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Article

Effects of Stand Age and Environmental Factors on Soil Phytolith-Occluded Organic Carbon Accumulation of Cunninghamia lanceolata Forests in Southwest Subtropics of China

by
Qifen Huang
1,2 and
Maoyin Sheng
1,3,*
1
Institute of Karst Research, Guizhou Normal University, Guiyang 550025, China
2
Geography & Environmental Science College, Guizhou Normal University, Guiyang 550025, China
3
National Engineering Research Center for Karst Rocky Desertification Control, Guiyang 550025, China
*
Author to whom correspondence should be addressed.
Forests 2025, 16(2), 240; https://doi.org/10.3390/f16020240
Submission received: 14 December 2024 / Revised: 15 January 2025 / Accepted: 24 January 2025 / Published: 26 January 2025
(This article belongs to the Section Forest Soil)
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Abstract

:
The area of Cunninghamia lanceolata forests in China is expansive, the soil PhytOC(phytolith-occluded organic carbon) stock of Cunninghamia lanceolata forests is a vital carbon reservoir on the global scale. Soil from the Cunninghamia lanceolata forests was collected, and the soil physicochemical indexes and phytoliths and PhytOC content were measured to explore the accumulation characteristics of PhytOC in the 0–10, 10–20, and 20–30 cm soil layers at different stand ages. The results are as follows: (1) soil phytolith content (11.98–32.60 g·kg−1), PhytOC content (0.48–1.10 g·kg−1), PhytOC/TSOC (1.90%–6.93%), soil PhytOC stock (0.446–1.491 t·hm−2), and mature forest > middle–aged forest > Huitou-sha forest > young forest. The soil PhytOC accumulation was significantly affected by stand age. Huitou-sha is not an advantageous afforestation way of Cunninghamia lanceolata. (2) the soil physicochemical properties and stand conditions had significant effects on soil PhytOC accumulation. High–silicon, carbon-rich, acidic soil environment and appropriate thinning are conducive to phytolith formation and PhytOC sequestration. (3) the accumulation potential of soil PhytOC in the Cunninghamia lanceolata forest is relatively large, and its importance as a forest carbon sink cannot be ignored. Soil PhytOC stock in Cunninghamia lanceolata forests of different stand ages will lay a foundation for accurate estimation of forest carbon sink.

1. Introduction

The rapid escalation of atmospheric carbon dioxide levels, leading to climate change, stands as a salient worldwide environmental issue, resulting in issues like rising temperatures, extended growth periods, regular severe weather, land deterioration, and other environmental hazards [1]. There is an urgent need to discover a reliable method for carbon storage. PhytOC (phytolith-occluded organic carbon) is a biochemical carbon sequestration product which plant roots absorb soil soluble silicon and fix atmospheric CO2 in the process of photosynthesis and deposit it in plant cells or intercellular spaces [2,3,4]. This process, known as a stable terrestrial ecological carbon sink mechanism [5,6], is crucial in the carbon cycle of the terrestrial ecosystem [7].
Roughly 56% of the carbon storage in the overall terrestrial ecosystem is found in forest ecosystems [8], with the yearly carbon sequestration making up approximately two-thirds of the general terrestrial biogeochemical carbon sequestration [9]. More than 20% of the phytoliths in terrestrial ecosystems come from forest ecosystems, and the high net primary productivity of forest vegetation brings more phytolith decomposition into the soil [10,11,12,13,14,15]. Phytoliths can identify plant groups and the soil parent material and determine soil water morphology, etc., the accumulation and redistribution of phytoliths has a very important effect on PhytOC sequestration [16,17]. The annual output of PhytOC in Chinese forests is 1.94 ± 0.44 × 106 kgCO2·a−1, and the annual PhytOC sink value is about 1.7 ± 0.4 TgCO2a−1. Subtropical and tropical forest ecosystems have the highest PhytOC sink potential [18], and the dissolution rate of phytoliths in coniferous forests is low [19], whereas the stability of coniferous forests is higher than that of other vegetation types [20].
PhytOC in forest ecosystems has mainly been studied in silicon-rich bamboo forests [21,22] and some broad-leaved forests [23] and coniferous forests [24]. The soil PhytOC of Masson pine at different ages was found to increase with stand age increase and decrease with soil the depth increase, indicating the characteristics of soil PhytOC at different ages in subtropical region [24].
The one of most representative subtropical coniferous forest species is Cunninghamia lanceolata. Cunninghamia lanceolata (Chinese fir) is a unique fast–growing coniferous forest species in China, which is distributed throughout almost the entire subtropical zone [25]. The total area of the Cunninghamia lanceolata forest in China is 1138.66 × 104 hm2, and artificial planting area is 990.20 × 104 hm2, and the total standing volume is 7.55 × 108 m3. The total afforestation area and total standing volume of the Cunninghamia lanceolata forest, respectively, account for 17.33% and 22.30% of the area of China, ranking first in the 100 most important tree species in China [26]. Consequently, the Cunninghamia lanceolata PhytOC sink of the ecosystem is crucial for Chinese forests and is a key component of the global terrestrial carbon cycle.
Studies have revealed a pattern of soil phytolith content in Cunninghamia lanceolata in which it initially rose, followed by a decline and subsequent rise [27,28,29]. As the depth of the soil increased, there was a reduction in the PhytOC content, whereas the PhytOC/TSOC (TSOC; total soil organic carbon) ratio showed a salient rise with increased soil depth [18]. Early research into the soil PhytOC in Cunninghamia lanceolata forests has provided a certain understanding of soil PhytOC and its accumulation characteristics of different soil layers in Cunninghamia lanceolata mature forest, but only the correlation between phytolith, PhytOC, and silicon was analyzed. We added the impact analysis of soil physicochemical properties and stand conditions on the formation and accumulation of soil PhytOC in Cunninghamia lanceolata, and we also added pairwise correlation analysis of phytolith, PhytOC, PhytOC:TSOC, physicochemical factors, and stand factors. We added research on the accumulation characteristics and storage estimation of soil PhytOC in Cunninghamia lanceolata at different stand ages.
In our study, the first hypothesis was put forward: the stand age has salient effect on the accumulation of soil PhytOC in Cunninghamia lanceolata forest, and the content and accumulation of PhytOC in the young forest soil may be less than that in the mature forest. The second hypothesis was that there is a significant correlation between the accumulation of PhytOC in the Cunninghamia lanceolata forest and stand conditions and soil physicochemical properties. The stand conditions and soil physicochemical properties can significantly change the characteristics of soil PhytOC accumulation in Cunninghamia lanceolata forests. In order to verify the two hypotheses proposed in our study, samples from the 0–10, 10–20, and 20–30 cm soil layers of Cunninghamia lanceolata forests of different ages were collected, and the physicochemical properties of the soil samples, the soil PhytOC sequestration characteristics, and carbon sequestration potential of the Cunninghamia lanceolata forests at different ages were determined experimentally. This study aimed to provide a reference for carbon sequestration calculations and the scientific management of Cunninghamia lanceolata forests in the southwest subtropical region and the whole country.

