The Spatiotemporal Variability of Soil Available Phosphorus and Potassium in Karst Region: The Crucial Role of Socio-Geographical Factors

Abstract

The contents of soil available phosphorus (AVP) and potassium (AVK) in karstic mountainous agricultural areas have changed rapidly in recent decades. This temporal variation displays strong spatial heterogeneity due to these areas’ complex topography and anthropogenic activities. Socio-geographical factors can reflect the changes in the natural environment caused by human beings, and our objective is to enhance understanding of their role in explaining the changes of AVP and AVK. In a typical karst region (611.5 km2) with uniform soil parent material and low climatic variability, 255 topsoil samples (138 in 2012 and 117 in 2021) were collected to quantify the temporal AVP and AVK changes. Random forest (RF) and partial dependence plot analyses were conducted to investigate the responses of these changes to socio-geographical factors (distance from the nearest town center [DFT] and village density [VD]), topography, biology, and landscape pattern indexes. The mean values of AVP (48.25 mg kg−1) and AVK (357.67 mg kg−1) in 2021 were significantly (p < 0.01) higher than those in 2012 (28.84 mg kg−1 and 131.67 mg kg−1, respectively). Semi-variance analysis showed strong spatial autocorrelation for AVP and AVK, ranging from 7.29% to 10.95% and 13.31% to 10.33% from 2012 to 2021, respectively. Adding socio-geographical factors can greatly improve the explanatory power of RF modeling for AVP and AVK changes by 19% and 27%, respectively. DFT and VD emerged as the two most important variables affecting these changes, followed by elevation. These three variables all demonstrated clear nonlinear threshold effects on AVP and AVK changes. A strong accumulation of AVP and AVK was observed at DFT < 5 km and VD > 20. The AVP changes increased dramatically when the elevation ranged between 1298 m and 1390 m, while the AVK changes decreased rapidly when the elevation ranged between 1350 m and 1466 m. The interaction effects of DFT and VD with elevation on these changes were also demonstrated. Overall, this study examined the important role of socio-geographical factors and their nonlinear threshold and interaction effects on AVP and AVK changes. The findings help unravel the complex causes of these changes and thus contribute to the design of optimal soil phosphorus and potassium management strategies.