Spatial variation characteristics and influencing factors of soil nutrients in the alpine forest-grassland ecotone at Minjiang headwaters

The alpine treeline ecotone is considered a warning line in response to global climate change. Changes in the ecological characteristics of vegetation types and species populations in the interlaced forest and grass belt are affected by climate change, soil nutrient status, and eco-chemical measurement characteristics. The status of the main soil nutrients in this sensitive ecological zone and its altitude gradient characteristics remain unclear. In early June 2019, soil samples were collected from 0 − 10 and 10 − 20 cm soil layers in two high mountain forest-grassland ecotones of the Minjiang River source area in different vegetation zones (meadow area: alpine meadow, tree species line: subalpine shrub zone, tree line: arbor shrub zone, and closed forest: dark coniferous forest) to determine soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), soil pH, and soil moisture content. Soil carbon, nitrogen, and phosphorus and the spatial distribution characteristics of water content and its influencing factors were analyzed. The results showed that vegetation type was the main factor affecting the physical and chemical properties of the soil in the forest–grassland ecotone in the Minjiang River area. The contents of total carbon (TC), TN, and SOC in the meadow area were significantly lower than those in the other vegetation zones (P < 0.05), and there was no significant difference in the contents of TC, TN, and SOC in the tree species line, tree line, and closed forest (P > 0.05). The TP content in the tree line was significantly lower than that in the other vegetation zones (P < 0.05). There was a significant correlation among TC, TN, SOC, and soil variable water content (P < 0.01). The correlation of soil physical and chemical properties was the weakest in the tree species line. The soil stoichiometric ratio was the largest in the tree line, and the spatial heterogeneity of soil C∶P and N∶P was strong in this vegetation type belt. The results showed that although there was no significant difference in the content of C, N, and P in the tree species line, tree line, and closed forest (P > 0.05), the interaction between the tree line and vegetation on soil physical and chemical properties was significant (P < 0.05).