Effects of different fertilization methods on soil quality of rubber plantation on Hainan Island

Sustainable development of natural rubber (Hevea brasiliensis Muell. Arg.) industry is of great significance to economic development and ecological security in the tropics. Depletion of soil fertility is a key issue in rubber plantations that requires urgent resolution. Fertilization is regarded as a direct method for enhancing soil quality and improving the soil microenvironment, and the excessive application of chemical nitrogen fertilizers leads to nitrogen leaching and soil degradation. The combination of chemical nitrogen and organic fertilizers can enhance soil nutrition, enhance soil quality, and improve soil productivity. However, the effects of different fertilization methods on the soil quality of rubber plantations remain not full unclear. A 12-year field experiment was conducted on a rubber plantation in Hainan, China. Four fertilization treatments were implemented: chemical fertilizer application alone (NPK), the total amount of fertilizer applied was 1.0 kg/tree chemical fertilizer (N:P₂O₅:K₂O=15:9:6), a combination of 50% chemical N fertilizer and 50% co-composted organic N fertilizer application (NPKO), a combination of 50% chemical N fertilizer and 50% composted organic N fertilizer application (NPKM), and a rubber plantation in the same area with no fertilizer application as a control (CK). All fertilization (chemical or organic fertilizer) treatments had the same total N application rate. Soil samples were collected from the 0～10 cm and 10～20 cm soil layers, and the soil physicochemical properties, enzyme activities, and microbial carbon and nitrogen were analyzed. Principal component analysis was employed to establish a minimum data set method for a comprehensive evaluation of the soil quality under various fertilization treatments. Our results indicated that significant differences were observed in soil nutrient levels among the four fertilization treatments. 1) Compared with CK, NPKO, NPKM, and NPK treatments significantly increased soil pH, organic carbon, and total nitrogen contents in the 0～20 cm soil layer by 2.39%～12.92%, 17.72%～96.51% and 32.28%～97.20%, respectively. 2) The NPKO treatment significantly increased the total phosphorus, total potassium, and soil available nutrient content in the 0～20 cm soil layer, whereas none of the different fertilization treatments significantly affected the carbon, nitrogen, and phosphorus stoichiometric ratios. 3) Urease activity in the 0～10 cm soil layer and acid phosphatase, β-1,4-glucosidase, and β-1,4-N-acetamido-glucosidase activities in the 10～20 cm soil layer were significantly lower in the NPKM treatment than in the CK, whereas the remaining enzyme activities did not show significant changes in all treatments. 4) All three fertilization treatments significantly increased soil microbiomass carbon and nitrogen content, with NPKO and NPKM increasing more significantly than NPK. 5) The evaluation index systems of TDS and MDS were used to calculate the soil quality indices of the rubber plantations under the different fertilization treatments. The scatter plot of the soil quality index fitted to the total and minimum datasets showed a significant positive correlation (R² = 0.85, P< 0.001). MDS is expected to replace TDS better than the cultivated horizon soil quality index. Nine indicators were selected for the MDS evaluation index system using principal component analysis (PCA): SOC, TK, TP, ACP, NH₄⁺-N, CAT, NAG, C: N, and MBN. The soil quality index ranged from 0.31～0.78, averaging 0.52, with NPKO>NPKM>NPK>CK, generally at medium levels. Compared with the CK treatment, the soil quality index for NPK did not exhibit a significant improvement. However, the soil quality for NPKO and NPKM at depths of 0～10 cm and 10～20 cm increased by 75.19%, 110.76%, 36.20%, and 16.34%, respectively (P < 0.05). Consequently, the combined application of organic and chemical fertilizers can effectively alleviate soil acidification, improve soil nutrient status, and improve soil comprehensive fertility levels; the enhancement in soil nutrients and quality following the NPKO treatment was the most pronounced. Therefore, this study suggests that the combination of organic fertilizer and chemical fertilizer, particularly composted organic fertilizers, could serve as a viable fertilization strategy to enhance soil fertility and improve soil quality in rubber plantations on Hainan Island.