This study compares the dynamic effects of straw and biochar on soil acidity and phosphorus (P) availability in the rice growth period to reveal how straw and biochar affect the availability of phosphorus in soil and utilization of P for rice crop.
Materials and methods
In the pot experiment, rice straw, canola stalk, and corresponding biochars were mixed uniformly with the Ultisol. Soil samples were collected at four stages of rice growth to analyze the dynamic changes of soil acidity and P availability. The availability of phosphate in straw/biochar-amended soils were evaluated using a combination of chemical extraction and diffusive gradients in thin films (DGT) technique.
Results
Soil pH, KCl-P, Olsen-P, DGT-P, and Al-P deceased with the rice growth, while Fe-P increased. Biochar increased soil pH and P availability more than straw returning, especially in the mature stage, while the DGT-P only increased in the tillering stage. The DGT-induced fluxes in sediments (DIFS) model revealed that all treatments increased the capacity of soil solid phase supplementing P to pore water in the filling and mature stages. The content of total P in different rice tissues followed the order of grain?>?straw?>?root, and RB350 treatment had the highest P content in rice tissues. In the mature stage, soil pH had positive correlations with KCl-P and Olsen-P, and soil Fe-P had positive correlations with total P of root and straw.
Conclusions
Application of biochar made at 550 ℃ resulted in a larger increase in available P in soil, while biochar made at 350 ℃ had more effect on the chemical forms of P. The canola stalk biochar showed a larger influence on the P availability than rice straw biochar. Biochar treatments had a larger effect on inhibiting soil acidification and improving P availability than straw returning directly.
Identifying the spatial distribution and degree of heavy metal contamination in the soils is required for urban environmental management. Magnetic measurement provides a rapid means of determining spatial distribution and degree of soil pollution and identifying various anthropogenic sources of heavy metals. The purpose of this study was to characterize the magnetic signature of heavy metal contamination and identify the sources of heavy metals in urban soils from steel industrial city.
Materials and methods
A total of 115 urban topsoils from Anshan city, Northeast China, were collected and determined for magnetic properties and heavy metal concentration. Magnetic susceptibility (χlf) and saturation isothermal remanent magnetization (SIRM) were determined as proxy for ferrimagnetic mineral concentration. Magnetic minerals were identified by using Curie temperature, X-ray diffraction (XRD), and scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The Pearson’ correlation and matrix cluster analyses were used to establish the relationship between magnetic parameters and heavy metal concentrations.
Results and discussion
Urban topsoils exhibit characteristic magnetic enhancement. The magnetic measurement in particle size fractions indicates that 50–2 μm fraction has the highest low-field magnetic susceptibility (χlf), while <2 μm has the highest frequency-dependent magnetic susceptibility (χfd) value. The soil χlf and SIRM values are significantly correlated with the contents of metals (Fe, Pb, Zn, Cu, and Cr) and Tomlinson pollution load index (PLI), which indicates that χlf and SIRM could be served as better indicators for the pollution of heavy metals in the urban topsoil. Spatial distribution maps of χlf, SIRM, and PLI indicate that the pollution hotspots tend to associate with the regions within and close to steel industrial zones. XRD and Curie temperature analyses indicate that the main magnetic minerals of urban topsoils are magnetite (Fe3O4), hematite (α-Fe2O3), and metallic iron. Magnetic minerals mostly occur in the pseudo-single-domain/multidomain (PSD/MD) grain size range, which is the dominant contributor to the magnetic enhancement of topsoils. SEM observation reveals that magnetic particles in soils exist in irregular-shaped particles and spherule. Results reveal that heavy metals from industrially derived and traffic emissions coexist with coarse-grained magnetic phases.
Conclusions
It is concluded that the magnetic measurement could be regarded as a proxy tool to detect the level of heavy metal pollution and identify the source of heavy metals in urban soils. Magnetic properties provide a fast and inexpensive method to map the spatial distribution of long-term pollution from steel industrial origin on region scale.
Magnetic susceptibility (χ) was measured on subtropical soils formed on a range of parent materials in Zhejiang Province, China, to demonstrate the influence of parent material lithologies on the amount and vertical distribution of χ in the soils. We found that the χ values of soils vary by up to two orders of magnitude with their parent material lithologies. Soils formed on igneous rocks with high χ values showed high absolute χ and distinctly decreased χ compared with χ values of parent materials. The χ values of soils formed on basalt, andesite and granodiorite were >250×10−8 m3 kg−1. The χ values of soils formed on granite, and other neutral and acid igneous rocks ranged from 50×10−8 to 120×10−8 m3 kg−1. In most of the soils formed on sedimentary rocks, χ enhancement in upper horizons of soils was found compared with parent material χ. This enhancement was believed to be pedogenic formation of superparamagnetic (SP)–stable single domain (SSD) magnetic grains, which are characterized by high values of frequency dependent susceptibility (χfd) and anhysteretic remanent magnetization (ARM). Soils formed on sedimentary rocks with a very low initial χ and iron content showed a low χ value throughout the profile. It was found that there is a highly significant relationship between the χ values of soils and their dithionite–citrate–bicarbonate solution (DCB) soluble iron contents (Fed). The χ values of soils in chronosequences formed on alluvium and two marine deposits increased as soils became older. It is suggested that the χ values can be used as a tool for determining the relative age in chronosequence studies. 相似文献
