Chapter Two - Effects of biochar amendments on soil phosphorus transformation in agricultural soils
Introduction
Phosphorus (P) is one of the most important essential macronutrients in nature. Almost all of biochemical processes in fauna and flora require P (Bate et al., 2008). Most of the P in soils is driven from the weathering of natural phosphate rock. Solubilized P is further translocated to subsoils or water bodies and then immobilized into organic P or transformed into various inorganic P forms (e.g., precipitates, adsorbed complexes on soil components). The soil P cycle is complex and involves a variety of chemical and biochemical processes such as adsorption and desorption, precipitation and dissolution, and mineralization and immobilization. P transformation is influenced by the soil formation process, biological activity, redox status, soil solution chemistry (pH, ionic strength, activity of ligands) and many environmental factors (soil, temperature, etc.) (Pierzynski et al., 2005). Bioavailable P can be produced by organophosphorus hydrolysis and inorganic phosphate dissolution. P bioavailability is mediated by the action of microorganisms, root symbionts and plant roots that produce hydrolytic enzymes and organic acids (Maltais-Landry et al., 2014). Orthophosphates in soil solution may diffuse at the rhizosphere, or they may be intercepted by high-affinity transport proteins of arbuscular mycorrhizal (AM) fungi and then transferred to host plants (Gul and Whalen, 2016). Excess soluble P or fine-grain P in the soil enters the surface or groundwater via surface runoff or underground pore leaching (Chen et al., 2018; Morshedizad and Leinweber, 2017), which is also one of the main factors that causes eutrophication. Therefore, the recycling of P via various transformational processes in the soil is critical in maintaining ecosystem balance.
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; Sohi et al., 2010). The pyrolysis process causes the deamination and deoxygenation of short-chain carbonaceous substances in lignin, cellulose and hemicellulose, resulting in their gradual conversion into C compounds with aromatic or graphite structures (Cimo et al., 2014).
Because biochar has advantages that include a high surface area, high C content, and strong resilience to biochemical decomposition, it has been of interest to agronomists and environmental scientists who have studied biochar and its amendment effects to different agricultural systems. Numerous recent studies have shown that biochar has a positive role in increasing soil C, providing essential nutrients, improving soil physical conditions, reducing greenhouse gas emissions, and adsorbing heavy metals (Hagemann et al., 2018; Jia et al., 2018; Kong et al., 2018; Sekulic et al., 2018; Thangarajan et al., 2018; Xu et al., 2016a).
Once biochar is added to the soil, it reacts with soil particles in a series of reactions including acid–base, dissolution–precipitation, oxidation–reduction and adsorption–desorption reactions (Joseph et al., 2010). Moreover, the high C content and high concentration of nutrient elements within biochar can affect the activity of soil microbes. Once can expect that these reactions can influence the mineralization of organic P, solubility of P, absorption onto soil minerals and organic matter, and immobilization of inorganic P in the soil. Therefore, exploring the mechanisms by which biochar influences the soil P cycle is important in improving the biochar application methods, increasing the plant available P, reducing the P loss to minimize the nonpoint source P pollution in the aquatic environment.
Section snippets
Phosphorus content
The type of feedstock used in the preparation of biochar largely affects the total P content in biochar. Many studies have reported the total P and the bioavailable P in biochar are unique to the type of feedstock (e.g., manure, woody materials, herbaceous plants, sewage sludge, and animal bones) that was used to produce biochar. We have calculated the total P and soluble P content of various biochars in the existing literature (Fig. 1). The results show that the total P content follows the
Phosphorus release from biochar and its impact on the soil P content
The chemical composition of biochar varies widely depending on the pyrolysis conditions and the type of feedstock. Phosphorus that retained in biochar can be released and used by plants for growth (Fig. 4). The release of P from biochar is part of the biochar aging process. The exposed C rings with a high density of P electrons (Contescu et al., 1998) as well as the free radicals in biochar are readily oxidized (Montes-Moran et al., 2004). Once biochar particulates are in contact with pore
Effects of biochar amendment on the soil P pool
Evidence has shown that biochar can affect biogeochemical properties of soils (e.g., pH, cation exchange capacity (CEC), microbial biomass). The presence of complex ions (Nelson et al., 2011; Slavich et al., 2013), plant growth (Sun et al., 2016) and soil biota such as microbial diversity (Anderson et al., 2011; Lehmann et al., 2011), ultimately affect the fate of P and its forms in soils (Chintala et al., 2014; Morales et al., 2013; Nelson et al., 2011; Wang et al., 2014). The impact of
Effects of biochar on soil P availability
Numerous studies have shown that biochar application has either positive or negative effects on soil available P (Table 1). On the positive side, most studies have reported that biochar can increase bioavailable P in soils to some extent. For example, Liu et al. (2017) studied that the effects of rice hull biochar amendments to red clay soil and alkaline soil, and they found that applications of 40 t ha− 1 increased the availability of P by 52.63% and 33.37%, respectively. Bai et al. (2015)
Effects of biochar on soil phosphatase activities
It has been frequently reported that biochar can influence the growth of microorganisms in soils. The application of biochar can alter the structure of the soil biome, significantly reduce the proportions of soil fungi and bacteria, and increase the proportions of gram-positive and gram-negative bacteria (Noyce et al., 2015; Wang et al., 2017). In soils, P-solubilizing microorganisms (PSMs) can be divided into P-solubilizing fungi (PSF) (such as Penicillium spp.), PSB (such as Bacillus spp.),
Effects of biochar on soil P loss
The application of biochar to soils is known to either increase P concentrations in leachate and runoff from soils (Bradley et al., 2015; Guo et al., 2014; Kumari et al., 2014; Madiba et al., 2016; Novak et al., 2014a, Novak et al., 2014b; Schnell et al., 2012; Troy et al., 2014; Wang et al., 2015b; Yuan et al., 2016) or reduce P concentrations in leachate and runoff from soils (Agegnehu et al., 2015; Beck et al., 2011; Feng et al., 2017; Laird et al., 2010; Liang et al., 2014; Liu et al., 2015
Direct and indirect effects
When biochar is applied to the soil, P in the biochar become labile (Fig. 10), and this portion of P can be directly absorbed by plant roots (Qian et al., 2013). Phosphorus in biochar can be slowly transformed into labile P via mineralization by soil microorganisms for further use by plant roots (McBeath et al., 2007). However, the amendments of biochar are known to increase the loss of soil P (Bradley et al., 2015; Guo et al., 2014). Therefore, this process may eventually lead to the loss of
Effects of biochar on soil P immobilization
The interaction of biochar with soil P can directly or indirectly affect the P immobilization and bioavailability. However, whether the application of biochar can enhance the adsorption or desorption of P from soil particles is not clearly understood due to contradicting results.
Some scholars have reported that biochar can effectively reduce the P bioavailability (Chintala et al., 2014). A study by Dari et al. (2016) showed that the soil maximum P retention capacity (S-max) increased as the
Perspectives
In summary, our literature review has shown that the pyrolysis temperature had a significant impact on the P content, the P morphology and the P stability of biochar. As the pyrolysis temperature increases, the total P content increases. It has been reported that both slow and fast P release from biochar to soils, which are affected by soil pH, ionic strength, microbial biomass and P content in biochar. The addition of biochar to soils also affects the P speciation in soils, increases the
Acknowledgments
This work was supported by ACES international joint research program between University of Illinois at Urbana-Champaign and Zhejiang University, the National Natural Science Foundation of China [Grant number 41522108], the National Key Research and Development Program of China (2017YFD0800103), and the Natural Science Foundation of Zhejiang Province [Grant number LR16B070001].
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