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Journal of the Geological Society of Korea - Vol. 58 , No. 1
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Journal of the Geological Society of Korea - Vol. 58, No. 1, pp. 117-135
Abbreviation: J. Geol. Soc. Korea
ISSN: 0435-4036 (Print) 2288-7377 (Online)
Print publication date 01 Mar 2022
Received 21 Jan 2022 Revised 10 Feb 2022 Accepted 15 Feb 2022
DOI: https://doi.org/10.14770/jgsk.2022.58.1.117

Effects of land use change on soil moisture content at different soil depths
Rogers Wainkwa Chia1, 2 ; Jin-Yong Lee1, 3, ‡ ; Jiwook Jang1, 3 ; Sungbeen Lee1, 3
1Department of Geology, Kangwon National University, Chuncheon 24341, Republic of Korea
2Research Institute for Earth Resources, Kangwon National University, Chuncheon 24341, Republic of Korea
3Research on Microplastics in Groundwater (RMPG), Kangwon National University, Chuncheon 24341, Republic of Korea

토지이용 변화가 깊이별 토양수분에 미치는 영향
Rogers Wainkwa Chia1, 2 ; 이진용1, 3, ‡ ; 장지욱1, 3 ; 이성빈1, 3
1강원대학교 지질학과
2강원대학교 지구자원연구소
3강원대학교 지하수미세플라스틱연구팀
Correspondence to : ‡+82-33-250-8551, E-mail: hydrolee@kangwon.ac.kr

Funding Information ▼
Abstract

Land use is a paramount human-induced change that has redesigned the world's surface since the beginning of civilization. With the worldwide population increment in the last decade, a lot of natural forests have been converted into croplands, which damage soil properties. Over time, some of these established croplands have been transformed into forest plantations to reduce damages to soil properties. Nowadays, scientists are interested on how these land use changes affect soil moisture. Although many studies have been documented in this area, they have not critically studied how soil moisture varies with soil depth between different land use types. In this review, we discuss soil moisture variability with depth in a natural forest, mono- and mixed-croplands, orchard, and mono- and mixed- forest plantations. We observed that the shift in land use from a natural forest into either a mono cropland resulted in the reduction of soil moisture at the topsoil layers, or the establishment of a mixed cropland resulted in high soil moisture at the topsoil layers and the increment and decrease of soil moisture with depth. However, the conversion of cropland to a mono forest or orchard caused soil moisture to decrease with the depth from the top soil layer (0-20 cm) to the inner soil layer (0-40 cm). After the conversion of cropland into a mixed forest, the soil moisture decline was severe in the deeper soil layer of 30-90 cm. This information will be useful in understanding soil moisture, which is essential for the management of environmental soil water.

초록

토지이용의 변화는 문명이 시작된 이래로 지구 표면을 재설계해온 인류가 유발한 가장 중대한 변화이다. 지난 10년 동안 전 세계적으로 인구가 증가함에 따라 많은 자연림이 경작지로 전환되어 토성이 손상되었다. 시간이 지남에 따라, 이러한 경작지 중 일부는 토양에 주는 피해를 줄이기 위해 조림사업으로 복구되었다. 최근 연구자들은 이러한 토지이용 변화가 토양수분에 어떤 영향을 미치는지 관심을 가지고 연구하고 있는데 이러한 분야에 대한 많은 문헌과 논문이 발간되었지만, 토양수분이 다양한 토지이용의 유형들 사이에서 지표로부터의 깊이에 따라 어떻게 변하는지 논의하는 비판적인 연구는 미미하다. 이 리뷰에서는 자연림, 단일 및 혼합 경작지와 조림지의 토양 깊이에 따른 토양수분의 변동성에 대해 고찰한다. 그 결과 자연림에서 단일 경작지로의 토지이용변화는 표토층의 토양수분 함량을 감소시켰고, 혼합 경작지의 건설은 표토층에서 높은 토양수분 함량을 보이고 깊이에 따라 증가하거나 감소하는 양상을 보였다. 그러나 경작지가 단수림 혹은 과수원으로 그 토지이용이 변화됐을 때는 표토층(0-20 cm)에서 내부 토양층(0-40 cm)까지의 깊이에 따라 토양수분 함량이 감소하였다. 또한, 경작지에서 혼합림으로의 토지이용변화는 30-90 cm 깊이의 토양층에서 극심한 토양수분 함량 감소를 보였다. 이 결과들은 환경 토양수 관리에 필수적인 토양수분을 이해하는 데에 유용할 것이다.

