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Studying changes in soil humus composition and humic acid (HA) structural characteristics caused by agronomic practices provide insights into the pathways of soil organic carbon (C) stabilisation dynamics. This five-year field study evaluated the effects of straw returning modes on humus composition and HA structure. Treatments included (i) corn straw returned on the soil surface (NTS), (ii) corn straw incorporated into soil within 0-10 cm (MTS), (iii) corn straw incorporated into soil within 0-20 cm (CTS) and (iv) no corn straw applied (CT). Soil HA was characterised by Fourier transform infrared (FTIR) and fluorescence spectroscopies. The results demonstrated that corn straw returning improved humus C fractions in this order NTS > MTS > CTS > CT in 0-20 cm depth. The FTIR and fluorescence results demonstrated that corn straw returning enhanced aliphatic, hydroxyl, methoxyl and carboxyl groups and simplified HA molecular structure, indicating regenerated and newly formed HA. Among all treatments, NTS was more conducive in simplifying HA molecular structure and enhancing aliphatic and hydrophobic C. Hydrophobicity in aliphatic C is the driving force in the stabilisation of soil C, which is important for sustainable agriculture. Therefore, we conclude that NTS is the better practice to turn arable lands into a sink for C. Additional keywords: corn straw retention, fluorescence EEM, FTIR spectra, humic acid structure. Received 25 January 2020, accepted 27 March 2020, published online 21 May 2020, Introduction It is generally accepted that removal of crop residues and conventional tillage have a profound effect on soil organic carbon (SOC) quantity and quality. Warren Raffa et al. (2015) [...]

The freezing winter temperatures in Northeast China hinder the breakdown speed of straw, making it challenging to implement widespread straw return in rice fields, meaning that crop remnants are not efficiently utilised. This study involved a pot experiment conducted on rice plants that were treated with varying quantities of biochar: CK (no biochar); BC1 (5 t ha−1); BC2 (10 t ha−1); and BC3 (20 t ha−1). An investigation was conducted to examine the impact of biochar on the soil organic carbon (SOC), humus (HS) composition, humic acid (HA) structure, and rice yield of paddy fields. The findings demonstrated that the use of biochar led to a substantial rise in SOC and HA-C concentrations in the soil layer between 20 and 40 cm. Additionally, biochar's application enhanced soil humification. Notably, the treatment with BC3 (20 t ha−1) had the most pronounced impact. The O/C ratio in the HA and the relative strength of the peaks at 1620 cm−1 on the infrared spectra showed a more pronounced response to the BC3 treatment compared to the other biochar treatments. However, the application of the BC1 treatment at a rate of 5 t ha−1 and the BC2 treatment at a rate of 10 t ha−1 had a minimal impact on the fluorescence intensity of humic acid (HA). The application of the BC3 treatment increased the aromatic nature of the humic acid (HA) in paddy soil, leading to the formation of an intricate and enduring HA structure. Furthermore, the use of the BC3 treatment resulted in a notable enhancement in the quantity of spikes, spike weight, and number of grains per spike. Additionally, it positively impacted the accumulation of dry matter in the spike, leading to a substantial 13.7% increase in the rice yield. Applying biochar at a rate of 20 t ha−1 is a sensible and effective approach to enhance the soil organic carbon (SOC) content, enhance the stability of the humic acid (HA)'s structure, and raise the rice yield in the rice-growing area of Northeast China. This study's findings will establish a theoretical foundation for utilising soil biochar in the rice fields located in Northeast China. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is a necessary and the most limiting nutrient in crop production. Addition of organic amendments to the soil can improve the use of N fertilizer in crops. However, the response of N to corn straw and biochar application in soils with different fertility levels is yet not well studied. This study investigates N utilization and uptake by corn plant at seedling stage using 15N-label tracer method. Two soils with different fertility levels were selected: (i) high fertility soil (HF), and (ii) low fertility soil (LF). The HF and LF soils were amended with biochar (HFB and LFB) and corn straw (HFC and LFC) in an outdoor pot experiment. The results showed that N absorption in corn plant increased by 23.47 − 30.58% (HFB), and by 10.34 − 13.59% (HFC) compared with unamended HF soil, and increased by 26.09 − 41.53% under LFB and by 13.59 − 20.23% in LFC compared with unamended LF soil. The 15N utilization rate increased in the order of: HFB > HFC > HF and LFB > LFC > LF, illustrating that biochar and corn straw application increases the efficiency by which corn plant utilizes N. Biochar and corn straw application significantly reduce 15N losses in the soil and improve 15N utilization in corn, particular in low fertile soil. [ABSTRACT FROM AUTHOR]

