Due to continuously increasing anthropogenic activities, the global average temperature increased by 0.9 °C since the 19th century and it is expected to be further increased to 1.5 °C by 2050
[1]. Manifold and continuous increases in GHG emissions are highly affecting terrestrial, freshwater and marine ecosystems by causing substantial and irreversible losses
[2]. These GHGs block the transmission of infrared radiation that tries to escape from the atmosphere and thus trap heat, as in a ‘greenhouse’
[3]. The major GHG sources include burning fossil fuels, use of nitrogen fertilizers, soil management, flooded rice fields, land conversions, burning biomass, livestock production and manure management
[4]. Climate change is projected to have significant impacts on agriculture through direct and indirect effects on crops, soils, livestock and pests
[5]. Though climate change is a slow process involving relatively small changes in temperature and precipitation over long periods of time, these slow changes in climate nevertheless influence various soil processes, particularly those related to soil fertility. The effects of climate change on soils are expected mainly through alterations in soil moisture conditions and increases in soil temperature and CO
2 levels as a consequence
[6]. Global climate change is projected to have variable effects on soil processes and properties important for restoring soil fertility and productivity
[7]. The major effect of climate change is expected through an elevation in CO
2 and increases in temperature and salinity
[8].
Crop production is vulnerable to climate variability, and climate change-associated increases in temperature, increases in CO
2 and changing patterns of rainfall may lead to a considerable decline in crop production
[1]. Changes in temperature, moisture, wet–drying and freeze–thawing cycles, etc., can lead to alterations in the growth and physiology of soil microorganisms
[9]. Climate-induced changes in environmental parameters can indeed influence both the structure and function of soil microbial communities and modify, for instance, the level of interaction among microorganisms required for the degradation of organic pollutants in soil, soil organic carbon stocks, soil properties such as pH, cation exchange capacity (CEC), water holding capacity (WEC) and nutrients stock
[10][11][12]. Also, extreme weather events such as droughts, extreme heat waves and heavy rainfall leading to floods have increased in past decades, increasing leaching, soil erosion and runoff at alarming rates. Enhancing crop production to meet rising demands owing to the increasing population, against the background of the threats of climate change, is a challenging task. Therefore, we require more attention towards adaptation and mitigation research. In the past few decades, agricultural technologies have been successful in eradicating hunger from many parts of the world, but by virtue of chemical means and usage, which has raised more concern for the environment, health and future agriculture
[13]. In recent, high-input farming systems and technologies, chemical fertilizers (consisting of N, P or K) are applied excessively to provide the plant nutrient requirement for increasing agricultural productivity worldwide
[14]. The use of chemical fertilizers has caused more harm than good in long-term perspectives. Therefore, the modern agricultural sector needs more clean and green strategies for simultaneously improving crop productivity and mitigating climate change impacts.
In order to emerge in modern agriculture, green strategies need to simultaneously deal with improved soil health, crop yields and climate change-associated environmental challenges. For improvements in soil fertility and plant growth, several options are available that range from traditional soil amendments to innovative solutions. Two such innovative solutions gaining immense attention are biochar and biostimulants. The unique potential of these strategies is that, in addition to enhancing soil fertility and crop yields, at the same time, these innovations have the ability to mitigate climate change impacts on agriculture and the environment.
2. Biochar
Biochar is a solid black stable carbon material mainly composed of carbon, minerals, volatile matter and moisture
[15]. For thousands of years, the pyrolysis of biomass into biofuels and biochar has presented a potential capability to sequester CO
2 from atmosphere, as well as to amend soils in earth layers
[16]. Biochar, a porous solid material resulting from biomass carbonization in no-oxygen and low-temperature (400 °C) conditions, is considered a significant tool in the mitigation of climate change because of its role in reducing GHG emissions from soil and sequestration of carbon in more stable form of carbon materials
[17]. Biochar’s properties highly depend upon the pyrolysis temperature; biochar produced at high pyrolytic temperatures, such as more than 500 °C, has been reported to improve porosity and bulk density to a significant extent. Biochar produced at pyrolytic temperatures lower than 500 °C has been reported to have a higher impact on the fungal and bacterial diversity of soil. Especially in coarse-textured soil, it is reported to affect bacterial diversity, whereas in fine-textured soils, it affects fungal diversity more
[18]. Along with several beneficial properties, the application of biochar has been reported to cause short-term negative impacts on earthworm populations. Future research efforts are required to mitigate this impact in favor of beneficial earthworm activity in soil systems
[15].
