N2-Fixing Tree Species Help to Alleviate C- and P-Limitation in Both Rhizosphere and Non-Rhizosphere Soils in the Eucalyptus Plantations of Subtropical China

The extracellular enzyme activity (EEA) and enzymatic stoichiometry (EES) of soil are useful indicators of shifts in soil nutrition and microbial resource requirements. Nevertheless, it is uncertain how the limitation of soil microbial nutrients is altered by a Eucalyptus plantation mixed with a N2-fixing tree species. Our study examined the microbial nutrient limitation in two plantations: a pure Eucalyptus plantation (PP) and a mixed plantation (Eucalyptus and Erythrophleum fordii, MP) in rhizosphere and non-rhizosphere soils, beginning with two indicators, soil EEA and EES. In this study, the soil EEA was considerably (p < 0.05) greater in the MP contrasted to the PP, and the enzyme C:N:P ratios of the PP (1.12:1:1.10) and MP (1.07:1:1.08) both diverged from the global average EEA (1:1:1), and the deviation degree of the PP was greater. The results of the vector analysis demonstrated that the vector angle (VA) and vector length (VL) were considerably (p < 0.05) smaller in the MP contrasted to the PP. In comparison to the PP, the MP had a considerably (p < 0.05) poorer carbon quality index (CQI). Additionally, both microbial and soil properties have a considerable impact on soil EEA and EES, according to variance partitioning analysis (VPA) and redundancy analysis (RDA). In summary, our results show that the restriction of microorganisms on C and P in rhizosphere soils is usually weaker than that in non-rhizosphere soils and that the addition of N2-fixing tree species to Eucalyptus plantations can lessen but not completely remove the restriction of soil microorganisms on C and P. Future management practices involving mixed plantations with N2-fixing trees species could help decrease microbial nutrient limitation and promote sustainable plantations.