Effects of soil properties on greenhouse gas emissions

The increasing population and the related increase in people's demand for food and energy have brought along important environmental problems. Global warming, which occurs with the increase of greenhouse gases in the atmosphere, is one of these problems. Although the increase in greenhouse gas rates in the world's atmosphere is largely due to activities in the energy sector, there are also significant emissions into the atmosphere related to agriculture, forestry and land use. Emissions as a result of agricultural activities CO₂, CH_4- and in the form of N₂O gases.

After the settled life and agriculture started, soils became one of the main sources of atmospheric CO₂ and other greenhouse gases. It is estimated that the greenhouse gas emission from the soil to the atmosphere since the industrial revolution is 78 ± 12 Pg CO₂-C [1].

However, terrestrial ecosystems are in the position of a pool in terms of carbon, since carbon can be held both in biomass and directly in the soil. Emissions from soils to the atmosphere and retention of carbon in soil and biomass can be affected by some soil properties.

Greenhouse Gas Emissions Due to Soil Organic Carbon Content

One of the most important problems encountered in many areas of agriculture is the low levels of soil organic matter, which is one of the main components of the sustainability of agriculture. In areas with intensive agriculture, there is a decrease in soil organic matter content over time due to reasons such as monoculture cultivation, excessive tillage, burning of remaining plant residues. Increasing carbon content in agricultural areas can increase soil fertility and biodiversity and thus contribute to climate change mitigation by sequestering carbon from the atmosphere [2].

Adding various organic materials to the soil and regaining the lost carbon is one of the recommended practices to improve soil properties and even combat climate change. However, adding these materials to the soil can facilitate the release of CO₂. For this reason, it is important to know what the expected effect is from the organic material added to the soil, to know the carbon holding capacity of the soil, and to know the soil carbon balance status for the short and long term after the application. Therefore, the goal should be to increase the biologically stabilized soil organic matter in the long term in accordance with environmental conditions [3].

Soil organic carbon is an important component of the global carbon cycle (Figure 1), and its presence in soils will have a significant impact on the global climate with which it interacts in the future [4]. This effect is complex and may differ according to soil properties. For example, since the microbial activity will be higher in soils with high temperatures, the decomposition of organic matter accelerates and this causes an increase in the release of CO₂ and CH₄ into the atmosphere. On the other hand, increased carbon sequestration can encourage more plant vegetation.

Figure 1. Carbon content of soil, atmosphere and biota (adapted from Lal, 2004 [34]).

There are two basic approaches that are commonly used to reduce the amount of CO₂ in the atmosphere. The first of these is the reduction of emissions and the other is carbon sequestration. Soil plays an important role in both cases. Carbon sequestration means that CO₂ in the atmosphere is taken by plants through photosynthesis and transferred to the soil [5]. Soil organic matter content can be increased by mixing plant residues into the soil. Increasing carbon stocks in the soil means supporting the processes that form stabilized organic compounds. If the impact of global warming is to be reduced, supporting carbon sequestration by soils, which is the largest terrestrial carbon pool, will undoubtedly be an important strategy [3].

Carbon Storage and Soil Conditions

Increasing the organic matter content by adding organic material to the soil may vary depending on the soil properties and the type of material, apart from the amount of added material. Organic matter added to soils does not always mean that carbon is stored in the soil. For example, grassland and forest soils with good drainage conditions can store significant amounts of carbon, while arable land can be a source of carbon [32, 33].

The carbon storage capacities of soils are not unlimited and may differ depending on the soil type. In mineral soils, the presence of clay and silt particles increases the stability of soil organic carbon through adsorption, but its amount is limited [6].

