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Journal of Resources and Ecology >

2023 , Vol. 14 >Issue 1: 57 - 66

DOI: https://doi.org/10.5814/j.issn.1674-764x.2023.01.006

Soil Ecosystem

The Influence of Plantation on Soil Carbon and Nutrients: Focusing on Tibetan Artificial Forests

  • LIU Ruixuan ,
  • YAO Yuan ,
  • ZHANG Sheng , *
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  • Key Laboratory of Bio-Resource and Eco-Environment of Ministry of Education, College of Life Sciences, Sichuan University, Chengdu 610065, China
*ZHANG Sheng, E-mail:

LIU Ruixuan, E-mail:

Received date: 2021-08-20

  Accepted date: 2022-02-20

  Online published: 2023-01-31

Supported by

The Strategic Priority Research Program of the Chinese Academy of Sciences(XDA20020401)

The Second Tibetan Plateau Scientific Expedition and Research Program(2019QZKK0404)

The Fundamental Research Funds for the Central Universities(JY201912)

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Abstract

As terrestrial ecosystem carbon (C) sinks, plantation ecosystems play essential roles in species diversity protection, resource supply and climate change. Artificial afforestation is of great important in improving the ecological condition, economic development and production in Tibet. Forests can improve soil property changes, yet the understanding of how plantations influence soil C and nutrient conditions in Tibet is still insufficient. This review combines with previous studies to explore the characteristics of soil nutrients, involving nitrogen (N) and phosphorus (P) on Tibetan poplar plantations. Generally, plantations have better abilities in improving the soil C and N cycles, and enhancing the soil stability. In this review, we further analyze the factors, including the modality of land-use, afforested period, tree species, climate factors and soil properties, which may affect the soil C and nutrients. (1) The patterns of land-use affect the accumulation of soil organic matter, thus influence the accumulation of soil C and nutrients; (2) Soil C and N increase with the years of artificial forests, while soil P is on the contrary; (3) The effects of different tree species on soil C and nutrients vary widely; (4) In terms of climate, the C sink of Tibetan plantation soil is most likely to be affected by precipitation, while the nutrient is more likely to be influenced by temperature; (5) Among soil properties, the most related factor to C is soil texture. Furthermore, our review pointed out that future research on soil ecological functions should be focused on soil microbes on Tibet plantation. At the end, we concluded three major challenges for the future research. Therefore, this review contributes to a better understand the effects of plantation on soil C and nutrients on the Tibetan Plateau.

Key words： plantation; soil carbon; nutrient; the Tibetan Plateau

Cite this article

LIU Ruixuan , YAO Yuan , ZHANG Sheng . The Influence of Plantation on Soil Carbon and Nutrients: Focusing on Tibetan Artificial Forests[J]. Journal of Resources and Ecology, 2023 , 14(1) : 57 -66 . DOI: 10.5814/j.issn.1674-764x.2023.01.006