2. Materials and Methods

2.1. Research Area Overview

The research zone is in Chishui City, northwest of Guizhou Province in China (28°16′–28°46′ N, 105°36′–106°15′ E), and in the transitional area between the Yunnan Guizhou Plateau and Sichuan Basin, the primary geography types are plateaus and mountain canyons. The landscape is steep in the southeast and sloping in the northwest, with an elevation range of 221–1730 m. The Mesozoic Jurassic, Cretaceous, and Cenozoic Quaternary strata are exposed [30,31], the soil parent material is brown-red, with purple red fine sandy (mud) rock residual slope deposits, and the soil is mainly purple soil. The classification of “purple soil” in this study according to WRB is Cambisol [32]. According to the Munsell color system, the soil color index is 7.5R3/2 [33]. The purple soil in the study area developed from a brick red thin layer to medium-thick layer argillaceous feldspar sandstone and brick red thick layer to massive uneven feldspar quartz sandstone [31], characterized by loamy soil rich in mineral nutrients with high natural fertility, and is an important soil resource that is unique in China [34]. Soil stratification is as follows: Layer O (litter layer): 0–8 cm, granular and fragmentary structure. The texture is light soil. Layer A (humus layer): 8–35 cm, cracks, large blocky structure, sandy soil texture. Layer B (deposition layer): 35–57 cm, compact no structure, texture of purple soil. Layer C (parent material layer): >57cm, containing large sandstone. Bulk chemical composition of the initial soil-forming rock: SiO2 is the most, as its average content is 52.82%–75.4%, and the maximum is about 85%, followed by Al2O3, and the average content is 7%–16.65%. The research zone is located in the mid–subtropical humid monsoon climate zone, with an average annual temperature of 18.1 C and an average annual rainfall of 1292.3 mm. The vegetation is dominated by evergreen oak and mixed evergreen deciduous forest, and there is a large area of pine forest and Chinese fir forest and Phyllostachys edulis forest [35].
The research zone is in Chishui City, northwest of Guizhou Province in China (28°16′–28°46′ N,105°36′–106°15′ E), and it is in the transitional area between the Yunnan–Guizhou Plateau and Sichuan Basin, and the primary geography types are plateaus and mountain canyons. The landscape is steep in the southeast and sloping in the northwest, with an elevation range of 221–1730 m. The Mesozoic Jurassic, Cretaceous, and Cenozoic Quaternary strata are exposed [30,31]. Bulk chemical composition of the initial soil-forming rock: SiO2 is the most, as its average content is 52.82%–75.4%, and the maximum is about 85%, followed by Al2O3, and the average content is 7%–16.65%. Soil parent material is brown–red, purple red fine sandy (mud) rock residual slope deposits, and the soil is mainly purple soil. The classification of “purple soil” in this study according to WRB is Cambisol [32]. According to the Munsell color system, the soil color index is 7.5R3/2 [33]. The purple soil in the study area, characterized by loamy soil rich in mineral nutrients with high natural fertility and is an important soil resource that is unique in China [34]. Soil stratification is as follows: Layer O (litter layer): 0–8 cm, granular and fragmentary structure. The texture is light soil. Layer A (humus layer): 8–35 cm, cracks, large blocky structure, sandy soil texture. Layer B (deposition layer): 35–57cm, compact no structure, texture of purple soil. Layer C (parent material layer): >57 cm, containing large sandstone. The research zone is located in the mid-subtropical humid monsoon climate zone, with an average annual temperature of 18.1 °C and an average annual rainfall of 1292.3 mm. The vegetation is dominated by evergreen oak and mixed evergreen deciduous forest, and there is a large area of pine forest and Chinese fir forest and Phyllostachys edulis forest [35].
Cunninghamia lanceolata is categorized by age as follows: Young forest, with an age equal to or less than 10 years; middle-aged forest, with an age between 11 and 20 years; near–mature forest, with an age between 21 and 25 years; mature forest, with an age between 26 and 35 years; and over-mature forest, with an age equal to more than 36 years [36]. Huitou-sha is a kind of Cunninghamia lanceolata forest with a special growth process. It sprouted in the old pile base of a felled middle-aged forest or mature forest, and the soil nutrition at the early stage was higher than that of a young forest. Therefore, Huitou-sha grows quickly in the early stage, but its subsequent growth declines, and Huitou-sha forest do not easily grow to large trees [37].
Combining with the distribution of Cunninghamia lanceolata, the topography, and other conditions, Hushi Forest Farm, Guandu Forest Farm, and Tongxing Forest Farm in Chishui City were selected as sampling areas. Hushi Forest Farm: The total area of the forest farm is 10,399.33 hm2, and the area of the Cunninghamia lanceolata forest is 300 hm2. Guandu Forest Farm: The total area of the forest farm is 10,000 hm2, and the area of Cunninghamia lanceolata forest is 506.67 hm2. Tongxing Forest Farm: The total area of the forest farm is 4393.33 hm2, and the area of Chinese fir forest is 2333.33 hm2. The three forest farms began thinning 15 years, and thinning is conducted to cut down the trees that do not grow well. The sampling areas have four types of Cunninghamia lanceolata: young forest, middle-aged forest, mature forest, and Huitou-sha forest (Table 1). The arborous of the sampling area is Cunninghamia lanceolata; the shrubs are mainly Camellia japonica and Smilax lanceifolia Roxb.; and the herbs are mainly Athyrium filix-femina, Cyclosorus interruptus, Cyclosorus parasiticus, Dicranopteris pedata, Diplopterygium chinense, Lophatherum gracile, Rubus buergeri, Rubus hunanensis, and Pteridium aquilinum.