Keywords: soil moisture, mono cropland, mono forest plantation, mixed forest plantation, land use change
키워드: 토양수분, 단일 경작지, 단일 조림, 혼합 조림, 토지이용 변화
1. Introduction

In a great deal of human history, the earth’s surface was uninhabitable and most of its landscapes were covered by forestlands, grasslands, and shrubberies (Hannah and Max, 2019; Awuchi et al., 2020; Fazan et al., 2020). For the past few centuries, these landscapes have undergone massive changes because of human activities (Sheil, 2018; Liu, C.A. et al., 2019; Tuffour-Mills et al., 2020; Polizel et al., 2021; Rude et al., 2021). They have been converted into different land uses, with croplands as the major land use establishment (Zhou et al., 2019; Spangler et al., 2020; Viana and Rocha, 2020). Before human civilization, there were 60 million square kilometers of natural forest on the earth. Following global development and population growth, a lot of natural forests covering the earth’s surface are transformed into croplands. According to FAO. (2020) since the 1990s, about 420 million hectares of natural forest land has been transformed into croplands. Nowadays, there are only less than 40 million square kilometers left in these forests (Bologna and Aquino, 2020). According to Busch and Ferretti-Gallon. (2020) human-driven land use changes from natural forests to cropland, also known as deforestation, is believed to exist where crop products are more profitable. In the context of ecological regions, land use changes in tropical regions (e.g., Amazons) are more dramatic, where natural forest trees are cut down to grow soybeans, corn, and other crops, or to expand pastures (Staal et al., 2020). Humans rely strongly on the capacity of these established cropland soils following the clear-cutting of natural forest to maintain a stable crop productivity, which accounts for more than 95% of global crop production needed to feed the expanding world population (Acir and Gunal, 2020; Borrelli et al., 2020; Silver et al., 2021). However, the transformation of surface land, especially from natural forests to cropland, is a major burden to soil properties in the long run (Cai et al., 2019; Xiong et al., 2020; Li et al., 2021).

Soil naturally exists in a three-phase system of mineral solids, water liquids and gases (Susha et al., 2014; Alam et al., 2021; Horn, 2021). The maximum amount of water that penetrates the soil profile and then is added to its water phase, and finally stored in different soil layers from the surface soil to the soil layer above the groundwater is called soil moisture (Wenwu et al., 2018; Babaeian et al., 2019). Knowledge about this amount of seepage water stored in different soil layers (called soil moisture) has been widely used in biogeological and hydro ecological research to monitor croplands, soil behavior, predict severe climate events and manage the environment (Sungmin and Orth, 2021). Soil moisture controls the plant directly by regulating the availability of resources in soils needful for its growth (Yu et al., 2015; Wang et al., 2019). According to Liu et al. (2021) soil moisture is affected by changes in land use and crop cover.