1 Introduction

Forests are multifunctional ecosystems, they can remarkably affect the surrounding environments (Yang et al., 2019). There is a huge amount of land available for afforestation and reforestation to produce carbon (C) sinks, where C can be stored in terrestrial ecosystems to mitigate climate change around the world (Zomer et al., 2008). China has the largest total area of artificial forest in the world, up to 71 million ha (FAO, 2010; Cao et al., 2016). Plantations play indispensable roles in many aspects, such as vegetation restoration, water and soil conservation and climatic changes (Bull et al., 2006; Zomer et al., 2008). Improving forest acreage and accelerating resource cultivation are greatly important to properly alleviate the shortage of C sequestration.
The relationship between soil fertility and tree growth is close and complex, which is also the essentially ecological problem of plantation. For instance, due to proper nutrient management and water conservation measures, the productivity of mature plantation was better than that of young plantation (Dong et al., 2014). As the foundation of forest ecosystem and plant growth, soil is the most important power to restore and maintain forest ecological function. Among of the rich microbiologic populations in soil, bacteria and fungi are the most important and they play vital roles in the energy cycle. These microbes can directly or indirectly affect the forest ecosystem (Clemmensen et al., 2013; Talbot et al., 2013), and can be used as predicted indicators for nutrient availability and plant diversity (Leff et al., 2015). In turn, forests not only affect the composition of soil microbial community through litter and root exudates but also affect its structure and function by changing soil properties (Yannikos et al., 2014). Therefore, evaluating the influence of plantation on soil ecosystem is important.
Artificial afforestation is one of the largest changes for land-use types in Tibetan Plateau. The main purpose of artificial afforestation is to construct ecosystems on the windbreaks, sand-fixation and soil conservation to improve the production and living conditions. Based on the previous studies (Luo, 2018; Liu and Zhang, 2019), the mid-watershed of “One River and Two Tributaries”, which mainly refers to the Yarlung Zangbo River, Lhasa River, Nian-Chu River, and administrative area covering several counties and districts of Lhasa, Rikaze and Shannan, contains the most forestations in Tibet (Fig. 1). The Salicaceae trees are the dominant species in Tibet plantation, approximately accounting for 75.44% of the total area, while pure forests are the dominant planted patterns in Tibet, approximately accounting for 91.6% of the total plantation areas in Tibet (Luo, 2018; Chen and Zhang, 2020; Liu et al., 2021a). The area of artificial afforestation in Tibet has been continuously increased, however, we still do not know the ecological effects. Plant growth depends on soil, yet research about the changes in the physical and chemical properties of soil by artificial forestation is not sufficient. Meanwhile, there are few ways to systematically determine whether the soil sequesters C and experiences nutrient changes after afforestation in Tibet. This review focuses on previous studies on the artificial forests in Tibet (Fig. 2). We aim to: 1) Understand the status of soil nutrients and fertility of afforestation in Tibet; 2) Explore the change of soil C and nutrients after afforestation in the main afforested areas; 3) Point out the future research direction in Tibetan plantations.
Fig. 1 The main districts and counties of plantations in Tibet

Note: The red, pink, yellow, blue and white areas represent areas with the highest, high, moderate, low and the lowest carbon density of plantations, respectively. The original data used here comes from the report of Liu and Zhang (2019).

Fig. 2 The poplar plantations in the Lhasa River Basin.

(a) shows the aerial image of plantation plot beside the Lhasa River Basin; (b) shows the Lhasa River Basin; (c) and (d) show the poplar plantations in Tibet

2 Plantation-Soil relationship

The plantation has unique biological community under artificial conditions, which is characterized by the simple structure, variety composition and high nutrient demand (Yang et al., 2019). On the one hand, artificial forest can change the vegetation type. On the other hand, it also contributes to the local soil and climatic environments (Ritter et al., 2003; Marañón et al., 2015). Forest ecological function refers to the ecological environments and effects formed by forest ecosystem. The ecological processes are included of biodiversity protection, water and soil conservation, carbon fixation and oxygen release (Sun et al., 2011). These ecological functions mainly depend on the key ecological processes, such as water, carbon, nutrient and biological cycles (Sun et al., 2011). Studies have shown that the different forest ages (Li et al., 2011), biodiversity (Paillet et al., 2010) and forest types (Barlow et al., 2007) have varied ecosystem functions. Generally, the ecological function of mixed forests is better than that of pure forests, while middle-aged forests have better ecological functions than the young forests (Cui et al., 2019). Ma et al. (2012) showed that pure forest transformation could effectively improve soil water holding capacity, reduce bulk density, increase porosity and provide good soil fertility for local vegetation restoration in the semi-arid valley of Lhasa. Ye et al. (2012) also found that vegetation restoration of Populus szechuanica plantation with different afforestation measures was beneficial to improve soil properties and increase microbial quantity. So far, most studies on soil nutrients of plantation in Tibet focused on pure forest, but there were few studies on the effects of plantation patterns on soil nutrients.
The variation of soil properties in plantations may be closely related to the soil organic matter (SOM) content, the trees growth and biochemical processes. Forests can induce litter accumulation and root mass decomposition, thus increase soil organic C (SOC) content (Luo et al., 2014) and soil nutrients (Li et al., 2014). The composition of forest litters generally refers to the leaves, woody (branches and barks), reproductive organs (flowers) and some undistinguishable debris (Swift et al., 1979; Schlesinger et al., 2001). Leaves and branches are the main components of litters (Fig. 3). The yield and component of litterfall are the main factor in determining the nutrient return (Bigelow et al., 2015) and the efficiency of matter cycle (Coq et al., 2011; Wang et al., 2016; Hishinuma et al., 2017) for forest ecosystem. Litters provide elements needed for forest vegetation growth by inputting nutrient elements into the soil through nutrient return. For example, litters provided much mineral elements into forest soils (Meentemeyer et al., 1982; Zhang et al., 2014).
Due to the different external environments and internal characteristics of plantation, trees in the same area will experience different nutrition limitations (Yu et al., 2015). Plantations are greatly affected by soil total nitrogen (TN), total P (TP), pH and C: N ratio, which might be related to the soil conditions, vegetations and environmental conditions (Yang et al., 2019). The TN and TP are two of the most key element indicators for tree growth (Ilori et al., 2014), and the fast-growing trees would consume more soil nutrients than the low-growing trees (Mendham et al., 2003; Merino et al., 2004). Thus, the maintenance and improvement of soil fertility are the main purposes in establishing the relationship between soil properties and tree growth. In turn, artificial forests can also affect soil nutrients. For instance, the stands with less productive trees generally have lower soil fertility (Firn et al., 2007). Afforestation has a profound impact on the biological and chemical processes in terrestrial ecosystems. Among of that, the composition, structure and function of soil microbial communities were rapidly and significantly changed (Fu et al., 2015). Also, the dynamic feedback mechanism with soil C and N caused by soil SOM, temperature, water and pH have been found (Fierer et al., 2009; Cheng et al., 2013).

