Size Composition, Stability, and Distribution of Metal Nutrient Elements of Soil Aggregates of Eucalyptus Plantations with Different Thinning Intensities

Eucalyptus plantations suffer from soil degradation and reduced productivity due to short rotation cycles and multiple generations of replanting. This study investigated the effects of different thinning intensities (CK, 30%, 45%, and 60%) on the size composition, stability, and distribution of metal nutrient elements (K, Ca, Mg, Fe, Mn, Cu, and Zn) of soil aggregates in Eucalyptus plantations by collecting 0–20 cm soil samples and using the dry-sieving method to separate soil aggregates into four sizes (>2 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm). Our findings were as follows: (1) The majority of aggregates comprised larger sizes, predominantly exceeding 2 mm in diameter, which were the most abundant. (2) Compared with unthinned stands (CK) and stands that were thinned by 30%, those thinned by 45% and 60% demonstrated enhanced soil aggregate stability. (3) The stands that were thinned by 30% had the highest Mg and Fe content, whereas those that were thinned by 45% contained the highest levels of Ca, Mn, Cu, and Zn. Larger aggregates (>2 mm) harbored the greatest quantities of metal nutrients, whereas smaller aggregates (<0.25 mm) stored the least. (4) The primary determinants of the metal nutrient content were the soil’s pH and organic carbon levels. (5) The distribution of aggregate sizes played a pivotal role in influencing the nutrient reserves within the aggregates. Overall, this study demonstrated that the thinning intensity not only impacts the stability of soil aggregates in Eucalyptus plantations, but also influences the accumulation of metal nutrient elements within these aggregates, which confirms the significance of macroaggregates as a reservoir for metal nutrient elements. To preserve and enhance soil macroaggregates, it is recommended to implement measures such as reducing the amount of mechanical disturbance, increasing the amount of organic matter, optimizing the stand structure, mitigating water erosion risks, and promoting biological activity while conducting regular assessments of the aggregate stability.