A three-year field experiment was conducted to analyze the effects of straw enrichment and deep incorporation on the humus composition and the structure of humic acid (HA) in black soil. The differences in the HA structure between different straw returning methods were detected by three-dimensional fluorescence spectroscopy and 13C NMR technology. The purpose of this paper is to provide a theoretical basis and data support for improving the straw returning system. Four different treatments, including no straw applied (CK), straw mulching (SCR), straw deep ploughing (MBR), and straw enrichment and deep incorporation (SEDI: harvested the corn straw from four rows together with a finger-plate rake and then crushed and buried them in one row in the 20∼40 cm deep level in the subsoil with a wind-driven input cylindrical plough), were used in this study. Our results showed that compared to CK treatment, SEDI significantly increased the contents of organic carbon (SOC), soil humic acid carbon (HAC), fulvic acid carbon (FAC), and humin C content (HM-C) in the subsurface soil layer by 27.47%, 34.33%, 19.66%, and 31.49%, respectively. Among all the straw returning treatments, SEDI treatment had the most significant effect in increasing the contents of HEC, HAC, and FAC. Straw returning not only reduced the degree of condensation and oxidation of the HA structure but also increased the proportion of alkyl C and enhanced the hydrophobicity of the HA structure in subsurface soil. Moreover, SEDI treatment significantly increased the proportion of aliphatic C/aromatic C of the HA structure in subsurface soil and improved the aliphatic property of HA, which had a significant effect on the HA structure compared to other treatments. [ABSTRACT FROM AUTHOR]

In the view of studying humus formation in corn straw under the action of microorganism, three fungi (Trichoderma reesei, Trichoderma harzianum, Phanerochaete chrysosporium) were selected to compare the humification effects and structural changes of humic acid-like (HAL). The structural characteristics of HAL formed by fungi from corn straw were assessed by elemental, thermogravimetric analyzer, Fourier transform infrared (FTIR) and 13C CPMAS NMR spectroscopies. The results showed that Phanerochaete chrysosporium had a strong ability to increase aliphatic C (increasing to 60.02%) within 30 days, and utilised aromatic C in HAL during the humification process. The Trichoderma harzianum selectively preserved relatively large quantities of aromatic C in HAL and reduced aliphatic C to 56.49%. Trichoderma reesei enhanced aromaticity and I2920/I1620 ratio of HAL to 38.48% and 0.80, respectively, and intensified the humification effect (PQ=63.75%) within 30 days. Moreover, the elemental composition of HAL in Trichoderma reesei was closest to that of soil humic acid. We concluded that treating corn straw with Trichoderma reesei could reduce aliphatic C and enhance aromatisation, thermal stability, and accumulation of alkyl C, phenolic and aromatic C, which can promote the humification process of corn straw and contribute to the accumulation and storage of soil organic matter. [ABSTRACT FROM AUTHOR]

Biochar application to soil may impact soil nitrogen (N) dynamics, but the effects on N uptake and utilization by crop remain largely unknown, especially the effects of the rate of biochar application. To investigate the effects of biochar on soil 15N retention rate and 15N utilization efficiency (15NUE) by maize, a six‐month 15N isotope tracer technique combined with in situ pot experiment was conducted in Mollisol. The experiment included four treatments: no biochar applied (CK) and biochar applied at the rates of 12 t ha−1 (P12), 24 t ha−1 (P24) and 48 t ha−1 soil (P48). Compared with CK, biochar application reduced soil bulk density and 15N loss rate, and significantly improved total N and 15N retention amount in the 0–30 cm soil depth. The P24 treatment had the largest increase in 15N retention rate throughout the 0–40 cm depth. After biochar application, the 15N uptake and 15NUE were significantly increased in the grain and leaf, which promoted grain yields. Contrary to this, the P48 treatment appeared to lower 15N uptake and 15NUE compared with P12 and P24. In conclusion, biochar application improves the potential of the soil to retain N and the improvement in 15N uptake and utilization are more pronounced in maize leaves and grain. Moreover, biochar application promotes 15N utilization in maize plant and improves maize yield. However, when biochar application rate is high (i.e. P48 treatment), the 15N retention by the soil and 15N utilization by the maize are reduced markedly compared with P12 and P24. [ABSTRACT FROM AUTHOR]

Straw returning to the field has become a common way to utilize crop residues. This method does not only solve the problem of rational utilization of straw but also improves soil organic carbon (SOC) quality and quantity. So far, various methods of straw return exist, but the comparison of these methods has not been sufficiently studied, especially in the black soil of Northeast China. The purpose of this study is to evaluate the effects of different corn straw amendments applied under equal carbon (C) mass on soil humus composition and structural characteristics of humic acid (HA). The pot experiment was carried out by simulating corn straw return, which included four treatments: no corn straw applied (CK), corn straw (CS), humidified corn straw (HCS) and corn straw biochar (Bc). Soil HA was characterized by Fourier transform infrared spectroscopy (FTIR), elemental analyzer and thermogravimetric analysis (TG). The results showed that CS, HSC, and Bc treatments increased SOC content by 7.51%, 31.03%, and 33.65%, respectively, compared to CK. Bc treatment showed a greater degree of humification (79.16%) and increased the C content of HA by 10.87% and 7.37% compared with CS and HCS, respectively. The FTIR spectra and elemental results revealed that CS and HCS treatments increased the aliphaticity of the HA structure, while Bc increased the aromaticity of C in HA structure, rendering HA structure complicated. It can, therefore, be inferred that the application of biochar will improve humus compositions, SOC content and enhances the stability of HA structure. [ABSTRACT FROM AUTHOR]