Tree species identity affects nutrient accumulation and stoichiometric in soil aggregates in mixed plantations of subtropical China

The conversion of monocultural to mixed plantations is an increasing and widespread practice worldwide to improve forest productivity and soil fertility. Although tree species identity can affect soil ecological processes, there is still limited research on how it affects soil aggregate stability and nutrient variability. Here, we investigated the variation of 0–100 cm aggregate stability index (percent of aggregate destruction, PAD; fractal dimension, D; geometric mean diameter, GMD; mean weight diameter, MWD), associated carbon (C), nitrogen (N), phosphorus (P), and stoichiometry in mixed plantations from coniferous trees (Pinus massoniana Lamb.) and a variety of individual broadleaved trees (Camellia oleifera Abel, Manglietia chingii Dandy, Cercidiphyllum japonicum Sieb. et Zucc., Michelia maudiae Dunn, and Bretschneidera sinensis Hemsl.). We discovered that compared to coniferous plantations, most mixed plantations significantly improved the mechanical and water stability of topsoil aggregates. Deep-rooted tree species (e.g., Michelia maudiae Dunn) favored the development of subsoil macro-aggregates to improve aggregate water stability (e.g., PAD = 46.10, MWD = 1.23, GMD = 0.49, D = 2.81). Mixed planting led to the enrichment of soil C, N, and P in microaggregate, which increased by at least 37.14, 20.24, and 61.11 %, respectively. The reduced C:P and N:P in soil aggregates indicate that soil P limitation may be alleviated. Furthermore, tree species and soil depth indirectly affected aggregate stability to regulate the accumulation of C, N, and P. Overall, we emphasize the necessity of selecting tree species combinations with complementary ecological niches in mixed plantations to improve soil stability and support forest productivity.