Identification of Yield-Limiting Factors Associated with Spatiotemporal Differentiation of Soil Nutrients in Long-Term Continuous Sugarcane Cultivation Fields

This study aimed to reveal the spatiotemporal variation in characteristics of soil chemical properties and their relationship with sugarcane yield under long-term continuous cropping, identify the key soil factors limiting sugarcane productivity, and provide a scientific basis for sustainable soil management in sugarcane fields.

A space-for-time substitution approach was adopted with five continuous cropping duration treatments: CK (0 a), T1 (1 a), T2 (10-19 a), T3 (20-29 a), and T4 (>35 a). Sugarcane yield and soil chemical properties within the 0-60 cm profile (stratified into 0-20, 20-40, and 40-60 cm layers) were systematically analyzed, including pH, soil organic matter (OM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), available potassium (AK), and available micronutrients contents. The integrated fertility index (IFI) was calculated using factor analysis combined with correlation coefficient analysis and the Nemerow composite index method. Two machine learning algorithms-Extreme Gradient Boosting (XGBoost) and Random Forest (RF)-were employed to quantify the relative contribution of individual soil chemical indicators to sugarcane yield.

(1) Long-term continuous cropping significantly reduced sugarcane yield (P<0.05), with a 13.68% decline observed in the T4 treatment (>35 years) compared with the CK, and induced deterioration in soil chemical properties. (2) Soil acidification was intensified under continuous cropping, exhibiting significant layer-specific characteristics. Compared with the control (CK), the mean soil pH decreased to 4.60 after more than 20 years of continuous cropping. The subsurface layer (20-40 cm) exhibited the most severe acidification, with pH values significantly lower than the CK in the T3 and T4 treatment groups. (3) Soil nutrients exhibited obvious surface enrichment and accumulation of certain elements. The contents of soil organic matter, total nitrogen, total phosphorus, alkali-hydrolyzable nitrogen, and available phosphorus in the surface layer (0-20 cm) were significantly higher than in deeper layers. Prolonged continuous cropping led to significant accumulation of available potassium and available zinc, increasing by 320.47% and 164.13%, respectively, in the surface layer. (4) The soil integrated fertility index (IFI) decreased significantly with soil depth. Although long-term continuous cropping improved average fertility in the surface layer, it exacerbated the variability and spatial heterogeneity of fertility in deeper soil layers. (5) Both XGBoost and RF analyses consistently identified soil pH in the 20-40 cm layer as the primary limiting factor determining sugarcane yield, with a relative importance contribution substantially greater than that of surface available nutrients and other soil indicators.

Under long-term continuous sugarcane cultivation, soils exhibited a spatiotemporal soil pattern characterized by surface nutrient enrichment and severe subsurface acidification. Acidification in the 20-40 cm soil layer constitutes the core obstacle restricting sugarcane productivity. In practical production, management strategies should shift from surface-oriented nutrient application to deep amelioration targeting subsurface acidification.