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• Straw-soil mixture (SSM) increased decomposition rates more than straw only (STO) • Straw placement depths and sites had little effect on decomposition rates • Relative abundances of aromatics were higher in SSM than in STO • Relative abundances of aromatics in SSM increased more at a colder site or at a deeper depth Crop straw is often retained on the ground as mulch or incorporated into soil through tillage practices. These straw management practices may affect straw decomposition rates and subsequently decomposition products due to mixing straw with soil, straw placement depths and experimental sites. Using litterbags containing maize (Zea mays. L) straw only (STO) or straw-soil mixture (SSM), this study aimed to determine the influence of mixing straw with soil and straw placement depths on decomposition rates and decomposition products, and to understand the relation of chemical composition of straw residues with straw decomposition rates. The litterbags were placed at three soil depths (0, 15 and 30 cm) in the fields at two cold sites (Hailun and Harbin), Northeast China, which had similar precipitations but different temperatures. During the 17-month experiment, the decomposition rate constants were higher by 60 %–160 % at a comparable depth in SSM than in STO. The mixing straw with soil was a more important factor in controlling straw decomposition rates than placement depths and experimental sites at the studied region. The relative abundances of O–alkyl C groups in straw residues decreased during the experimental period, even in the winter, while those of aromatic groups increased profoundly. The relative abundances of O–alkyl C groups in straw residues were lower by 0.4 %–6.9 % and those of aromatic groups were higher by 0 %–4.6 % in SSM than in STO. These findings suggest that the conventional litterbag method underestimated straw decomposition rates. Incorporating straw into soil could enhance maize straw decomposition and the formation of more stable soil organic matter (SOM) in the cold high-latitude studied region. [ABSTRACT FROM AUTHOR]

Soil pH is an essential indicator for assessing soil quality and soil health. In this study, based on the Chinese farmland soil survey dataset and meteorological dataset, the spatial distribution characteristics of soil pH in coastal eastern China were analyzed using kriging interpolation. The relationships between hydrothermal conditions and soil pH were explored using regression analysis with mean annual precipitation (MAP), mean annual temperature (MAT), the ratio of precipitation to temperature (P/T), and the product of precipitation and temperature (P*T) as the main explanatory variables. Based on this, a model that can rapidly estimate soil pH was established. The results showed that: (a) The spatial heterogeneity of soil pH in coastal eastern China was obvious, with the values gradually decreasing from north to south, ranging from 4.5 to 8.5; (b) soil pH was significantly correlated with all explanatory variables at the 0.01 level. In general, MAP was the main factor affecting soil pH ( r = -0.7244), followed by P/T ( r = -0.6007). In the regions with MAP < 800 mm, soil pH was negatively correlated with MAP ( r = -0.4631) and P/T ( r = -0.7041), respectively, and positively correlated with MAT ( r = 0.6093) and P*T ( r = 0.3951), respectively. In the regions with MAP > 800 mm, soil pH was negatively correlated with MAP ( r = -0.6651), MAT ( r = -0.5047), P/T ( r = -0.3268), and P*T ( r = -0.5808), respectively. (c) The estimation model of soil pH was: y = 23.4572 - 6.3930 × lgMAP + 0.1312 × MAT. It has been verified to have a high accuracy ( r = 0.7743, p < 0.01). The mean error, the mean absolute error, and the root mean square error were 0.0450, 0.5300, and 0.7193, respectively. It provides a new path for rapid estimation of the regional soil pH, which is important for improving the management of agricultural production and slowing down soil degradation.

An abandoned petroleum well which had been exploited for about twenty years in Songyuan city of Jilin Province, China, was selected to study the compositions and characteristics of soil humus using revised humus composition method and Simon-Kumada method. Soil samples were collected at 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5 and 10.5 m apart from the well head. Results show that the petroleum contents increase from 0.08 g/kg (10.5 m to the well head) to 153.3 g/kg (0.5 m to the well head). With the increase in petroleum content, the contents of soil organic carbon and water soluble organic carbon increase; for total soil humus, the contents of extractable humus (HE) and humic acid (HA) decrease whereas that of humin (HM) increase; the percentage of HA/HE (PQ 72.0%-8.05%) decrease and HM/HE ratio (31.4-76.7) increase; for different combined humus, the contents of loosely combined humus (HI) and stably combined humus (HII) have a decrease tendency while that of tightly combined humus (HIII) increase; the HI/HII ratio (0.19-0.39) shows an increase tendency, whereas HI/HIII ratio (0.032-0.003) and HII/HIII ratio (0.096-0.009) decrease; the PQs of HI (3.21%-1.42%) and HIII (58.1%-35.5%) also decrease, and the range of PQ change is less in HI than in HII; the color coefficient (deltalogk) of water soluble organic matter (WSOM) decreases, whereas no obvious change for HA. The above results indicate that petroleum hydrocarbon promotes the formation of HM but not HA. The decrease in HA is mainly due to the restraining effect of petroleum hydrocarbon on the formation of stably combined HA. Petroleum hydrocarbon leads molecular structure of WSOM more complex but no effect on molecular structure of HA.