3 The influence of plantations on soil C sequestration

The soil system is the largest C pool of the terrestrial ecosystem, acting a vital role in global C cycling. The C stores in soil surpasses the total of vegetation biomass and atmospheric pools (Cao et al., 2018). The soil C pool is composed by SOC and soil inorganic C (SIC). SOC, which accounts for at least three times as much C as the CO2 pool in the atmosphere, is the largest C pool in the terrestrial C pool (Amundson, 2001). Forest vegetation and corresponding soil mainly account for 60% of the C reserved in the terrestrial ecosystems (Winjum et al., 1992). Forests occupy by 80% of the land and 40% of the underground C in the terrestrial C pool, respectively (Dixon et al., 1994; Luo et al., 2018). Thus, the relationship between soil C and forest is important and complex (Fig. 3).
Fig. 3 The relationship between plantation and soil ecological system

Note: Plantation and soils can affect each other through the tree species, litters and soil microbes.

Here, we show some factors may affect the soil C in the plantation in Tibet or Qinghai-Tibet Plateau. The modality of land-use can influence C sequestration in soil. Shi and Yu (2003) compared the SOC of poplar plantations under different land-use patterns along the Lhasa River Valley. The results indicated that: 1) The conversion of farmland and the establishment of artificial forest network, which had the function of C fixation, were beneficial to the accumulation of SOM and SOC; 2) Litters played an important role in the C cycles (Fig. 3). This may be due to the large amount of CO2 by which plantation absorbed from the air, the strong ability to transport C into the underground part, and the large amounts of litters. Thus, poplar plantation has a positive effect to C sequestration for the soil.
The afforested period can also impact soil C dynamics and sequestration. It was reported that SOC increased with stand ages in Picea asperata plantations on the Eastern Tibet Plateau (Cao et al., 2020). Also, Yu and Jia (2014) found SOC increased with tree ages in Salix cheilophila in the east of Qinghai Province. On the other regions, for example, at the eastern margin of the Qinghai-Tibet Plateau (Northwest Sichuan), soil-vegetation C reserves increased with the afforested years for S. cupularis forests (He et al., 2018). According to the results reported by Liu and Zhang (2019), the main districts and counties with different C density of poplar plantations in Tibet were listed in Fig. 1. The results indicated that mature plantations tended to have higher carbon density than the young ones, and the proportion of artificial macrophanerophytes and shrubs was increased (Table 1). Thus, soil C increase with the years of plantations in the Qinghai-Tibet Plateau. This may be because: 1) Litters and root exudates can be inputted sustainably during the stand growth; 2) Different growth and development of forests with different ages may lead to different C sequestration.
Table 1 The average tree ages and evaluation indexes of five plantations’ zones
Types Average ages of
plantations (yr)		 Proportion of artificial
macrophanerophytes (%)		 Proportion of artificial
shrubs (%)		 Average carbon density of
plantations (t ha-1)
A 13.01 2.26 0.13 21.23
B 11.79 0.31 0.02 14.43
C 9.90 0.27 0.02 10.73
D 7.90 0.33 0.29 6.38
E 6.29 0.07 0.01 3.76

Note: Five types based on the carbon density of plantations. The A, B, C, D and E represent areas with the highest, high, moderate, low and the lowest carbon density of plantations, respectively. Source: Liu and Zhang, 2019.

