RESUMEN
Understanding the effects of different levels of nitrogen fertilizer applications on soil respiration rates and soil biochemical properties is of great importance for providing a theoretical basis for accurate assessments of the soil respiration intensity and carbon recycling in grassland ecosystems. A field experiment was performed from April 2017 to March 2018, in which four different levels of nitrogen applications were investigated, including 0 kg·hm-2 (N0), 60 kg·hm-2 (N1), 120 kg·hm-2 (N2), and 180 kg·hm-2 (N3). The seasonal changes in the soil respiration rate, soil temperature, and soil moisture in the alfalfa grassland under different levels of nitrogen applications were observed, and soil biochemical characteristics were observed after each harvest in the growing season. The results showed that soil respiration rate of the alfalfa grassland displayed significant seasonal variation under different nitrogen levels. In particular, the soil respiration rate reached a peak during the last 10-day period of July and then decreased to the minimum in mid-December. During the growing season of alfalfa, the soil respiration rate of the alfalfa grassland increased with the increases in the nitrogen application rate. The mean soil respiration rates of the N1, N2, and N3 treatments were 0.97, 1.04, and 1.07 g·(m2·h)-1, respectively, and these values were 10.2%, 18.2%, and 21.6% greater than that of N0[0.88 g·(m2·h)-1], respectively. The results from ANOVA testing indicated that nitrogen applications had no significant effect on the soil respiration rate during the non-growing season of alfalfa (P>0.05). According to the statistical analysis, the soil respiration rate had a significant exponential positive relationship with soil temperature during the growing season, non-growing season, and entire year of alfalfa grassland observations under different nitrogen application rates (P<0.01); the coefficients of determination were ranked as follows:growing season (0.46-0.62) < non-growing season (0.66-0.76) < whole year (0.80-0.86). Soil temperature (T) and soil moisture (W) interacted with each other and ultimately affected the soil respiration (RS), and by using a two-factor linear model of soil temperature and soil moisture, a better fit was obtained for the change in the soil respiration rate. Both of the two factors explained 68%-80% of the variation in the seasonal soil respiratory rate during the growing season of alfalfa. Nitrogen fertilization decreased the soil pH and available phosphorus content (AP) to varying degrees, but it increased the available potassium (AK), soil organic matter (SOM), and soil urease (URE) and invertase activity (INV). Total nitrogen (TN) and available nitrogen (AN) showed different trends under different nitrogen levels. The TN and AN contents increased considerably in soils; however, when the nitrogen rate was higher than N2 (120 kg·hm-2), TN and AN decreased with the increases in the nitrogen application rate. According to the correlation matrix analysis between soil respiration and soil biochemical properties during the growth period of alfalfa, data showed that the soil respiration rate (RS) was significantly and negatively correlated with soil pH (P<0.01), and it was significantly and positively correlated with soil TN and URE (P<0.01). Simultaneously, there was a significant positive correlation between the soil respiration rate (RS) and SOM (P<0.05), and there was a significant negative correlation with INV (P<0.05). The soil nutrient and enzyme activities of the alfalfa grassland explained the variations in the soil respiration rate under different nitrogen application levels to varying degrees.