2.2. Sample Selection and Sample Collection

There are three forest farms, so three sampling points for each stand age were set, and three 20 × 20 m quadrats in each sampling point were set (each sample quadrat is spaced at least 20 m apart), so the total number of quadrats is 36 (Figure 1, Table 1). Soil samples from the 0–10 cm, 10–20 cm, and 20–30 cm soil layers were collected from each quadrat, so the total number of soil samples was 108. There were 27 soil samples for each stand age. In each quadrat, 5 soil samples were collected in an “S” shape in three locations and mixed evenly to obtain the composite soil samples. The cultivation and environment of Cunninghamia lanceolata in the sampling quadrats were investigated in detail, and the samples were collected in July 2023. We examined the count of Cunninghamia lanceolata trees in each composite sample quadrat and measured the diameter at breast height (DBH) and tree height (TH). We documented the density of the canopy in these quadrats; the makeup of the understory shrub and grass vegetation; and environmental parameters like topography, slope, and aspect, as well as the soil types. The ring knife served to assess the bulk density of the soil.

2.3. Measure of Soil Physicochemical Properties

The soil sample processing consisted of manually removing plants, animal remains, and gravel at the lab, followed by natural air drying at ambient laboratory temperatures and ball-milling. Then, the obtained soil samples were passed through a 0.149 mm sieve [38] and stored in clean airtight bags for measuring physicochemical properties (Figure 2 and Figure 3). We measured the soil bulk density, field moisture capacity, natural moisture content, and capillary porosity using the soil ring knife method [39,40]. Total porosity and non–capillary porosity were calculated according to Formulas (1) and (2) [41]. Soil pH determination was measured using the potentiometric method. The water–soil ratio was 2.5:1 [42]. Total soil organic carbon was measured through potassium–dichromate external heating, with 0.2 mol/L ferrous–sulfate standard solution titration [43]. Soil total nitrogen was digested using selenium powder–copper sulfate–potassium sulfate–sulfuric acid and measured with a Kjeldahl apparatus. Total soil phosphorus was measured by sulfuric acid–perchloric acid digestion and the molybdenum antimony colorimetric method. Soil available silicon was determined using the lithium metaborate melting–nitric acid buffer extraction–molybdenum blue colorimetric method [44].
Total porosity = 93.947 − 32.995 × bulk density
Non-capillary = total porosity − capillary porosity

2.4. Computational Formula of Phytolith and PhytOC

Soil phytoliths were extracted using the wet oxidation method [45]. The PhytOC was determined using visible spectrophotometry [46].
phytolith content (g·kg−1) = phytolith mass (g)/soil sample mass (kg)
PhytOC content (g·kg−1) = PhytOC mass (g)/soil sample mass (kg)
PhytOC   stock = ∑ i = 1 n ( 10,000 × H i × BD i × C i )
PhytOC stock (kg·hm−2), where Hi is the soil depth of layer i, BDi is the soil bulk density of layer i, and Ci is the soil PhytOC content of layer i.

