May , 2022, Volume : 3 Article : 11
Biochar: An Effective Amendment to Improve the Phosphorus Utilization Efficiency
Author : Mohsina Anjum
ABSTARCT
Phosphorus is Biochar has attracted widespread attention in crop production due to its beneficial effect on nutrient availability, in particular, soil phosphorus (P) pool. Numerous studies suggested the use of biochar to increase the availability of P while limiting P loss to the environment. Understanding the soil P transformation greatly improves the application of biochar and P utilization efficiency. This article briefly discusses the effect of biochar on P speciation, availability, and its effect on loss risk to the environment.
Keywords: Biochar, phosphorus, P pools, P utilization efficiency, P leaching, soil microbial activity
Phosphorus (P) is one of the essential nutrients for crop nutrition. Due to its scarcity, the management of P fertilizer is considered a challenge for the 21st century. The soil P cycle is very complex and most of the soil P is derived from weathering of primary minerals. Soluble P is thereafter translocated to the subsoil and then immobilized into organic P or transformed into various inorganic P forms. P availability is controlled by various chemical and biochemical processes. Further, excess soluble P can be of environmental concern impacting the quality of water bodies (Pradhan et al., 2021). Therefore, P recycling is critical to maintaining a balance in the ecosystem. Biochar is a type of aromatized insoluble black carbon that has been pyrolyzed from the biomass of feedstock at 300-700°C under anoxic or completely oxygen-free conditions (Kookana et al., 2011). Application of biochar has several advantages such as high surface area and high carbon (C) content, besides being resistant to decomposition (Lehmann et al., 2011). It has been of interest to agronomists and environmental scientists who have studied biochar and its amendment effects on different agricultural systems. Moreover, higher carbon content and nutrients concentration in the soil can promote the activity of microbes. Therefore, understanding the effect of biochar on P speciation is important to improve availability and minimize the loss of P.
Effect of biochar on soil P pools
Evidence has shown that biochar can affect soil P speciation and its bio-availability by affecting the different soil properties (pH, cation exchange capacity, microbial biomass) (Wang et al., 2014) (Fig. 1). Interaction of biochar with native soil P may be attributed to changes in pH, P distribution, and ionic strength after the amendment of biochar. Biochar addition was shown to increase extractable forms of P (Olsen P and Mehlich-3 P). In addition to available forms of P, biochar increases the labile forms of P. These two forms of P are easily used by the plant rather than conversion to more stable forms of P (Wang et al., 2014). Xu et al. (2014) reported increased Ca-P and decreased Fe-P content with the addition of biochar. Increased concentration of water-soluble P has been reported by Xu et al. (2016). However, he did not find a significant effect of biochar on NaOH-Pi, and HCl extractable Pi.
P pools varied with the texture of the soil increased concentration of NaHCO3-P or NaOH-P was observed in clay loam soil in contrast to loamy sand soil (Chathurika et al., 2016). Soil reaction also has a significant effect on P pools with an increased amount of Ca-P in acidic soil compared to alkaline soil. However, biochar reduced the content of Fe-P irrespective of the soil reaction (Xu et al., 2014). In acid soil, most of P is fixed by Al and Fe oxides while in alkaline soil, it is precipitated by Calcium. Nevertheless, the addition of biochar modifies the soil pH thereby, influencing its adsorption to soil minerals (Biederman and Harpole, 2013). The fact that biochar addition increased soil microbial biomass P (MBP) may be attributed to the increased availability of carbon and other nutrients (Zhu et al., 2017). Zhai et al. (2015) reported that an increase in MBP might be ascribed to improved soil quality. However, the response of MBP to biochar application varied with the location and application of P fertilizer.
