Response of soil structure and fertility to long-term fertilization in alpine grasslands revealed based on fractal theory

Grasslands support various ecological functions, such as climate regulation, soil and water conservation, carbon turnover, and nutrient cycling. However, nearly a half of global grasslands have been degraded to varying degrees, especially on the Qinghai–Tibet Plateau (QTP), an extremely fragile ecosystem. As a primary measure of grassland restoration, fertilization largely affects soil aggregate size distribution (ASD) and particle size distribution (PSD). Fractal theory provides a powerful tool for analysis and characterization of soil ASD and PSD. However, our current understanding of soil ASD and PSD responses to fertilization remains limited. Therefore, this study investigated the response of soil structure and fertility to long-term fertilization in alpine grasslands on the QTP based on fractal theory. Long-term fertilization caused significant changes in soil ASD, which varied depending on fertilization practice, particularly in the topsoil (0–5 cm). Soil texture, which is influenced by soil parent material, exhibited a continuous vertical distribution along the depth and was not altered by fertilization. In addition, the results further confirmed that large microaggregates with specific pore structures contributed more to soil organic carbon (SOC) sequestration than did other size aggregates. The generalized dimension spectra showed varying increases (6.27–34.93 %) in the complexity or fineness (ΔD) of the soil PSD after fertilization. Capacity dimension (D0) decreased significantly with NP addition and was correlated strongly with plant species diversity and richness, whereas the information dimension (D1), D1/D0, and correlation dimension (D2) exhibited significant correlations with soil nutrients. During long-term fertilization, soil texture dominated the vertical distribution of the soil erodibility factor, and characteristic microaggregates were the key factors influencing change in soil microstructural stability. Overall, the findings highlight that selection of appropriate fertilization methods (single or combined applications) is vital for regulation and improvement of soil structure and fertility in alpine grasslands.