2.5. Statistical Analysis

The significance of variable differences was assessed using one-way ANOVA and Duncan’s innovative multiple range method (LSR). We drew these maps with the help of Origin 7.5, R software (Version 4.3.0), and ArcGIS 10.8 software. We analyzed the relationship between the soil PhytOC and the soil physicochemical properties and stand conditions using linear regression models. The correlations between the soil PhytOC and the soil physicochemical properties and stand conditions were analyzed using the R software ggplot package basing on Pearson’s correlation coefficients. We performed redundancy analysis (RDA) and plotted the results using R software ggplot package.

3. Results

3.1. Soil Available Silicon Content and Phytolith Content

The range of available silicon content in Cunninghamia lanceolata was 35.01–53.70 mg·kg−1 in young forest, 45.69–63.36 mg·kg−1 in middle-aged forest, 50.52–84.33 mg·kg−1 in mature forest, and 40.62–58.57 mg·kg−1 in Huitou-sha forest, which increased with the increase of stand age and generally showed the following pattern: young forest < Huitou-sha < middle-aged forest < mature forest. As the depth of the soil increased, there was a noticeable reduction in the average content of available silicon, indicating a marked enrichment on the surface, with the discrepancy being statistically significant in different soil layers (p < 0.05, (Figure 4a)).
The mean values of the phytolith content in Cunninghamia lanceolata of different stand ages were 11.98–24.29 g·kg−1 in young forest, 20.70–28.36 g·kg−1 in middle-aged forest, 25.62–32.60 g·kg−1 in mature forest, and 17.22–21.51 g·kg−1 in Huitou-sha forest. There were salient differences in the phytolith content in the different soil layers and stand ages (p < 0.05) (Figure 4b). The overall performance pattern is as follows: mature forest > middle-aged forest > Huitou-sha forest > young forest. The content of soil phytoliths in the mature forest was the highest of all the forest types, which were, respectively, 28.56 g·kg−1 (0–10 cm), 32.60 g·kg−1 (10–20 cm), and 25.62 g·kg−1 (20–30 cm). In young forests, in the 0–10 cm and 20–30 cm soil layers, the phytolith contents were lowest, respectively, at 18.48 g·kg−1 and 11.98 g·kg−1. In the 10–20 cm soil layer, the content of the Huitou-sha forest was the lowest, recorded as 21.51 g·kg−1. Initially, the soil phytolith content rose, followed by a decline as the soil depth increased. The phytolith content in the 10–20 cm soil layer was markedly greater compared to that in the 0–10 cm and 20–30 cm soil layers. The phytolith content in the 0–10 cm and 20–30 cm soil layers showed no notable discrepancy between the middle-aged and mature forests, but a significant disparity was observed in the phytolith content within each layer between the young forests and Huitou-sha forests.

3.2. Soil PhytOC Content and the Ratio of Soil PhytOC Content to Total Soil Organic Carbon

The PhytOC content in the 0–10, 10–20, and 20–30 cm soil layers varied at 0.22–0.37 g·kg−1 in the younger forest, 0.34–0.63 g·kg−1 in the middle-aged forest, 0.57–1.10 g·kg−1 in the mature forest, and 0.29–0.38 g·kg−1 in the Huitou-sha forest, indicating a rising trend as the stand age increased. In the mature forest, the average PhytOC content in the 10–20 cm soil layer exceeded those in the middle-aged and Huitou-sha forests, and the PhytOC content was the lowest in the young forest. The soil PhytOC content rose initially and then fell as the soil depth increased, revealing a pattern of 10–20 cm > 0–10 cm > 20–30 cm, with notable differences observed in the different soil layers and stand ages (p < 0.05) (Figure 4c).
The ratio of PhytOC to the total soil organic carbon (PhytOC/TSOC) in the different soil profiles of different stand ages ranged from 1.95 to 3.86% in the young forest, 2.03 to 4.62% in the middle-aged forest, 2.39 to 6.94% in the mature forest, and 1.99 to 3.74% in the Huitou-sha forest. There were salient differences in the different soil layers and stand ages (p < 0.05; Figure 4d). The average values of PhytOC/TSOC in the 0–10, 10–20, and 20–30 cm soil layers were in the order of mature forest> middle-aged forest> Huitou-sha tree > young forest, and the PhytOC/TOC in each soil layer first increased and then decreased with the increase of soil depth.

3.3. Soil PhytOC Stock of Cunninghamia Lanceolata Stands

In the 0–10, 10–20, and 20–30 cm soil layers of the different Cunninghamia lanceolata stands; the PhytOC stock was 0.29–0.46 t·hm−2 in the younger forest; 0.41–0.72 t·hm−2 in the middle-aged forest; 0.77–1.47 t·hm−2 in the mature forest; and 0.37–0.42 t·hm−2 in the Huitou-sha forest. There was a notable difference in the diverse stand ages and soil layers (Figure 5). Along the vertical direction of the soil, there was an initial rise in the soil PhytOC stock, followed by a decline as the soil depth increased, revealing the following pattern: 10–20 cm > 0–10 cm > 20–30 cm.

