## 1 Introduction  
Soil nitrogen is the key parameter supporting plant growth and development. There are many forms of nitrogen in soil, with soil water-soluble nitrogen being one of the more important substances that plants can absorb directly [1]. However, the content of water-soluble nitrogen in soil is low.  
Therefore, it is necessary to accurately determine soil water-soluble nitrogen, so as to provide a scientific basis for rational fertilization [2]. The acid hydrolysis [3], the alkali hydrolysis diffusion [4], and the alkaline hydrolysis distillation [5] methods are the traditional chemical methods for the determination of soil water-soluble nitrogen. Although these methods are highly sensitive, the disadvantages of tedious measurement, long detection time, and expensive reagent limit their development [6]. In recent years, spectral technology has been widely applied to soil nitrogen detection, among which near infrared spectroscopy has certain advantages and application prospects [7; 8]. However, near infrared spectroscopy cannot overcome the effect of moisture on soil nitrogen detection, which limits its practical application in nitrogen detection to a certain extent, especially the detection of water soluble and hydrolytic nitrogen [9; 10].  
Surface-enhanced Raman spectroscopy (SERS) has attracted wide attention in recent years. The principle is that the molecular signals in the chemical compound can be enhanced when the detected materials surface adsorb some nanoscale rough metals (such as gold, silver, and copper) [11]. Beside this, SERS also has the advantages of simple pretreatment, convenient equipment, and fast detection speed, which makes it suitable for the rapid screening of complex compounds [12]. Some researchers have applied SERS to nitrogen molecular analysis. [PERSON] et al. [13] successfully applied SERS based on silver colloids to trace the determination of nitrogen-containing drugs. [PERSON] et al. [14] monitored nitrogen assimilation at the levels of large cells and single cells by SERS combined with the stable isotope detection technique. The results indicated that the nondestructive approach could identify active nitrogen assimilation cells for genomic analysis and provide a theoretical basis for further comprehending the nitrogen metabolism of environmental microorganisms. [PERSON] et al. [15] found that the first and second order Raman spectroscopy (RS) of disordered graphite and polyacetylene could be applied to detect and analyze different nitrogen content using 514.5 and 413.1 nm laser excitation. Furthermore, the adsorption and orientation of nitrogen compounds on the surface of iron in an acidic solution by SERS was studied and discussed in [PERSON]'s research [16].  
In the study of soil water-soluble nitrogen in agriculture, many domestic and foreign scholars have carried out relevant research. However, there are few studies on the application of SERS in the determination of soil water-soluble nitrogen. In this paper, the purpose of this study is to apply the SERS technique to detect soil water-soluble nitrogen quickly and accurately. Moreover, the enhancing effects of silver colloid, gold colloid, and Opto Trace Raman 202 (OTR 202) were compared. As a result, we established the regression equation between SERS characteristic peaks intensity and soil water-soluble nitrogen, which helped to improve the efficiency of soil water-soluble nitrogen detection and provide a scientific guidance for urea fertilization.