Abstract
The purpose of this study is to analyze the effects of different straw returning modes on the structure of humic acid (HA) in soil by fluorescence spectroscopy and infrared (IR) spectroscopy. Four different straw returning modes, including straw returning to topsoil (St), straw returning to subsoil (Ss), straw mixing with topsoil (Smt), and straw mixing with subsoil (Sms), were used in this study; the soil HA was analyzed after 12 months of corn straw returning by a combination of fluorescence spectroscopy and IR spectroscopy. Based on the results, it was established that IR spectroscopy can estimate the complication and oxidation degree of soil HA and also evaluate its aliphaticity and aromaticity. Monodimensional fluorescence spectroscopy could preliminarily determine the changes in the humification of HA through the fluorescence intensities. The intensity ratio of I456/I380 calculated from synchronous-scan fluorescence spectra could be used to evaluate the humification degree of soil HA. The total luminescence spectra of HAs provided more information on the fluorophores in the structure, including the amount and peak position of lignin-like structures and phenol-like or naphthol-like structures. Among the four straw returning modes, Ss is the most beneficial for reducing the oxidation degree and increasing the aromatization and humification degree of subsoil HA. It is believed that fluorescence spectroscopy and IR spectroscopy are relatively simple and sensitive methods for analyzing soil HA.
Highlights
Crop straw is a good source of soil organic fertilizers [1,2,3,4]
Five treatments were set up in the experiment: CK: no straw was applied; straw returning to topsoil (St): corn straw was evenly placed in the topsoil (5–20 cm), and the surface was covered with topsoil; Ss: corn straw was evenly placed in the subsoil (20–40 cm), and the upper soil was covered successively; straw mixing with topsoil (Smt): corn straw was uniformly mixed with topsoil, and the topsoil was covered; straw mixing with subsoil (Sms): corn straw was evenly mixed with subsoil, and the upper soil was covered successively. ree points were randomly selected for each treatment to collect soil samples on April 15, 2017, and the soil samples were air-dried to remove organic residues prior
At the ratio of 2920/1620, St > Sms > Smt > Ss > CK; straw returning resulted in a decrease in the degree of aromatization of the topsoil humic acid (HA), which was associated with the decomposition of the straw applied to the topsoil to produce simple compounds
Summary
Crop straw is a good source of soil organic fertilizers [1,2,3,4]. At present, straw returning is the most convenient and practical straw utilization technology [5, 6]. e effect of straw returning on soil organic matter (SOM) is reflected in the change in organic carbon content and, more importantly, can improve the quality of SOM [7]. Tian et al [9] buried wheat straw in the topsoil, and the organic carbon content in the topsoil increased significantly. Bhattacharyya et al [11] studied that rice straw mixing with topsoil can markedly increase the total carbon content of the soil. Lenka and Lal [12] carried out a straw-covered no-tillage rotation for 15 consecutive years, during which the organic matter content of the 0– 10 cm soil layer increased by about 50%. There are some other studies on straw returning, which were mostly concentrated on the soil organic carbon (SOC) content of the topsoil [13, 14]. More than 50% of SOC content is stored in the 30–100 cm soil layer [15]; the fertility of the subsoil has a certain impact on crop yield [16]
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