Characteristics of soil respiration on typical cropland slope in mollisol region of Northeast China

The characteristics of soil respiration in cropland slopes are different from those in cropland plains. In order to accurately evaluate greenhouse gases emission in Northeast China farmland, it was necessary to study the patterns and factors influencing soil respiration in different cropland slopes. The study was conducted on a typical cropland slope in a mollisol region in Northeast China. An entire cropland slope was divided into four slope positions based on change in gradient-summit, shoulder-slope, back-slope and toe-slope. The LI-8100 probe was used to monitor soil respiration throughout the maize growing period. Water content and temperature of the surface soil (0-20 cm soil layer) were respectively measured using soil moisture meter and thermometer. The relationship between soil carbon dioxide (CO₂) flux and the impact of temperature, soil moisture content were then analyzed for different slop positions (summit, shoulder-slope, back-slope and toe-slope). The study laid the basis for scientific evaluation of soil respiration and carbon balance in mollisol in cropland slopes. The results showed that CO₂ flux in summer was higher than in spring and autumn. Peak soil respiration occurred during the period from maize silking to tasseling, which was from July 3 to August 23 in the study area. During this period, the cumulative respiration was 58.7%-59.9% of total soil respiration for the whole growing period of maize across the four slope positions, indicating it was the main period of soil respiration in mollisol regions in Northeast China. The peak rate of soil respiration was significantly different in each slope position (P < 0.05). It was significantly higher in the toe-slope position7.56 μmol(CO₂)·m^-2·s^-1 than at the summit5.60 μmol(CO₂)·m^-2·s^-1, shoulder-slope6.08 μmol(CO₂)·m^-2·s^-1 and back-slope6.47 μmol(CO₂)·m^-2·s^-1. The results showed that soil respiration rates among different slope positions in typical mollisol regions were significantly different, especially in summer. CO₂ flux had a positive, linear correlation with soil temperature for all the slope positions (P < 0.05). The exponential equation best expressed the relationship between soil temperature and soil respiration rate. Then temperature sensitivity (Q₁₀) was the strongest for the back-slope position, compared with the others. There was an insignificant correlation between soil respiration and soil volumetric water content. These results indicated that change in soil temperature significantly influenced soil respiration rate. The cumulative emission of CO₂ in toe-slope523.97 g(CO₂-C)·m^-2 was 18.5%, 22.8% and 34.9% higher than in back-slope443.13 g(CO₂-C)·m^-2, shoulder-slope426.81 g(CO₂-C)·m^-2 and summit388.50 g(CO₂-C)·m^-2, respectively. This was caused by the migration and redistribution of organic carbon, the change in soil bulk density, and the related effect on soil under the different environmental factors. The results of the study suggested that it was necessary to consider the respiration characteristics of different slope positions in order to limit any deviations in the monitoring results based on cropland plains respiration in the scientific evaluation of soil respiration on cropland slopes.