Fertilization Improves the Yield of Sapindus saponaria by Affecting Leaf-Soil-Microbial C-N-P Content and Stoichiometry

Abstract

The purpose of this study was to evaluate the effects of different nitrogen (N), phosphorus (P), and potassium (K) fertilization ratios on the carbon (C), N, and P contents and their ecological stoichiometric characteristics in the leaf-soil-microbial system of Sapindus saponaria and elucidate their relationship with yield. A "3414" experimental design was employed in a 6-year-old Sapindus saponaria woodland located in Fujian Province of China. Fourteen N-P-K fertilization treatments with three replicates were established. Leaf, soil, and microbial samples were collected and analyzed for C, N, and P contents. Redundancy Analysis (RDA), Partial Least Squares Path Modeling (PLS-PM), and the entropy-weighted technique of ranking preferences by similarity to optimal solutions (TOPSIS) were utilized to assess the relationships among variables and determine optimal fertilization strategies. It was found through research that different fertilization treatment methods have a significant impact on both the soil nutrient content and the C, N, and P contents of soil microorganisms. Compared with the control group, soil organic C, total N, and total P, and microbial C, N, and P contents increased by 14.25% to 52.61%, 3.90% to 39.84%, 9.52% to 150%, 6.65% to 47.45%, 11.84% to 46.50%, and 14.91% to 201.98%, respectively. Results from Redundancy Analysis (RDA) indicated that soil organic C, total N, and total P exerted a significant influence on the leaf nutrients. PLS-PM demonstrated that fertilization indirectly affected leaf nutrient accumulation and yield by altering soil properties, with soil total phosphorus and leaf phosphorus being key determinants of yield. Additionally, soil microbial entropy impacted yield by regulating microbial biomass stoichiometric ratios. The entropy-weighted TOPSIS model identified the N₂P₂K₂ treatment (600 kg/ha N, 500 kg/ha P, and 400 kg/ha K) as the most effective fertilization strategy. Optimizing N-P-K fertilization ratios significantly enhances leaf nutrient content and soil microbial biomass C, N, and P, thereby increasing Sapindus saponaria yield. This research clarifies the underlying mechanisms through which fertilization exerts an impact on the C-N-P stoichiometry within the leaf-soil-microbial system. Moreover, it furnishes a scientific foundation for the optimization of fertilization management strategies in Sapindus saponaria plantations.

Keywords: C–N–P stoichiometry; Sapindus saponaria; TOPSIS; fertilization; microbial nutrient limitation.

Figure 1

Characteristics of organic C, total N, and total P contents and their stoichiometric ratios of Sapindus saponaria soils under different N–P–K fertilization treatments: (A) leaf C content, (B) leaf N content, (C) leaf P content, (D) LC:LN, (E) LC:LP, and (F) LN:LP. Different lowercase letters indicate significant differences between fertilization treatments at the 0.05 statistical significance level.

Figure 2

Characteristics of SOC, TN, and TP contents and their stoichiometric ratios of Sapindus saponaria soils under different N–P–K fertilization treatments. (A) SOC, organic carbon; (B) TN, total nitrogen; (C) TP, total phosphorus; (D) C:N, carbon/nitrogen ratio; (E) C:P, carbon/phosphorus ratio; (F) N:P, nitrogen/phosphorus ratio. HSD1: 0–20 cm. HSD2: 20–40 cm. A significant difference at the 0.05 level exists between fertilization treatments denoted by different lowercase letters.

Figure 3

Characteristics of MBC, MBN, and MBP and stoichiometric ratio of Sapindus saponaria soil under different N–P–K fertilization treatments. (a) MBC: microbial carbon content; (b) MBN: microbial nitrogen content; (c) MBP: microbial phosphorus content; (d) MBC:MBN: microbial carbon to nitrogen ratio; (e) MBC:MBP, microbial carbon to phosphorus ratio; (f) MBN:MBP, microbial nitrogen to phosphorus ratio. At the 0.05 significance level, distinct lowercase letters denote substantial differences among fertilization treatments.

Figure 4

RDA of soil and microbial biomass C, N, P, and stoichiometric ratios of Sapindus saponaria under different fertilization treatments: (a) 0 to 20 cm soil layer and (b) 20 to 40 cm soil layer.

Figure 5

Correlations between carbon, nitrogen, and phosphorus contents of leaves, soil, and microbiomass of Sapindus saponaria in 0–20 cm (a) and 20–40 cm (b) soil layers. LC: leaf blade carbon content; LN: leaf blade nitrogen content; LP: leaf blade phosphorus content; SOC: organic carbon; TN: total nitrogen; TP: total phosphorus; C:N: SOC to TN ratio; C:P: SOC to TP ratio; N:P: TN to TP ratio; LC:LN: leaf blade carbon to nitrogen ratio; LC:LP: leaf blade carbon to phosphorus ratio; LN:LP: leaf blade nitrogen to phosphorus ratio. MBC: microbial biomass carbon content; MBN: microbial biomass nitrogen content; MBP: microbial biomass phosphorus content; MBC:MBN: microbial biomass carbon to nitrogen ratio; MBC:MBP: microbial biomass carbon to phosphorus ratio; MBN:MBP: microbial biomass nitrogen to phosphorus ratio.

Figure 6

PLS–PM analysis for 0 to 20 cm and 20 to 40 cm soil layers: (a) 0–20 cm layer soil and (b) 20–40 cm layer soil. Fertilization denotes the amount of fertilizer applied. QN and QP represent microbial entropy nitrogen and phosphorus; MBX:MBP, MBC:MBP, and MBN:MBP ratios; and X:P: C:P, and N:P ratios. Production denotes the yield of the fruit. Blue lines indicate positive impacts, and red lines indicate negative impacts. Straight line thickness indicates relative magnitude of impact. The table of numbers on the lines were path coefficients. *, p < 0.05; **, p < 0.01; ***, p < 0.001. R² denotes adjusted fit.