Nitrogen-fixing tree species restructure soil nitrogen networks and enhance subsoil nitrogen accumulation in Eucalyptus mixed plantations

Title : Nitrogen-fixing tree species restructure soil nitrogen networks and enhance subsoil nitrogen accumulation in Eucalyptus mixed plantations

Abstract:

Eucalyptus monocultures often experience nitrogen (N) depletion and soil fertility decline. Mixing with N-fixing tree species is a promising solution, yet the mechanisms driving soil N transformation across the soil profile remain poorly understood. Here, we compared non-fixing plantations, including pure Eucalyptus plantations (PF) and an Eucalyptus mixture with the non-N-fixing broadleaf species Michelia macclurei (MF1), with Eucalyptus based N-fixing mixtures containing Alnus formosana and Dalbergia odorifera (MF2 and MF3). We assessed litter quality and soil physicochemical properties, N-related enzymes, N transformation rates, and bacterial communities in topsoil (0-20 cm) and subsoil (20-40 cm). Our results revealed that, compared with non-fixing plantations, N-fixing plantations exhibited lower litter C:N ratios and substantially higher inorganic N (IN), microbial biomass N, NO_3^--N, and NH_4⁺-N, indicating improved substrate quality and enhanced IN regeneration. Crucially, N-fixing tree species created favorable conditions that activated a robust N cycle in subsoil. This was evidenced by higher rates of ammonification, nitrification, and mineralization, coupled with increased protease and urease activities. Microbiologically, N-fixing mixtures assembled a more efficient N-transformation network, enriching keystone N-fixing and nitrifying taxa (e.g., Bradyrhizobium, Rhizobium, Nitrospira). Contrastly, non-fixing plantations were dominated by ammonifying / denitrifying taxa (Pseudomonas, Bacillus), leading to a more open N cycle. Overall, this study demonstrates that N-fixing tree mixtures can fundamentally reverse N depletion in Eucalyptus plantations by enhancing substrate quality and, most importantly, by activating a sustained N regeneration system in the subsoil. These results provide mechanistic evidence that N-fixing plantations can sustain soil fertility in plantations.