On the other hand, the C deposited rate and distribution in soils are also varied from tree species (Jandl et al., 2007), leading to an obvious difference in soil SOC (Schulp et al., 2008). For instance, studies showed that the soil C pool decreased with the coniferous trees planting, while it increased with the hardwood trees planting (Guo et al., 2002; Paul et al., 2002; Berthrong et al., 2009).
In terms of climate, it is generally believed that precipitation has far-reaching influence on soil C sequestration in plantations, while temperature is not significant (Luo, 2018). When the annually average rainfall is less than a certain area value, soil is prone to C accumulation, while when the rainfall is more than a certain area value, soil is prone to C loss. Thus, the effect of Tibetan plantations on soil C sequestration may not be affected by the temperature but precipitation.
Soil properties, e.g. clay mineral type, soil bulk density, physical structure and nutrient status, play key roles in soil C accumulation (Hagedorn et al., 2003). Among of them, the most related factor is soil texture. However, the results on the effect of soil texture on SOC after afforestation are inconsistent. For example, Six et al. (2002) and Krull et al. (2003) pointed out that SOC content increased with the increase of soil powder and clay content. Paul et al. (2002) also considered that clay soils had greater C sequestration potential in the long run. However, on the contrary, Thuille et al. (2006) found that C loss occurred in high-clay soils after grassland afforestation. In general, most studies had indicated that the higher clay soil, the better effect on C sequestration. As there are too many and complex factors affecting the soil C pool of plantations, both natural and man-made, it should be considered synthetically. Anyway, compared with the control plots, the soil SOC was increased in the plantation of Tibetan Plateau (Luo, 2018).

4 The influence of plantations on soil nutrient

The assessment of plantation soil nutrients is fundamental for forest management (Ilori et al., 2014). Not only C, but also N and P can give vital impacts on the soil environment changes by afforestation. The C sink process depends on the N availability (Luo et al., 2006; Allison et al., 2010), while the SOC accumulation requires the different C:N ratio (Hessen et al., 2004). Thus, the effects of artificial forests on N and P in Tibet are discussed in this paper.
Soil nutrient dynamics are affected by a series of complex processes and regulated by multiple factors (Liu et al., 2010; Laughlin, 2011; Deng et al., 2016), including of the initial land type and property, climatic factors, afforested years and tree species. The initial land type determines the basic level of soil nutrients (Shi et al., 2016). The forestland is mainly non-arable land in Tibet, e.g. river terraces and slopes. The SOM structure remodels the soil microenvironments, while the microbial activities will be accelerated by afforestation. A large amount of soil nutrients is released from the SOM and absorbed by tree roots to synthesize biomass. In turn, the tree returns the nutrients into the soil in the form of litters and root exudates (Fig. 3).
Not like soil C, soil nutrients were significantly affected by temperature in Tibetan forests, then was moisture. The annually average temperature and soil enzymes activities are lower in Tibetan Plateau compared with the low altitude regions, leading to a slow litters’ returning rate. The higher the altitude, the lower the temperature will be. Luo (2018) have pointed out that the available P (AP) was particularly related to air temperature and altitude in Tibetan Plateau, while in most soil layers, the change of TN reserves was not that case. As for the forestland in Tibetan Plateau, although the afforested areas are dominated by wasteland, with the altitude decreasing from northwest to southeast, the rainfall and temperature increased gradually, leading to an accumulation of biomass (Luo, 2018). Therefore, the soil nutrient fertility was gradually better from northwest to southeast with the increased temperature.
The soil N of plantation is also affected by afforested years (Li et al., 2012; Chang et al., 2014). More researchers have indicated that soil N would be increased with the increase of planted age, while soil P is contrary. For example, Luo (2018) indicated that the reserves of TN and available N (AN) increased with plantation age, while the TP and AP decreased in Tibetan plantation. Similarly, the concentration of TN increased with stand age in P. asperata plantations on the Eastern Tibetan Plateau, however, the soil TP concentration was increased first and then decreased (Cao et al., 2020). Other studies also showed that soil N content increased within a certain afforested period, while P was on the contrary (Zhao et al., 2007; Chang et al., 2014; Zeng et al., 2014). Therefore, the afforested year has a great effect on soil nutrients in Tibet, especially on soil N and P.
Soil nutrient requirements and feedbacks are also different among tree species (Firn et al., 2007). The input amount and decomposed rate of litters are varied from tree species (Guo et al., 2004; He et al., 2011), which results in the different feedback to soil nutrients. The varied soil nutrients could be contributed to the litter compounds (Li, 2012). Therefore, it is important to match species with the sites in Tibetan Plateau. For instance, selecting N-fixed tree species can potentially increase the availability of soil N (Resh et al., 2002; Dan, 2005), and further improve soil fertility (Resh et al., 2002). Additionally, the selection of tree species also should be considered the environmental adaptability. The suitable tree species can bring better benefits to improve soil nutrients and soil development. It has been indicated that the plantation of poplars could reserve AP better than willows in Tibetan Plateau (Luo, 2018). Therefore, tree species selection is important to improve soil nutrients in Tibetan Plateau.

