Responses of available phosphorus in different slope aspects to seasonal freeze-thaw cycles in Northeast China

•Researched on influence of topography on soil AP during seasonal FTCs.•Sunny slope was much more sensitive to FTCs than that in shady slope.•Good relationships exist between AP and SOM in both sunny slope and shady slope.•There were no clear relationships between AP and SMC in field. Freeze-thaw cycles (FTCs) are one of the main factors that alter soil physicochemical properties, consequently affecting soil productivity. However, there is limited research regarding the responses of available phosphorus (AP) in different slope aspects to seasonal FTCs. In this study, two slopes with different aspects, sunny (Ssn) and shady (Ssd), were selected in Meihekou, Northeast of China, located at a high latitude with the seasonal monsoonal climate. Spatio-temporal changes in AP, soil organic matter (SOM), and soil moisture content (SMC) on Ssn and Ssd were analyzed during the seasonal freeze-thaw period at seven sampling times from October 2015 to March 2016. Each sampling included different slope positions and soil depths (5 [D5], 10 [D10], 15 [D15], and 20 cm [D20]). Seasonal FTCs significantly (p < 0.01) affected AP content on both Ssn and Ssd. The AP trends over time were similar between Ssn and Ssd, i.e., initial increase and subsequent decrease, peaking in February. Ssn was considerably more sensitive to FTCs, especially in the surface soil. Overall, after seasonal FTCs, the AP content increased to 464.9 % and 68.2 % on Ssn and Ssd, respectively. Regarding soil depth, AP content increased the maximum at soil depth D5 on Ssn and Ssd with an increment rate of 563.2 % and 122.0 %, respectively. Regarding slope positions, AP content of the down slope increased the maximum (64.9 mg kg−1), with a 774.6 % increment rate on Ssn, while on Ssd, it increased the maximum on the middle slope (23.4 mg kg−1), with an increment rate of 114.3 %. SOM content of Ssd was significantly higher than that of Ssn (p < 0.01), and SOM content of the lower slope was significantly higher than that of the upper slope, which was more apparent on Ssd. For the entire slope, good power function relationships existed between AP and SOM on both Ssn (r2 = 0.59) and Ssd (r2 = 0.80). However, because of the continuous change in SMC, there were no clear relationships between AP and SMC in the field. Our findings demonstrate that slope aspect is a key driver of the different responses of AP during the seasonal freeze-thaw process, and that slope information could be useful for understanding the funct