Effects of land-use changes on carbon stocks in soil

The biggest reason for the global climate change, the effects of which we have felt much more clearly in recent years, is the increase in the rates of greenhouse gases in the atmosphere. Carbon dioxide is the gas with the highest percentage of these greenhouse gases. The rise of atmospheric carbon (CO₂) concentrations at increasing rates after the industrial revolution is due to anthropogenic (human activities).

After the oceans, terrestrial ecosystems are the largest carbon storage areas. Soil is the largest carbon reserve in the terrestrial biosphere and stores three times as much carbon as atmospheric carbon. Even the slightest change in soil carbon affects atmospheric carbon because there is a constant flow between atmosphere and soil carbon. Depending on the differences in land use, the carbon in the soil stocks also changes. When the direction of this change is from the soil to the atmosphere, the amount of atmospheric greenhouse gas increases.

Changes in land use and land surface cover can contribute to climate change by affecting the world's energy balance, as well as being affected by climate change. While more than half of the land changes occur directly with human activities, the remaining part is associated with indirect factors such as climate change [1]. Human-induced changes in land surface cover are of great importance for the structure and function of ecosystems, but equally important for providing food and shelter for humans. In this module, the interaction between land use changes and climate change is examined.

Effects of Different Uses of Terrestrial Lands on Carbon Capture

There is uncertainty about exactly how the land on earth is used. On the other hand, it is a known fact that these lands are constantly being transformed. There are 13 billion hectares of non-glacial land worldwide, of which one-third (4.3 billion hectares) consists of permanent grassland and agricultural land [2]. In general, the proportional distribution of non-glacial terrestrial areas in the world has been tried to be summarized in Figure 1.

Figure 1. Estimation of global uses of non-glacial land (adapted from Breewood and Garnett 2020 [2])

It has long been known that changes in land surface cover can change the amount of carbon stored in the soil, and as a result, atmospheric carbon may be affected. However, a large number of studies examining changes in land use change and soil C stocks have shown that there can be significant disagreements about the direction and magnitude of the change that occurs as a result of some use differences.

The transition from one ecosystem to another can occur spontaneously in nature, but mostly as a result of human activities. Each soil has a carbon carrying capacity and an equilibrium carbon content depending on the type of plants on it and the climatic characteristics of the region [3]. In land use changes, there are differences in carbon input and output to the soil and this balance is disturbed. At the end of the process, soil C stocks will have changed either as a carbon source or as a carbon sink. There are many studies on this subject.

In a literature reviewing study done by Deng et al., [4], based on the synthesis of 103 new publications from 29 countries, they tried to reveal the effects of land use changes on soil carbon stocks. In their results they stated that as a general average, land use transformations significantly reduced soil carbon stocks (0.39 Mg.ha^-1.year^-1). Other results found by the researchers are summarized in Table 1. While the soil carbon stocks increased in the transition from agricultural land and forest areas to meadow pasture in soil carbon stocks, the change in carbon stocks appeared as a decrease in areas converted from meadows and forests to agricultural land. It is also stated that carbon stocks decrease in places where there is a transformation from forest to forest. However, researchers reported that there was no change in soil carbon stocks in areas converted from agricultural land and grasslands to forest. In addition, it was also stated in the results of the same study that the changes in carbon stocks fluctuated over time due to changes in land uses.

Table 1. Effects of land use changes on soil C uptake

In a similar study, Dawson and Smith [5] made a comparison of the results of many researches and emphasized in their findings that the carbon content of the soil decreased with the conversion of forest and pasture lands into agricultural land, whereas carbon increases were observed in the opposite case. The results compiled by the researchers from the data they obtained are given in Table 2.

Table 2. Potential changes in soil carbon stocks due to land use change (adapted from Dawson and Smith (2007) [5]. Positive values show gains negative values show losses.)

As can be seen from the charts, forests and meadows have a vital importance in the storage of carbon. Considering the area they cover on the world, they play an important role in reducing emissions by ensuring carbon sequestration in the global carbon cycle.

Effects of Land Use Changes in Meadow Pasture Areas

Pasture are of vital importance for the world in terms of their biodiversity and ability to prevent erosion. In addition, they are among the most important terrestrial carbon sinks. Grassland-pasture areas are one of the main elements of the global carbon cycle and it can be said that they hold approximately 0.5 Pg CO₂ annually, although there is uncertainty in their efficiency and cycle times [6]. It is reported that the carbon stock in the soil is 634 Pg, together with the vegetation on the meadows and grasslands located in the tropical and temperate climate regions globally [7]. Grasslands, which are an important carbon sink, have a vital role in reducing the effects of climate change.