5 The influence of plantations on soil stability

Soil texture is closely related to SOM over a wide range of scales. The composition of SOM is very complex, consisting of a mixture of large molecules, microbial and plant fragments (Lehmann et al., 2008). The main body of SOM in artificial forests of Tibet is litter. Microbial residues make up most of the organic matter, and the biomolecular structure consists mainly of the cell envelope of dead fungi and bacteria (Liang et al., 2019). After afforestation, SOM increased in plantation soils compared with the control land (Luo, 2018), which provides a variety of nutrients for the microbes and trees. Under the combined effect of microbes, trees and soil animals, the litters were returned into the soil and formed new complex substances. These iterative processes increase the relative stability of soil texture.
In Tibet, plantations are mainly in the valley, therefore, they can effectively improve soil water holding capacity, reduce bulk density, increase porosity and improve fertilizer retention ability, which is conducive to the restoration of the degraded ecosystem and vegetation reconstruction in the semi-arid area (Ma et al., 2012). The increase of soil macroporosity and macropore volume is related to the shrub root network (Hu et al., 2019), because the soil bulk density near the roots is higher than that of far away the roots (Zuazo and Pleguezuelo, 2009). Thus, the level of root network reflects soil voids. The larger the voids, the lower the soil stability. After afforestation in Tibet, soil bulk density and soil silt clay content were downward, while soil moisture, pH and sand content were upward (Luo, 2018). However, study on the roots of plantation in Tibet is limited to date. Therefore, more comprehensive evidence (e.g. plant litters and secretions.) in the impact of Tibetan plantation on soil stability should be given in the future.