3.4. Correlation Between Soil PhytOC and Environmental Factors

There was a notable positive relationship between the soil phytoliths, PhytOC content, and PhytOC:TSOC ratio with DBH and TH (p < 0.01) and a significant negative correlation with the stand density (SDI) and canopy density (CD) (p < 0.01). A majority of the soil physicochemical elements showed correlations with the soil phytoliths, PhytOC content, and PhytOC:TSOC (Figure 6). Within this group, there was a notable positive association between the soil phytoliths, PhytOC content, and PhytOC:TSOC with the soil ASi and TSOC (p < 0.01), as well as a salient positive association with the soil total phosphorus (TP), pH, total nitrogen (TN), N:P, C:P, and natural moisture content (NMC) (p < 0.01). Conversely, a significant negative correlation was observed with the C:N, total porosity (STP), and non-capillary porosity (NCP) (p < 0.05). No notable association was found between the soil phytoliths and other factors such as bulk density (BD), field moisture capacity (FMC), and capillary porosity (CP) (p > 0.05). There was a notable positive correlation between the soil PhytOC and bulk density (BD) (p < 0.05), whereas no salient connection was found between the field moisture capacity (FMC) and capillary porosity (CP) (p > 0.05).

3.5. RDA of Soil PhytOC Accumulation in Cunninghamia Lanceolata Stands

In this study, we conducted a redundancy analysis (RDA) to delve deeper into how the soil physicochemical properties and stand conditions influenced the accumulation of soil phytoliths and PhytOC in Cunninghamia lanceolata. The first and second axes can summarize well the influence of the physicochemical properties and stand conditions on the accumulation of soil PhytOC, with a cumulative percentage of 99.81%, indicating that these two axes represented most of the information of 18 factors in Cunninghamia lanceolata (Figure 7). RDA1 accounted for 97.95%, and the main factors were Asi, TSOC, CD, TN, TP, TH, DBH, N:P, NCP, C:P, pH, and C:N. The proportion of PC2 is 1.86%, which is mainly composed of BD. Monte Carlo experiments showed that 18 soil physicochemical properties and stand conditions significantly affected the accumulation of soil PhytOC in Cunninghamia lanceolata. The order of importance is as follows: Asi, TSOC, CD, TN, TP, TH, DBH, N:P, NCP, C:P, pH, C:N, SDI, BD, NMC, STP, FMC, and CP (Table 2). The long arrows representing Asi, TSOC, TP, TH, pH, DBH, and N:P formed acute angles with the PhytOC and phytoliths. The analysis revealed that the primary beneficial elements influencing the soil PhytOC accumulation were these seven factors. The arrows for NCP, SDI, and CD being long and forming obtuse angles with the PhytOC and phytoliths and PhytOC:TSOC arrows suggested that these three elements were the negative factors for soil PhytOC accumulation.

4. Discussion

4.1. Accumulation Characteristics of Soil PhytOC in Cunninghamia Lanceolata Forests of Different Stand Ages

The phytolith content of the Cunninghamia lanceolata forest in our study area ranged from 11.98 to 32.60 g·kg−1, and the PhytOC content ranged from 0.22 to 1.10 g·kg−1, which increased with the stand age increase. The results were consistent with those of Phyllostachys edulis forests [23,24], Masson pine forests [24], and Quercus acutissima forests [25]. Therefore, the effect of the stand age on the soil phytoliths and PhytOC accumulation in the Cunninghamia lanceolata forest was significant, with the pattern of mature forest > middle-aged forest > young forest, which was consistent with hypothesis one of this study. The soil in the forest ecosystem does not produce phytoliths, and the phytolith was released into the soil by the decomposition of litter. The accumulation rate in soil mainly depends on the input of litter under the same conditions [47,48,49]. The annual total litter amount of Cunninghamia lanceolata of different ages was 418 g.m−2 per year in mature forest, 310 g·m−2 per year in middle-aged forest, and 106 g·m−2 per year in young forest [50]. Therefore, litter input increased with the increase in stand age, and soil phytoliths and PhytOC also increased with the increase in stand age.
In our study, the age of Huitou-sha was 2 (15 for old pile) and the age of the young forest was 9. The PhytOC and phytolith values of Huitou-sha were higher than those of the young forest. This phenomenon is due to the fact that the soil C, N, P, and Si in the Huitou-sha forest are higher than those in the young forest, and the litter cover amount is also higher than that of the young forest. The initial growth rate of Huitou-sha is rapid; but, in general, after 3–5 years, the growth rate is significantly less than that of the Cunninghamia lanceolata forest planted with seedlings, and the storage of the PhytOC is bound to lag behind that of the Cunninghamia lanceolata forest of the same stand age. After the old piles are cut down, there is no need for land preparation, digging, and fertilization, which can save afforestation costs. However, considering the slow growth process of Huitou-sha in the later stage and its poor wood quality, Huitou-sha is rarely used for afforestation in forest farms at present.