Conversion of grassland pastureland to farmland is one of the biggest changes in land use, accelerating both soil erosion and the loss of soil organic carbon. Since soil carbon can be released more easily in pasture areas that have been transformed into arable areas, it causes an increase in atmospheric carbon. However, there are too many variables that affect organic carbon loss in such transformations. In a study that tried to reveal the effects of land use changes on soil carbon stocks with a meta-analysis, Soil carbon stocks decreased by 10% when converted from pasture to plantation, while this loss was 59% in conversion from pasture to field. In addition, soil organic carbon increased by 8% in areas converted from natural forest to pasture, and by 19% in areas converted from field to pasture [8].

In another meta-analysis study conducted by examining 81 samples, it was stated that soil organic carbon decreased after the transformation of meadow pasture areas and this decrease occurred at a soil depth of 0-60 cm and remained the same at deeper depths [9]. The same researchers emphasized that the decrease in organic carbon in the soil continues over time and reaches equilibrium at the end of 20 years. As a result of the study, it was stated that the organic carbon loss mainly occurs as a result of precipitation, physical and chemical properties of the soil and changes in soil microorganisms.

Apart from the direct loss of pasture lands, it is a very common situation in the world that they lose their productivity, especially due to wrong practices. Soil carbon stocks naturally decrease in areas damaged in this way. The most damaging practice in this regard is improper grazing. Satellite observations made in Mongolia, one of the countries with the largest grasslands in the world, have shown that significant biomass losses have occurred over time. The decrease in biomass of approximately 60% as a result of observations made over a period of 21 years in almost the entire steppe region of the country is associated with decreased precipitation and increased temperature, as well as increased goat production due to the demand for cashmere products in the country and the resulting overgrazing [10].

It will be possible to increase productivity in these areas with the right rangeland management to protect and improve the rangelands. This will cause an increase in the carbon stocks of meadow pastures, which are important carbon sinks. As a result of an examination made on the data obtained as a result of many researches, it has been stated that an increase in C levels ranging from 0.105 to 1 Mg per hectare is achieved annually with improved grazing management, fertilization in meadow pasture areas, enrichment of grazing lands with leguminous species, and irrigation [11].

Intensive grazing increases soil organic carbon content by 6-7% in C₄ - C₃ mixed pastures dominated by C₄ plants, while soil organic carbon content in grasslands dominated by C₃-plants showed an average decrease of 18% [12]. Researchers emphasized that the C₃ and C₄ dominant grasslands show such strong differences in soil organic carbon content related to grazing that the focus should be placed on this.

To assess the carbon storage potential of soils, the global effects of improved grassland management practices and conversion to pastures were evaluated by analyzing the results of 115 studies [13]. According to the results, methods such as fertilization, improved grazing management, conversion of arable land to pasture, use of natural vegetation, sowing of legumes, irrigation and earthworm incorporation were evaluated among the improved rangeland management practices, and it was revealed that soil C content increased with improved management in 74% of the studies. While the increase in carbon sequestration rates continued for 40 years, it was generally highest at 10 cm soil depth. C retention rates by type of remediation range from 0.11 to 3.04 Mg C.ha⁻¹.year^-1 with an average of 0.54 Mg C.ha⁻¹.year^-1.

Effect of Land Use Differences on Carbon Stocks in Forested Areas

Forests are one of the most important carbon sinks in terrestrial ecosystems in terms of both soil and biomass. Globally, about half of the carbon held on land is located in forests (1146 x 10¹⁵ g), while 2/3 of this amount is retained in soil pools [14]. Soils contribute to net carbon sequestration on a global scale, as the amount of carbon entering the soil by decomposition (61.4 x 10¹⁵g) on an annual basis slightly exceeds the amount released by respiration (60 x 10¹⁵ g) [15].

Although soil carbon sequestration is an important phenomenon in terms of the global carbon cycle, the use of some products obtained from forests is also needed. Forestry is an important industry sector today and it is expected that changes will occur in the carbon pools of the forests depending on the different forestry practices. Cutting and spacing of trees are among them.

In a meta-analysis in which 432 data were analyzed and evaluated on a global scale, it was stated that there may be an 8±3% decrease in soil carbon after cutting, and this decrease is expected to be 30±6% in litter [16]. In the study, it was stated that the decrease in the amount of carbon could be different according to tree species, and the decrease in coniferous and coniferous species was around 20%, while it was around 36% in leafy species. Researchers also stated that the reduction in soil carbon may vary according to soil class. While no significant change was observed in soils with low organic matter content, it was stated that the decrease was more significant in soils with higher organic matter content. For example, while the change in alfisols and spodosols was not significant, the decrease in soil carbon content in inseptisols and ultisols was 13% and 7%, respectively.

