Biochar interaction with chemical fertilizer regulates soil organic carbon mineralization and the abundance of key C-cycling-related bacteria in rhizosphere soil
Introduction
The soil carbon (C), made up of organic and inorganic fractions, is the largest C pool in the terrestrial ecosystem [1]. It is approximately 3.3 times that of the atmospheric C pool [2]. It makes up approximately two-thirds of all C in the terrestrial ecosystems and is actively exchanged with the atmosphere [3]. As a key factor used to assess soil quality and plant productivity, soil organic C (SOC) plays a crucial role in improving soil physicochemical properties, soil nutrient cycling, photosynthesis in plant leaves, and soil biological properties [4]. However, various human activities, such as intense land use and excessive long-term use of N-based chemical fertilizers, could increase soil carbon loss and increase its release as carbon dioxide (CO2) into the atmosphere. CO2, along with nitrous oxide (N2O) and methane (CH4), contributes to over 90% of global climate warming from anthropogenic activities [5]. Due to the inevitability of increasing plant yields to meet food demand, the use of N fertilizers has increased drastically, accounting for 72% of soil N input in China [6] and 77.8% in the US in 2018 [7]. Given the tight connections between C and N cycles in the terrestrial ecosystem, the increasing use of chemical N fertilizers could alter the ecosystem's C cycles [8]. Therefore, exploring management practices that could reduce the mineralization and loss of labile C fractions remains a major concern in soil management and global environmental health.
Biochar is an aromatic C-rich solid byproduct obtained when biomass is pyrolyzed in an oxygen-limited environment [9]. It has gained attention in agriculture for its role in improving soil physical, chemical, and biological properties [10]. As a C-rich material, the application of biochar can increase the availability of recalcitrant and labile C compounds that can undergo degradation after their incorporation into the soil [11]. Published reports have shown that biochar could exert either positive, negative, or no effects on soil fertility and C storage potential [[12], [13], [14]]. Labile organic C inputs contained in straw and litter, for example, can increase not only SOC concentration and nutrient availability but could also stimulate microbial activity [15,16]. However, most practical soil applications of biochar are made along with inorganic fertilizers to improve N retention and optimize crop production [9]. While biochar could increase SOC stocks, its interaction with chemical fertilizers could alter soil C dynamics by modifying C cycling enzyme activity and soil microbial communities involved in SOC degradation. Similarly, inorganic N addition to soils could affect soil physicochemical properties [17,18], soil microbial community composition and function [19,20], and soil organic matter stability [21]. Notably, soil C degrading organisms are the biotic controlling factors for SOC turnover, thereby significantly regulating soil C-cycling and CO2 emission [22]. Some taxa possess higher C-cycling abilities among the soil bacteria via increased decomposition of organic C sources in the soil, which they incorporate into their biomass and respire as CO2 [23]. They possess the RuBisCO form II enzyme gene (cbbM), which enables them to utilize C more efficiently [24]. Since these bacteria have great control over soil C cycling and act as the primary regulators of C exchange in the soil-atmosphere system [25,26], their regulatory activities can shift under altered environmental conditions and soil management practices [27].
With the varying effect of sole biochar and inorganic fertilizer on soil properties and bacterial distribution, predicting the identities of C-cycling-related 16S rRNA genes when biochar is co-applied with chemical fertilizers in the soil systems is of utmost importance. The co-application of biochar and fertilizers has become a common practice, but their effect on C-cycling and the abundance of major C-cycling-related 16S rRNA genes has not been adequately documented in soil-plant systems. Therefore, the changes in soil C fractions over time and how they regulate the distribution of different key C-cycling-related bacterial communities in rhizosphere soil under biochar + fertilizer interactions require adequate understanding. The use of rhizosphere soil in our study is important because it differs from the bulk soil due to changes in its biogeochemistry, which are influenced by root exudates containing labile C that supports microbial activity [28,29]. Therefore, this study will provide the needed insight into C-cycling in practical agricultural soils, unlike the numerous available reports without the effect of plant rhizosphere. Hence, this study could contribute to ecosystem models for predicting the C budget of terrestrial ecosystems in soil-plant systems.
We hypothesize that: Compared to sole biochar, combined biochar and fertilizer would reduce the mineralization of SOC, with variation among biochar types. Also, N fertilization in BF would provide substrates for N-cycling-related bacteria, which may compete for dominance and C sources with the C-cycling-related bacteria to regulate C-cycling. Similarly, the expected increase in root exudates associated with increased plant nutrition, biomass, and photosynthesis under BF could influence the abundance of rhizosphere C-cycling related bacterial groups.
Therefore, this study's objectives were to understand the effects of different biochar + fertilizer interactions on the mechanisms regulating the dynamic relationship between soil C fractions, C-cycling enzyme activity, the diversity, and functional abundance of key bacterial communities regulating C cycling in the plant-soil system. These interactions on plant photosynthetic CO2, nutrient concentration and biomass of maize would also be observed.
Section snippets
Production and characterization of biochar
The biochars used for this study were produced from bamboo, rice straw, cow manure, and pig manure. The raw feedstocks were air-dried, crushed, and pyrolyzed at 500 °C ± 10 °C (at a holding time of 2 h). Slow pyrolysis of the biomasses was carried out using a bench-scale pyrolysis unit (SSBP-50004, Jiangsu, China.) under oxygen-limited conditions. The internal oxygen was purged out from the reactor by introducing a stream of nitrogen gas at 0.5 L min−1 for 2 min prior to pyrolysis. After
Properties of biochar and the effects of biochar/biochar and fertilizer interactions on soil chemical properties
Higher contents of total N and P were observed in the pig manure biochar (PMB) compared to cow manure biochar (CMB), rice straw biochar (RSB), and bamboo biochar (BB) (Table S1). The CMB had the highest pH content, while the BB had the highest total C content and specific surface area. In addition to the high ash content in RSB, this biochar also had higher peaks of C and O-containing functional groups (Fig. 1). Despite its high C content, the bamboo biochar had the weakest peaks/intensities of
Influence of biochar + fertilizer interactions on soil C fractions and C-cycling enzyme activity
The DOC pool is regarded as the primary form of labile C and a significant indicator of the lability of SOC [37]. The initial increase in DOC and DIC in BF relative to biochar could be attributed to an increase in soil nutrients provided by the added fertilizers, which stimulated microbial activities' for the degradation of the introduced labile C in biochar. Increases in soil nutrients after fertilization and the provision of suitable habitats for microbial growth and protection from predators
Conclusion
The use of biochar sources containing higher intensities of C-containing functional groups (as in CMB, PMB, and RSB) rather than their total C (as in BB) content regulated their influence on the mineralization of SOC. Combined biochar and fertilizer (BF) increased plant nutrient uptake, biomass, and photosynthesis. However, the assumed deposition of the plant photosynthetic CO2 as roots exudates were insignificant in altering the abundance of rhizosphere C-cycling-related bacterial community.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was funded by the Fujian Forestry Science and Technology Promotion Project (2020TG17) and the University-Industry Cooperation Project of Fujian Provincial Department of Science and Technology (2016N5005).
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