Spatial variation of soil fertility and its influencing factors in Myrica rubra region: A case study in Xianju county and Linhai city

【Objective】Soil fertility, the key factor of fruit production, is usually influenced by diverse factors in major producing areas. The study aimed to clarify the vital factors closely associated with the spatial heterogeneity of soil fertility by different comprehensive evaluation systems. The specific goals were to alleviate the common problem of soil acidification to provide a theoretical basis for realizing the precise management of soil nutrients in the main districts of Myrica rubra production.【Methods】 According to the distribution of M. rubra main production area and suitable planting area, representative producing areas of Xianju county and Linhai city, Taizhou city, Zhejiang province were selected as study sites. 100 top soil (0-20 cm) samples were collected based on an intensive grid of 1 km×1 km.The spatial heterogeneity of soil fertility was studied by geostatistical methods. The soil fertility status of each region was evaluated by the integrated soil fertility evaluation index method, and the impact factors of soil fertility were explored by Pearson correlation analysis and Structural Equation Model (SEM).【Results】The results of soil physicochemical experiments showed that the average pH, organic carbon (OC), alkali- hydrolyzable nitrogen (AN), available phosphorus (AP) and available potassium (AK) of M. rubra soils in the study area were 4.48, 18.21 g · kg- 1 , 115.01 mg · kg- 1 , 15.57 mg · kg- 1 and 239.23 mg·kg-1 , respectively. Compared with the average value of soil nutrients in the main producing areas of M. rubra in China, the soil pH and AP were lower than the average value (4.95 and 24.65 mg·kg-1 ) respectively, and the OC and AK were higher than the average value (19.47 g·kg-1 and 62 mg·kg-1 ). The soil acidification was serious in our study sites, and the content of AP was generally lower than the average levels. The coefficients of variation of the five fertility indicators were 12.02%, 25.98%, 35.51%, 100.35%, and 53.74%, respectively. Except for the strong variation of AP, the others were all moderate, and all fertility indicators were greatly affected by fertilizer application. The nutrient- impoverished study area with low OC, AN, AP and AK were mainly distributed in the east region of Linhai City, and the nutrient-rich area with high OC, AN, AP and AK were distributed in the west region of Xianju County. The district with high pH was located in the East of Xianju county, while the district with low pH was located in the West of Xianju county and Linhai city. The convergent management of M. rubra forest had resulted in significant aggregation in high nutrient zones and acidification problems. The integrated soil fertility evaluation index showed that 91% of the soil samples in M. rubra forest reached medium and high fertility levels, only 9% of the soil samples were at low fertility level. Specifically, the Western part of Xianju county and the Western part of Linhai city were relatively fertile, while the Eastern part of Xianju county and the Eastern part of Linhai city were relatively impoverished. The results of the Pearson correlation analysis showed that the tree age was significantly positively correlated with the soil OC (p＜0.05), altitude was significantly and positively correlated with the soil AN (p＜0.01), while the correlation between slope and AK was significantly negative (p＜0.05). The results of the Structural Equation Model showed that the tree age had a significant positive effect on OC (p＜0.05), the altitude had a significant positive effect on AN (p＜0.05), a significant negative effect on AK (p＜ 0.05), and slope aspect had a significant negative effect on AK (p＜0.05). With the growth of M. rubra trees, the soil pH showed an overall trend of decreasing, indicating that the soil acidification caused by excessive intensive management and fertilization application was becoming increasingly serious. As the aging of M. rubra trees, the standing environment continued to be stable, and the carbon source was continuously replenished by anthropogenic activities. Consequently, the accumulation rate of organic matter was greater than the decomposition rate, and the soil OC showed a rising trend, so that the organic matter kept accumulating. The results of various research methods showed that the anthropogenic fertilization application was the dominant factor affecting the soil nutrients, and elevation, slope direction and tree age were also important factors affecting the soil fertility.【Conclusion】The increase of the fertilizer input and reinforce field management in the East M. rubra producing areas of Xianju county and Linhai city is suggested to relieve soil acidification and fertility imbalance. While in the West of Xianju county and Linhai city, it is better to reduce the fertilizer application appropriately. Besides, in our study area, more phosphorus fertilizer is needed because the overall soil phosphorus level is lower than the average value of main M. rubra production areas soils in China. However, in order to alleviate the problem of soil acidification caused by the excessive fertilizer application in the Western region of Xianju county and Linhai city, application of the quicklime, organic fertilizer and physiological alkaline fertiliz-er is needed for soil improvement.