more and more croplands were reclaimed in mountainous areas. However, land conversion to cropland can lead to evident changes of SOC. The innovation of this study was to quantify the changes of SOC for different land covers in montane soils. Eight hundred twenty-nine topsoil samples were collected from dryland in Northeast China, and each soil sample was collected at least 200 m off the road at every 5-10 km interval, and five sites were mixed as one soil sample. Landsat 8 images were acquired from the bare soil period, and land cover data for 1985-2020 were obtained from Yang and Huang (2021). The surface reflectance of the 829 topsoil samples (0-20 cm) was extracted from Landsat 8 images. Then, a SOC prediction hybrid model was built after k-means clustering based on measured SOC using the potassium dichromate heating method, and a probability hybrid model was developed for SOC mapping. Land conversion to cropland was assigned to eight periods (i.e., five-year intervals from 1985 to 2020) based on land cover data, and the conversion percentages of different land cover types to cropland were calculated. Seven mountainous sub-regions were selected in Northeast China based on distribution of cropland and mountains to analyze the effects of land conversion to cropland on SOC. Our study showed that (1) the hybrid model had a higher accuracy than the global model, and the hybrid model led to the R-2 of 0.77 and RMSE of 4.66 g kg(-1) for validation samples. (2) Land conversion to cropland was mainly from forest in the 1980 s (proportion of 67.13%) and from grassland after 2015 (proportion of 52.24%). (3) SOC decreases with an average rate of 0.30 g kg(-1) per five years from 1985 to 2020, and the slope of land conversion to cropland increases with an average rate of 0.35 degrees per five years from 1985 to 2020; (4) SOC of land conversion to cropland from forests decreased by 2.04 g kg(-1), and wetland has an evident decrease trend (decrease by 1.84 g kg(-1)) and an increasing trend from barren (increase by 4.07 g kg(-1)). Our results suggested that land conversion to cropland has a negative effect on SOC in montane soils and should control land conversion to cropland to protect natural resources.