Effects of Agricultural Potassium Fertilizer Application on Soil Carbon Cycle

: This review examines the impact and regulatory mechanisms of potassium fertilizer on the soil carbon cycle, discussing how potassium fertilizer affects soil carbon storage and flow through various pathways. As a vital agricultural nutrient, potassium not only plays a crucial role in crop growth but also influences several aspects of the soil carbon cycle, including carbon sequestration, microbial activity, soil respiration, and the structure and enzymatic activities of the soil. The article begins by introducing the importance of the soil carbon cycle and the application of potassium in agriculture, followed by an analysis of how potassium fertilizer promotes soil organic carbon storage, including increasing the amount of plant residue returned to the soil. It then discusses the impact of potassium fertilizer on the structure and function of soil microbial communities, which play a key role in the soil carbon cycle. The article also explores the influence of potassium fertilizer on soil respiration, including its regulatory effects on microbial respiration and root respiration, and how potassium improves soil structure to affect soil respiration. Additionally, the impact of potassium fertilizer on the activity of enzymes related to carbon cycling is discussed in detail, as these enzymes play a central role in the decomposition of organic matter and nutrient cycling. Finally, the article summarizes the potential role of potassium fertilizer in improving soil health, enhancing carbon sequestration capacity, and addressing climate change, calling for more research to understand the mechanisms of potassium fertilizer and to provide a scientific basis for sustainable soil management strategies.


Introduction
The soil carbon cycle represents a critical component of the Earth's system, governing the balance and movement of carbon between the soil, vegetation, and atmosphere.It is a complex network of processes that include the decomposition of organic matter, respiration of soil organisms, and photosynthesis [1, 2].The significance of the soil carbon cycle cannot be overstated, as it plays a pivotal role in maintaining soil fertility, supporting plant growth, and regulating atmospheric carbon dioxide levels, thereby influencing global climate patterns (Figure 1).
In the realm of agriculture, potassium fertilizer is commonly applied to enhance crop yields and improve plant health.Potassium is an essential macronutrient that aids in various plant physiological processes, including water uptake, enzyme activation, and photosynthesis.While the benefits of potassium for plant growth are well-documented, its effects on the soil carbon cycle are multifaceted and merit comprehensive examination [3,4].
Figure 1.The Soil Microbial Carbon Network [5] This review paper aims to elucidate the effects and regulation mechanisms of potassium fertilizer on the soil carbon cycle.Through a synthesis of current scientific literature, how potassium application influences soil carbon sequestration, soil respiration, soil structure, and the activity of carbon cycling enzymes.By understanding these interactions, can begin to unravel the broader implications of potassium fertilizer use in sustainable agriculture practices and its potential impact on the global carbon budget.

Potassium Fertilizer and Soil
Carbon Sequestration

Influence on Plant Biomass and Residue Return
Potassium is an essential nutrient that affects plant growth and development.Adequate K nutrition can increase plant biomass production, which upon decomposition, contributes to the soil organic carbon pool.The return of plant residues to the soil is a primary pathway through which potassium influences soil carbon sequestration.Studies have indicated that K fertilization can enhance the quantity and quality of plant residues returned to the soil, thereby increasing the potential for carbon storage.

Interaction with Soil Microorganisms
The decomposition of organic matter and subsequent carbon sequestration is largely mediated by soil microorganisms [6].Potassium can affect the activity and composition of soil microbial communities, which in turn influences the rate of organic matter decomposition and stabilization [7].For instance, K availability has been linked to the activity of specific microbial groups that are involved in the decomposition of complex organic compounds.

Effect on Soil Aggregation
Soil structure, particularly soil aggregation, is closely related to the stabilization of organic matter and hence, carbon sequestration [8, 9].Potassium can influence soil physical properties, including aggregate stability.Aggregates provide a physical barrier that protects organic matter from rapid decomposition, and K fertilization has been associated with improved aggregate formation and stability.