Soil Aggregate Size on Greenhouse Gas Emissions

Depending on the aggregate size in soils, there may be differences in gas transport and emissions. In a study conducted for this purpose, the effect of soil aggregate size on greenhouse gas emissions was compared in two soils with sandy loam and clayey loam texture [7]. Soils with aggregate sizes of less than 0.5 mm, 0.5-1 mm, 1-2 mm and 2-4 mm were used in the study. According to the results obtained, pore sizes significantly affected the release of CO₂ and CH₄ from the soil, but did not show any effect on the release of N₂O. The highest CO₂ emission occurred in clayey loam textured soil with the smallest aggregate size. On the other hand, in soils with sandy loam texture, the highest CO₂ release occurred in soils with the largest aggregate size. In the study, it has been reported that there are increased CH₄ flows in soils with small aggregates. As a result of their study, the researchers stated that tillage practices, which greatly affect the pore properties of soils, may have a direct effect on greenhouse gas emissions. It has been reported that CO₂ emission decreases with increasing aggregate size in a clay loam soil [8], whereas CH₄ release is higher in soils with small aggregate size [9].

Soil pH on Greenhouse Gas Emissions

Soil pH is an expression that represents the amount of H⁺ ions in the soil solution that is in dynamic equilibrium with a negatively charged solid phase. H⁺ ions, which are strongly attracted to negatively charge solid surfaces, can replace other cations present there. pH, which is closely related to soil and plant biochemical processes, is the pH of the soil solution [10].

There are different ideas related to the soil pH on greenhouse gas emissions. pH, which has an important effect on the physical, chemical and biological properties of the soil, inevitably also affects the carbon flow in the soil [11]. Alkaline soils have a significantly higher CO₂ content and the organic carbon in these soils is not very chemically stable for mineralization. Addition of calcium agents to the soil can alter soluble organic carbon concentrations and stimulate a limited soil organic carbon mineralization [12].

In a study with acidic tropical forest soils, the pH of the soil was increased from 3.9 to 5.7 by adding Ca(OH)₂ and CaSO₄, and as a result, it was understood that the mineralization of carbon in organic fertilizer was supported [13]. Contrary to what has been reported, in a different study, it was found that soil respiration decreased and organic carbon accumulation in the soil was promoted due to the increase in pH as a result of calcium addition in organic tundra soils [14]. Researchers stated that more studies should be done in natural ecosystems to elucidate the effect of calcium-induced pH changes on soil organic carbon.

In a study examining the effects of soil pH on long-term CO₂ emissions, emission measurements were made in soils with pH values of 3.65, 5.00, 6.90 and 8.55 [15]. In the study, the subjects in which nitrogen fertilizer was not applied and the subjects that the fertilization was made in the form of nitrate and ammonium were combined with plots at different pH values. According to the results obtained, the CO₂ emissions were not affected by ammonium and nitrate fertilization at different soil pH values. With the nitrogen fertilizer applications, the amount of dissolved organic carbon decreased at all pH values. Researchers reported pH increase and nitrogen fertilization increases CO₂ emission and heterotrophic microbial respiration in soil plays a dominant role in CO₂ emissions.

Soil Moisture and Greenhouse Gas Release from Soil

While plant growth in non-irrigated areas is limited by the available soil moisture, an increase in above-ground parts and root development occurs with irrigation. This situation also increases the carbon entry into the soil. By fertilizing and irrigating arable lands and pasture lands, carbon entry into the soil is increased. With the increase in soil moisture, the microbial activity also increases. As a result of both the increase in biomass and the increase in carbon input, and the increase in microbial activity, the acceleration of the decomposition process of organic carbon also increases the CO₂ release from the soil [16].

In a study examining the effect of restricted irrigation on greenhouse gas emissions by years where a drip irrigation system is used, the highest CO₂ emission values were obtained in the area with the most irrigation and the emission values measured after irrigation were higher than the values measured before irrigation [31]. Similar results were obtained in terms of N₂O emission values measured in the study, and the highest emission values were achieved in the subject with the most irrigation. N₂O values after irrigation were higher than before irrigation, as were CO₂ values. As a result of the study, the researchers stated that irrigation practices significantly increased greenhouse gas emissions. However, in limited irrigation applications, lower CO₂ emission has been seen due to the slowdown of microorganism activities [17].

