Functional Changes of Rhizosphere and Non-Rhizosphere Soils Under the Decline of Pinus sylvestris var. mongolica Plantations

Abstract

The decline of Mongolian Scots pine (Pinus sylvestris var. mongolica) plantations in the "Three-North" shelterbelt region is closely linked to soil degradation. This study compared rhizosphere and non-rhizosphere soils across different stand ages, focusing on nutrient availability, microbial biomass, enzyme activities, and soil particle morphology. Results showed that SOC and TN accumulated with age, whereas AP, AK, and pH declined in older stands, indicating progressive acidification. Results demonstrated that SOC and TN increased with stand age, whereas AP, AK, and pH exhibited a marked decline in the older stands (stands aged ≥ 40 years), reflecting progressive acidification and nutrient depletion. Rhizosphere soils consistently displayed higher SOC, TN, microbial biomass, and enzyme activities than non-rhizosphere soils, largely driven by root exudation and enhanced microbial turnover. The increasing C_mic/N_mic ratio with age suggested a fungal-dominated microbial community, which may exacerbate stand decline by fostering pathogenic fungi. Scanning electron microscopy revealed pronounced particle fragmentation and surface roughness with increasing stand age, particularly in rhizosphere soils, indicating root-driven physical and biochemical weathering. These findings highlight the synergistic effects of stand development and rhizosphere processes on soil structure and fertility, providing a theoretical basis for the sustainable management and restoration of declining plantations.

Keywords: Pinus sylvestris var. mongolica; microbial biomass; non-rhizosphere soils; rhizosphere soils; soil nutrients; stand age.

Figure 4

(a) Total rhizosphere effect of soil index in Pinus sylvestris var. mongolica plantations at different stand ages (10a–60a, years). (b) Rhizosphere effect of soil property index in Pinus sylvestris var. mongolica plantations. AP: available phosphorus; TK: total potassium; AK: available potassium; PRA: protease activity; SA: sucrase activity; MBN: microbial biomass nitrogen; SOC: soil organic carbon; TP: total phosphorus; UA: urease activity; HN: alkali-hydrolyzable nitrogen; MBC: microbial biomass carbon; CA: catalase activity; SRI: soil respiration intensity; SPA: soil phosphatase activity and TN: total nitrogen. Values are means + standard deviations (n = 3). Different lowercase letters above bars indicate significant differences among treatments (p < 0.05).

Figure 5

Tree diagram of cluster analysis of microbial indicators of rhizosphere (a) and non-rhizosphere (b). Redundancy analysis of soil nutrients and microbial indicators of rhizosphere (c) and non-rhizosphere (d). AP: available phosphorus; TK: total potassium; AK: available potassium; PRA: protease activity; SA: sucrase activity; MBN: microbial biomass nitrogen; SOC: soil organic carbon; TP: total phosphorus; UA: urease activity; HN: alkali-hydrolyzable nitrogen; MBC: microbial biomass carbon; CA: catalase activity; SRI: soil respiration intensity; SPA: soil phosphatase activity and TN: total nitrogen. Dots of different colors represent different stand ages; the length of the arrowed lines indicates the extent to which a nutrient factor affects microbial indicators, with longer lines signifying greater impact and shorter lines indicating lesser impact.

Figure 6

(a) The rhizosphere soil structure of different stand ages (10a–60a, years) under the scanning electron microscope. (b) The non-rhizosphere soil structure of different stand ages (10a–60a, years) under the scanning electron microscope.

Figure 1

Nutrient content of rhizosphere and non-rhizosphere soils of Pinus sylvestris var. mongolica at different stand ages (10a–60a, years). (a) SOC: soil organic carbon; (b) pH; (c) AP: available phosphorus; (d) TP: total phosphorus; (e) HN: alkali-hydrolyzable nitrogen; (f) TN: total nitrogen; (g) AK: available potassium; (h) TK: total potassium. Values are means + standard deviations (n = 3). Capital letters denote statistically significant differences between rhizosphere and non-rhizosphere soils at the same sampling location and stand age (p < 0.05), while lower-case letters denote statistically significant differences among soil samples taken from the same position across different stand ages (p < 0.05).

Figure 2

Changes in microbial biomass nitrogen (MBN) (a), microbial biomass carbon (MBC) (b), microbial biomass carbon to nitrogen ratio (C/N ratio) (c), and soil respiration intensity (SRI) (d) in rhizosphere and non-rhizosphere soils of different stand ages (10a–60a, years). Values are means + standard deviations (n = 3). Capital letters denote statistically significant differences between rhizosphere and non-rhizosphere soils at the same sampling location and stand age (p < 0.05), while lower-case letters denote statistically significant differences among soil samples taken from the same position across different stand ages (p < 0.05).

Figure 3

Soil enzyme activity in rhizosphere and non-rhizosphere soils of Pinus sylvestris var. mongolica at different stand ages (10a–60a, years). (a) SA: sucrase activity, (b) UA: urease activity, (c) CA: catalase activity, (d) PRA: protease activity (e) SPA: soil phosphatase activity and (f) Soil enzyme index. Values are means + standard deviations (n = 3). Capital letters denote statistically significant differences between rhizosphere and non-rhizosphere soils at the same sampling location and stand age (p < 0.05), while lower-case letters denote statistically significant differences among soil samples taken from the same position across different stand ages (p < 0.05).

Figure 7

Location of study area in Zhanggutai Town, Zhangwu County, Liaoning Province, China. Site conditions of Pinus sylvestris var. mongolica plantations of different ages (10a–60a, years).