4.2. Effects of High-Silicon, Carbon-Rich, and Acidic Soil Environments on the Accumulation of Soil PhytOC

The effects of soil physicochemical factors on the soil phytolith and PhytOC accumulation were studied. The soil phytolith and PhytOC contents were positively correlated with the soil ASi and TSOC. It was significantly correlated with soil environment factors such as the pH, TP, and TN. These were consistent with hypothesis two of this study. ASi and TSOC provide essential Si and C elements for the formation process of soil phytoliths and PhytOC, and they are also basic nutrients to promote plant growth [51]. The increase in available silicon content can improve the yield and sequestration rate of PhytOC [52,53,54]. Secondly, the accumulation rate of PhytOC and the PhytOC storage in soil also increased significantly [26,46]. Soil carbon as a component element of PhytOC directly promotes the sequestration and accumulation of PhytOC in Cunninghamia lanceolata forests. The content of soil organic carbon in the study area was high, which was consistent with the results of relevant studies in this area [24,45]. The soil conditions of high silicon and rich carbon in the study area are favorable for the accumulation and enrichment of the soil PhytOC. Therefore, the artificial application of exogenous silicon [46,52] and litter cover [55] can effectively improve the function of the soil carbon sink in the Cunninghamia lanceolata forest ecosystem, which is of great significance for carbon sinks in terrestrial ecosystems.
Related studies have shown that phytoliths are stable when the soil pH is 2–8. When the soil pH exceeds 8 or falls below 2, the amount of phytoliths dissolved increases and the stability decreases [56,57,58,59]. One can see that soil phytoliths are more stable and difficult to decompose in acidic and alkaline environments, and the encapsulated organic carbon is also stored. Consequently, the findings corroborate the aforementioned deductions, showing a notable link between soil pH values and soil PhytOC, with the mildly acidic soil conditions in the southwest subtropical area being more favorable for the soil PhytOC accumulation.
The soil phytolith and PhytOC contents were significantly negatively correlated with the stand density (SDI) and canopy density (CD). In the Cunninghamia lanceolata of the study area, thinning began about 16 years ago, and the poorly growing trees were cut down, the stand density and canopy density reduced, and the growth space was left for the remaining Cunninghamia lanceolata trees, which was conducive to the growth of the Cunninghamia lanceolata forest and the formation of soil PhytOC. Thus, appropriate artificial thinning management was conducive to the accumulation of soil phytolith and PhytOC in the Cunninghamia lanceolata forest. That is consistent with the findings of research on Masson pine forests [24].

4.3. Accumulation Potential of Soil PhytOC in Cunninghamia Lanceolata Forests

The phytolith and PhytOC content and PhytOC storage of the Cunninghamia lanceolata forest were lower than those of the Phyllostachys edulis forest [27,28,29]; however, the planting area of the Cunninghamia lanceolata forest is expansive, with a total area of 1138.66 × 104 hm2, much higher than that of the Phyllostachys edulis forest (467.78 × 104 hm2). The total PhytOC stock of the 0–30 cm soil in the Cunninghamia lanceolata forest was about 2.264 × 107 t·hm−2, and the total PhytOC stock of the 0–30 cm soil in the Phyllostachys edulis forest was 3.413 × 106 tC [22]. The total PhytOC stock in the Cunninghamia lanceolata forest ecosystem is rich, and its importance as a forest ecosystem carbon sink cannot be ignored.

5. Conclusions

The PhytOC content (11.98–32.60 g·kg−1), PhytOC content (0.48–1.10 g·kg−1), PhytOC/TSOC (1.90%–6.93%), and soil PhytOC stock (0.446–1.491 t·hm−2) of the Cunninghamia lanceolata forests we studied in the southwest subtropics of China increased significantly with the stand age increase. The cultivation of Huitou-sha is not conducive to Cunninghamia lanceolata afforestation and PhytOC storage, so this planting way is not recommended. The PhytOC accumulation in the Cunninghamia lanceolata forests was significantly correlated with the soil physicochemical factors (Asi, TSOC, TN, TP, and pH) and stand conditions (CD, TH, and DBH). Compared to the Eastern Cunninghamia lanceolata forest in China, the soil PhytOC stock of the Cunninghamia lanceolata forest in southwest subtropics of China was higher, which benefited from the high-silicon, carbon-rich, mildly acidic soil cumulative environment and appropriate thinning measures. The soil PhytOC accumulation of the Cunninghamia lanceolata forest in Southwest China is plentiful, which plays an important role in the carbon sinks of the global forest ecosystem. This study was the first to analyze the characteristics of soil PhytOC accumulation in Cunninghamia lanceolata forests of different stand ages, and the results are of great significance for accurately estimating the soil PhytOC stocks of Cunninghamia lanceolata forests of different stand ages, strengthening the carbon sink construction of Cunninghamia lanceolata forests and their sustainable management. In addition, this study only focused on the topsoil of 0–30 cm and did not analyze the deeper soil layers and did not analyze the near-mature and over-mature forests. In the future, we will comprehensively explore the characteristics of PhytOC accumulation in different soil layers of Cunninghamia lanceolata forests of various stand ages.

Author Contributions

Q.H.: Writing—original draft, Visualization, Investigation, Formal analysis, and Data curation. M.S.: Writing—review and editing, Supervision, Project administration, Methodology, Investigation, Data curation, and Conceptualization. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support for this study was provided by the Science and Technology Program of Guizhou Province (Qainkehe Zhicheng [2024]yiban120; Qainkehe Zhicheng [2023]yiban210), the Project of Central Finance Guides Local Scientific and Technological Development (Qiankehe Zhongyindi [2023]028), and the Guizhou Normal University Academic New Talent Fund Project (Qianshi Xinmiao [2022]21).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author, M.S., upon reasonable request.