While gas emissions from the soil are affected with the amount of irrigation water applied, there may be changes in the amount of emissions related to the quality characteristics of the water used in irrigation too. Salinity value of irrigation water is another parameter that effects the emission rates. In a study conducted with irrigation waters with different salinity rates, the effect of salts on soil organic carbon mineralization was investigated by monitoring the amount of CO₂ emission from the soil [18]. According to the results obtained, it was observed that the rate of formation of CO₂-C decreased with the increase in the salinity of the irrigation water. In addition, according to the researchers, when the salt source is NaCl, the decrease in carbon mineralization is higher than when it is of CaCl₂ and MgCl₂.

Soil Temperature and Greenhouse Gas Emission from Soil

Soil temperature can be effective in the emission of greenhouse gases due to its effects on microbial activity along with moisture. Under favorable soil temperature conditions, CO₂ release increases due to the oxidation of organic matter [19]. An increase in soil temperature accelerates organic matter decomposition, increases microbial activity and root growth, and as a result accelerates the carbon mineralization process and CO₂ emissions increase [20].

In a study the effect of soil temperature and moisture on the concentration of N₂O and CO₂ in an organic soil from two different depths were investigated [21]. According to the results obtained in the study, N₂O concentration increased in parallel with the increase in soil moisture at a depth of 10-30 cm. CO₂ concentration in soil increased with the increase in soil temperature. This increase occurred both at a depth of 10-30 cm and at a depth of 50-100 cm.

Similarly, in another study, it was noted that CO₂ emission increased with the increase in soil temperature and the rate of increase reached 59% [20].

However, since the microbial activity in the soil can be active between certain temperature values, it has been reported that a linear relationship cannot always be obtained between CO₂ emissions and the soil temperature [22].

Changes in Greenhouse Gas Emissions Related to the Tillage

As a result of tillage practices, there can be significant changes in greenhouse gas transfers between the atmosphere and the soil. Greenhouse gas emissions arising from tillage can be caused by the use of fossil fuels consumed by the vehicles used directly, as well as the effects it has on soil properties.

Although the effects of no-till agriculture, especially on erosion control are now considered fundamental, scientists continues to discuss the effectiveness of no-till practices in reducing CO₂ emissions today. It is noteworthy that in many studies, different results have been obtained depending on soil, environmental characteristics and time.

Tillage can affect the distribution and conversion of soil carbon content by opening or closing new transfer channels between the soil surface and the atmosphere. In addition, changes in some soil properties such as aggregate stability, water holding capacity and temperature as a result of tillage can change the decomposition dynamics of organic matter [23].

Four different applications were compared in a study aiming to determine the possible effects of reduced tillage practices on CO₂ emissions from soils [24]. These applications are 12 cm deep tillage with chisel plow, 5 cm deep tillage with chisel plow, mulching of straw on the surface and fallow land. Tillage with chisel plow at a depth of 10-12 cm in the summer period has increased CO₂ emission about 2 times compared to other applications. This application was followed by shallow (5 cm deep) tillage, mulching and fallow land respectively.

In a study comparing the effects of direct sowing with traditional tillage on CO₂ emissions, soybeans were grown by direct sowing vs cultivating the soil with chisel, disc harrow and cultivator, and CO₂ emissions from the soil have been measured for 55 days. As a result of the study, it was determined that the lowest emission occurred in direct sowing application [25].

With a meta-analysis evaluating 56 studies targeting CO₂ emissions directly from the soil, it was aimed to reveal the status of emissions from no-till and conventional tillage practices [26]. The researchers summarized their findings as follows:

Soil CO₂ emissions were found to be almost twice as high in the first hours after tillage compared to uncultivated soils. The variability of the CO₂ emissions from soil was associated with soil texture, crop rotation, time and soil organic carbon content rather than tillage practices. Emission reduction in no-till applications has been realized especially in clay textured soils where crop rotation is performed, soil carbon content is low and in arid climate conditions. According to the results obtained by the researchers, the effect of no-till applications on CO₂ emissions is open to debate and that the reduction of emissions mostly occurs under certain soil and environmental conditions.