The conversion of a natural forest into cropland is one of humanity’s largest impacts on the soil (Haas et al., 2020; Yin et al., 2020; Wilken et al., 2021). A recent assessment indicates that if the current land use change from a natural forest into a cropland trend is persists, approximately 75% of the remaining natural forest will be cleared due to the expansion of cropland establishments in the next 25 to 50 years (Byerlee et al., 2014; FAO., 2020). In the past, majority of land use changes from a natural forest into a cropland, involved the cultivation of a mono cropland site whereby farmers grow only single crop yearly, such as wheat, corn, rice, rapeseed, sugar cane, and cotton, a phenomenon known as monocropping (Merten et al., 2020; Malaiarasan et al., 2021). The establishment of a mono cropland site following deforestation is extensively used in industrial crop cultivation (Bronson, 2019; Salaheen and Biswas, 2019). Also, some mono cropland sites are useful for wastewater treatment and water quality improvement (Marín-Muñiz et al., 2020). Another advantage of a mono cropland site is in the improvement of planting productivity and yield increment (Liu et al., 2018; Slattery and Ort, 2021). However, some studies show that soil moisture content is reduced after the conversion of natural forests to mono cropland. Bizuhoraho et al. (2018) showed that soil moisture was lost upon changing land use from natural forest to cropland. Then, Tellen and Yerima (2018) showed about 23% loss of soil moisture content at low altitude and 42% of soil moisture content loss after change in land use from a natural tropical forest into a mono cropland of corn. Currently land use has drifted to mixed croplands and few studies have started focusing on land use change from a natural forest into a mixed cropland system as separate entities. Results from these studies show that soil moisture either increased or decreased following a change in land use from a natural forest into a mixed cropland, which is contrary to the establishment of a mono cropland after land use change from a natural forest into a mono cropland (NongKling and Kayang, 2017; Chen, C. et al., 2019; Whang, J. et al., 2021). These discrepancies and uncertainties suggest the need for a study to better understand the pattern of how changing land use from natural forests into mono- and mixed-cropland can impact soil moisture entirely.

In the past decades, many croplands have been converted into either mono or mixed forest plantations (Harper and Sochacki, 2019; Yue et al., 2020; Zhou et al., 2020; Vincent et al., 2021). One of its main purposes is to prevent or repair unhealthy soil, as well as the decline in soil moisture due to land use changing from natural forest to cropland (Yu et al., 2019; Ghosh and Mishra, 2020; Jourgholami and Labelle, 2020). At the same time, in some areas, for economic reasons, some croplands have been transformed into orchards (Martin-Gorriz et al., 2020; Vu and Shen, 2021; Whang, R. et al., 2021). Many research reports point out that the soil moisture content can be improved after land use is switched from a cropland to a forest plantation (Cunningham et al., 2015; Cao et al., 2018; Vopravil et al., 2021). However, it is necessary to report appropriately without any uncertainty how these land use changes from cropland to mono or mixed forest plantation, affect the soil moisture at different soil depths. Soil moisture in different soil layers is usually related to different hydrological processes and ecological functions (Duan et al., 2016; Zhang et al., 2019; Yang, T. et al., 2021).

The soil undergoes a vertical exchange of resources, causing the physical and chemical properties of the soil to change from surface soil to underground soil (Adugna and Abegaz, 2015; Han et al., 2018). According to Fang et al. (2016) surface soil moisture is usually affected by precipitation and infiltration and is a reliable source of water for crop growth, while deep soil moisture is the soil reservoir. However, most studies only focus on the O layer (0-20 cm) and A layer (0-30 cm) soil depth, and deeper soils are often overlooked or paid less attention. Accurate and continuous measurement of soil moisture with depth can be very difficult, because on-site measurement of moisture in deeper soil layers can be expensive, and there is no direct relationship between surface and underground soil moisture (Gao, L. et al., 2015; Gao, X. et al., 2017; Yanni et al., 2018). Also, another reason may be that the top and subsoil layers are more sensitive to global changes and other disturbances than deeper soil layers (Tian et al., 2017; Chen, S. et al., 2018; Yanni et al., 2018).

Although few studies have been able to report changes in soil moisture content with depth (Tuo et al., 2018; Nan et al., 2019; Lan et al., 2021; Xu et al., 2021). For sustainable environmental management, it is necessary to fully understand the changes in soil moisture content with depth between different land use types. Hence, a needful study that reviews the most recent reports on changes in soil moisture content at different land use types and depths. So, the objective of this study was to evaluate recent knowledge on how soil moisture content varies with depth following land use change from a natural forest to into a mono or mixed cropland and then reforestation with either a mono or mixed forest plantation. This information will help broaden our understanding of the impact of land use on soil moisture dynamics in deforestation and reforestation areas between different soil depths and is necessary for proper environmental management and future research.

