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Mangrove afforestation is considered an important measure in the "natural-based solution" for mitigating climate warming through sequestering massive carbon (C) into vegetation biomass, yet how the planted mangrove species facilitate soil C sequestration remains unclear. Here, we investigated the stock, source, and fraction of soil organic carbon (SOC) over 1 m depth after 20-year afforestation of five mangrove pure plantations (Acrostichum aureum, Acanthus ilicifolius, Aegiceras corniculatum, Kandelia obovate, and Excoecaria agallocha) on Qi'ao Island, South China. The results showed that SOC stocks did not significantly differ among the five plantations, with an average value of 16.7 kg C m−2. Based on the two-end-member mixing model with plant–soil C stable isotope signatures, the autochthonous (mangrove-derived) C source accounted for 20.2–34.1% of SOC but varied significantly among the plantations. The SOC stock in particulate fraction (1.2–2.0 g C kg−1) and mineral-associated fraction (14.3–16.0 g C kg−1) also significantly differed among the plantations. The similar SOC stock but different source contributions and C fractions among the plantations observed here may have important implications for mangrove afforestation to optimize stand structure and maximize C sequestration. [ABSTRACT FROM AUTHOR]

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]

Plant leaf nutrient concentrations and resorption are sensitive to fertilization, yet their co-responses under different nutrient-limited conditions have not been well studied. We conducted a meta-analysis from a global dataset of 43 reports, including 130 observations of studies with plant leaf nitrogen (N) or phosphorus (P) concentrations and nitrogen resorption efficiency (NRE) or phosphorus resorption efficiency (PRE), in response to fertilization under different nutrient-limited conditions divided by the thresholds of leaf N:P ratio values of 10 and 20. The results showed that N fertilization generally increased leaf N concentration and decreased NRE, with greater magnitudes under N-limited conditions. P fertilization also generally increased leaf P concentration and decreased PRE, with greater magnitudes under P-limited conditions. N fertilization decreased leaf P concentration and increased PRE only under the N-limited condition. Under the P-limited or N and P co-limited conditions, however, N fertilization increased leaf P concentration and did not change PRE. Moreover, P fertilization did not change leaf N concentration under all nutrient-limited conditions but significantly increased NRE under the N-limited or N and P co-limited conditions. These findings suggest that plants cope with fertilization-induced N limitation vs. P limitation at the leaf level with different nutrient-use strategies. [ABSTRACT FROM AUTHOR]

As the secondary nutrients in soils, exchangeable base cations can form cationic bridges with clay particles and organic matters to promote the formation of soil aggregates, and to protect soil organic carbon (Corg) and nutrients from loss. To have a comprehensive understanding toward the dynamics of soil exchangeable base cations in the tea planting process, it requires to evaluate these cations' distribution in soil aggregates at various sizes. In this study, soil samples (0–20 cm) from the tea plantations at various ages (8, 17, 25, and 43 yr.) in the hilly regions of southern Guangxi, China were divided into >2, 1–2, 0.25–1, and <0.25 mm aggregate fractions through a dry-sieving procedure. Then, the cation exchange capacity (CEC), total exchangeable bases (TEB), and different individual cations, such as exchangeable potassium (K+), sodium (Na+), calcium (Ca2+), and magnesium (Mg2+), in soil aggregates were detected. Our results suggested that the mean weight diameter (MWD), that is an indicator evaluating the stability of soil aggregates, in the 17-yr. tea plantations appeared to be the highest among all the tea plantations, indicating that more stable soil aggregates could be formed in the 17-yr. tea plantations in comparison with the rest plantations. During tea planting of 35 yr. (from 8 to 43 yr.), the increasing rate of soil CEC stock was 33.42 cmol m−2 yr.−1, while the loss rates of soil exchangeable K+, Na+, Ca2+, Mg2+, and TEB were 0.89, 0.52, 3.86, 1.23, and 6.51 cmol m−2 yr.−1, respectively. Interestingly, the loss rates of soil exchangeable base cations in the middle phase (from 17 to 25 yr.) were greater than those in other phases. Therefore, it is of critical importance to implement a sustainable management scheme to maintain soil aggregate stability and to minimize the loss of soil exchangeable base cations in the tea planting process, especially after 17 yr., in the hilly regions of southern Guangxi, China. [ABSTRACT FROM AUTHOR]

Pinus yunnanensis var. Tenuifolia is an important species of timber and grease in southern China, but the characteristics of the soil microbial community in P. yunnanensis var. natural secondary forests are still poorly understood. Using a fumigation-extraction method and phospholipid fatty acid (PLFA) analysis, we study microbial biomass and community composition in the topsoil (0-10 cm) of three types of secondary forests (PYI, PYII, PYIII) dominated by P. yunnanensis var. to varing degrees. Microbial biomass carbon and nitrogen, total PLFA, and PLFA contents of bacterial, fungal, and arbuscular mycorrhizal fungi were significantly lower in PYI than PYII or PYIII, and there were significant differences in the monounsaturated/saturated fatty acid ratio among the tested forests. Principal component analysis indicated that the soil microbial community structure of the tested forests differed significantly. The changes in soil microbial biomass and community composition were positively correlated with soil water content, pH, organic matter (SOM), total nitrogen (TN), and total phosphorus. Season did not significantly affect the soil microbial community structure, but significantly affected soil microbial biomass, SOM, and TN, which were higher in the dry season than in the wet season. [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]

It is essential to reveal the variations in the activities of carbon (C)-cycling enzymes in soil aggregates of different sizes, so as to shed more light on the soil organic C (C org) dynamics in the tea plantation ecosystems. In this study, the activities of cellobiohydrolase, xylanase, β-glucosidase, and invertase were detected in soil aggregates obtained at the depth of 0–20 cm from tea plantations of various ages (8, 17, 25, and 43-yr.) in hilly region of southern Guangxi, China. Soil aggregates were divided into >2, 2–1, 1–0.25, and <0.25 mm fractions through a dry sieving procedure. This study hypothesized that the potential of soil C org decomposition would be higher at early stage (in the initial 17-yr.) than that at late stage (after 17-yr.). Our results suggested that, irrespective of the tea planting age, soil aggregate-related C org contents were mainly distributed in the >2 and 2–1 mm fractions, and soil aggregates at the >2 mm sizes had the highest activities of C-cycling enzymes. Notably, the C-cycling enzyme activities in soil aggregates were closely correlated with C org availability, and C org contents might not exert a vital part. On the other hand, tea planting for a long time could promote soil C org to be accumulated. At early stage, the accumulated soil C org was primarily derived from the >2 mm fractions; meanwhile, the C-cycling enzyme activities in the >2 mm fractions were also markedly elevated, indicating that the accumulated C org in soils at early stage was more easily decomposed and showed a higher potential of decomposition than that at late stage, thus supporting our hypothesis. Overall, the youthful tea plantations in the initial 17-yr. showed an increasing trend with respect to the potential of soil C org decomposition; however, when tea plantations became mature, the potential of soil C org decomposition was reduced in hilly region of southern Guangxi, China. • Soil aggregates at the >2 mm sizes were main carriers of soil C-cycling enzymes. • Soil C-cycling enzyme activities in tea plantations showed an order of 17 > 25 > 8 > 43-yr. • Long-term tea planting was beneficial for the accumulation of soil organic C. • The accumulated soil organic C in the first 17-yr. was easy to be decomposed. [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.)