Spatial distribution characteristics of soil organic matter and its influencing factors in small watershed of feldspathic sandstone region

Abstract: Feldspathic sandstone region is one of the main coarse sand source regions in the middle reach of Yellow River. Feldspathic sandstone region features complex and diverse landform with ravines and gullies criss-cross. It is a key region for implementing soil and water conservation and ecological restoration in China. Studying the spatial distribution characteristics of soil organic matter (SOM) had great significance to guide vegetation construction and reduce the input of coarse sand into Yellow River. Baojiagou small watershed is the most typical watershed among Feldspathic sandstone regions with the basin area of 12.67 km2. Thanks to the ecological immigration policy, the watershed is uninhabited and unaffected by a human presence. According to the results of the survey of the current situation of land use types in Baojiagou small watershed, forest and grassland are the main types of land use. Sampling points were set up by the grid method. In July and of 2018, soil samples were collected from 0-20, >20-40 and >40-60 cm in the plots under each samples. SOM contents were analyzed using sulfate digestion method with potassium dichromate and soil mechanical composition was measured by laser particle-size analyzer. Vegetation coverage was analyzed using photographic measurement. In this research, we studied the relationships between SOM and environmental factors in Baojiagou watershed through geostatistics analysis and constrained ordination. The results showed that soil organic matter content range from 0.98 to 15.45 g/kg. Ranking of SOM content according to topographic factors was: valley floor>slope> ridge. Exponential model fitted best to the semi-variance functions of soil organic matter content in the study areas, and the spatial variability of soil organic matter was at medium level, with the nugget effect ranging from 55.1 % to 57.5 %. SOM contents in the center parts of the study area were relatively high, whereas those in the edge parts were low. Redundancy analysis (RDA) demonstrated that the first axis and the second axis explained 96.1% and 2. 8% of the total variables of the SOM, The first two axes explained 99.3% of the relationship between the SOM and environmental factors. The correlation coefficient order of each environmental factor and the first ranking axis was the vegetation coverage > soil erosion degree > soil particle diameter > slope position > elevation > slope. Vegetation coverage and soil erosion degree had the greatest correlation with the first ranking axis, which was defined as vegetation-soil erosion factor, and slope factor had the largest correlation coefficient with the second ranking axis, which was defined as slope factor. Vegetation coverage was the dominant factor affecting the spatial distribution characteristics of SOM, soil erosion degree was the main factor affecting the vertical spatial distribution characteristics of SOM in feldspathic sandstone region. This paper suggests that in the shallow soil layer area, it is not suitable to carry out large scale land preparation and afforestation work and sealing measures should be taken. Trees can be planted in gully areas with deep soil and good water and fertilizer conditions.