In order to measure and evaluate the effect on soil CO₂ emissions, the results of 46 studies conducted in different climate and soil conditions were evaluated with a meta-analysis and it was noted that the effect of tillage on soil CO₂ emissions was inconsistent [27]. According to the findings obtained, the researchers stated that, on average cultivated soils emit 21% more CO₂ than uncultivated soils. It has been stated that this difference increases to 29% in sandy soils where soil moisture and carbon content are low in arid climate conditions. However, it has also been reported that there is no difference in emission related to the tillage type in clay soils with high soil organic carbon content. According to the researchers, it has been noted that no-till farming can be an effective measure in reducing CO₂ emissions from soils in arid conditions.

Mulching is one of the effective applications on carbon sequestration, and this effect may vary depending on the soil properties and the application time of the mulch material. Mulching helps reduce the negative effects of climate change by offsetting greenhouse gas emissions and increasing organic carbon sequestration in soil [1]. However, continued residue addition can bring soils to carbon saturation level, resulting in a situation where greenhouse gas emissions from soils exceed the amount of carbon sequestration [28].

In a study in which mixing plant residues to the soil was compared with the mulch application, two different soil types were used and carbon in the upper soil layer was monitored [30]. According to the results of the study carried out for two years, the mixing of corn residues to the soil in clay soils caused CO₂ flow to increase 38% compared to mulching, this increase was 16% in sandy soils. Soil moisture content was maintained and the decomposition of organic carbon on the soil surface increased when mulching is used in sandy soils. Therefore, researchers pointed out that mulching may deplete soil carbon stocks in sandy areas in the long run.

In another study, wheat straw used as mulch material was applied as 0, 8 and 16 Mg per hectare every year [29]. In this study, 244 kg N application per hectare and no nitrogen application was combined with mulch applications. At the end of the 15-year trial, important findings were obtained, some of which are as follows:

With long-term mulch applications, there was an increase in the macro aggregates, especially those larger than 250 μm, in the soil. This increase was largely due to mulch applications. This caused the accumulation of high carbon and nitrogen concentrations in macro aggregates. In the study, it was stated that the carbon in the micro aggregates was relatively preserved, whereas the carbon in the macro aggregates and between the micro aggregates was not stabilized and was prone to microbial decomposition. Increasing carbon and nitrogen concentrations of macro aggregates significantly affected CO₂, CH₄ and N₂O flows and higher emission values were measured in mulch applications. Compared to the control, soil organic carbon concentration in the first 10 cm depth in soil increased by 32% and 90% with 8 and 16 Mg mulch applications, respectively and approached soil organic carbon capacity with 15 years of mulch applications. Finally, the global warming potential was calculated in the study and the results are given in Table 2 as CO₂ equivalent. Accordingly, with the annual 16 Mg mulch application, higher greenhouse gas flow was realized and the highest net global warming potential value was calculated.

Table 2. Global warming potential calculated on a yearly basis (adapted from Lenka & Lal, 2013 [29])

References

1. Lal. R. (2011). Soil Health and Climate Change: An Overview. In: Singh, B., Cowie, A., Chan, K. (eds) Soil Health and Climate Change. Soil Biology, vol 29. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-20256-8_1

2. Kavvadias. V. Papadopoulou. M. Vavoulidou. E. Theocharopoulos. S. Malliaraki. S. Agelaki. K., … Psarras. G. (2018). Effects of Carbon Inputs on Chemical and Microbial Properties of Soil in Irrigated and Rainfed Olive Groves. Soil Management and Climate Change, 137–150. doi:10.1016/b978-0-12-812128-3.00010-0

3. Navarro-Pedreño. J. Almendro-Candel. M.B. Zorpas. A.A. (2021). The Increase of Soil Organic Matter Reduces Global Warming, Myth or Reality? Sci 2021, 3, 18. https:// doi.org/10.3390/sci3010018