2. Impact of land use change from a natural forest into a cropland on soil moisture
2.1 Effects of a monocrop land establishment on soil moisture content

Mono cropland sites are highly interactive with soils, and a lot of studies have been done to examine what could be the possible impact of this form of land use on soil (Ahmed et al., 2019; Sadiq et al., 2021). These studies found out that continuing monocropping for a couple of years can reduce the amount of nutrients in soils (Zhang et al., 2016; Salaheen and Biswas, 2019). While studies which focus on the impact of a mono cropland establishment following a change in land use from a natural forest into mono cropland field documented a reduction in surface soil moisture (Table 1). This reduction in the mono cropland top soil's moisture content after land use change from a natural forest can be associated with rapid evaporation of soil water from the crop field’s topsoil layer layers due to the reduction in the vegetation cover (Liang et al., 2021; Zong et al., 2021; Fig. 1). According to Huang et al. (2021) the presence of the forest canopy and excessive leave litter on the forest floor, function as shade to the forest floor's topsoil layers, and this serve as a vapor barrier that controls soil moisture lost through evaporation from the topsoil layers but in the mono cropland, the huge canopy or abundant leave litter absent or smaller. The low soil moisture in the top crop fields may also be due to crop planting activities (e.g., ploughing) exposing the arable land to temperature, so the soil moisture quickly evaporates. (Kavian et al., 2014; Jeřábek et al., 2017). Natural forest soils have high organic matter content, which improves the water absorption and water storage capacity of the soil. However, once mono croplands are established after a natural forest is clear-felled, the soil organic matter will be reduced, and the soil moisture will also be reduced (Tolimir et al., 2020).

Table 1. 
Impact of land use conversion from natural forest to cropland on soil moisture at variable soil depths.



Fig. 1. 
Land use change from natural forest to mono cropland resulted in the decrease of soil moisture in the 0-40 cm soil layer, and the increase of soil moisture from 40-80 cm to deeper soil moisture but the establishment of a mixed cropland in a native forest resulted to high topsoil moisture that increase with depth.

Although most studies have shown that after land use changes from a natural forest into the mono cropland, the soil moisture in the topsoil of cropland will decrease, the subsoil layers do not follow this pattern. Conversely, the soil moisture content in the subsoil layer (e.g., 40-60 cm) contrasted with the content in the topsoil, is relatively higher (Fig. 1). This can be linked to the fact that the roots density of most crop plants does not grow to the 40-60 cm soil layer, unlike forest trees with deeper root density distribution which absorbs soil moisture from this layer and reduces the soil moisture content (Yu et al., 2019; Guo et al., 2020). According to Han et al. (2018) high soil moisture at this layer could be attributed to a phenomenon called the freeze-thaw cycle, where by soil moisture from deeper soil layers migrate to soil layers above with lower soil temperature, accumulate and render the soil layers below moisture deficient. Also, soil moisture increased with depth (e.g., 0-60 cm) can be attributed to the increase infiltration of soil moisture into deeper soil depths (Sainju et al., 2017; Zhao et al., 2018; Dai et al., 2019).

2.2 Effects of a mixed cropland setting on soil moisture content

For some time now, as an attempt by farmers to resolve the environmental issues and soil nutrient deficiency, there is a shift from a mono cropland system into a mixed cropping field system (Kordi et al., 2020; Schulz et al., 2020). A good practical example is in an arid area, where water shortage is a threat to crop cultivation sustainability, and farmers turn to mixed cropping to mitigate this situation (Chen et al., 2018a). Mixed cropping, also referred to as intercropping, is the oldest form of systemized agricultural production and involves the growing of two or more species or cultivars of the same species together in the same field (Lizarazo et al., 2020). This form of growing crops comes with a lot of advantages such as reduced pest and disease incidence, reduced soil erosion, more biomass production, resource use efficiency, and yield stability (Weltzien and Christinck, 2017; Chen et al., 2021).

