Long-term impacts of extensive terracing on soil aggregates and associated C-N-P in the Camellia oleifera orchard of southern China

In recent years, the primary development way of orchards has been large-scale mechanized terracing, which significantly disturbs soil profile and reduces soil quality. Soil aggregates and associated nutrients contents can be used as diagnostic markers of soil quality. However, the variation of these markers in Camellia oleifera orchards with land preparation methods (LPM) and plantation ages (PA) and the pathways by which they are affected remain unclear. Here, soil samples were collected from Camellia oleifera orchards developed on terrace and slope with different plantation ages (2, 5,10, 30 years) and soil depths (0-20 cm, 20-40 cm) to quantify them. The results showed the aggregate stability, SOC, TN, TP, C/P, and N/P ratios varied significantly with different LPM and PA. These indicators were significantly lower than the slope at the early stage of terrace construction. In particular, the MWD, aggregate-related C and N of terrace were 23.16 %, 45.92 %, and 23.53 % lower than the slope, respectively. With increasing plantation age, the difference between the terrace and slope gradually decreased. In 30-year-old plantations, some indicators of terrace even exceeded the slope, e.g., the MWD and aggregate-related C of terrace were 0.85 % and 1.32 % higher than the slope, respectively. This trend was more evident in the surface soil (0-20 cm). Meanwhile, it also showed that the newly built terraces were limited by C and N but gradually alleviated with increasing plantation age. The path analysis showed that the LPM had a greater effect on soil quality, mainly through aggregate composition and nutrients. Overall, although the soil quality of terraces would gradually improve with increasing plantation ages, it required a long time and economic cost. This study helps to deepen the understanding of soil nutrients change patterns after terracing and provides the theoretical basis for optimizing terracing.