4. Govers. G. Merckx. R. van Oost. K. van Wesemael. B. (2012). Soil Organic Carbon Management for Global Benefits: A Discussion Paper. In Proceedings of the Workshop organised by the Scientific and Technical Advisory Panel of the Global Environmental Facility ‘Soil Organic Carbon Benefits: A Scoping Study’, Nairobi, Kenia, 10–12

5. Lal. R. Negassa. W. Lorenz. K. (2015). Carbon sequestration in soil. Curr. Opin. Environ. Sustain. 2015, 15, 79–86.

6. Ingram. J.S.I. Fernandes. E.C.M. (2001). Managing carbon sequestration in soils: Concepts and terminology. Agric. Ecosyst. Environ. 2001, 87, 111–117.

7. Mangalassery. S. Sjögersten. S. Sparkes. D. L. Sturrock. C. J. & Mooney. S. J. (2013). The effect of soil aggregate size on pore structure and its consequence on emission of greenhouse gases. Soil and Tillage Research, 132, 39–46. doi:10.1016/j.still.2013.05.003

8. Drury. C.F. Yang. X.M. Reynolds. W.D. Tan. C.S. (2004). Influence of crop rotation and aggregate size on carbon dioxide production and denitrification. Soil and Tillage Research 79, 87–100

9. Kimura. S.D. Melling. L. Goh. K.J. (2012). Influence of soil aggregate size on greenhouse gas emission and uptake rate from tropical peat soil in forest and different oil palm development years. Geoderma 185–186, 1–5.

10. Rengel. Z. (2011). Soil pH, soil health and climate change. Soil health and climate change, 69-85.

11. Brady. N.C. Weil. R.R. (2002). The Nature and properties of soil [M], 13th ed. Springer Nehterlands, 2002, P249

12. Nambu. K. van Hees. P. A. Jones. D. L. Vinogradoff. S. & Lundström. U. S. (2008). Composition of organic solutes and respiration in soils derived from alkaline and non-alkaline parent materials. Geoderma, 144(3-4), 468-477.

13. Motavalli. P. P. Palm. C. A. Parton W. J. Elliott. E. T. & Frey. S. D. (1995). Soil pH and organic C dynamics in tropical forest soils: Evidence from laboratory and simulation studies. Soil Biology and Biochemistry, 27(12), 1589–1599. doi:10.1016/0038-0717(95)00082-p

14. Hobbie. S.E. Gough. L. (2002). Foliar and soil nutrients in tundra on glacial landscapes of contrasting ages in northern Alaska. Oecologia 131, 453–462

15. Wang. L. Han. Z. & Zhang. X. (2010). Effects of soil pH on CO2 emission from long-term fertilized black soils in Northeastern China. Sci Res, 2010, 58-61.

16. Entry. J. A. Mills. D. Mathee. K. Jayachandran. K. Sojka. R. E. & Narasimhan. G. (2008). Influence of irrigated agriculture on soil microbial diversity. Applied Soil Ecology, 40(1), 146–154. doi:10.1016/j.apsoil.2008.03.012

17. Sinaie. S. Sadeghi-Namaghi. H. Fekrat. L. (2019). Effects of Elevated CO2 and Water Stress on Popu-lation Growth of The Two-spotted Spider Mite, Tetranychus urticae Koch (Acari: Tetranychi-dae), on Sweet Pepper Under Environmentally Controlled Conditions. Journal of Asia-PacificEntomology, 2019; 22 (1), 96-102

18. mancer . H. & Bouhoun . MD (2018). Effect of irrigation water salinity on the organic carbon mineralization in soil (laboratory incubation). In AIP Conference Proceedings (Vol. 1968, No. 1, p. 020007). AIP Publishing LLC.

19. Tüfenkçi . S. native . C. & Cakmakci . T (2021). From Soil to CO2 Release One view . biosystem Engineering II (pp.117-128), Ankara: Academician bookstore .

