Soil C-N-P Stoichiometric Characteristics at the Aggregate Scales in Eucalyptus Plantations with Different Stand Types in Subtropical China

Studying the patterns of change in soil aggregate stability and ecological stoichiometry in different forest types is of significant importance to the research on the distribution, limitations, balance, and cycling of soil organic carbon, nitrogen, and phosphorus (C-N-P). However, the impacts of pure Eucalyptus forests and mixed artificial forests on the stability of aggregates and the stoichiometric characteristics of organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) remain unclear. Samples were collected from the 0-20 cm and 20-40 cm soil layers of a Eucalyptus-Mytilaria laosensis mixed forest, a Eucalyptus-Erythrophleum fordii mixed forest, and a pure Eucalyptus forest. Soil samples were dry sieved into four types of aggregates according to particle size (> 2 mm, 1-2 mm, 0.25-1 mm, and < 0.25 mm). The OC, TN, and TP contents and composition were measured to evaluate the soil structure stability; soil OC, TN, and TP stock potential; and ecological stoichiometric characteristics. Our results showed that the structural stability of the soil and the contents and stocks of OC, TN, and TP were significantly higher in mixed forests than in the pure Eucalyptus forest: the 0.25- to 1-mm-sized aggregates had the highest soil OC, TN, and TP contents, and the aggregates > 2 mm in size had the greatest contribution to the soil OC, TN, and TP stocks in bulk soil. The aggregates > 2 mm in size generally had the lowest C/N, C/P, and N/P ratios (0-20 cm). The mean weight diameter (MWD) was the main factor affecting the stoichiometric characteristics of the 0-20 cm soil layer. The Eucalyptus-M. laosensis mixed forest promoted soil-aggregate stability and improved soil fertility. Increasing the proportion of soil aggregates with large sizes (> 2 mm) is key to improving soil ecological environments. Thus, selecting suitable tree species mixed with Eucalyptus can play a vital role in alleviating the reduction of soil aggregate stability, thereby causing the accumulation of soil C, N, and P contents and protecting soil productivity, quality, and health in subtropical China.