Responses of soil total phosphorus to freeze and thaw cycles in a Mollisol watershed

Abstract

Freeze-thaw cycles (FTCs) change soil physiochemical properties and biogeochemical processes and inevitably influence the spatial distribution of soil phosphorus (P). In this study, a field investigation was conducted on the Mollisol soil of the Guangrong watershed from 2016 to 2018 to clarify the effect of FTCs on the spatial distribution of total phosphorus (TP) at soil depths of 0–5, 5–10 and 10–20 cm. A laboratory incubation experiment was conducted to reveal the effect of FTCs on TP in soil profiles (0–30 cm) in Mollisols. The results showed that (1) the spatial autocorrelation (nugget to sill ratio) of TP decreased by 5.92% after the FTCs while the spatial variance (coefficient of variation) increased by 17.19%. The TP in 85% of the watershed area decreased after the FTCs, and the mean content of TP decreased by 9.53%. These changes were mainly influenced by land use types, topographical factors, soil properties and human activities. (2) TP increased in the intersection between farmland and forestland (0.01 g kg−1) after the FTCs, while TP decreased both in the areas near the hydrological channel (0.06 g kg−1) and at the outlet of the watershed (0.13 g kg−1). Compared with down-slope tillage, maize planting, south-facing slopes and low vegetation coverage areas, the cross-slope tillage, soybean planting, north-facing slopes and high vegetation coverage areas reduced TP losses by 56.1%, 19.5%, 115.8% and 141.9%, respectively, in the 0–20 cm soil layer. (3) The effects of FTCs on TP in soil profile were mainly influenced by the soil moisture content, FTC frequency and soil bulk density in the laboratory incubation experiment. TP variability showed a V-shaped trend as the number of FTCs increased, and decreased as the initial moisture content increased at 1.1 g cm−3 of soil bulk density. (4) An equation based on the TP and mean value of normalized difference snow index, normalized difference vegetation index, ferrous minerals index and normalized difference soil moisture index from the freezing period to the thawing period was built to roughly predict the TP after FTCs (R2 = 0.72). Generally, the TP tended to decrease during both the freezing stage and the thawing stage in the field. Areas with down-slope tillage, existing maize fields, south-facing positions, low vegetation coverage and high snow coverage should be focused on to reduce P losses during FTCs.