Effect of biochar on P availability
Biochar application has both beneficial and harmful effects on available P. Liu et al. (2017) reported that the application of 40 t biochar ha-1 increased the availability of P by 52.6%. There have been reports of an above 1000 fold increase in available P with the addition of poultry litter-based biochar at a rate of 10 t ha-1 (Bai et al., 2015). However, some studies reported no significant effect on the availability of P with biochar addition (Ahmed and Schoenau, 2015). In contrast, the reduced effect of biochar on P availability has been shown in previous studies (Li et al., 2017). Though the negative impact of biochar on P availability is not fully investigated, it is evident that soil properties (pH, P distribution, microbial activity) and characteristics of biochar affect the bio-availability of soil P.
The indirect effect of biochar on P availability can be explained by its interaction with organic matter or minerals. Biochar enhances the mineralization of organic C and P by adsorbing organic chelating molecules and their breakdown into lower molecular weight organic acids (Lou et al., 2016). Desorption or dissolution and mineralization are the main factors controlling the release of P from biochar. Coprecipitation of P with Al, Ca, Fe and Mg has also been shown to increase P bio-availability. Soil pH influenced the availability of P by changing sorption characteristics of the soil. Increased pH with the addition of biochar increases the negative charge on the mineral surface, thereby keeping more P in the soil solution (Cui et al., 2011). Biochar coupled with high native P may enhance the rate of P release from reactive metal oxide fraction (Cui et al., 2011). However, P utilization efficiency is low in soils with higher native P (de Vasconcelos et al., 2017).
Biochar indirectly affects P availability by altering microbial activity. The micro-organism grows in the porous particle of biochar and breaks the organic substance adsorbed on the soil surface (Joseph et al., 2010). The extent of organic molecule breakdown varied with the chemical energy needed for breaking the chemical bond of the enzyme. The presence of beneficial microbial communities like symbiotic fungi enhances P utilization efficiency in soils even in soils with low P content.
Effect of biochar on risk loss of P
With concern to the environmental risk of P loss, biochar has been shown to either increase or decrease P concentration in leachate. Liard et al. (2010) reported a ~ 69 % reduction in P concentration in leachate due to biochar addition, while Sonnie et al. (2014) reported reduced leaching of P with the application of biochar. Prior literature suggested the beneficial role of biochar in reducing the loss of P through leaching or runoff.
Studies have shown that changes in pH and ionic strength with the addition of biochar are the main factors controlling P leaching in soil. Low ionic strength with high pH is shown to increase the rate of colloid release (Kumari et al., 2014). Madiba et al. (2016) observed five times greater leaching loss of P with the combined application of biochar and P fertilization in comparison to the biochar alone. However, no effect of biochar on the leaching of P was observed at a greater rate of biochar application (50 kg ha-1).
The release of P from biochar to solution increases the water-soluble P concentration in leachates. Feng et al. (2017) suggested the use of coated biochar (e.g., nano-ceria) and reported a significant reduction in P loss (27%) in leachate. The application method of biochar can also influence the loss of P from soil. Top dressed soil with biochar showed greater loss of total P with its amount varying from 14-20%, whereas it was <3% in soil into which biochar was mixed (Schnell et al., 2012).
Conclusion
Amendment with biochar has been shown to increase the availability of P in soil. Biochar promotes soil quality and may have a beneficial effect microbial community of soil. Results from the previous studies suggested that biochar can be an effective amendment for improving P use efficiency, in particular soils with low P. Reduction in soil solution P in leachate makes biochar an effective amendment in the crop production to boost P use efficiency, reduces P loss and environmental risk. Keeping the above beneficial effects in view, adsorbed phosphorus on the biochar can act as a slow-release P fertilizer. P is the ultimate limiting nutrient and currently, 90% of the current usage of phosphorus resources (>80% of fossil P resources) is as fertilizer in agriculture. However, improper biochar application may decrease crop productivity and can deteriorate soil quality. Further, research is needed to find out the optimum rate of application of biochar for different crops to avoid negative effects. However, for an optimum recommendation and package of practices, further research about the retention-release patterns and the sorption capacity of biochars to fully understand the P retention in soils. This is essential, to avoid water pollution because of the continuous release of P from biochar to downstream water bodies.
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