不同施氮水平对紫花苜蓿草地土壤呼吸和土壤生化性质的影响

为探讨不同施氮水平对紫花苜蓿草地土壤呼吸速率和土壤生化性质的影响及其关系,本研究于2017年4月至2018年3月采用田间试验和室内分析相结合的方法,设置了无氮(N0,0)、低氮(N1,60 kg·hm~(-2))、中氮(N2,120 kg·hm~(-2))和高氮(N3,180 kg·hm~(-2))这4个施氮水平,监测了不同施氮水平下紫花苜蓿草地土壤呼吸速率及土壤水热的季节变化,并于紫花苜蓿生长季内不同茬次刈割后测定了土壤生化性质.结果表明:(1)不同施氮水平下紫花苜蓿草地土壤呼吸速率均表现出明显的季节性变化特征,在7月下旬达到峰值,12月中旬降至最低;随施氮量的增加紫花苜蓿生长季内土壤呼吸速率逐渐增强,N1、N2和N3施氮水平下的土壤呼吸速率均值分别为0.97、1.04和1.07 g·(m~2·h)~(-1),与N00.88 g·(m~2·h)~(-1)]相比,土壤呼吸速率分别增加了10.2%、18.2%和21.6%;施氮对紫花苜蓿非生长季内土壤呼吸速率无显著影响(P0.05).(2)不同施氮水平下紫花苜蓿生长季、非生长季和全年的土壤呼吸速率与土壤温度拟合指数模型均达极显著水平(P0.01),且指数模型的决定系数R~2值表现为生长季(0.46～0.62)非生长季(0.66～0.76)全年(0.80～0.86).(3)施氮在一定程度上降低了紫花苜蓿草地土壤的pH值和速效磷(AP),而提高了速效钾(AK)、土壤有机质(SOM)、土壤脲酶(URE)和土壤蔗糖酶活性(INV).土壤全氮(TN)和碱解氮(AN)含量在不同施氮水平下表现出不同的变化趋势,当施氮量在0～120 kg·hm~(-2)时,TN和AN随施氮量的增加而增加,继续增施氮肥超过N2(120 kg·hm~(-2))水平时则略有下降.(4)通过紫花苜蓿生长季内土壤呼吸与其土壤生化性质之间的相关矩阵分析可知,土壤呼吸速率(R_S)与土壤pH值呈极显著负相关(P0.01),与TN和URE呈极显著正相关(P0.01),与SOM呈显著的正相关(P0.05),与INV呈显著负相关(P0.05).综合考虑土壤生化特性对不同施氮条件下紫花苜蓿草地土壤呼吸速率的影响,可为草地生态系统土壤呼吸强度研究提供理论依据.

Effects of Different Levels of Nitrogen Fertilization on Soil Respiration Rates and Soil Biochemical Properties in an Alfalfa Grassland

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 N00.88 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 (R_S), 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 (R_S) 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.