Soil nitrogen content is a critical parameter in agricultural and environmental studies, as it reflects the fertility of the soil and its potential to support plant growth. Total nitrogen in soil primarily exists in organic forms, while inorganic nitrogen constitutes only 1% to 5% of the total. The dynamics of soil organic matter and nitrogen are influenced by factors such as climate, vegetation, and farming practices, with hydrothermal conditions playing a significant role in their accumulation and decomposition.
To determine total nitrogen in soil, two main methods are commonly used: dry combustion and wet digestion. The Kjeldahl method, a widely accepted wet digestion technique, was first developed by Danish chemist Johan Kjeldahl in 1883. This method involves digesting the soil sample with concentrated sulfuric acid, which converts organic nitrogen into ammonia. The ammonia is then distilled, absorbed in boric acid, and titrated to calculate the total nitrogen content.
For cases where nitrate and nitrite nitrogen must also be considered, the sample is first treated with potassium permanganate to oxidize nitrite to nitrate, followed by reduction using iron powder to convert all nitrogen forms to ammonium. This ensures that all nitrogen fractions are accounted for in the final measurement.
The semi-micro Kjeldahl method is particularly popular due to its accuracy and ease of use in standard laboratories. It requires specific equipment, including a distillation unit, a semi-micro burette, and reagents like 10 mol/L NaOH, boric acid, and mixed indicators. Proper preparation of these reagents is essential for reliable results.
In the procedure, 1.0000 g of air-dried soil (passed through a 0.149 mm sieve) is digested with sulfuric acid and an accelerator mixture containing K₂SO₄, CuSO₄·5H₂O, and selenium. The digestion process continues until the solution turns gray-green, indicating complete breakdown of organic matter. After cooling, the sample is distilled, and the ammonia is absorbed in boric acid. Titration with a standard H₂SO₄ or HCl solution determines the nitrogen content.
For samples containing nitrate and nitrite, an additional step is required before digestion. Potassium permanganate is added to oxidize nitrite, and reduced iron powder is used to reduce nitrate to ammonium. This ensures accurate quantification of all nitrogen forms.
Another important parameter is hydrolyzable nitrogen, which includes both inorganic and some easily decomposable organic nitrogen. This fraction is often measured using the alkaline solution dispersion method. In this approach, the soil sample is treated with a dilute sodium hydroxide solution, allowing volatile nitrogen compounds to diffuse into boric acid. The amount of ammonia is then determined via titration with standard HCl.
The method involves weighing 2.00 g of air-dried soil, placing it in a diffusion dish, and adding 2 mL of 2% boric acid with an indicator. Alkaline glycerol is applied around the outer chamber, and the dish is sealed with frosted glass. After incubation at 40°C for 24 hours, the ammonia is titrated with a 0.01 mol/L HCl solution. A blank test is conducted simultaneously to correct for any errors in reagent or procedural steps.
Proper calibration of all instruments and reagents is crucial for accurate results. The volume of the standard acid used in titration is recorded, and the nitrogen content is calculated using a standard formula. Results from parallel determinations should not differ by more than 5 mg/kg for hydrolyzable nitrogen and 0.005% for total nitrogen when the content is above 1.0 g/kg.
Attention to detail, such as ensuring proper mixing of reagents, maintaining consistent temperatures during digestion, and avoiding contamination, is essential for obtaining reliable data. These procedures provide valuable insights into soil fertility and help guide sustainable agricultural practices.
Upper Limb External Fixation
The wrist joint external fixation fully considers the physiological characteristics of the wrist joint and the structural stability of the biomechanics, so that the metacarpophalangeal joint can be completely bent freely, and the tendons and ligaments of the fingers and wrists can be relatively relaxed. Moreover, the insertion method, length and stress of the bone traction needle are also considered, and the overall force of the bone traction needle is relatively uniform.
The ulna radius external fixator fractures is relatively simple, generally unilateral fixation, or simple combination fixation, generally should not fix the upper and lower radioulnar joints. Radial external fixation stenting has a good effect on the treatment of fracture. Premature removal after surgery is not appropriate, which is easy to cause dislocation and affect fracture healing. Functional exercise can be carried out gradually after four weeks of plaster external fixation, when the wrist joint function should gradually return to normal.
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