The aboveground primary production is a major source of carbon (C) and nitrogen (N) pool and plays an important role in regulating the response of ecosystem and nutrient cycling to natural and anthropogenic disturbances. To explore the mechanisms underlying the effect of spring fire and topography on the aboveground biomass (AGB) and the soil C and N pool, we conducted a field experiment between April 2014 and August 2016 in a semi-arid grassland of northern China to examine the effects of slope and spring fire, and their potential interactions on the AGB and organic C and total N contents in different plant functional groups (C 3 grasses, C 4 grasses, forbs, Artemisia frigida plants, total grasses and total plants). The dynamics of AGB and the contents of organic C and N in the plants were examined in the burned and unburned plots on different slope positions (upper and lower). There were differences in the total AGB of all plants between the two slope positions. The AGB of grasses was higher on the lower slope than on the upper slope in July. On the lower slope, spring fire marginally or significantly increased the AGB of C 3 grasses, forbs, total grasses and total plants in June and August, but decreased the AGB of C 4 grasses and A. frigida plants from June to August. On the upper slope, however, spring fire significantly increased the AGB of forbs in June, the AGB of C 3 grasses and total grasses in July, and the AGB of forbs and C 4 grasses in August. Spring fire exhibited no significant effect on the total AGB of all plants on the lower and upper slopes in 2014 and 2015. In 2016, the total AGB in the burned plots showed a decreasing trend after fire burning compared with the unburned plots. The different plant functional groups had different responses to slope positions in terms of organic C and N contents in the plants. The lower and upper slopes differed with respect to the organic C and N contents of C 3 grasses, C 4 grasses, total grasses, forbs, A. frigida plants and total plants in different growing months. Slope position and spring fire significantly interacted to affect the AGB and organic C and N contents of C 4 grasses and A. frigida plants. We observed the AGB and organic C and N contents in the plants in a temporal synchronized pattern. Spring fire affected the functional AGB on different slope positions, likely by altering the organic C and N contents and, therefore, it is an important process for C and N cycling in the semi-arid natural grasslands. The findings of this study would facilitate the simulation of ecosystem C and N cycling in the semi-arid grasslands in northern China.
Hydroxypropyl methyl cellulose (HPMC), a potential soil improver with the ability to hold water, has practical applicability in alleviating soil and water nutrient loss. By using Cl− as tracer ions, we evaluated the effects of different HPMC viscosities on soil water infiltration characteristics and conservative solute migration characteristics. Hydroxypropyl methyl cellulose with viscosities of 2 × 104, 6 × 104, 10 × 104, and 20 × 104 mPa s−1 was selected. Experimental results revealed that the cumulative infiltration amount, wetting front migration distance, and infiltration rate decreased significantly with increased HPMC viscosity. The addition of HPMC delayed the soil solute transport process, and HPMC with a higher viscosity caused greater soil solute transport delay. Both the convection‐dispersion equation (CDE) and the two‐region model (TRM) accurately simulated solute transport, but the simulation accuracy of the TRM was higher. Results of parametric fitting based on the TRM indicated that the average pore water velocity, hydrodynamic dispersion coefficient, and dispersion degree decrease with increased HPMC viscosity. The average pore water velocity decreased by 13.6–28.2% relative to the control group (CG). Compared with the CG, the water content ratio in movable water of HPMC groups changed irregularly with an increase in viscosity. The mass exchange coefficient increased significantly compared with the CG, with an irregular trend. These data will be useful in the application and promotion of HPMC as a soil improver.
Under natural rainfall, the surface runoff erosion of sloping farmland tends to remove large quantities of soil particles and constitutes nonpoint source pollution. The existing sediment and nutrient loss models focus on estimating the total amount of pollutants in the long term. The Existing mathematical models that describe the nutrient loss process on slopes have some shortcomings, which have not accounted for the effect of infiltration on nutrient concentrations in the exchange layer before runoff starts. Here, an approximate semianalytical model of sediment yield and nutrient loss was based on surface runoff processes. Simulated rainfall experiments were performed to calibrate the model's parameters and verify its reliability. The established model incorporated raindrop splashing, diffusion, and water infiltration effects on nutrient transfer in the exchange layer. Raindrop splashing played a leading role in nutrient translocation from the exchange layer to runoff. The simulated runoff, sediment, and nutrient matched their measured values reasonably well (R2 > 0.8; Nash–Sutcliffe efficiency > 0.347). The model's sediment yield items were more sensitive to runoff erosion than splash erosion. The raindrop‐induced water transfer rate in the nutrient loss simulation dramatically affected the peak nutrient loss rates, whereas the depth of the exchange layer clearly affected the overall range of change in nutrient loss rate and boosted the total nutrient loss. Therefore, measures such as vegetation coverage or deep fertilization should be adopted to weaken raindrops’ kinetic energy and reduce nutrient concentrations in the exchange layer to prevent agricultural nonpoint source pollution in the Loess Plateau.
Presently, rescue equipment and method for uncontrolled blowout in offshore drilling are very rare, of which the consequences were very serious. With analysis of mechanics of materials based on shut-in water hammer pressure and wellhead pressure while rescuing, and fluid mechanics of uncontrolled blowout based CFD, a rescue equipment for uncontrolled blowout in offshore drilling has been invented, and proposing a rescue method. There are several emergency scenarios have been made. Take the “deep horizon oil rig” in the gulf of Mexico for example, the flow field properties around this equipment could be calculated by software to prove the reliability of it. The equipment is a conceptual invention which could be designed in different types according to the size and strength of original wellhead. This equipment could prevent well blowout and collect leaped oil.
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