Soil Quality Assessment in Reclaimed Coastal Paddy Fields: A Case Study from Eastern China

Assessing the soil quality of coastal reclamation areas is fundamental to alleviating land resource scarcity in coastal cities and ensuring food security. However, it remains unclear how to evaluate soil quality in reclaimed areas and identify factors driving its variation. In this study, paddy soils from four typical reclaimed coastal areas in China (Yueqing, YQ; Longgang, LG; Rui’an, RA; Longwan, LW) were investigated to construct a minimum data set via principal component analysis and to calculate soil quality indices (SQIs). YQ had higher contents of soil organic carbon (SOC: 22.82 g kg−1), total nitrogen (TN: 0.14 g kg−1), total water-soluble salts (TWS: 3.09 g kg−1), cation exchange capacity (CEC: 21.77 cmol(+) kg−1), and available Fe (44.23 mg kg−1), Mn (36.63 mg kg−1), Cu (30.31 mg kg−1), and Zn (8.79 mg kg−1) than the other sites. RA exhibited significantly higher activities of β-glucosidase (BG: 32.79 nmol g−1 h−1), xylanase (XYL: 5.88 nmol g−1 h−1), N-acetyl-β-D-glucosaminidase (NAG: 18.00 nmol g−1 h−1), leucine aminopeptidase (LAP: 27.01 nmol g−1 h−1), and acid phosphatase (PHOS: 54.50 nmol g−1 h−1) than the other sites (p < 0.05). LW had the greatest bacterial and fungal abundances, whereas LW displayed the highest fungal diversity. RA showed the highest SQI (SQIw: 0.49; SQIa: 0.48), followed by LW (SQIw: 0.47; SQIa: 0.47), LG (SQIw: 0.43; SQIa: 0.41), and YQ (SQIw: 0.38; SQIa: 0.41). Random forest analysis indicated that soil enzyme activities (XYL, NAG, BG, CB, PHOS), nutrients (TN, SOC, AN), fungal abundance and diversity, and TWS were key SQI predictors (Welch ANOVA p = 0.016), with XYL being the strongest predictor (p < 0.05). Collectively, soil quality in coastal reclamation areas is co-regulated by microbial metabolic activity, nutrients, and salinity, with soil enzyme activity serving as an important indicator for its assessment, thereby deepening the understanding of soil quality dynamics within these areas and enabling their sustainable management. However, the microbial mechanisms underlying these patterns across broader environmental gradients remain to be explored.