Biochar has been considered as a stable-carbon source for improving soil quality and long-term sequestration of carbon. However, in view of ecological environmental feedback and the tightly coupled system of carbon-nitrogen cycling, further attention has shifted to the effect of biochar on soil net nitrogen mineralization (SNNM). Recently, ecological evaluations of biochar were mostly based on laboratory incubation or pot experiments, ignoring external and uncontrollable natural factors. Therefore, the essential characteristics of local environments were not accurately described.
Materials and methods
In this paper, a nonlinear stochastic model of SNNM based on least squares support vector machine (LS-SVM) was set up to study the effect of biochar on nitrogen cycling in a field experiment. In order to explore this effect in natural conditions, partial derivative (PaD) sensitivity analysis of LS-SVM was firstly proposed, evaluated by the data from a known equation, and then applied to open the “black-box” stochastic model of SNNM.
Results and discussion
Comparing with the sensitivity analysis of artificial neural networks (ANNs), the RD values of LS-SVM PaD1 algorithm were almost the same as those of ANNs PaD1 algorithm. However, the RSD values of LS-SVM PaD2 algorithm were closer to the given equation. In the SNNM model, RD values of LS-SVM PaD1 algorithm of initial nitrogen, time, and precipitation were 21, 15, and 14 %, and the biochar RD value was only 0.51 %, implying that biochar did not influence SNNM directly. However, the cumulative RSD of the PaD2 algorithm of biochar with the other factors was 15.05 %, the maximum of the interactions, implying that it could greatly enhance the tendency for SNNM by interacting with other factors.
Conclusions
PaD sensitivity analysis of LS-SVM was a stable and reliable data mining method. In the SNNM model, initial nitrogen, time, and precipitation were the main controlling factors of the SNNM model. Biochar did not directly influence SNNM; however, it could greatly enhance the tendency for SNNM by interactions with other factors by decreasing the inhibitory effect of initial nitrogen on SNNM and modifying soil condition to change the effect of other factors on SNNM.
Appropriate land management is important for improving the soil quality and productivity of the saline-sodic farmland. A recent study has revealed that flue gas desulfurization (FGD) gypsum and lignite humic acid application enhanced the salt leaching and crop production. The purpose of this study was to investigate the effects of applied FGD gypsum and lignite humic acid (powder) on the soil organic matter (SOM) content and physical properties.
Materials and methods
This study was based on a field experiment of five consecutive rapeseed-maize rotations in a saline-sodic farmland soil (Aquic Halaquepts) at coastal area of North Jiangsu Province, China. The soil is sandy clay loam texture with pH of 8.43 and clay content of 185 g kg?1. Six treatments included three FGD gypsum rates (0, 1.6, and 3.2 Mg ha?1) and two lignite humic acid rates (0 and 1.5 Mg ha?1). The amendments were incorporated into 0–20 cm soil depth manually every year. Soil samples were collected from each treatment and analyzed for soil organic matter, water-stable aggregates (wet sieving method), bulk density (clod method), water retention capacity (pressure plate apparatus), total porosity (calculated from bulk density and particle density), and microporosity (calculated from water content at 0.01 MPa).
Results and discussion
After 5 years, the SOM and soil physical properties were significantly (P?<?0.05) affected by the application of FGD gypsum and lignite humic acid, especially at the 0–20 cm soil depth. The highest amount of SOM with best soil physical condition was observed in the field which was treated with FGD gypsum at 3.2 Mg ha?1 with lignite humic acid, and the SOM, total porosity (TP), microporosity (MP), mean weight diameter (MWD), water-stable macroaggregate (WSMA), and available water content (AWC) were increased by 22.8, 6.34, 23.2, 48.1, 55.5, and 15.8 %, respectively, while the bulk density (BD) was decreased by 5.9 % compared to no amendments applied. The generalized linear regression analysis showed that the SOM explained 42.9, 55.0, 48.5, and 54.2 % of the variability for BD, MWD, WSMA, and MP, respectively.
Conclusions
This study illustrates the benefits of applying FGD gypsum and lignite humic acid for increasing the soil organic matter content and improving the soil physical properties and suggests a great potential for ameliorating saline-sodic farmland soil (Aquic Halaquepts) by using combined amendment of FGD gypsum with lignite humic acid.
The degeneration of fluvo-aquic soils due to long-term excessive fertilization is increasing in the Huang-Huai-Hai Plain, China. Products from crop straw and livestock breeding wastewater, biochar, and biogas slurry provide safe and efficient biomass resources for soil quality improvement. We assumed that biochar and biogas slurry could improve soil structure and soil water retention capacity for their special characteristics. The present study aimed to compare the effects of biochar and hoggery biogas slurry treatments on improvements to soil physical properties and water-holding capacity, and their different driving mechanisms.
Materials and methods
This study was based on a field experiment of four consecutive winter wheat–summer maize rotations on the Huang-Huai-Hai Plain, China. Using the principle of equal nitrogen inputs, three treatments were conducted: conventional farming fertilizers, biochar, and hoggery biogas slurry. The differences in indicators such as soil bulk density, total porosity, aggregate structure, saturated hydraulic conductivity, and hydraulic property parameters between different treatments were compared and analyzed. The driving factors generating these differences were also discussed.
Results and discussion
Compared to conventional fertilization, soil bulk density decreased under biochar and hoggery biogas slurry treatments, whereas soil total porosity increased after hoggery biogas slurry treatment. In the 0–20-cm soil layer, biochar treatment increased the content of >2-mm macrosoil aggregates and hoggery biogas slurry treatment increased the content of 0.25–0.5 or 1–2-mm soil aggregates. The soil saturated hydraulic conductivity in the 0–20-cm soil layer did not change significantly with biochar application, but increased with hoggery biogas slurry treatment. The application of biochar and hoggery biogas slurry improved the water-holding capacity, increasing the field capacity by 15.34 and 13.83 %, and the available water content by 16.20 and 25.87 %, respectively, in the 0–20-cm soil layer.
Conclusions
Both biochar and hoggery biogas slurry treatments significantly improved soil structure and water-holding capacity. Biogas slurry treatment significantly increased soil saturated hydraulic conductivity, soil aggregate content, while biochar treatment significantly decreased bulk density and increased total porosity of the soil.