Several recent studies have reported on the role of biochar in improving the efficiency of fertilizer use and thus reducing the economic and environmental burden of manufacturing given requirements of fertilizers
[19][20][21]. The biochar-based efficient use of fertilizer can avoid the manufacturing fraction of fertilizers and associated GHG emissions
[22]. Biochar-based fertilizers can increase crop productivity significantly and further increase crop productivity in soils which are not responsive to common fertilizers (
Figure 1A)
[23].
Biochar has a gigantic ability to sequester CO
2 while preventing the release of carbon back into the atmosphere after its decomposition (
Figure 1B)
[24]. With this practice, about 2.5 gigatons of CO
2 can be sequestered annually
[25]. The slower decomposition of biochar in comparison to biomass stems from its potential to mitigate climate change impacts, as it lowers the rates of photo-synthetically fixed carbon returning back into the atmosphere
[26]. The difference in the rates of decomposition of biochar and raw biomass critically determines the net carbon stock available in soil that has evolved over time
[26]. Biochar presents larger soil carbon stocks with prolonged lifetimes in comparison to raw biomass
[27]. The embedded carbon of biochar in this case is considered as a redistribution of carbon from biomass sources, with the ability to persistently derive larger carbon sequestration and influence on net GHG balances
[28]. Despite several potential benefits of biochar, the applications of biochar still have some bottlenecks to be resolved. The health risks associated with the inhalation of black carbon particles released during biochar formation is one of the health and environmental concerns that requires more research attention and management.
Biochar is composed of a mixture of compounds with varying decaying kinetics in soil, and over time, the decay rates of biochar slow down
[29]. Also, microorganisms cannot digest biochar completely; therefore, biochar-based amendments of soil are considered a source of permanent agents for carbon sequestration in soil
[30]. Depending on the physicochemical properties of biochar, it can offer a sustainable way towards suitable feedstock for the circular economy paradigm. The application of biochar as an enrichment of soil can offset the CO
2 emission of land by 12% annually. Moreover, along with performing a critical role in improving soil health and crop productivity, it has the ability to minimize 1/8 of annual CO
2 emissions
[25]. This mitigation strategy could possibly reverse the net global warming and can significantly aid in carbon-negative technology development for the sustainable future of human civilization.
Biochar affects the rate of native soil organic matter by significantly varying the stocks of non-pyrogenic soil carbon
[31]. The ways by which biochar can impact the soil organic matter includes reducing in the amounts of detritus in the soil in comparison to adding biomass to soil directly
[32], increasing in the yields of plants’ biomass
[33] and altering the rates of stabilization, humification and soil organic matter
[34]. Biochar is also reported to have an impact on the improvement of crop yields via nutrient provision, alterations in the pH of soil, enhancing the cation exchange capacity (CEC) of soil, improving the efficiency of fertilizer use and enhancing the water holding capacity in the drainage of clayey or sandy soils (
Figure 1). Biochar application has also been reported as improving the microbe-mediated chemical reactions and enzymatic activity of soil (
Figure 1C)
[35]. Soil applications of biochar also have the potential to minimize soil runoff and erosion. A systematic meta-analysis revealed the mitigation of soil erosion by 16% and runoff by 25% upon biochar-based soil amendment. This effect was found to be stronger in tropical zones over subtropical
[36].
Figure 1. Potential role of biochar in soil amendments and crop productivity via various mechanisms; (
A): increase in crop productivity via biochar-induced root zone amendment of crop
[23], (
B): biochar effect on climate under cultivated field by inducing positive effects on increasing soil organic carbon, soil inorganic carbon and water retention and decreasing emissions and evapotranspiration
[24], (
C): mechanism of biochar-associated improvement of soil nitrogen
[35], (
D): overall role of biochar in improving soil properties and crop productivity.
