A sedimentation tank which can remove fine sediment with low cost and high efficiency is of great significance for the wide application of drip irrigation techniques with the Yellow River water. In this study, the settling process of an inclined-tube gravity sedimentation tank which has high removal efficiency for fine particles in practice was thoroughly investigated. The sediment concentration distribution in the tank was measured by an optical back-scattering turbidimeter. The sediment thickness at the tank bottom was also measured. In addition, the size grading of sediment deposited at different positions on the tank bottom and at different heights in the inclined tubes was also measured by a laser particle size analyzer. It was found that the removal efficiency of fine sediment was 64.7–69.7% in the inclined-tube gravity sedimentation tank, which was higher than that of the sedimentation tank without inclined tubes (with a sediment removal rate of 20.7–32%). The sediment was mainly deposited in the flow adjustment area and settlement area with inclined tubes. A suitable height for the inclined tubes was 70–90 cm. In addition, the water inlet, baffle, and overflow weir in the tank negatively affected the fine sediment settling in two experiment cases. The experimental results enhance our understanding of the sedimentation characteristics in the tank, and indicate the direction for the subsequent structural optimization of the tank.

Background: Di-nitrogen oxide (N₂O) emissions from soil may lead to nonpoint-source pollution in farmland. Improving the C and N content in the soil is an excellent strategy to reduce N₂O emission and mitigate soil N loss. However, this method lacks a unified mathematical index or standard to evaluate its effect. Methods: To quantify the impact of soil improvement (C and N) on N₂O emissions, we conducted a 2-year field experiment using biochar as carbon source and fertilizer as nitrogen source, setting three treatments (fertilization (300 kg N ha⁻¹), fertilization + biochar (30 t ha⁻¹), control). Results: Results indicate that after biochar application, the average soil water content above 20 cm increased by ∼26% and 26.92% in 2019, and ∼10% and 12.49% in 2020. The average soil temperature above 20 cm also increased by ∼2% and 3.41% in 2019. Fertigation significantly promotes the soil N2O emissions, and biochar application indeed inhibited the cumulation by approximately 52.4% in 2019 and 33.9% in 2020, respectively. N₂O emissions strongly depend on the deep soil moisture and temperature (20–80 cm), in addition to the surface soil moisture and temperature (0–20 cm). Therefore, we established an exponential model between the soil moisture and N₂O emissions based on theoretical analysis. We find that the N₂O emissions exponentially increase with increasing soil moisture regardless of fertilization or biochar application. Furthermore, the coefficient a < 0 means that N₂O emissions initially increase and then decrease. The a_RU < a_CK indicates that fertilization does promote the rate of N₂O emissions, and the a_BRU > a_RU indicates that biochar application mitigates this rate induced by fertilization. This conclusion can be verified by the sensitivity coefficient (SC_B of 1.02 and 14.74; SC_U of 19.18 and 20.83). Thus, we believe the model can quantify the impact of soil C and N changes on N₂O emissions. We can conclude that biochar does significantly reduce N₂O emissions from farmland.

The removal of nutrients by overland flow remains a major source of non-point pollution in agricultural land. In this study, a mathematical model of ammonium nitrogen transport from soil solution to overland flow was established. The model treated the mass transfer coefficient (k_m) as a time-dependent parameter, which was not a constant value as in previous studies, and it was evaluated with a four-slope gradient and three rainfall intensities. The kinematic-wave equation for overland flow was solved by an approximately semi-analytical solution based on Philip’s infiltration model, while the diffusion-based mass conversation equation for overland nutrient transport was solved numerically. The results showed that the simulated runoff processes and ammonium nitrogen concentration transport to the overland flow agreed well with the experimental data. Further correlation analyses were made to determine the relationships between the slope gradient, rainfall intensity and the hydraulic and nutrient transport parameters. It turned out that these parameters could be described as a product of exponential functions of slope gradient and rainfall intensity. Finally, a diffusion-based model with a time-dependent mass transfer coefficient was established to predict the ammonium nitrogen transport processes at the experimental site under different slope gradients and rainfall intensities.

The nutrient loss caused by soil erosion is the main reason for soil degradation and environmental pollution, and polyacrylamide (PAM) as a common soil amendment has a great influence on runoff and erosion processes at the slope. In order to investigate the mechanism of nutrient transport with runoff, a field experiment was conducted and a simple mathematical model was developed in this study. Four PAM application rates (0, 1, 2, and 4 g·m⁻²) and two rainfall intensities (50 and 80 mm·h⁻¹) were applied in the field experiment. The results revealed that runoff rate of 2 g·m⁻² PAM application treatments decreased by 5.3%-10.6% compared with the control groups, but it increased by10.9%-18.7% at 4 g·m⁻² PAM application treatments. Polyacrylamide application reduced ammonium nitrogen concentrations of runoff by 10.0% to 44.3% relative to the control groups. The best performance with correlation coefficient (R²) and Nash–Sutcliffe efficiency (NSE) showed that the ammonium transport with runoff could be well described by the proposed model. Furthermore, the model parameter of the depth of the mixing layer (hm) linearly increased with an increase in flow velocity, but exponentially decreased with an increase in PAM application rate.

Micro-drip irrigation promotes high water and nutrient use efficiency in crops.We aimed to develop a new micro-drip irrigation technology that can reduce the effects of chemical fertilizer input on soil quality while maintaining agricultural productivity. There were four treatments: micro-drip irrigation with 240 (LF), 300 (MF), and 360 kg ha^-1 fertilization (HF) and subsurface drip irrigation (SDI) with 240 kg ha^-1 fertilization (CK). Micro-drip irrigation decreased N concentration around the roots by >24% compared with SDI, and increased the urease, alkaline phosphatase activities, and partial nitrogen productivity by 31.5%, 37.7%, and 22.1%, respectively. Although the HF treatment increased cucumber yield by 20%–45% compared with LF and MF, the urease and alkaline phosphatase activities decreased by 11.4%–15.3% and 11.5%–23.0%, respectively. Furthermore, the HF treatment significantly increased the N concentration around the roots. There was a negative correlation between soil enzymatic activities and N concentration. Overall, micro-drip irrigation mitigated the inhibition of soil enzymatic activities and increased nutrient uptake and cucumber yield. However, the negative effects of high fertilization on soil quality and N loss were unavoidable. The optimal fertilization level for greenhouse cucumber cultivations was 300 kg N hm^-2 under micro-drip irrigation.