Effect of intercropping on balancing effect of absorption and desorption characteristics of phosphorus in red soil

The migration and environment effect of phosphorus in soil are affected by its’ adsorption and desorption. Although the excessive application of phosphorus fertilizer causes phosphorus fixation and loss, reasonable intercropping promotes the absorption and utilization and decreases fixation of phosphorus. This study investigated the adsorption and desorption of phosphorus in red soil under intercropping and phosphorus application, it is signicant for promoting the efficient utilization of red soil phosphorus and balancing environmental effects. In this study, a two-factor split-plot block experiment was adopted through field trials, in which the first factor was the planting pattern, namely maize||soybean intercropping and maize monoculture; the second factor was phosphorus application levels: P0 (0), P60 60 kg (P₂O₅)·hm⁻², P90 90 kg (P₂O₅)·hm⁻², and P120 120 kg (P₂O₅)·hm⁻². This study aimed to explore the effects of intercropping and application of phosphorus on the adsorption and desorption of phosphorus in red soil, and to quantitatively analyze the relative contribution of intercropping and phosphorus application to phosphorus adsorption and desorption by using the structural equation model, and to reveal the key intercropping effect factors on the adsorption/desorption of phosphorus in red soil by using the aggregated boosted tree methods. Results showed that: 1) the Langmuir isothermal adsorption equation was most suitable for fitting phosphorus adsorption in red soil. The adsorption amount of soil phosphorus increased first and then tended toward saturation with the increase in phosphorus concentration in the equilibrium solution, while the adsorption amount of phosphorus decreased gradually with the increase in phosphorus application. 2) Phosphorus adsorption and desorption in red soil were significantly affected by planting pattern, phosphorus application, and the interaction between planting pattern and application of phosphorus (P<0.01). Compared with monoculture, the maize||soybean intercropping increased the adsorption and desorption of phosphorus by 22.9% and 9.2%, respectively (P<0.05). Under four application rates of phosphorus, compared with monoculture, the adsorption of phosphorus in intercropping increased significantly by 13.0%, 19.4%, 41.5%, and 23.9% (P<0.05), respectively. The desorption of phosphorus increased significantly by 90.2% and 194.4% in P0 and P60 intercropping (P<0.05), but decreased by 52.1% and 34.1% in P90 and P120 intercropping, respectively (P<0.05). 3) Under different planting patterns and phosphorus application levels, the adsorption of soil phosphorus had a significant negative correlation with soil pH, organic matter, resin phosphorus, available phosphorus, total phosphorus, and degree of phosphorus saturation (P<0.01), and a significant positive correlation with free iron oxide, free alumina, and phosphate sorption index (P<0.01). However, the desorption of phosphorus from red soil had a significant negative correlation with a standard phosphorus requirement (P<0.01). The adsorption and desorption of phosphorus in the red soil were mainly affected by pH, organic matter, and free iron oxide. Intercropping of maize and soybean changed soil pH and contents of organic matter and free iron oxide, resulting in differences in the phosphorus adsorption and desorption from that of maize monoculture in red soil, improving the soil phosphorus buffering capacity. At a low phosphorus level, intercropping can accelerate a large amount of phosphorus desorption for plants to absorb and utilize; at high phosphorus levels, intercropping can promote phosphorus adsorption and effectively slow down the loss of phosphorus.