Dissolved organic carbon, a critical factor to increase the bioavailability of phosphorus during biochar-amended aerobic composting

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Highlights

  • The activation mechanism on P attributed to DOC was investigated.

  • NaHCO3-Pi, NaOH-Pi and HCl-Pi were main P-fractions.

  • Biochar promoted the composting humification and increased the P-bioavailability.

  • DOC prevented phosphate binning with calcium ions and increased NaHCO3-P content.

ABSTRACT

Considerable research efforts have been devoted to increase phosphorus (P) availability during aerobic composting. However, there is little discussion weather the dissolved organic carbon (DOC) controls the transformation among P-fractions. Thus, we investigated the changes in DOC compositions and P-fractions during biochar-amended composting (wet weight basis, 5% and 10%). TP content continuously increased since the ‘concentration effect’ during aerobic composting. NaHCO3-Pi, NaOH-Pi and HCl-Pi were main P-fractions, and biochar can improve P-bioavailability by transforming NaOH-Pi and HCl-Pi into NaHCO3-Pi. Structure equation models (SEMs) indicated that biochar enhanced the P-bioavailability through regulating the decomposition of DOC. Our results at least hint that the activation mechanism on P under the influence of DOC during biochar-amended composting.

Introduction

Aerobic composting is a well-developed method to achieve organic stabilization, pathogens inactivation and nutrient cycling under controlled conditions (Jiang et al., 2019; Wang et al., 2014a; Wang et al., 2014b), which shows capacity to transform refractory phosphorus (P) into plant-available fractions through acidification, chelation, exchange and polymerization reactions (Wei et al., 2016a). Biochar, characterized as specific surface area, high porosity structure and cation exchange capacity (Awasthi et al., 2020), has been introduced to make P-activation more effective. Wei et al. (2020) reported that biochar was a satisfactory amendment to improve labile inorganic phosphorus associated with the indirect influence on nitrogen forms during cow manure composting. Wei et al. (2016b) demonstrated that available P-fractions increased during kitchen waste and sawdust co-composting, especially in the treatment modified with biochar. Ngo et al. (2013) showed that phosphorus was labile in organic substances, and composting transformed organic phosphorus into water-soluble phosphorus in manure compost, vermicompost and their mixture with biochar.

Microbial-controlled decomposition of dissolved organic carbon (DOC) is a typical composting process, which drives dynamic changes in physico-chemical parameters, and may further affect the P-availability during aerobic composting. DOC, as a complex organic mixture, usually contains small molecular compounds, such as amino acids, sugars and organic acids, as well as macromolecular compounds, including enzymes, polyphenols and amino-sugars (Jiang et al., 2019), which can adsorb dissociative phosphate through functional groups (hydroxy, carboxyl and aliphatic) (Akdeniz, 2019; Guo et al., 2020), thus preventing phosphorus from binding to heavy metal oxides and their hydroxides to immobilize (Song et al., 2015). Easily-degradable DOC can provide energy source for organophosphorus-degrading and inorganic phosphate-solubilizing bacteria (Yang et al., 2020), resulting in the dissolution of inorganic phosphorus and the mineralization of organic phosphorus during composting process. Humic acid-like substances, polymerized from protein-like and fulvic acid-like substances (Grigatti et al., 2017; Wei et al., 2016c), can combine with phosphorus to form stable-enhanced organic mixtures during humification process. Biochar can decrease composting density and improve aeration condition, as well as provide comfortable shelters for microorganisms (Liu et al., 2017a; Sanchez-Monedero et al., 2018), thus promoting DOC decomposition in the heating and thermophilic phases, and DOC humification in the colling and mature phases (Jindo et al., 2016; Wang et al., 2014a). Whether this result will affect the interaction mechanism between DOC and phosphorus needs to be further explored.

To address aforementioned research questions, we hypothesized that biochar can activated phosphorus through regulating DOC decomposition during swine manure (SM) composting mixed with maize straw (MS). Thus, this study was conducted with the following aims: 1) to identify the dynamic changes in DOC components and structures using three-dimensional excitation-emission matrix (EEM) with fluorescence region integral (FRI) analysis during biochar-amended composting; 2) to ascertain the P-availability with a modified sequential fractional extraction method; 3) to establish the interaction pathways between physico-chemical parameters and P-fractions in composting microenvironment with structure equation models (SEMs). These results potentially reveal the activation mechanism on phosphorus attributed to DOC during aerobic composting.

Section snippets

Composting procedure

Aerobic fermentation was conducted in medium-scale PVC reactors (Volume: 100 L). Fresh SM and MS were homogenized (V/V, 1:3) to obtain mixtures with a low C:N (C:N ≈ 15) in order to deal with more SM for relieving the environmental pressure. Two piles were mixed with biochar (wet weight basis, 5% and 10%, labeled as BL and BH treatments, respectively), resulting in experimental treatments, while one pile free of biochar served as control (CK). Each treatment was conducted in triplicate.

DOC content and components

As an active component, DOC could reflect the microbial activity and composting process (Song et al., 2015). DOC content continuously decreased in three treatments during composting process, and was significantly higher in CK treatment than that in BL and BH treatments during maturation phase (Table 2). This result indicates that biochar promotes the DOC decomposition during aerobic composting, which was consisted with Hagemann et al. (2018), who reported that lower DOC content was found in

Conclusions

In present research, dynamic changes in DOC composition and P-fractions during composting processes was estimated. Fluorescence spectra showed that an increase in humic acid-like substances was along with a decrease in tyrosine-like, tryptophan-like and fulvic acid-like substances, showing that biochar might promoted the composting humification and maturity. TP content continuously increased since the ‘concentration effect’ during aerobic composting. With respected to P-fractions, NaHCO3-Pi,

Acknowledgments

This research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA23070502) and the National Key Research and Development Program of China (No. 2018YFD0500205).

References (33)

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    Thus, the microorganisms that produced ALP probably mainly utilized unstable NaHCO3-IP during metabolism. The DOC and NaHCO3-P contents had a significant positive correlation (Fig. 3), and Cui et al. (2021) also found that DOC was a key factor that affected the transformation of NaHCO3-P during composting. In this study, adding biochar increased the pH (Appendix A Table S1) and ALP activity (Fig. 2a) but reduced the DOC content (Appendix A Table S1), which may explain the decrease in the NaHCO3-P contents and increases in the NaOH-IP and HCl-P contents of the compost products under WSB and BC.

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