Anthropogenic carbon in the agroecosystems: Coexistence effects of biochar and microplastics on soil ammonia volatilization

Yuanyuan Feng Feng

The existence of the life on this planet is majorly based on the Carbon (C), Nitrogen (N) and Water Cycles that sustain a productive and sustainable ecosystem that promotes life. The agricultural practices across the globe at present produces enough food for a population of about 8 billon. The resources involved in these agricultural processes and practices are enormous; with 187 million tonnes of fertilisers being applied to soils, about 4 tonnes of chemicals to fight pests and 2.7 trillion cubic meters of water to promote growth. This is largely not manageable and sustainable in the future, especially with a growing global population expected to be 9.8 billion by 2050. Addition of C into the agroecosystem to promote growth is well established, but the impact of other antropogenic sources of C needs to be evaluated for its impact on these abovementioned cycles. The impact of pollution by microplastics (MPs), a type of anthropogenic source charcoal, on the N cycle in soil ecosystems is attracting widespread attention. Meanwhile, dissolved organic matter (DOM)-rich biochar (pyrochar (PBC) and hydrochar (HBC)) is being attempted as a soil conditioner to reduce gaseous N losses from agricultural soils. Exogenous substances (e.g., PBCs, HBCs and MPs) may alter soil DOM by releasing DOM (i.e., rapidly mineralized and extractable carbon) into the soil, thereby affecting soil microbial and enzymatic activity and regulating plant nitrogen uptake, which in turn alters ammonia (NH3) emissions from agricultural soils. My PhD research selected straw-derived PBC and HBC, as well as polyethylene (PE), polyethylene terephthalate (PET) and polyacrylonitrile (PAN), to investigate changes in soil NH3 volatilization and DOM fractions in the presence and co-occurrence of these anthropogenic sources of carbon, while observing crop yields and nitrogen uptake. The objectives of the study were: 1) to assess the effects of anthropogenic carbon coexistence on NH3 volatilization in agricultural soils; 2) to compare the differences in soil properties, particularly soil DOM fractions, bacterial community and crop growth under anthropogenic carbon coexistence; and 3) to explore the mechanisms of anthropogenic carbon effects on NH3 volatilization in agricultural soils. The results disclosed that both MPs and MPs+HBC unexpectedly mitigated the accumulated NH3 volatilization in paddy soils at a concentration of 0.5% (w/w) of MPs compared to the control group (CK) without MPs or HBC addition. Among them, PAN was more effective than PE in mitigating NH3 volatilization. MPs+HBC increased NH3 volatilization by 37.8-46.2% compared to MPs, indicating that the coexistence of MPs and HBC weakened the mitigating effect of MPs on NH3 volatilization. Furthermore, the inhibition of soil urease activity by MPs or MPs+HBC ranged from 10.6-19.0%. Furthermore, the results of studies on N cycle-related microorganisms closely associated with NH3 volatilization showed that MPs+HBC altered bacterial community structure and species diversity, and enhanced predicted gene abundance throughout the rice growing season. In the wheat season, HBC, MPs, and HBC + MPs reduced cumulative NH3 volatilization by 45.4%, 66.7–67.4% and 29.8–58.7%, respectively, compared to the control without MPs or HBC. Soil DOM fulvic acid-like and humic aid substance abundances of the other treatments were increased by 18.5–56.2% and 12.7–39.3%, respectively, compared to the control. Besides, soil NH3 fluxes were positively correlated to soil NH4+-N concentration at basal fertilization and soil NO3--N concentration at first supplementary fertilization. HBC, MPs and HBC + MPs treatments promoted soil urease activity and plant N uptake by 6.5–24.2% and 31.9–74.3%, respectively, relative to the control. Studies on the coexistence of PBC and MPs showed that PBC alone and MPs with PBC (MPs + PBC) reduced 5.5% and 11.2–26.6% cumulative NH3 volatilization than the control, respectively, in the rice season. The increased nitrate concentration and soil cation exchange capacity were dominant contributors to the reduced soil NH3 volatilization in the rice season. PBC and MPs + PBC persistently reduced 44.5% and 60.0–62.6% NH3 losses than CK in the wheat season as influenced by pH and nitrate concentration. Moreover, PBC and MPs + PBC increased humic acid-like substances in soil dissolved organic matter by an average of 159% and 180% than CK, respectively, in rice and wheat seasons. The increased adsorption of soil ammonium roots and the promotion of crop root growth were the main mechanism of NH3 reduction. Our findings deepen the understanding of the processes by which anthropogenic source charcoal affects organic matter fractions and N cycling in agricultural soils. These studies provide an important opportunity to advance the understanding of the impact of MPs on the agricultural environment and provide a theoretical basis for the rational agricultural application of HBCs and PBCs in soils.

Anthropogenic carbon in the agroecosystems: Coexistence effects of biochar and microplastics on soil ammonia volatilization

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