20. Jabro. J. D. Sainju. U. Stevens. W. B. & Evans R. G. (2008). Carbon dioxide flux as affected by tillage and irrigation in soil converted from perennial forages to annual crops. Journal of Environmental Management, 88(4), 1478–1484. doi:10.1016/j.jenvman.2007.07.012

21. Nosalewicz. M. Stępniewska. Z. Nosalewicz. A. (2013). Effect of soil moisture and temperature on N2O and CO2 concentrations in soil irrigated with purified wastewater. Int. Agrophys., 27(3), 299-304. https://doi.org/10.2478/v10247-012-0098-3

22. Gonzalez-Mendez. B. Webster. R. Fiedler. S. et al., (2015). Short-term Emissions of CO2 and N2O in Response to Periodic Flood Irrigation With Waste Water in The Mezquital Valley of Mexico. Atmospheric Environment, 2015; 101, 116-124

23. Lu. X. Lu. X. Tanveer. S. K. Wen. X. & Liao. Y. (2015). Effects of tillage management on soil CO2 emission and wheat yield under rain-fed conditions. Soil Research, 54(1), 38-48.

24. Mühlbachová. G, Kusá. H. Růžek. P. Vavera. R. (2022). CO2 emissions in a soil under different tillage practices. Plant Soil Environ.. 2022;68(6):253-261. doi: 10.17221/110/2022-PSE.

25. Akbolat. D. Barut. Z. Turgut. M. M. et al,. (2016). Soil CO2 Emissions Under Conventional and Con-servational Tillage Methods in Soybean Cultivation in Cukurova Plain of Turkey. AgronomySeries, 2016; 59, 15-20

26. Bregaglio. S. Mongiano. G. Ferrara. R. M. Ginaldi. F. Lagomarsino. A. & Rana. G. (2022). Which are the most favourable conditions for reducing soil CO2 emissions with no-tillage? Results from a meta-analysis. International soil and water conservation research, 10, 497-506. doi: 10.1016/j.iswcr.2022.05.003

27. Abdalla. K. Chivenge. P. Ciais. P. & Chaplot. V. (2016). No-tillage lessens soil CO₂ emissions the most under arid and sandy soil conditions: results from a meta-analysis. Biogeosciences, 13(12), 3619–3633. doi:10.5194/bg-13-3619-2016

28. Six. J. Conant R.T. Paul E.A. Paustian K. (2002). Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant and Soil 241, 155–176.

29. Lenka. N. K. & Lal. R. (2013). Soil aggregation and greenhouse gas flux after 15 years of wheat straw and fertilizer management in a no-till system. Soil and Tillage Research, 126, 78–89. doi:10.1016/j.still.2012.08.011

30. Nishigaki. T. Sugihara. S. Kilasara. M. & Funakawa S. (2020). Carbon dioxide flux and soil carbon stock as affected by crop residue management and soil texture in semi‐arid maize croplands in Tanzania. Soil Use and Management. doi:10.1111/sum.12680

31. Gültekin. R. Avağ. K. Görgişen. C. Öztürk. Ö. Yeter. T. & Alsan. P. B. (2023). Effect of deficit irrigation practices on greenhouse gas emissions in drip irrigation. Scientia Horticulturae, 310, 111757.

32. Ciais. P. Soussana. J.F. Vuichard. N. Luyssaert. S. Don. A. Janssens. I. Piao. S. Dechow. R. Lathière. J. Maignan. F. et al. (2010). The greenhouse gas balance of European grasslands. Biogeosci. Discuss. 2010, 7, 5997–6050.

33. Schils. R. Kuikman. P. Liski. J. van Oijen. M. Smith. P. Webb. J. Alm. J. Somogyi. Z. van der Akker. J. Billet. M. et al. (2008). Review of Existing Information on the Interrelations Between Soil and Climate Change. Final Report ClimSoil Prepared for DG Environment, Contract Number 070307/2007/486157/SER/B1, Technical Report-2008-048. Available online: https://ec.europa.eu/environment/archives/soil/pdf/climsoil_report_dec_2008.pdf (access date 14 February 2022)

34. Lal. R. (2004). Soil carbon sequestration impact on global climate change and food security. Science 2004, 304, 1623–1627.