Acknowledgments

We much appreciated the help of Li Liu, Mengxia Luo, Lukang Song, and Changxu Chen in the soil sample collections, and we thank the anonymous reviewers for their constructive comments and suggestions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Forests 16 00240 g001
Figure 1. Location information of the sample quadrats of different stand ages. Note: DEM color represents elevation change. The higher the color on the top, the higher the altitude, and the lower the color on the bottom, the lower the altitude. SMY, young forest; SMZ, middle-aged forest; SMC, mature forest; SMH, Huitou-sha forest.
Figure 1. Location information of the sample quadrats of different stand ages. Note: DEM color represents elevation change. The higher the color on the top, the higher the altitude, and the lower the color on the bottom, the lower the altitude. SMY, young forest; SMZ, middle-aged forest; SMC, mature forest; SMH, Huitou-sha forest.
Forests 16 00240 g001
Forests 16 00240 g002
Figure 2. Soil TSOC, TN, TP, and pH in Cunninghamia lanceolata forests of different stand ages. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus. Sample size n = 108.
Figure 2. Soil TSOC, TN, TP, and pH in Cunninghamia lanceolata forests of different stand ages. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus. Sample size n = 108.
Forests 16 00240 g002
Forests 16 00240 g003
Figure 3. Soil physical properties and C:N, C:P, and N:P in Cunninghamia lanceolata forests of different stand ages. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TH, tree height. Sample size n = 108.
Figure 3. Soil physical properties and C:N, C:P, and N:P in Cunninghamia lanceolata forests of different stand ages. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TH, tree height. Sample size n = 108.
Forests 16 00240 g003
Forests 16 00240 g004
Figure 4. Soil available Si content (a), soil phytolith content (b), soil PhytOC content (c), and PhytOC:TSOC ratio (d) in the different stand ages of Cunninghamia lanceolata. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). SMY, young forest; SMZ, middle-aged forest; SMC, mature forest; SMH, Huitou-sha forest. Sample size n = 108.
Figure 4. Soil available Si content (a), soil phytolith content (b), soil PhytOC content (c), and PhytOC:TSOC ratio (d) in the different stand ages of Cunninghamia lanceolata. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). SMY, young forest; SMZ, middle-aged forest; SMC, mature forest; SMH, Huitou-sha forest. Sample size n = 108.
Forests 16 00240 g004
Forests 16 00240 g005
Figure 5. Content of soil PhytOC stock in the different stand ages of Cunninghamia lanceolata. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). SMY, young forest; SMZ, middle-aged forest; SMC, mature forest; SMH, Huitou-sha forest. Sample size n = 108.
Figure 5. Content of soil PhytOC stock in the different stand ages of Cunninghamia lanceolata. Note: Different capital letters represent the significance between different soil layers of same ages (p < 0.05). Different lower-case letters represent the significance between different ages of same soil layers (p < 0.05). SMY, young forest; SMZ, middle-aged forest; SMC, mature forest; SMH, Huitou-sha forest. Sample size n = 108.
Forests 16 00240 g005
Forests 16 00240 g006
Figure 6. The correlation analysis of soil PhytOC accumulation and environmental factors in Cunninghamia lanceolata stand soil. Note: Asi, available silicon; BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; DBH, diameter at breast height; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus; TH, tree height. Sample size n = 108. *, ** and *** respectively indicate that the correlation is significant at 0.05, 0.01 and 0.001 levels.
Figure 6. The correlation analysis of soil PhytOC accumulation and environmental factors in Cunninghamia lanceolata stand soil. Note: Asi, available silicon; BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; DBH, diameter at breast height; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus; TH, tree height. Sample size n = 108. *, ** and *** respectively indicate that the correlation is significant at 0.05, 0.01 and 0.001 levels.
Forests 16 00240 g006
Forests 16 00240 g007
Figure 7. RDA of environmental factors on the soil PhytOC accumulation. Note: Asi, available silicon; BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; DBH, diameter at breast height; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus; TH, tree height. Sample size n = 108.
Figure 7. RDA of environmental factors on the soil PhytOC accumulation. Note: Asi, available silicon; BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; DBH, diameter at breast height; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus; TH, tree height. Sample size n = 108.
Forests 16 00240 g007
Table 1. Basic information of sample quadrats in the study.
Table 1. Basic information of sample quadrats in the study.