6 The influence of plantations on soil microbes

Soil microbes can take influences on the degradation of SOM, the incorporation of humus into mineral nutrients and the change of soil physical structure, which play critical roles in forest ecosystem (Xu et al., 2008; Huang et al., 2013; Koranda et al., 2013). Some studies reported that soil N availability might affect to forest soil microbial growth (Wagai et al., 2011; Xiao et al., 2019). For instance, bacteria generally need higher N and grow well under N-rich soil conditions. On the other hand, the relative change of soil C and N concentrations will affect total microbial lipid (Wagai et al., 2011). Compared with mineral soils, the Tibetan forests organic layer had higher nutrients, which lead to a higher microbial biomass and activity (Zhang et al., 2017; Luo et al., 2020).
For Tibetan forests, soil microbial diversity may be easily changed by tree community and diversity (Luo et al., 2020). On the one hand, plant communities can accelerate nutrient cycles by plant rhizosphere sediments (Grayston et al., 1998). On the other hand, the higher diversity of plant community, the more soil microbial community will be, which will improve the niche availability of soil microorganisms (Fichtner et al., 2014). Additionally, the environmental stresses (e.g. heavy metal pollution) and soil environmental heterogeneity (Vellend et al., 2017; Liu et al., 2021b) can also influence soil microbial diversity.
Soil microorganisms can mediate the activities of extracellular enzymes (Li et al., 2018; Feng et al., 2019). For example, hydrolases (e.g., N-acetylglucosidase and β-glucosidase), which are produced by fungi, are responsible for obtaining C, N and P to support primary metabolism and oxidases (e.g. polyphenol oxidases), and further to improve the availability of soil nutrients (Sinsabaugh et al., 1994, 2008). According to the reports of Tabatabai (1994) and
Parham and Deng (2000), the activities of β-1, 4-glucosidase, β-1, 4-N-acetylglucosaminidase and phosphomonoesterase were related to the microbial acquisition of C, N and P respectively. While the enzymes of soil cellulase, catalase, protease and invertase were related to SOM or microbes (Li et al., 2018). Additionally, soil pH, soil types, soil texture and soil moisture also greatly affect to soil microbial diversity in plantation (Huang et al., 2014; He et al., 2017; Wu et al., 2020; Yang et al., 2020).
Climatic factors, especially the hydrothermal conditions also affect the soil microbial community of plantation (Luo, 2014). The temperature and humidity of monsoon are conducive to litters decomposition, meanwhile plants secrete a large amount of organic C and other degradable substrates into the soil during the vigorous growth. Thus, the available substrates in the soil are abundant to support the growth of microbes (Schimel et al., 1999; Liu et al., 2000; Rogers and Tate, 2001). Studies had showed that the large amount of plant litters in dry season coupled with good soil aeration and the dissolved oxygen content in the soil, could benefit to the growth of aerobic microbes (Niu et al., 2011). However, other studies had pointed out that there was a significantly negative correlation between microbial biomass and temperature, while a positive correlation with plantations (Gao, 2017). The contrary results may be due to the different vegetation type, soil type and climatic zones. In addition, the soil microbial communities are varied from season. In summer, as the high temperature, tree rhizosphere exudates are released into the soil to stimulate the growth of bacterial communities. In autumn, with the stop of tree growth, soil microbes are mainly fungi. While, in winter, the activity of many warm-friendly microbes are stopped and the cold-friendly microbes are in large numbers (Nemergut et al., 2005). Therefore, the soil microbes in Tibetan plantations will be changed with the seasonal fluctuation, and it is worthy to study further in depth in the future.
Compared with previous studies on Tibetan Plateau, we know that soil microorganisms of plantations are closely related to C, nutrients, soil physical and chemical properties and climate changes. Thus, future study should be focused on the following aspects: 1) The relationship between tree species and ages with soil microbial diversity. 2) The variation of soil microbial activity, abundance and diversity with elevation gradient by the climatic change.

7 Conclusions and outlook

In this study, the characteristics of plantation soil nutrients and fertility in Tibetan plantations are summarized. The changed factors of soil C and nutrients in plantations after afforestation in key afforested areas are discussed. The artificial forests can enhance soil C and N cycles and optimize the soil stability. The modality of land-use, afforested period, tree species, climatic factors and soil properties greatly affect the soil C and nutrients. Land use patterns can affect the accumulation of SOM, thus influence the accumulation of soil C and nutrients. However, it is not clear which land use pattern is most suitable for soil C storage and nutrients accumulation in Tibet plantations. On the one hand, the contents of soil C and N are positively correlated with planted ages, while P is contrary. On the other hand, the effects of different tree species on soil C and nutrient are varied. Additionally, C sink is mainly affected by precipitation while nutrients (N and P) are mainly affected by temperature. Overall, plantations can improve soil water holding capacity, reduce bulk density, increase porosity and improve fertilizer retention ability in Tibetan Plateau.
There are several challenges to be addressed in the future research: 1) We should establish a more systematic mechanism on the dynamic changes of soils in key afforested areas in Tibet. 2) It is necessary to study the soil changes and the relationship with environmental factors in plantations to reveal the most favorable species. 3) It should be considered the effect of globally climatic change on plantation and the characteristics of forest stand on soil nutrients and microbes in Tibetan Plateau. Overall, this review illuminates the potential effects of Tibetan artificial forests on soil ecological function. Tibetan plantation can improve the level of soil C, N and P contents and enhance the soil stability. In future, more attention should be given on the soil microbial populations and structures in Tibetan plantations.

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