Objective We studied the spatio-temporal evolution characteristics of soil fertility indicators and the variation of soil fertility grades in Changyang Tujia Autonomous County, aiming to provide a scientific basis for sustainable agriculture production in this region.

Methods We collected soil pH, organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK) data from Changyang County during 2007−2015, 2018, and 2022. The weights of each indicator were determined using the correlation coefficient method. Membership function curves were chosen and the turning points of the membership functions were identified. The fuzzy mathematics (Fuzzy) comprehensive evaluation method was employed to calculate the comprehensive soil fertility index (IFI) based on the weight values and membership values of each soil fertility indicator. The spatio-temporal evolution characteristics of each soil fertility indicator and the comprehensive soil fertility index (SFI) were analyzed using geostatistics and geographic information system (GIS) technologies.

Results From 2007 to 2022, spanning a 15-year period, the average values of soil pH, organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK) increased by 0.47 units, 1.24 g/kg, 0.57 g/kg, 38.67 mg/kg, and 119.44 mg/kg, respectively, all showing moderate variability. Between 2018 and 2022, the area of neutral farmland expanded by 29.41%, regions with SOM levels≥Grade II rose by 14.04%, with TN≥Grade II decreased by 17.92%, with AP≥Grade I and AK≥Grade II increased by 81.13% and 42.45%, respectively. The nugget coefficients for pH and TN fell within the 25% to 75% range, indicating moderate spatial correlation, whereas those for SOM, AP, and AK were below 25%, suggesting strong spatial correlation. In 89% of Changyang County's farmland, the soil fertility index (SFI) reached Grade III or higher, with the area of farmland having an SFI≥Grade II increasing by 7.87% from 2018 to 2022. SOM and TN were identified as the primary factors contributing to variations in soil fertility. The spatial distribution patterns of SFI were comparable to those of SOM and TN, with farmland having an SFI≥Grade II predominantly concentrated in the northern, western, and central regions, while the eastern and southern regions displayed medium to lower soil fertility levels.

Conclusion Over the 15-year period, the farmlands in Changyang County have transitioned from acidic to neutral, with SOM, AP, AK, and soil fertility levels consistently increasing. The soil fertility index has reached relatively high levels. However, the distribution of high-fertility farmlands across the county is uneven. SOM and TN are the key factors driving the improvement in the Integrated Fertility Index (IFI). Therefore, significant consideration should be given to applying organic amendments to enhance SOM levels, thereby promoting high nitrogen sequestration and fertility potential.