Objectives Aiming at the needs of soil degradation control and trace metal activity regulation in the black soil region of Northeast China, we studied the synergistic regulatory effects of long-term organic material application on soil aggregate structure and the fraction distribution of copper (Cu) and zinc (Zn), and the threshold response laws between aggregate size classes and trace metal fractions were proposed.

Methods A long-term black soil field experiment, located at the Teaching and Research Base of Jilin Agricultural University, had lasted for 15 years when the research was carried out. The experiment had five treatments, including a chemical fertilizer control (CK) and four organic material treatments at the base of same NPK rate with CK: chicken manure (JF), mushroom dreg (JZ), leymus chinensis (MC) and aspen leaves (SY). In the first year (2010), eight years (2017) and 15 years (2024) of the experiment, 0−20 cm layer soil samples were collected, and soil aggregates were separated into four size classes (>2 mm, 0.25−2 mm, 0.053−0.25 mm, and <0.053 mm) using the wet sieving method. The BCR sequential extraction method was employed to analyze the fraction distribution of Cu and Zn in aggregates of each size class, and the RF model was used to identify the key driving factors of aggregate quality and their nonlinear responses.

Results Compared with CK, all organic material treatments increased the total contents of Cu and Zn in aggregates of all sizes, with JF treatment showing the most significant increase. Compared with CK, except for JF and JZ treatments in silt-clay aggregates, all organic material treatments reduced the acid-extractable Cu content by 23.09%−72.10% in aggregates of all size classes; Except for SY treatment in macro-aggregates and silt-clay aggregates, the reducible Cu content increased by 6.68%−208.87%; MC and SY treatments drove the transformation of Cu to stable fractions in all aggregate size classes, with the Cu transfer factor decreased by 9.79%−51.77%. Long-term organic material application generally increased the oxidizable Zn and residual Zn contents in aggregates of all size classes. In small- and micro-aggregates, JF and JZ treatments reduced the Zn transfer factor by 4.54%−20.16%, promoting Zn transformation to stable fractions, while all organic material treatments in other size classes drove Zn transformation to available fractions. The soil aggregate composition was dominated by small aggregates, accounting for 48.89%−56.70% of the total, and the proportion of macro-aggregates was significantly increased by 53.83%−157.05%, compared with CK. The regulation of Cu and Zn fractions and the optimization of aggregate structure under different organic materials combined to form a spatial synergistic retention mechanism of “physical interception-chemical fixation”. RF model (R²=0.94) showed that aggregate size class was the core driving factor affecting aggregate quality, with a contribution rate of 36.41%. The key thresholds were 7% for the proportion of acid-extractable Zn and 53% for the proportion of residual Zn, and the proportion of reducible Cu exhibited an “N-shaped” dose-response characteristic.

Conclusion Long-term organic material application can increase the total contents of Cu and Zn in black soil, regulate their fractions, improve the proportion of macro-aggregates, and optimize soil structure. The fraction differentiation dominated by the chemical properties of different organic materials, combined with the physical structure of aggregates, collectively constitutes the spatial synergistic retention mechanism of “physical interception-chemical fixation”. RF model can quantify the key thresholds and nonlinear responses in this mechanism, identify the specific laws of Cu and Zn fractions affecting aggregate composition, and reliably predict aggregate quality based on the proportion of each fraction.