The disappearance of forest areas causes these areas, which act as a carbon sink, to turn into carbon sources. It is claimed that 2.1 billion tons of CO₂ emissions occur annually due to the degradation of tropical forests, of which 53% is due to timber production, 30% to firewood and 17% to forest fire [17].

In a study conducted to measure the net annual changes in the aboveground carbon density of living vegetation on the tropical forests of the world, 12-year data between 2003 and 2014 were evaluated using MODIS satellite data, and it was stated that the tropical forests of the world were carbon sources with annual net carbon emissions of 425.2 ± 92.0 Tg. [18]. In the study, the net carbon emission was found to be 861.7 ± 80.2 Tg C year^-1, while the gain was calculated as 436.5 ± 31.0 Tg C year^{-1 .} It was stated that the carbon gains obtained were due to forest growth, while the losses were due to deforestation and the subsequent decrease in carbon density in the remaining forests. The data obtained in the study are given in Table 3 on the basis of continents.

Table 3. Changes in aboveground carbon storage (Tg C year -1) in tropical forests of the world.

In forest areas, the change in soil organic carbon depending on the changes in land use is an inevitable result. However, there are many factors that affect the change in soil organic carbon stocks. For example, in a study examining the effects of conversion from forestry to plantation, it was reported that there was a very small change in soil carbon stocks in a forest with broad-leaved species, while the reduction in carbon stocks reached 15% in an area transformed from conifers [8]. The change in soil carbon has been affected by the precipitation intensity, and losses reaching 56% have been experienced in areas with precipitation over 1500 mm. Soil carbon loss in areas transformed from forest to plantation continued throughout the process and was found to be 20% and stabilized after the 40th year. In the results of the same study , it was stated that the loss of soil organic carbon in the lands converted from forest to agricultural land was 42% and this occurred in the first 60 cm of the soil.

In another study examining the effect of the conversion of agricultural areas to forest areas, Pinus on the formerly grain and vine cultivated areas. The results in which soil carbon storage was determined by modeling were shared by establishing radiate plantations [19]. While it was reported that organic matter accumulation occurred, mostly in the topsoil layers, during afforestation, annual carbon accumulation per square meter was given as 19 g in vine-grown areas and 41 g in grain-grown areas.

Mangrove areas , which consist of trees and shrubs in coastal areas and are important carbon sinks for the world with their carbon storage potential of up to 25.5 million tons per year [20], are also constantly decreasing. It is estimated that between 0.15 and 1.02 billion tons of carbon is emitted annually due to degradation of mangroves, swamps and seagrass meadows [21].

Changes in Carbon Stocks Due to Urbanization

Due to increasing urbanization, changes are seen in the land cover and as a result, there are significant losses in terrestrial carbon stores. Especially in areas less affected by urban expansion, habitat losses reach higher levels. It is clear that there has been rapid urbanization on a global scale in recent years.

A study evaluating the effects of urbanized land on carbon dynamics in South America over a 63-year period between 1945 and 2007, using a process-based dynamic model was done. In this study it was calculated that the carbon emission to the atmosphere was 0.21 Pg due to urbanization during this period [22]. According to the researchers, the type of land vegetation that existed before urbanization and the time elapsed over land transformation determine the extent of the effects of urbanization on carbon dynamics. After the rapid decline in carbon storage during land conversion, an urban ecosystem gradually accumulates carbon and can compensate for the initial carbon loss in 70-100 years. According to the researchers, the carbon sequestration rate of the urban ecosystem decreases over time and almost disappears after two centuries.

In another study that tried to determine the effect of rapid urbanization on terrestrial carbon resources, the effect of land use changes on carbon sinks and emissions from 2000 to 2012 in Guangzhou, China was analyzed [23]. Urbanization in the city of Guangzhou increased by 119% during the study period, and carbon emissions significantly exceeded their carbon sink capacity. While the decrease in the total carbon sink was 30%, the total carbon emissions increased from 13.73 × 10⁶ tons to 39.67 × 10⁶ tons. Researchers warned that "urban growth limit" and " ecological red lines" should be determined for land use in state policies in order to ensure the economic use of land .

The emission of carbon to the atmosphere by keeping it in the oceans and terrestrial ecosystems has become very important today. Land use is of great importance in terms of carbon sequestration in soils. Grasslands and forests generally have higher carbon sequestration potential than arable land. Avoiding land use changes that will increase net carbon emissions in terrestrial areas will become increasingly vital for the control of global warming.

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