Abstract
Soil aeration is a crucial factor that regulates crop residue decomposition, and the chemical composition of decomposing crop residues may change the forms and availability of soil nutrients, such as N and P. However, to date, differences in the chemical composition of crop straw residues after incorporation into soil and during its decomposition under anaerobic vs. aerobic conditions have not been well documented. The objective of the present study was to assess changes in the C-containing functional groups of wheat straw residue during its decomposition in anaerobic and aerobic environments. A 12-month incubation experiment was carried out to investigate the temporal variations of mass, carbon, and nitrogen loss, as well as changes in the chemical composition of wheat (Triticum aestivum L) straw residues under anaerobic and aerobic conditions by measuring C-containing functional groups using solid state nuclear magnetic resonance (NMR) spectroscopy. The residual mass, carbon content, and nitrogen content of the straw residue sharply declined during the initial 3 months, and then slowly decreased during the last incubation period from 3 to 12 months. The decomposition rate constant (k) for mass loss under aerobic conditions (0.022 d-1) was higher than that under anaerobic conditions (0.014 d-1). The residual mass percentage of cellulose and hemicellulose in the wheat straw gradually declined, whereas that of lignin gradually increased during the entire 12-month incubation period. The NMR spectra of C-containing functional groups in the decomposing straw under both aerobic and anaerobic conditions were similar at the beginning of the incubation as well as at 1 month, 6 months, and 12 months. The main alterations in C-containing functional groups during the decomposition of wheat straw were a decrease in the relative abundances of O-alkyl C and an increase in the relative abundances of alkyl C, aromatic C and COO/N-C = O functional groups. The NMR signals of alkyl C and aromatic C in decomposing wheat straw residues under anaerobic condition were higher than those under aerobic conditions. The higher mass percentages of lignin and the higher signals of aromatic C and alkyl C functional groups in decomposing wheat residues under anaerobic conditions than under aerobic conditions were due to the slower decomposition rates of aryl C and alkyl C in wheat straw residues under anaerobic conditions.
Highlights
3.8 billion tons of crop residues are produced annually in the world [1]
The decomposition rate constants (k) of wheat straw in the anaerobic and aerobic conditions were 0.014 d-1 and 0.022 d-1, respectively, and the half lives (t1/2) of wheat straw decomposition in the anaerobic and aerobic condition were 122.2 d and 72.8 d, respectively (Table 1). These results indicated that wheat straw decomposition in the anaerobic condition was slower than in the aerobic condition
Our results indicate that the decomposition rates declined as the ratios of lignin-to-carbohydrate increased because of the loss of cellulose and hemicellulose
Summary
3.8 billion tons of crop residues are produced annually in the world [1]. Crop residues can be returned to the soil for nutrient recycling, and they are an important source of organic matter to improve soil physical, chemical and biological properties [2, 3]. In addition to organic carbon, crop residues contain around 3.0 to 8.2 kg of nitrogen, 0.2 to 0.6 kg of phosphorous, and 7.2 to 23.3 kg of potassium per ton dry matter [4]. Significant differences in the oxidation-reduction potential and soluble oxygen concentration between anaerobic and aerobic conditions lead to differences in the activity of soil microbial communities [18,19,20]. The accumulation of partially degraded lignin residues in soil organic matter (SOM) of an anaerobic soil has been shown to be greater than that of a comparable aerobic soil [22]. To date, many details about plant litter decomposition in anaerobic and aerobic environments remain unclear
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