Abstract
The study of organic matter in ancient paddy soils is helpful for understanding the influence of human activities on soil carbon sequestration and global climate change. However, little information on the spatial distribution and structural characteristics of the humic substances (HS) in ancient paddy soils is available. The spatial distributions of humic acids (HAs) and fulvic acids (FAs) in ancient paddy soils and modern cultivated paddy soils at the Shanlonggang site on the Liyang Plain were investigated, and the associated structures were characterized by using 13C nuclear magnetic resonance (NMR). The 13C NMR spectra revealed the following carbon types in HAs and FAs in both types of paddy soil in order of decreasing abundance: O-alkyl carbon (ranging from 39.7 to 51.8% and from 42.6 to 50.9%, respectively) ≥ alkyl carbon (ranging from 16.8 to 23.5% and from 15.7 to 22.4%, respectively) ≈ carboxyl carbon (ranging from 13.3 to 19.3% and from 16.9 to 22.0%, respectively) > aromatic carbon (ranging from 12.8 to 23.5% and from 10.0 to 17.2%, respectively). Moreover, the degree of aromaticity of HA was higher than that of FA in both soil samples. The humic constituents of the buried ancient paddy soils were less aromatic and oxidized than those of the modern cultivated paddy soils. The organic carbon in the ancient paddy soils was also less aromatic and oxidized than that in the modern cultivated paddy soils, suggesting that the structures of the HS in the ancient paddy soils were relatively simple. The results of this study provide new insights into the effect of secondary paddy soil formation on the spatial distribution, structural characteristics, and stability mechanisms of the HS in ancient paddy soils.
Highlights
Since the discovery of paddy remains and paddy farming implements in Hemudu (You 1978; Greenland 1997), the origin and spread of paddies have become the focus of research in archeology, agronomy, genetics, soil science, and environmental studies
The buried ancient paddy soil profile presented a slight decrease from 7.90 to 6.15 g kg-1. These results indicate that the accumulation rate of organic carbon in the modern cultivated paddy soils was faster than that of the buried ancient paddy soils, and the organic carbon content of the buried ancient paddy soils tended to become stable with increasing years of farming
The changes in the organic carbon content range in the buried ancient paddy soils were less than those in the modern cultivated paddy soils at the Shanlonggang site, and the organic carbon contents in the two soil profiles both decreased gradually with increasing soil depth
Summary
Since the discovery of paddy remains and paddy farming implements in Hemudu (You 1978; Greenland 1997), the origin and spread of paddies have become the focus of research in archeology, agronomy, genetics, soil science, and environmental studies. The distribution of soil organic carbon (SOC) and the chemical structures and functional group compositions of humic substances (HS) in different paddy soils that are typical in China have been explored with the aid of advanced solid-state 13C nuclear magnetic resonance (NMR) techniques (Xu et al 2017; Ci et al 2009). HS are the main component of soil organic matter (SOM) and play an important role in soil structure modification and soil fertility improvement, and functional groups of soil clay fractions play a key role in determining the adsorption affinity and retention of dissolved organic carbon in soils (Mandeep et al 2017). The quality of HS could be reflected by the FA/HA ratio, and the stability of HS in soils is closely related to their distribution in soils and structural characteristics (Egli et al 2008; Dou et al 2008; Mikutta et al 2009)
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