Recently, a couple of studies have reported high soil moisture at the topsoil layer and an increase in soil moisture with depth following the change in land use from a natural forest into the mixed cropland compared to deforestation from a natural forest into a mono cropland (Wang, T. et al., 2021; Table 1). The high amount of soil moisture following intercropping could be associated with more shade from plant leaves and small evapotranspiration (Ndiso et al., 2017; Fig. 1). In most cases in a mixed cropping system in the topsoil layer of 0-30 cm, soil moisture turns to be high in the topsoil layer because of a large amount of canopy that lessened the quantity of soil moisture lost through evaporation and the deep system, which helps in the extraction of soil moisture in deeper soil layers and minimized soil water extraction in the topsoil (Nyawade et al., 2019; Fig. 1).

3. Impact of land use change from a cropland into a forest plantation on soil moisture
3.1 Effects of a mono forest plantation establishment on soil moisture content

To meet the hydrological international goal of mitigating climate change as stipulated by the “Kyoto Protocol,” many croplands have been converted into plantations through investment in the clean development mechanism and reforestation (Hou et al., 2019; Bossio et al., 2020; Park et al., 2020). Even before that, the phenomenon of establishing a mono tree plantation on a cropland can be traced back to the year 1368, when single tree species of Pinus sylvestris were cultivated in the Lorenze Forest in Germany and the 18th and 19th centuries in other parts of Europe because of the scarcity of timber, with the aim to enhance the rapid production of plants within a short period (Liu et al., 2018; Brooking, 2020; Podrázský et al., 2020).

The advantages of converting croplands into a mono forest plantation are restoration of deforested watersheds and destroyed environment (Hong et al., 2020; Nichol and Abbs, 2021). When cropland is converted to a mono plantation, in most cases, soil moisture decreases with increasing depth (Fig. 2). The decrease in soil moisture with depth can be linked to the fact that high soil moisture content is absorbed by plant roots at deeper soil layers by the deep root system of trees to be utilized for plant growth and this issue becomes intense as the plant grows older (Jian et al., 2015; Liang et al., 2018; Zhu et al., 2018; Fig. 2). Also, according to Luo et al. (2020) the decrease in soil moisture with depth could be associated with the ability of the large forest canopy of mono forest plantations to retain heavy rainfall could penetrate deeper soil layers and light rain with short pulses to only soaked the topsoil.


Fig. 2. 
Land use change from a cropland into a mono forest plantation resulted in the decrease of soil moisture with depth but the shift of land use from a cropland into a mixed forest plantation resulted to an increase and decrease in soil moisture with depth.

3.2 Effects of a mixed forest plantation establishment on soil moisture content

As of 2015, following the observation that reforestation with a mono forest plantation comes along with a lot of environmental drawbacks and impacts, about three-quarters of the world’s forest industry has shifted to a mixed forest plantation system (e.g., Pinus (42%) and Eucalyptus species (26%) (Marron and Epron, 2019). In this case, mixed forest refers to a plantation reforestation system where two or more tree species are planted in the same area (Tembata et al., 2020; Messier et al., 2021; Waitz and Sheffer, 2021). The goal of mixed forests is usually achieved by mixing a small number of different tree species or some specific tree species and strains (Lu et al., 2018; Branclion and Holl, 2020).

Many studies have documented the positive impact of mixed forest plantations on the environment, but most studies only focus on above-ground environmental factors, and less on underground (Xie et al., 2021). Studies on the effects of mixed forest plantation facilities on the properties of underground soil (e.g., soil moisture) indicate that soil moisture decreases with depth after the establishment of a mixed forests (Table 2). The excess deficit or utilization of soil moisture in deeper soil layers and subsequently lower soil moisture in deeper soil layers compared to the shallow depths after the establishment of a mixed forest plantation could be associated to the fact that nearly all mixed tree species consist of deep root systems, which rely more on deeper soil water content for tree growth than the surface soil water (Bello et al., 2019). Also, according to Yao et al. (2016) the decrease in soil moisture with depth in mixed forests may be related to the obstruction of precipitation (e.g., rainfall) and evaporation, which are due to the excess leaf shade of the plants in the mixed forest.