Moreover, biochar also plays an important role in the mitigation of climate change impacts through several other secondary mechanisms, such as it playing a role in the reduction in nitrous oxides and methane emissions in soil. Pyrolysis of biomass to biochar can avoid processes such as decomposition and combustion of biomass, which contribute to the emission of NOx and methane in the atmosphere. According to a recent study, the application of biochar could lower emissions by 50–80% in an acid savanna oxisol and by 70–80% in slightly acidic to neutral soil. Another study estimated the annual soil nitrous oxide emissions avoided by the application of biochar as a reduction factor RN of 25%. A study from China, which is a significant GHG emitter, revealed the potential of biochar to offset the total CH
4 and N
2O emissions from China’s crop land via pyrolysis of waste to biochar
[37]. Biochar application, in combination with dicyandiamide, has been reported to reduce the cumulative N
2O emissions by 69–70% and CO
2 emissions by 30–43%. This reduction in emissions was found to be associated with damage to bacterial network complexity
[38]. Similarly, a few studies have reported on the reduced methane emissions of soil via the application of biochar
[22]. However, further research input is required to estimate the associated reduced fraction of nitrous oxide or methane emissions in various soil conditions.
In summary, biochar has a high potential to mitigate climate change’s impact on soil, agriculture, and ultimately, on crop yields. Biochar possesses more potential benefits over hazards in comparison to other soil management and mitigation technologies. However, a careful analysis is required for the production of biochar at large scale, as it could attract companies, industries and money makers to stock carbon in a stable form for trade, thereby resulting in further food insecurities. With the advent of research in this field, there is also a dire need for careful policy making, designs, protocols, project monitoring and advice for agriculture extensions to maximize the output and to avoid any negative outcomes associated with poor irrigation practices and implications.
3. Biostimulants
More recently, biostimulants have been reported as one among various significant and potential mitigation strategies in assisting plants to develop resistance against several environmental abiotic stresses resulting from rapidly changing climatic conditions
[39]. Various recent studies have shown the tremendous potential of biostimulants in agriculture by providing aid to plants against climate change-induced stresses such as salinity, drought, temperature, etc.
[40]. Biostimulants are one of the emerging biological strategies with potential to mitigate climate-induced biotic and abiotic stresses in plants, without compromising on soil health, plants’ growth and the environment. Biostimulants are microbes, organic compounds or amalgamations of the two that could help in the regulation of plant growth and certain behaviors via alterations at the molecular, biochemical, physiological and anatomical levels
[41]. Biostimulants can act as a promising mitigation strategy in recent crop production scenarios, as they are reported to function through various modes of action due to their diverse nature and the varying composition of these bioactive compounds
[42]. Biostimulants can be broadly categorized into various classes such as botanical extracts including seaweed/algae extracts, amino acids, protein hydrolysates, vitamins, antioxidants, cell-free microbial products, anti-transpiration agents, chitin and fulvic and humic acid, along with their derivatives
[43][44]. The application of biostimulants in very small amounts could have the potential to induce resistance against stresses, and this quality of biostimulants makes this class different from applications of fertilizers and manures to soil. Studies have also revealed their ability to contribute towards the maintenance of the ecological balance in agro-ecosystems by reducing the use of chemical fertilizers, pesticides and heavy metals in agricultural practices
[45]. Based upon the immense potential of biostimulants, the European Commission has planned to substitute 30% of chemical fertilizers with organic-based inputs by the end of 2050
[39]. Along with their tremendous ability of augmentation in the levels of production, biostimulants have also been reported for their role in reducing greenhouse gas emissions by decreasing fertilizer consumption in the agricultural sector
[46]. A recent study reported that extracts of seaweed can significantly reduce the release of greenhouse gases by supplementing synthetic fertilizer input in sugarcane cultivation and observed a potential offset of 260 kg CO
2 equivalent/Mg cane production/ha from 5% foliar application of seaweed extract
[47]. Biostimulant-induced responses differs among plant species, depending on morphological modifications to gene expression, their mode of application and phyto-hormone responses
[48].
In summary, the use of biostimulants is an emerging mitigation green strategy that has a tremendous ability to counter water scarcity or drought and soil and water salinity-associated stresses in plants but is also a safe practice to maximize the productivity and nutritional values of crops. However, the biostimulants-associated mode of action has been frequently characterized in model studies only, and its understanding is limited under field conditions. There is significant room for research into the applications of biostimulants in cropping systems under field conditions to understand the impact of external factors on the practical applications of biostimulants in agriculture.