Periphyton has the potential to increase phosphorus use efficiency in paddy fields

Water quality and periphyton functional response to input of dissolved manure-derived hydrochars (DHCs)

Dissolved organic matter (DOM) plays a critical role in the global carbon cycle and provides food and energy for aquatic organisms. Recently, hydrochar, as a solid carbonaceous substance derived from hydrothermal carbonization, has been increasingly used as a soil amendment. Upon entering the soil, dissolved components (DHCs) were released from hydrochar as exogenous DOM, finally entering the aquatic ecosystems by runoff, which participates in environmental geochemical processes. However, relevant reports revealing the response of the aquatic ecosystem to the input of DHCs remain insufficiently elucidated. For the first time, the fundamental features of DHCs and their influence on water quality and aquatic biological function were investigated in this study. DHCs at 260 °C (DHC260) had lower yields, a greater [C/N], worse biodegradability, and larger humic acid relative amounts than did DHCs at 180 °C (DHC180). The DHC structural alterations in periphyton-incubated aquatic ecosystems suggested that protein substances were more easily degraded or assimilated by periphyton, especially for DHC180, with rates of decrease of 34.5-63.5%. The increased chemical oxygen demand (COD) degradation in the DHC260 treatments was most likely due to humic acid substances with higher COD equivalents. Furthermore, DHC260 caused phosphorus to accumulate in periphyton, reducing aquatic phosphorus concentration. Notably, the abundances of Flavobacteria and Cyanobacteria associated with water blooms increased 12.7-25.5- and 1.3-8.3-fold, respectively; consequently, the promotional impact of DHCs on algal blooms should be considered. This result extends the nonnegligible role of DHCs in aquatic ecosystems and underlines the need to regulate the hydrochar application process.

Contribution of periphytic biofilm of paddy soils to carbon dioxide fixation and methane emissions

Rice paddies are major contributors to anthropogenic greenhouse gas emissions via methane (CH₄) flux. The accurate quantification of CH₄ emissions from rice paddies remains problematic, in part due to uncertainties and omissions in the contribution of microbial aggregates on the soil surface to carbon fluxes. Herein, we comprehensively evaluated the contribution of one form of microbial aggregates, periphytic biofilm (PB), to carbon dioxide (CO₂) and CH₄ emissions from paddies distributed across three climatic zones, and quantified the pathways that drive net CH₄ production as well as CO₂ fixation. We found that PB accounted for 7.1%-38.5% of CH₄ emissions and 7.2%-12.7% of CO₂ fixation in the rice paddies. During their growth phase, PB fixed CO₂ and increased the redox potential, which promoted aerobic CH₄ oxidation. During the decay phase, PB degradation reduced redox potential and increased soil organic carbon availability, which promoted methanogenic microbial community growth and metabolism and increased CH₄ emissions. Overall, PB acted as a biotic converter of atmospheric CO₂ to CH₄, and aggravated carbon emissions by up to 2,318 kg CO₂ equiv ha^-1 season^-1. Our results provide proof-of-concept evidence for the discrimination of the contributions of surface microbial aggregates (i.e., PB) from soil microbes, and a profound foundation for the estimation and simulation of carbon fluxes in a potential novel approach to the mitigation of CH₄ emissions by manipulating PB growth.

Interactions between periphytic biofilms and dissolved organic matter at soil-water interface and the consequent effects on soil phosphorus fraction changes

Dissolved organic matter (DOM) plays vital roles in carbon and other nutrient transformation at soil-water interfaces (SWI) in paddy fields. It is associated with the growth and withering of periphytic biofilms. However, the interactions between DOM and periphytic biofilms remain largely unknown. In this study, a microcosm experiment with different initial DOM contents elucidated that the biomass, and biomass nitrogen and phosphorus contents were greatly influenced by humic-like substances (C2 and C3), while the growth of periphytic biofilms increased the contents of humic-like (C1 and C2) and tryptophan-like substances (C5) in soil. Moreover, the decomposition of periphytic biofilms significantly increased soil pH, DOM, C2, C3 and C5 contents, but caused decrease in Eh, with consequent reduce in water soluble phosphorus (WSP) and release of algal available phosphorus (AAP). Results from this study revealed how DOM interacts with periphytic biofilms and the consequent effects on changes of bioactive phosphorus fractions, and provide practical information for designing periphytic biofilm based biofertilizer from the perspective of soil DOM.

Biological soil crusts alleviate the stress of arsenic on rice germination and the underlying immobilization mechanisms

The high concentration of arsenic (As) in paddy soil has seriously threatened the growth of rice and human food safety. Biological soil crusts (BSC), which are ubiquitous in paddy fields, have been shown a high ability to capture trace metal elements. In the present study, we investigated the effectiveness and mechanism of BSC for immobilizing As, and tested their potential to alleviate the stress of As on rice germination. It is found that BSC can remove 77.8% of arsenic in solution with 3.5 mg L^-1 initial As concentration. The As content in BSC reached 514.5 mg kg^-1 after 216 h exposure, and the entrapped As was mainly distributed in BSC as a non-EDTA-exchangeable fraction, which might be intracellularly accumulated. Proteobacteria and Bacteroidetes were the dominant phyla in BSC after being exposed to As, playing a significant role in tolerating As and As biogeochemical cycling. The presence of BSC notably promoted the germination rate (18.3%) and dry biomass (103.4%) of rice seeds under 3.5 mg L^-1 As stress while reducing the As content in plant roots (8.2-34.3%) and shoots (8.7-47.6%). These findings demonstrate that BSC have a great entrapping effect on As and highlight the importance of BSC in alleviating the stress on rice germination by As, providing a potential nature-based and low-cost strategy to decontaminate paddy fields polluted with As.

Periphytic microbial response to environmental phosphate bioavailability - relevance to P management in paddy fields

Periphyton occurs widely in shallow-water ecosystems such as paddy fields and plays critical parts in regulating local phosphorus cycling. As such, understanding the mechanisms of the biofilm's response to environmental P variability may lead to better perceptions of P utilization and retention in rice farms. Present study aims at exploring the biological and biochemical processes underlying periphyton's P buffering capability through examining changes in community structure, phosphorus uptake and storage, and molecular makeup of exometabolome at different levels of P availability. Under stressed (both excessive and scarce) phosphorus conditions, we found increased populations of the bacterial genus capable of transforming orthophosphate to polyphosphate, as well as mixotrophic algae who can survive through phagotrophy. These results were corroborated by observed polyphosphate buildup under low and high P treatment. Exometabolomic analyses further revealed that periphytic organisms may substitute S-containing lipids for phospholipids, use siderophores to dissolve iron (hydr)oxides to scavenge adsorbed P, and synthesize auxins to resist phosphorus starvation. These findings not only shed light on the mechanistic insights responsible for driving the periphytic P buffer but attest to the ecological roles of periphyton in aiding plants such as rice to overcome P limitations in natural environment. Importance The ability of periphyton to buffer environmental P in shallow aquatic ecosystems may be a natural lesson on P utilization and retention in paddy fields. This work revealed the routes and tools through which periphytic organisms adapt to and regulate ambient P fluctuation. The mechanistic understanding further implicates that the biofilm may serve rice plants to alleviate P stress. Additional results from extracellular metabolite analyses suggest the dissolved periphytic exometabolome can be a valuable nutrient source for soil microbes and plants to reduce biosynthetic costs. These discoveries have the potential to improve our understanding of biogeochemical cycling of phosphorus in general and to refine P management strategies for rice farm in particular.