Sample QuadratsnumbersEnvironment of Sample QuadratsBasic Information of Cunninghamia lanceolata Forests
Elevation
(m)		Slope
Location		Slope
(°)		Stand Ages
(a)		Group of Stand AgeMean Diameter
at Breast Height
(cm)		Basal Area
(cm2)		Mean Tree Height
(m)		Stand Density
(Individual hm−2)		Canopy
Density
(%)
SMY-H1949Middle219Young9.907555.308.60245585
SMY-H2952Middle219Young9.977709.348.64247088
SMY-H3950Middle229Young10.007802.908.69248586
SMY-G1955Middle239Young10.037897.178.73250087
SMY-G2951Middle249Young10.408541.508.78251588
SMY-G3953Middle229Young10.288445.088.82254590
SMY-T1960Middle219Young10.418608.988.87253087
SMY-T2956Middle219Young10.318544.518.91256088
SMY-T3966Middle219Young10.268494.888.96257089
SMZ-H11021Middle2717Middle-aged12.403992.8116.3482774
SMZ-H21022Middle2717Middle-aged12.464090.0416.3583971
SMZ-H31024Middle2717Middle-aged12.444047.7716.5083372
SMZ-G11016Middle2817Middle-aged12.494139.1516.4984575
SMZ-G21014Middle2817Middle-aged12.534224.8616.6985774
SMZ-G31016Middle2817Middle-aged12.554208.6916.6385173
SMZ-T11026Middle2717Middle-aged12.714377.5516.7486378
SMZ-T21030Middle2917Middle-aged12.524306.7216.5087577
SMZ-T31033Middle2917Middle-aged12.414202.3616.6386976
SMC-H11005Middle2028Mature18.407175.7820.4167575
SMC-H21010Middle2028Mature18.347393.1020.3970068
SMC-H31008Middle2228Mature18.627620.5620.3870072
SMC-G1957Middle2128Mature18.646818.7020.4062571
SMC-G2958Middle2228Mature18.456947.6220.4165072
SMC-G3957Middle2128Mature18.766906.7820.4362570
SMC-T1945Middle2428Mature18.637083.8420.4165066
SMC-T2945Middle2428Mature18.657645.1420.4070065
SMC-T3944Middle2428Mature18.867539.0520.4067564
SMH-H11023Upper402Huitou-sha2.05126.021.8095585
SMH-H21024Upper402Huitou-sha2.08131.771.9597084
SMH-H31027Upper402Huitou-sha2.13142.462.10100086
SMH-G11002Upper422Huitou-sha2.14141.642.2598586
SMH-G21002Upper422Huitou-sha2.09139.222.40101589
SMH-G31005Upper422Huitou-sha2.21162.562.55106088
SMH-T1990Upper402Huitou-sha2.23160.832.70103086
SMH-T2993Upper402Huitou-sha2.27173.942.85107588
SMH-T31000Upper402Huitou-sha2.30173.583.01104587
Note: SMY, young forest; SMZ, middle–aged forest; SMC, mature forest; SMH, Huitou-sha forest; H, Hushi Forest Farm; G, Guandu Forest Farm; T, Tongxing Forest Farm.
Table 2. RDA analysis and Monte Carlo test of environmental factors on the PhytOC accumulation of Cunninghamia lanceolata stand soil.
Table 2. RDA analysis and Monte Carlo test of environmental factors on the PhytOC accumulation of Cunninghamia lanceolata stand soil.
Environmental FactorsRDA AnalysisMonte Carlo Test
RDA1RDA2RDA3Order of Importancer2p
ASi1.486−0.146−0.0251.0000.9830.001 ***
TSOC1.471−0.1240.1922.0000.9620.001 ***
CD−1.3840.2870.3873.0000.8710.001 ***
TN1.3740.2770.3824.0000.8570.001 ***
TP1.311−0.2450.1615.0000.7780.001 ***
TH1.289−0.220−0.4746.0000.7500.001 ***
DBH1.2740.064−0.3437.0000.7200.001 ***
N:P1.2650.4720.3718.0000.7710.001 ***
NCP−1.1680.097−0.1349.0000.6070.001 ***
C:P1.1040.2810.18610.0000.5620.001 ***
pH1.094−0.320−0.07011.0000.5580.001 ***
C:N−1.063−0.408−0.35112.0000.5460.001 ***
SDI−0.8250.7030.02713.0000.4400.001 ***
BD0.7240.9420.67514.0000.4800.001 ***
NMC0.718−0.538−0.03215.0000.3100.001 ***
STP−0.605−0.5290.17016.0000.2400.001 ***
FMC−0.424−0.6690.14417.0000.2040.001 ***
CP−0.015−0.4850.12818.0000.0660.029 *
Note: Asi, available silicon; BD, bulk density; CD, canopy density; CP, capillary porosity; C:N, TSOC:TN; C:P, TSOC:TP; DBH, diameter at breast height; FMC, field moisture capacity; NCP, non-capillary porosity; NMC, natural moisture content; N:P, TN:TP; PhytOC, phytolith-occluded organic carbon; STP, total porosity; SDI, stand density; TSOC, total soil organic carbon; TN, total nitrogen; TP, total soil phosphorus; TH, tree height. Sample size n = 108. * and *** respectively indicate that the correlation is significant at 0.05 and 0.001 levels.
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Huang, Q.; Sheng, M. Effects of Stand Age and Environmental Factors on Soil Phytolith-Occluded Organic Carbon Accumulation of Cunninghamia lanceolata Forests in Southwest Subtropics of China. Forests 2025, 16, 240. https://doi.org/10.3390/f16020240

AMA Style

Huang Q, Sheng M. Effects of Stand Age and Environmental Factors on Soil Phytolith-Occluded Organic Carbon Accumulation of Cunninghamia lanceolata Forests in Southwest Subtropics of China. Forests. 2025; 16(2):240. https://doi.org/10.3390/f16020240

Chicago/Turabian Style

Huang, Qifen, and Maoyin Sheng. 2025. "Effects of Stand Age and Environmental Factors on Soil Phytolith-Occluded Organic Carbon Accumulation of Cunninghamia lanceolata Forests in Southwest Subtropics of China" Forests 16, no. 2: 240. https://doi.org/10.3390/f16020240

APA Style

Huang, Q., & Sheng, M. (2025). Effects of Stand Age and Environmental Factors on Soil Phytolith-Occluded Organic Carbon Accumulation of Cunninghamia lanceolata Forests in Southwest Subtropics of China. Forests, 16(2), 240. https://doi.org/10.3390/f16020240

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