Modulation of Root Exudation and Rhizosphere Interactions
The rhizosphere, the zone of soil influenced by root activity, is a hot spot for carbon sequestration.Potassium affects root growth and exudation patterns, which can alter rhizosphere processes and microbial interactions.Root exudates are a significant source of organic carbon in the soil, and changes in their quantity and composition due to K fertilization can have implications for carbon sequestration.

Implications for Soil Carbon Models
Understanding the role of potassium in soil carbon sequestration is crucial for developing accurate soil carbon models and predicting the long-term effects of fertilization on carbon dynamics.Current models often overlook the nuanced effects of K on the soil carbon cycle, and incorporating these effects can improve model predictions and inform sustainable fertilization practices.

Potassium Fertilizer and Soil Respiration
Soil respiration is a key process in the soil carbon cycle, representing the flux of carbon dioxide (CO 2 ) from the soil to the atmosphere.It is primarily driven by the metabolic activities of roots and soil organisms as they decompose organic matter and respire.The rate of soil respiration is a crucial indicator of soil health and fertility, and it is influenced by various factors, including temperature, moisture, and nutrient availability.

Impact of Potassium on Microbial Respiration
Microbial respiration is a significant component of total soil respiration.Potassium fertilizer can influence microbial activity by altering the soil's chemical environment [10].Adequate potassium levels can enhance microbial efficiency in breaking down organic compounds, leading to increased CO 2 emissions.Conversely, potassium can also stimulate the growth of microbial populations that utilize CO 2 for biomass production, potentially reducing net CO 2 flux.

Potassium's Role in Root Respiration
Root respiration accounts for a substantial portion of soil respiration, and it is directly affected by potassium availability.Potassium is vital for plant physiological processes, including respiration.It can affect root growth patterns, root density, and the rate of root respiration.Studies have shown that potassium-deficient plants may have reduced root respiration rates, which could lead to lower overall soil respiration.

Influence on Soil Physical Properties
The physical properties of soil, such as texture and structure, can impact soil respiration rates.Potassium fertilizer can influence these properties by affecting soil aggregation and porosity.Improved soil structure can enhance gas exchange between the soil and atmosphere, potentially increasing soil respiration rates.

Interaction with Other Nutrients
Potassium does not act in isolation; it interacts with other soil nutrients, such as nitrogen (N) and phosphorus (P), which also play roles in the soil carbon cycle.The balance of N, P, and K is critical for optimizing plant growth and microbial activity.The synergistic effects of these nutrients can influence soil respiration patterns and should be considered when evaluating the impact of potassium fertilizer.

Implications for Carbon Budgeting and Climate Change
Understanding the effects of potassium fertilizer on soil respiration is essential for accurate carbon budgeting and predicting the potential impacts of agricultural practices on climate change.As soil respiration is a major source of atmospheric CO 2 , managing potassium levels in agricultural soils could be a strategy for controlling CO 2 emissions and mitigating climate change [11].

Potassium Fertilizer and Soil Structure
Soil structure is a key factor in the functioning of the soil carbon cycle, influencing water and air movement, root growth, and microbial habitats.The arrangement of soil particles into aggregates determines the soil's ability to support plant life and store carbon.Potassium (K) fertilizer has a significant impact on soil structure, which in turn affects the soil carbon cycle.

Role of Potassium in Aggregate Formation
Potassium plays a crucial role in the formation and stabilization of soil aggregates.Aggregates are groups of soil particles that bind together, and their stability is essential for good soil structure.Potassium helps to flocculate clay particles, leading to the formation of stable aggregates.This process is important for creating a favorable environment for organic matter incorporation and carbon sequestration.

Influence on Soil Porosity and Water Retention
The presence of potassium affects soil porosity and water retention capacity.Well-structured soils with adequate potassium levels tend to have improved porosity, which facilitates the infiltration and storage of water.This is beneficial for plant growth and can lead to increased biomass production and, consequently, greater organic carbon input into the soil.

Interaction with Soil Biota
Soil structure is not only a physical attribute but also a biological one.The activities of soil fauna, such as earthworms, and microorganisms are influenced by the presence of potassium [12].These organisms contribute to the breakdown of organic matter and the formation of stable soil aggregates.Potassium fertilizer can enhance the biological activity in the soil, promoting the formation of biogenic aggregates and thus impacting the soil carbon cycle.

