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Summary: Introducing N2‐fixing tree species into Eucalyptus plantations could replace nitrogen (N) fertilization to maintain high levels of N consumption and productivity. However, N enrichment may exacerbate phosphorus (P) limitation as Eucalyptus robusta Smith is extensively planted in P‐poor tropical and subtropical soils.We conducted a field experiment in a pure plantation of Eucalyptus urophylla × grandis to investigate the impacts of N fertilization and introduced an N2‐fixing tree of Dalbergia odorifera T. Chen on soil P transformation.Nitrogen fertilization significantly enhanced soil occluded P pool and reduced the other P pools due to acidification‐induced pH‐sensitive geochemical processes, lowering Eucalyptus leaf P concentration with higher N : P ratio. By contrast, introduced N2‐fixing tree species did not change soil pH, labile inorganic P pool, and Eucalyptus leaf N : P ratio, even enhanced organic P pools and reduced occluded P pool probably due to altering microbial community composition particularly stimulating arbuscular mycorrhiza fungal abundance.Our results revealed differential responses and mechanistic controls of soil P transformation in Eucalyptus plantations with N fertilization and introduced N2‐fixing tree species. The dissolution of occluded P pool along with organic P accumulation observed in the mixed plantations may represent a promising future to better manage soil P availability. [ABSTRACT FROM AUTHOR]

N2-fixing species play a crucial role in mixed-plantations as they improve stand productivity. To quantify the N transfer from N2-fixing species to Eucalyptus (Eucalyptus urophylla × E. grandis) in N2-fixing species/Eucalyptus plantations, we established a pot experiment and confirmed the occurrence of this process under natural conditions. The 15N was traced in labeled species as well as in neighboring tree species after labeling, and the growth was evaluated in short-term natural trials. Our results showed that a bidirectional N transfer occurred. The amount of net N transfer was 21.8–127.0 mg N plant−1, which was equal to 1.5–21.2% of the total nitrogen (TN) that accumulated in Eucalyptus plants under pot conditions, was transferred from Dalbergia odorifera to Eucalyptus. The amount of N transferred significantly decreased with the increasing N application rate but increased with time after labeling. Compared with the results for the Eucalyptus monocrop, the soil N concentration (including NO3−-N and NH4+-N) greatly improved when D. odorifera was introduced together with Eucalyptus under both field and pot conditions. Furthermore, the results under field conditions were consistent with the results of the pot experiment. The dry matter (DM) yield (14.5–16.4%) and the N content (5.1–9.6%) in Eucalyptus increased when mixed together with D. odorifera, but the N content in and DM yield of D. odorifera slightly decreased. It is concluded that the N transfer between Eucalyptus and D. odorifera is a much more important dynamic process than previously recognized, and Eucalyptus and legume intercropping is a successful management practice because N transfer provides a significant amount of N required for Eucalyptus productivity. [ABSTRACT FROM AUTHOR]

• The combination of 15N dilution and 15N leaf labeling method was used. • We verified the nitrogen transfer from legume trees (Dalbergia odorifera) to eucalyptus. • The findings provide some evidence in support of eucalyptus and legume trees mixed systems. There is substantial evidence that legume-derived nitrogen (N) is transferred to neighboring nonlegumes in herbaceous mixtures and intercropping systems. However, considering that woody species are key sources and major consumers N, information regarding whether N transfer from leguminous to nonleguminous woody species in mixed plantations was detected. To detect N transfer in a Eucalyptus urophylla × Eucalyptus (E.) grandis and Dalbergia odorifera mixed system, two pot experiments were set up, one with 15N dilution and the other with 15N leaf labeling of Dalbergia odorifera. Seedlings were fertilized with CO (15NH 2) 2 urea, and the effects on interspecific N transfer and biological N fixation (BNF) efficiency were observed. Results showed that: (1) The values of competition ratio of Eucalyptus urophylla × E. grandis to Dalbergia odorifera (CR ED) greater than one indicate that Eucalyptus urophylla × E. grandis was a stronger competitor and had the absolute advantage over Dalbergia odorifera , N application significantly increased the competition ratio of Eucalyptus urophylla × E. grandis to Dalbergia odorifera. An increase in the dry matter yields of Eucalyptus urophylla × E. grandis was observed in the NB treatment (20.36–33.16%), but the dry matter yields of Dalbergia odorifera decreased by 10.02–19.57%. (2) N was transferred from Dalbergia odorifera to Eucalyptus urophylla × E. grandis at rates of 9.60% and 6.50% in the 3 and 6 g pot−1 urea treatments with isotope dilution markers, respectively. N transfer rates of 13.66% and 7.88% were observed in the NB and MB treatments with 15N leaf labeling. (3) Interspecific root interaction increased N use efficiency and increased N 2 fixation by Dalbergia odorifera ; N fixation increased by 8.51% and 4.34% at the N 1 and N 2 levels under the NB treatment. The biomass and N use efficiency of Eucalyptus urophylla × E. grandis and Dalbergia odorifera has grown together were effectively improved compared to those of monocultures. The mechanism for our results may depend on interspecific N transfer, i.e., N transfer from Dalbergia odorifera to Eucalyptus urophylla × E. grandis , the stimulation of Dalbergia odorifera BNF and its increased use of atmospheric N sources may improve the exploitation of mixed plantations. [ABSTRACT FROM AUTHOR]

Introducing N 2 -fixing tree species into Eucalyptus plantations could replace nitrogen (N) fertilization to maintain high levels of N consumption and productivity. However, N enrichment may exacerbate phosphorus (P) limitation as Eucalyptus robusta Smith is extensively planted in P-poor tropical and subtropical soils. We conducted a field experiment in a pure plantation of Eucalyptus urophylla × grandis to investigate the impacts of N fertilization and introduced an N 2 -fixing tree of Dalbergia odorifera T. Chen on soil P transformation. Nitrogen fertilization significantly enhanced soil occluded P pool and reduced the other P pools due to acidification-induced pH-sensitive geochemical processes, lowering Eucalyptus leaf P concentration with higher N : P ratio. By contrast, introduced N 2 -fixing tree species did not change soil pH, labile inorganic P pool, and Eucalyptus leaf N : P ratio, even enhanced organic P pools and reduced occluded P pool probably due to altering microbial community composition particularly stimulating arbuscular mycorrhiza fungal abundance. Our results revealed differential responses and mechanistic controls of soil P transformation in Eucalyptus plantations with N fertilization and introduced N 2 -fixing tree species. The dissolution of occluded P pool along with organic P accumulation observed in the mixed plantations may represent a promising future to better manage soil P availability., (© 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.)