Table 2. 
Effects of land use change from a cropland into a mono/mixed forest plantation on soil moisture at variable soil depths.


On the other hand, some studies report that the establishment of mixed forests causes soil moisture to increase with increasing depth. These increase in soil moisture content with depth following the establishment of a mixed forest plantation could be linked to a couple of factors such as the type of tree species used for the establishment of the mixed forest plantation, weather, root system, and diversity of plant communities (Chaturvedi and Raghubanshi, 2018; Maxwell et al., 2020; Liu et al., 2021; Fig. 2). According to Tang et al. (2018) the establishment of mixed forest plantations with tree species such as Pinus tabuliformis and Hippophae rhamnoides can trigger more soil water use in shallow soil layers than in deeper soil layers. Only when the precipitation in the dry season decreases, can the soil moisture be used in the deeper soil layer (Amazonas et al., 2018). In some very rare cases, the mixing of tree species (e.g., Platanus orientalis and Hippophae rhamnoides) can end up in low tree density and diversity (Liu et al., 2018; Fig. 2). According to Gao et al. (2018) low plant density results in low transpiration rate, so soil moisture accumulates with increasing depth.

4. Impact of change in land use from cropland into orchards on soil moisture content

In a nutshell, the earth’s entire surface is facing soil deterioration which prevails because of the huge augmentation in land use change from natural forest into crop fields (Béliveaua et al., 2017; Owuor et al., 2018; Fentie et al., 2020). In addition to severe soil degradation, land use changes are accompanied by many negative social impacts, such as reduced soil nutrients, low crop productivity, and long-term forest-dependent communities may fall into poverty (Oyetunji et al., 2020; Ullah et al., 2020). Somehow, to reduce the excessive burden on cropland in some way, since 1990, most of the croplands established on deforested lands have been converted into orchards to produce high-value fruits (Chen, C. et al. (2019); Hossain et al., 2020). In China, where soils have faced the largest form of soil degradation as erosion caused by land use changed from the natural forest into wheat and maize croplands, in 1990 to 2013 changed croplands into orchards to reduce pressure on crop fields (Lu et al., 2016; Chen, Z. et al., 2019).

Several studies have shown that as land use is changed from a cropland to an orchard, soil moisture decreases with depth from the top to the bottom soil layer (0-20 cm and 20-40 cm) (Table 3). The decrease of soil moisture from the top to subsoil layers in an orchard field after the conversion of a crop field into an orchard can be attributed to; water infiltration from the top soil layer into the deep soil layers bottom, soil moisture evaporation at the top soil layers and transpiration of the orchard fruit tree (Su and Shangguan, 2019; Tang et al., 2019). In addition, the reduction in soil moisture can be attributed to land use treatments, including the use of heavy mechanical tillage in orchards and the application of agrochemicals in orchards (Bogunovic et al., 2020; Fig. 3). Some studies have shown that as land use changes from cropland to orchard, soil moisture at a depth of 40-100 cm increases (Table 3). According to Zhang, Z. et al. (2018) most orchard fruit trees have long root systems (e.g., Pyrus communis orchard, Malus domestica orchard). Due to root pressure, this helps to absorb water from deeper soil layers and accumulate them in this underground soil layer (Zhang, Y. et al., 2018a; Zhang, J. et al., 2018b; Li et al., 2019b). In addition, the precipitation water infiltrated from the top soil layer may accumulate in this layer instead of infiltrating into deeper soil layers, which will subsequently increase the soil moisture in this layer. (Zhang et al., 2018c; Fig. 3). Soil moisture decreased in the deeper soil layers (e.g., 100-240 cm) (Liu et al., 2019b; Fig. 3). The decrease in soil moisture in the deeper soil layer of the orchard field can be attributed to upward migration of soil moisture especially from the water table in this layer to subsoil layers above due to the root pressure of the orchard trees (Song et al., 2017; Li et al., 2019b).