Effect on Root Architecture and Function
Root architecture and function are closely linked to soil structure.Potassium availability can influence root growth patterns, root hair development, and the overall root system architecture.A well-developed root system can improve soil structure through the formation of root channels and the exudation of organic compounds that help stabilize soil aggregates.

Implications for Soil Carbon Dynamics
The impact of potassium on soil structure has significant implications for soil carbon dynamics.Improved soil structure can lead to increased carbon storage in the form of stable organic matter within aggregates.Additionally, the enhanced water retention and biological activity in wellstructured soils can further promote carbon sequestration.

Potassium Fertilizer and Carbon Cycling Enzymes
Enzymatic processes are at the heart of the soil carbon cycle, catalyzing the transformation of organic matter into forms that can be utilized by plants and microorganisms.The activity of carbon cycling enzymes is a critical factor in the decomposition of organic matter, nutrient cycling, and overall soil health.Potassium (K) fertilizer can significantly influence these enzymatic activities, thereby affecting the soil carbon cycle.

Influence on Enzyme Activity
Potassium is known to act as a cofactor for many enzymes involved in plant and microbial metabolism.Adequate levels of potassium can enhance the activity of enzymes that decompose organic matter, such as cellulases and ligninases.This can lead to increased turnover of organic matter and potentially affect the balance of carbon storage and release in the soil.

Interaction with Other Soil Nutrients
The activity of carbon cycling enzymes is not only influenced by potassium but also by the availability of other nutrients such as nitrogen (N) and phosphorus (P).The interplay between these nutrients can modulate enzyme production and activity.For example, the presence of potassium may enhance the efficiency of N and P in promoting enzyme activity, leading to a more dynamic soil carbon cycle.

Effect on Microbial Carbon Use Efficiency
Microbial carbon use efficiency (CUE) is a measure of how effectively soil microorganisms convert organic carbon into microbial biomass.Potassium can affect CUE by influencing the microbial uptake of carbon and the allocation of resources towards growth or respiration.Higher CUE values indicate more carbon being stored in microbial biomass rather than being respired as CO 2 , which can have implications for soil carbon sequestration.

Role in Abiotic Stress Tolerance
Potassium also plays a role in abiotic stress tolerance in plants.By enhancing the plant's ability to withstand stress, potassium can indirectly affect the quantity and quality of organic matter inputs into the soil.This, in turn, can influence the activity of carbon cycling enzymes and the overall soil carbon cycle.

Implications for Soil Fertility and Climate Change Mitigation
Understanding the effects of potassium fertilizer on carbon cycling enzymes is essential for managing soil fertility and developing strategies for climate change mitigation.By optimizing potassium levels in agricultural soils, it is possible to influence enzymatic activities that promote carbon sequestration and reduce greenhouse gas emissions.

Conclusion
Throughout this review, we have explored the multifaceted role of potassium fertilizer in the soil carbon cycle.From enhancing soil carbon sequestration to influencing soil respiration, and from impacting soil structure to modulating carbon cycling enzymes, potassium emerges as a key player in soil carbon dynamics.The evidence presented underscores the importance of potassium not only for plant nutrition and yield but also for its broader environmental implications.
The integration of potassium within sustainable agricultural practices offers a promising avenue for enhancing soil health and mitigating climate change.By optimizing potassium use, we can improve soil structure, increase microbial efficiency, and promote carbon sequestration.This, in turn, contributes to the resilience of agricultural systems against climate variability and supports the global effort to reduce atmospheric CO2 levels.
Future research should aim to unravel the complex interactions between potassium and other soil nutrients, as well as their collective impact on the soil carbon cycle.Longterm field studies and advanced modeling approaches are needed to quantify the effects of potassium fertilizer on carbon dynamics and to refine our strategies for sustainable soil management.
In conclusion, potassium fertilizer stands out not only as an essential nutrient for crop production but also as a significant regulator of the soil carbon cycle.Its judicious use can lead to healthier soils, higher crop productivity, and a better environment for future generations.