Impacts of Oak Mulch Amendments on Rhizosphere Microbiome of Citrus Trees Grown in Florida Flatwood Soils

Abstract

Rhizosphere interactions are an understudied component of citrus production. This is even more important in Florida flatwood soils, which pose significant challenges in achieving sustainable and effective fruit production due to low natural fertility and organic matter. Citrus growers apply soil amendments, including oak mulch, to ameliorate their soil conditions. Thus, the aim of this research was to evaluate the effects of oak mulch on citrus nutrient uptake, soil characteristics, and rhizosphere composition. The plant material consisted of 'Valencia' sweet orange (Citrus × sinensis) trees grafted on 'US-812' (C. reticulata × C. trifoliata) rootstock. The experiment consisted of two treatments, which included trees treated with oak mulch (300 kg of mulch per plot) and a control. The soil and leaf nutrient contents, soil pH, cation exchange capacity, moisture, temperature, and rhizosphere bacterial compositions were examined over the course of one year (spring and fall 2021). During the spring samplings, the citrus trees treated with oak mulch resulted in significantly greater soil Zn and Mn contents, greater soil moisture, and greater rhizosphere bacterial diversity compared to the control, while during the fall samplings, only a greater soil moisture content was observed in the treated trees. The soil Zn and Mn content detected during the spring samplings correlated with the significant increases in the diversity of the rhizosphere bacterial community composition. Similarly, the reduced rates of leaching and evaporation (at the soil surface) of oak mulch applied to Florida sandy soils likely played a large role in the significant increase in moisture and nutrient retention.

Keywords: citrus; flatwoods; microbiome; soil amendments; soil nutrient.

Figure 1

Experimental plots. (A) Authors John Santiago and Lukas Hallman applying oak mulch in September 2020 to 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with and without oak mulch. (B) Aerial view of the experimental design of the field study.

Figure 2

Macro- and micronutrient contents and concentrations measured in April 2021 in leaves (A,B) and soil (C,D) of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with and without oak mulch. Graphs (A,C) are macronutrients; (B,D) are micronutrients. Bars are ± standard deviation of the mean (n = 3). Treatments with * were considered significantly different by Student’s t-test with Tukey’s honestly significant difference (p < 0.05).

Figure 3

Macro- and micronutrient contents and concentrations measured in October 2021 in leaves (A,B) and soil (C,D) of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with and without oak mulch. Graphs (A,C) are macronutrients; (B,D) are micronutrients. Bars are ± standard deviation of the mean (n = 3).

Figure 4

pH (A), temperature (B), cation exchange capacity (C.E.C.; (C)), and moisture (D) measured in April 2021 from soils of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with oak mulch and the control. Bars are ± standard deviation of the mean (n = 3). Treatments with * were considered significantly different by Student’s t-test with Tukey’s honestly significant difference (p < 0.05).

Figure 5

pH (A), temperature (B), cation exchange capacity (C.E.C.; (C)), and moisture (D) measured in October 2021 from soils of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with oak mulch and the control. Bars are ± standard deviation of the mean (n = 3). Treatments with * were considered significantly different by Student’s t-test with Tukey’s honestly significant difference (p < 0.05).

Figure 6

Diversity of rhizosphere bacteria of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock from samples taken in April 2021. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with oak mulch and the control. Alpha diversity (A) was measured by Shannon index of rhizosphere bacteria orders among treatments. Plotted in (A) are boxes (interquartile), median (line within each box), and whiskers (lowest and greatest values). A principal coordinate analysis (PCoA) based on Bray–Curtis dissimilarity matrix of rhizosphere bacterial samples can be found in (B), where colors indicate treatment and include covered (blue) and uncovered (orange). Treatments with * were considered significantly different (p < 0.05).

Figure 7

Diversity of rhizosphere bacteria of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock from samples taken in October 2021. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with oak mulch and the control. Alpha diversity (A) was measured by Shannon index of rhizosphere bacteria orders among treatments. Plotted in (A) are boxes (interquartile), median (line within each box), and whiskers (lowest and greatest values). A principal coordinates analysis (PCoA) based on Bray–Curtis dissimilarity matrix of rhizosphere bacterial samples can be found in (B), where colors indicate treatment and include covered (blue) and uncovered (orange).

Figure 8

Canonical correspondence analysis (CCA) of rhizosphere bacterial communities of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock from samples taken in April 2021. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with oak mulch and the control. Each dot represents the rhizosphere bacterial community within a sample, while the colors indicate treatment (trees grown with oak mulch in blue and trees grown with the control in orange). Measured parameters include soil phosphorus (P), cation exchange capacity (C.E.C.), soil temperature, soil moisture, and soil pH. Treatments with * were considered significantly different (p < 0.05).

Figure 9

Canonical correspondence analysis (CCA) of rhizosphere bacterial communities of 7-year-old ‘Valencia’ sweet orange (Citrus × sinensis) trees grafted on ‘US-812’ (Citrus reticulata × Citrus trifoliata) rootstock from samples taken in October 2021. Trees were planted in flatwood soils located in Fort Pierce, FL, USA, and treated with oak mulch and the control. Each dot represents the rhizosphere bacterial community within a sample, while the colors indicate treatment (trees grown with oak mulch in blue and trees grown with the control in orange). Measured parameters include soil phosphorus (P) content, cation exchange capacity (C.E.C.), soil temperature, soil moisture, and soil pH. Treatments with * were considered significantly different (p < 0.05).