Quantifying sensitive soil quality indicators across contrasting long-term land management systems: Crop rotations and nutrient regimes

Managing the land properly can help conserve the soil which is critical for sustaining our life and the global society. However, measuring the quality of the soil explicitly is still a challenge. Therefore, this study was conducted to identify suitable soil quality indicators among contrasting land managements (i.e., simple vs. complex crop rotations; manure vs. balanced fertilization) at two long-term experimental fields. The fractal structure of soils was documented by the mass-diameter relationship of soil aggregates using 3D laser scanning. Hydraulic conductivity (K), pore size fractions, and soil physical S parameter (S-index) were determined from moisture retention curves using a HYPROP system. Soil microbial community structure was characterized using phospholipid fatty acid analysis. Our results demonstrated an improved hierarchical fractal aggregation in soils under perennial legumes and grasses (Dm = 0.97) compared to nonfractal aggregation under fallow phases (Dm = 0.99). In addition, across nutrient managements, only balanced fertilization exhibited significantly enhanced fractal aggregation. Moreover, complex crop rotations and balanced fertilization also improved S-index, saturated water content (sat. WC), and plant available water (PAW) compared to their counterparts (Ps < 0.05). Similarly, significant differences between simple and complex rotations were evident for microbial community composition and biomass carbon (MBC) with 1.5 times higher MBC in the soils under complex rotation compared to both the adjacent forest soil and the simple 2-yr rotation. Our results proved that complex crop rotations including perennials enhanced soil quality, and this outcome was associated with higher crop productivity. Both balanced fertilization and manure contributed to improving soil functions: where cattle manure had a stronger positive effect on nutrient cycling, while balanced fertilization beneficially influenced water relationships and soil physical condition. Our results suggest that the Dm, S-index, PAW, soil organic C, and MBC are robust indicators useful for evaluating management options that also influence agricultural productivity.