Microbial inoculants reshape structural distribution of complex components of humic acid based on spectroscopy during straw waste composting

Highlights

• Actinomycetes increased the degradation rate of straw compost by nearly 15%.

• Actinomycetes promoted the transformation from fulvic acid to humic acid.

• Actinomycetes reshape the compositional features of humic acid.

• Structural distribution of humic acid is a key factor affecting humification.

Abstract

This study aimed to explore the mechanism of actinomycetes inoculation to promote humification based on spectroscopy during straw waste composting. Results showed that inoculating actinomycetes could significantly increase the humification index and humification ratio, which were 2.53% and 21.79% respectively (P < 0.05). A spectroscopic analysis suggested that actinomycetes promoted the relative content of complex components of humic acid and reshaped the structural distribution of two sub fluorescence peaks in it. Furthermore, variance partitioning analysis demonstrated that compared with the intensity, the high-quality uniform distribution of fluorescence peaks had a greater contribution to the improvement of humification. In addition, structural equation model further verified that actinomycetes inoculation promoted the transformation of fulvic acid to humic acid, and then promoted the formation of humic acid. Therefore, actinomycetes inoculation can promote the humification of straw compost by reshaping the complex components of humic acid.

Introduction

The annual output of main crop straw has stabilized at about 8.00 × 10⁸ tons * year⁻¹ since 2013 in China. Therefore, the resource treatment of straw waste is a research hotpot (Xu et al., 2017, Yao et al., 2021). The common environmental-friendly straw treatment methods mainly include developing biomass energy and applying it to feed or commercial organic fertilizer (Cai et al., 2021, Qin et al., 2021). Among them, composting is an universal way because of its lower cost and environmental harmless characteristics (Malik et al., 2015, Mengqi et al., 2021). Compost products contain a large number of humic-like substance (HLS), which can effectively improve soil fertility and promote plant growth (Wei et al., 2022). Therefore, increasing the content of humic acid in compost and promoting its humification degree is vital for improving the quality of compost products (Bai et al., 2020, Qu et al., 2022).

The lignocellulosic polymers in straw degraded slowly during composting due to its complex structural characteristics (Bernal et al., 2009, Liao et al., 2018). The small-molecule sugars produced by degradation of lignocellulosic polymers are important precursors for HLS generation during straw composting (Wu et al., 2021). Therefore, the degradation efficiency of lignocellulose limits the generation of HLS during composting (Wang et al., 2019). Microbes are usually the main driving factors in the biological pathway of the degradation of lignocellulose polymer and the formation of HLS during straw composting (Guo et al., 2013). Previous studies have shown that exogenous microbial inoculation could secrete extracellular enzymes to promote the degradation of lignocellulose polymers, thereby accelerating the formation of HLS (Cheng, 2018). At present, the microbial inoculants which applied to straw composting generally include bacteria, fungi and actinomycetes (Tian et al., 2017, Wu et al., 2019). Compared with bacterial and fungal agents, actinomycetes have the characteristics of relatively high cellulase activity and temperature resistance (Walia et al., 2017). Therefore, actinomycetes are widely used in straw-composting-improvers to promote the degradation of lignocellulosic polymers (Greff et al., 2022, Harindintwali et al., 2020, Li et al., 2018). Previous studies have shown that actinomycetes are used to inoculate straw compost to increase the activity of lignocellulose polymers degrading enzymes and promote the degradation of cellulose polymers (Wang et al., 2016, Wei et al., 2019). However, there are few reports explored how actinomycetes affect the formation of HLS and further affect the humification of straw waste compost. In particular, the changes in the structure and composition of HLS during straw composting under the actinomycetes inoculation also need to be further clarified. Therefore, this study aimed to reveal the specific promotion mechanism of improving humification by actinomycetes inoculation.

The structure and composition characteristics of HLS are important indicators to determine the quality of HLS and the humification degree of compost products (Fuentes et al., 2020). Therefore, the study of the structure of HLS is helpful to reveal the mechanism of the effect of actinomycetes inoculated on the humification of straw composting. Excitation emission matrix combined with parallel factor analysis (EEM-PARAFAC) model are widely applied to analyze the structure of HLS (Ding et al., 2022, Guo et al., 2018). The PARAFAC can subdivide the complex HLS into multiple small components, so as to reveal the subtle changes of HLS structure during composting (Yu et al., 2020). Therefore, this study employed the technology of EEM-PARAFAC to reveal how actinomycetes inoculation alter the structure of HLS. This helps us understand the specific promotion mechanism of improving humification by actinomycetes inoculation.

In this study, actinomycetes was inoculated to enhance straw composting of four common crop sources (soybean straw, rice straw, wheat straw and corn straw). The aims are to explore (1) the effect of actinomycetes inoculation on the HLS content and the humification degree during straw composting; (2) the changes of structure and composition characteristics of sub fluorescence peaks in HLS by inoculating actinomycetes; (3) the relationship between environmental factors and different fluorescence components of HLS; (4) the key factors influence the humification degree by inoculating actinomycetes during straw composting.