Nitrogen is one of the most abundant elements in nature and is mainly involved in the nitrogen cycle of the biosphere. However, this element will transform into a variety of nitrogen-containing organic matter in plants after entering the plant. Nitrogen is arguably the representative of organic matter. With the development of science and technology and the increasing material needs of people, the cyclical influence of humans on nitrogen is becoming more and more obvious. With the use of fertilizers based on nitrogen, it also has a greater impact on crops. People need a more comprehensive understanding of nitrogen and related products and the forms and contents present in the soil.

        Carbon is the first of six large elements necessary for plants and is a life element. Very important. If the soil does not effectively supply carbon to the crop, the crop will be chronically ill-healed. If the carbon content is increased, the microorganisms in the soil can obtain good breeding conditions. The large-scale reproduction of microorganisms in the soil will further improve the soil's biological fertility and physical fertility, thereby increasing the utilization of mineral nutrients such as N, P, K in the soil and forming a virtuous circle. It can be seen that carbon and phosphorus are essential nutrients in plant growth. Better research and measurement of their content, as well as the effectiveness of measurement methods, is crucial.

        Now we can measure C, N, P elements by laser spectroscopy. This method has the advantages of no sample preparation and fast measurement speed, and has been widely used in various fields. When the laser acts on the surface of the sample, a plasma containing the sample material is induced in a very short time. During the plasma generation, an emission spectrum with information of the sample element is emitted, and by detecting these emission spectra, the elements of the sample are obtained. information. This technology is called Laser Induced Breakdown Spectroscopy (LIBS), commonly known as laser spectral elemental analysis technology, and the detection limit can reach ppm level. With the cooling of plasma, the condensed sample particles can be transported to ICP-MS. It can measure trace, trace elements or isotopes in samples, and the detection limit can reach ppb level. The measured elements cover most of the elements in the periodic table, up to more than 100.

        The Earth and Environment Division of the Los Alamos National Laboratory in the United States measured C, N, and P elements for soil. A soil sampler was taken, and the soil sample of the section was taken, uniformly mixed and then compressed. Experiments and measurements were performed using the LIBS system. The experiment uses a 1064 nm laser with a frequency of 10 Hz and a maximum energy of 100 mJ. The experimental process uses 100% energy output. 100-200 laser pulses are performed according to different measurement elements and experimental areas.

The figure above shows the process of soil sampling

The figure above shows the characteristic strong peak position of carbon and the standard curve of C/(Al+Si).

The figure above shows the ratio of C/Si. The experiment was performed in a 1mm2 area with 200 laser pulses.

 

        The figure above shows the position of the strong peak of the nitrogen element and the calibration curve of the sand clay mixture. The nitrogen content is between 0.3 and 6%. The strongest characteristic peak of nitrogen appears at 746.83 nm. During the experiment, the sample was sampled 5 times with 100 laser pulses at a time. The detection limit can reach 0.3-0.8%. The detection limit can be reduced to 0.1-0.3% if appropriate argon is introduced.

        The figure above shows the characteristic strong peak position of phosphorus and its calibration curve. The content of phosphorus in the sample is about 300-4300ppm. During the experiment, the sample was sampled 5 times with 200 laser pulses each time. The strongest characteristic peak of phosphorus appears at 255.32 nm. The detection limit of the phosphorus element by experiment is about 300-1000 ppm.

The figure above shows the auxiliary effect of helium and argon on the experiment.

 
        The measurement method of soil non-metallic elements was established by the above experiment, and it was fast and effective. The detection limit of non-metallic elements is slightly higher than that of metal elements, but it can still be measured well in experiments. The strongest characteristic peak of carbon appears at 247.856 nm, and 193.091 nm can also be used, where there is no interference from other elements. The strongest characteristic peak of nitrogen appears at 746.83 nm. The strongest characteristic peak of phosphorus appears at 255.32 nm. In addition, the introduction of some gases is more helpful for the experiment, such as the sensitization of argon, the detection limit of nitrogen can be reduced to 0.1-0.3%. In the measurement of carbon and phosphorus, helium gas can be introduced to reduce noise. In order to better measure these non-metallic elements, the sampling amount can be appropriately increased, for example, using a slow linear scanning experiment, usually in which case the platform moving speed is set to 0.2-0.5 mm/s, so that the experimental method has Two advantages, one is that you can get more pulse points on the surface of the sample, the data is added to effectively increase the intensity value, and also avoid some negative effects that may be caused by only one experimental point.

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