Tree plantation systems influence nitrogen retention and the abundance of nitrogen functional genes in the solomon islands

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Abstract

Tree mono-plantations are susceptible to soil nutrient impoverishment and mixed species plantations have been proposed as a way of maintaining soil fertility while enhancing biodiversity. In the Solomon Islands, mixed species plantations where teak (Tectona grandis) is inter-planted with a local tree species (Flueggea flexuosa) have been used as an alternative to teak mono-plantations and are expected to increase soil microbial diversity and modify microbial biogeochemical processes. In this study, we quantified the abundance of microbial functional genes involved in the nitrogen (N) cycle from soil samples collected in teak, flueggea, and mixed species plantations. Furthermore, we measured soil properties such as pH, total carbon (C) and total N, stable N isotope composition (δ15N), and inorganic N pools. Soil pH and δ15N were higher under teak than under flueggea, which indicates that intercropping teak with flueggea may decrease bacterial activities and potential N losses. Higher C:N ratios were found under mixed species plantations than those under teak, suggesting an enhancement of N immobilization that would help preventing fast N losses. However, inorganic N pools remained unaffected by plant cover. Inter-planting teak with flueggea in mixed species plantations generally increased the relative abundance of denitrification genes and promoted the enrichment of nosZ-harboring denitrifiers. However, it reduced the abundance of bacterial amoA (ammonia monooxygenase) genes compared to teak mono-plantations. The abundance of most denitrification genes correlated with soil total N and C:N ratio, while bacterial and archeal nitrification genes correlated positively with soil NH4+ concentrations. Altogether, these results show that the abundance of bacterial N-cycling functional guilds vary under teak and under mixed species plantations, and that inter-planting teak with flueggea may potentially alleviate N losses associated with nitrification and denitrification and favor N retention. Mixed plantations could also allow an increase in soil C and N stocks without losing the source of income that teak trees represent for local communities.

Figures

  • FIGURE 1 | Smallholder tree plantation systems in Solomon Islands. The field site is located within the Pacific region (A), in Solomon Islands (B), at Kolombangara, Western Province (C), where teak (D), flueggea (E), and mixed-species (F) plantations were established in 2009.
  • TABLE 1 | P-values obtained from a two-way analysis of variance (ANOVA) to detect the effects of plantation type, sampling time, and their interaction on the measured soil variables at Kolombangara, Solomon Islands.
  • TABLE 2 | Soil chemical characteristics measured in different plantation types and at different sampling times at Kolombangara, Solomon Islands.
  • TABLE 3 | P-values obtained from a two-way analysis of variance (ANOVA) to detect the effects of plantation type, sampling time, and their interaction on the abundances of the functional genes involved in soil N cycling at Kolombangara, Solomon Islands.
  • FIGURE 2 | Abundance of the total bacterial community (A) and of the narG (B), nirK (C), nirS (D), nosZ (E), nifH (F), AOA (G), and AOB (H) genes in soil under plantations of teak (diamonds), flueggea (squares), and mixed species (triangles) at three sampling dates. Gene abundances are expressed in log (gene copy numbers ng−1 DNA). Asterisks indicate significant differences between plantation types (Tukey HSD test, P < 0.05).
  • FIGURE 3 | Principal component analysis (PCA) biplot showing the clustering of microbial functional gene abundances per plantation type (A) and per sampling date (B). T: teak; F: flueggea; M: mixed; Dec12: December 2012; May13: May 2013; Dec13: December 2013.
  • FIGURE 4 | Relative abundance of the narG (A), nirK (B), nirS (C), nosZ (D) genes, nosZ/(nirK+nirS) ratio (E), relative abundance of the nifH gene (F), and AOA/AOB ratio (G) in soil under plantations of teak (diamonds), flueggea (squares), and mixed species (triangles) at three sampling dates. Asterisks indicate significant differences between plantation types (Tukey HSD test, P < 0.05).
  • TABLE 4 | Pearson coefficients for correlation between functional gene abundances and soil characteristics, Kolombangara, Solomon Islands.

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Reverchon, F., Bai, S. H., Liu, X., & Blumfield, T. J. (2015). Tree plantation systems influence nitrogen retention and the abundance of nitrogen functional genes in the solomon islands. Frontiers in Microbiology, 6(DEC). https://doi.org/10.3389/fmicb.2015.01439

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