## 1 Introduction  
Soil nitrogen is the key parameter supporting plant growth and development [1]. Thus, it is of great importance to obtain soil nitrogen information quickly and accurately for precision fertilization and agricultural production [2]. However, many conventional soil analytical techniques, such as Dumas combustion, are often complex because of the multi-component interactions [3]. The use of near-infrared (NIR) spectroscopy to estimate soil nitrogen content shows a greater advantage and prospects for wider application [4]. In recent years, many scholars have used NIR spectroscopy to detect soil nitrogen and have improved the detection accuracy with respect to soil water content removal, soil spectral data processing, characteristic bands selection, and algorithm optimization. Firstly, NIR spectroscopy could also be used as a rapid, inexpensive, and non-destructive technique to predict some physical, chemical, and biochemical properties of soil [5]. Soil nitrogen was detected with the multiple linear regression method at spectral bands of 1702, 1870, and 2052 nm using NIR spectroscopy [6]. [PERSON] et al. detected N, P, K, organic matter (OM), and pH content in a loamy mixed soil. The results showed that the correlation coefficients between the measured and the predicted values of N, OM, and pH were 0.93, 0.93, and 0.91 respectively [7]. Secondly, the effects of soil water content on soil nitrogen detection were researched. The prediction performance of carbon and nitrogen content affected by sample grinding was studied, and the results showed that the prediction accuracy was the highest with oven-dried, 0.2 mm ground soil samples [8]. Moreover, the wavelet decomposition and continuous removal methods were used to reduce the interference of soil moisture on soil total nitrogen detection by NIR spectroscopy in [PERSON]'s research, where both of those two methods performed well [9]. Third, the characteristic bands of soil nitrogen using NIR spectroscopy were also studied. The selected bands at 556, 1642, and 2491 nm were found to be the optimum prediction model of soil nitrogen content of black soil in Northeast China [10]. However, [PERSON] et al. proposed that the 1450, 1850, 2250, 2330, and 2430 nm bands could be selected as the characteristic bands to estimate organic nitrogen content in soil based on visible NIR spectroscopy [11]. Besides, the 1902, 2364, 1826, and 2098 nm bands were selected as the characteristic bands to predict soil nitrogen using NIR spectroscopy in [PERSON]'s research [12]. The determination coefficient of soil total nitrogen prediction reached 0.94 using the characteristic bands of 1100 and 2300 nm with the partial least squares regression(PLSR) model [13]. Moreover, temperature also affects soil nitrogen detection by NIR spectroscopy [14]. Particularly, drying temperature has an influence on water removal and the activity of urease [15]. The effects of soil water content on soil nitrogen detection by NIR spectroscopy have been studied a lot, while the effects of drying temperature on the detection of nitrogen by NIR spectroscopy have seldom been studied. In the current studies, drying is generally used to remove soil moisture but the optimum drying temperature was rarely considered. Coarse samples were dried at 20 \({}^{\circ}\)C for 48 h for the prediction of C and N content using visible near-infrared spectroscopy, and the comparative method was used to detect their potential mineralization in heterogeneous soil samples [16]. Flat-dry samples were dried at 35 \({}^{\circ}\)C for 12 h to explore the effects of soil sample pretreatments and standardized rewetting [17]. Soil was subjected to oven drying at 60 \({}^{\circ}\)C for 24 h, after which it was ground and sieved with a 0.002 m sieve [18]. Soil samples were dried at 80 \({}^{\circ}\)C for 8 h to detect soil nitrogen with different pretreatments using an NIR sensor in [PERSON]'s research [19; 20].