Soil fertility responses to dry-season irrigation with rubber-processing wastewater in a tropical rubber plantation

Xishuangbanna is a major natural rubber production region in China, characterized by extensive monoculture rubber plantations, and a distinct dry-wet seasonal climate. To alleviate dry-season water stress and its constraints on rubber tree growth, a three-year field experiment was conducted in a mature rubber plantation using rubber-processing wastewater as an alternative irrigation source. The experiment included a dry-season wastewater irrigation treatment applied from December to the following March (WW treatment), and a non-irrigated control (CK treatment). The results showed that soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and pH consistently declined with increasing soil depth; however, vertical stratification was less pronounced under WW treatment. Compared with CK, the WW treatment significantly increased soil nitrate nitrogen (NO3−−N), and available sulfur (AS) concentrations (p < 0.05). Wastewater irrigation reduced the soil C/N and C/P ratios, and slightly increased the N/P ratio in 0–20 cm, indicating a shift in nitrogen–phosphorus supply balance. Comprehensive nutrient scoring, integrating multiple soil fertility indicators, showed a modest improvement in nutrient status under WW treatment. Overall, dry-season wastewater irrigation altered soil nutrient distribution and stoichiometric relationships, and resulted in limited short-term improvement in soil fertility in rubber plantations. However, SOC, TN, TP, and total potassium (TK) remained at relatively low levels, suggesting that wastewater irrigation alone is insufficient to substantially enhance soil nutrient status. These findings highlight the need for long-term field studies and integrated soil fertility management strategies to evaluate and ensure the sustainability of wastewater irrigation in rubber plantation systems.