Table 3. 
Impact of land use change from a cropland into orchard on soil moisture at variable soil depths.



Fig. 3. 
The shift in land cover from cropland into an orchard resulted in the decrease of soil moisture from the 0-40 cm soil layer, an increase in soil moisture from the 40-100 cm, and a decrease in soil moisture from 100 cm to 240 cm soil layer.

5. Conclusion and future studies

With the recent human urge to growth, and food deficiencies needed to feed its expanding populations, the earth’s surface has experienced an array of land use changes. These land use changes degrade the soil especially soil moisture, which is an important ingredient for ecosystem plant development and proper functioning. The pattern of land use change at first consisted primarily of cutting down natural forest trees to set up mono croplands. As time went by the productivity of these mono croplands dropped. So, in some areas, these mono croplands are converted into mixed croplands, while in other areas for economic reasons, mono croplands are transformed into orchards. Fast forward to the 19th century, with the emergence of climate change and soil degradation mitigation concepts and agenda, most croplands are converted into mono forest plantations and/or mixed forest plantations. Although a lot of studies are done to report how these variable land use changes impact soil moisture, very few of these studies focus on how these variable land use changes affect soil moisture with depth. So, available studies on land use changes from a natural forest into mono and mixed croplands and then mono- and mixed-forest plantation type on soil moisture with depth in the last decade were selected, inferred and its findings were reported on tables. From the studies, it can be recognized that the transformation of natural forests into mono croplands subjected the soil moisture reduction in the topsoil layers of the mono cropland. However, this was not the case in scenarios whereby natural forests were converted into mixed croplands. The review paper is useful in showing that it is preferable to set up mixed cropland after clearing a natural forest than mono cropland. Establishing mixed cropland increases the soil moisture in the topsoil layer and with depth for a short period of time. Persistent crop cultivation reduces soil nutrients and soil moisture in both mono and mixed croplands. In situations where croplands were converted to orchards mainly for economic reasons, a decrease in soil moisture with depth was noticeable. Soil moisture reduction with depth can only be an issue if the plants in the area (e.g., semiarid regions) depend solely on soil moisture belowground for growth. The reduction of soil moisture with depth can be less severe if the cropland area is converted into a mixed forest plantation instead of mono forest plantations. Heavy rainfall can easily replenish the decrease in soil moisture with depth.

Through this review, it can be noticed that more studies are needed to fill the remaining gaps in this research area. The following studies are necessary to fill this gap;

  • 1) We need more studies on soil mechanisms responsible for soil moisture variability at the top, sub, and deeper soil layers;
  • 2) We need more studies on soil moisture content at deeper soil layers below 100 cm. Most studies of soil moisture variability on croplands only sample the soil layers less than 100 cm. If more studies are done which study soil moisture in soil layers beyond the 100 cm deeper in croplands. This helps to accurately compare the soil moisture variability with depth between the croplands and other land use types which sample beyond the 100 cm soil layer;
  • 3) It will be interesting to compare the changes in soil moisture between a single orchard and a mixed orchard establishment. Most research on measuring soil moisture only focuses on the establishment of a single orchard. Perhaps mixed orchard fruits can increase soil moisture and soil productivity
  • 4) We need more studies to crosscheck how soil moisture in different soil layers could be replenished through light to heavy precipitations being lost. especially in areas where soil moisture decreases with soil depth; and
  • 5) We need more studies with a more elaborate pattern on how soil moisture varies following the establishment of a mono cropping or mono plantation. These studies show soil moisture is either high or low at the surface and either increase or decreases with depth.
Acknowledgments

This work was funded by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2019R1A6A1A03033167 and NRF-2021R1F1A1064027). It was also supported by Korea Environment Industry and Technology Institute (KEITI) through Measurement and Risk assessment Program for Management of Microplastics Program (or Project), funded by Korea Ministry of Environment (MOE) (2020003110010).

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