4.1 Effects of covering soil thickness and controlling factors on root respiration
As a technical artificial soil, the study area was obviously different with the natural soil (Zhang et al. 2007). The coal gangue layer changed the hydraulic properties and organic carbon content in reclaimed soil (Chen et al. 2018a; Chen et al. 2018b), which will affect the activity of the root microorganisms of plants in the topsoil. Thus, the covering soil thickness will affect the effects of coal gangue on the root respiration in reclaimed soil, which was proved in our study. However, the effects of coal gangue on the root respiration did not show significant regularity as the covering soil thickness increasing. The root respiration of site B was the highest, but at site A, the daily and monthly amplitude range and daily average value of the root respiration were lower compared with site B and D. It indicated that plants were obvious influenced by coal gangue with a thin covering soil thickness, which against the growth of plant roots. With the increasing of covering soil thickness, the negative effects of coal gangue will decrease. Although plants were not affected by the negative effects of the coal gangue with thicker soil, the positive factors that were beneficial to microbial activity in the root system also disappeared at the same time. Thus, the covering soil thickness at 25-45cm was the most conducive to plant growth. In addition, the root respiration of site C should be greater than site A, but surprisingly the root respiration was the smallest. It may because the artificial disturbance reduced plant metabolism and even leading to the death of surface grasses, which caused significant reduction of root respiration. In addition, root respiration is mainly influenced by root biomass, soil temperature and water content (Jia et al. 2013). Many studies showed that root respiration had a significant positive correlation with root biomass (Lai et al. 2017; Li et al. 2013). In our study, the temperature and soil water content of each site was similar. Thus, the root respiration was mainly influenced by root biomass in our study. In this study, the root biomass of site C was significantly less than other study site (Fig. 8), which proved that artificial disturbance reduced the root biomass and leaded to root respiration decrease. In addition, the root biomass of site B was higher than other sites, which indicated that the thickness at 25-45cm had a great influence on root growth in reclaimed soil.
Root respiration is a complex biological process, which makes up of two parts: plant root respiration and plant root microbial respiration. Besides the root biomass, root respiration also influences by soil temperature and water content (McConnell et al. 2013; Zhao et al. 2011). In our study, the root respiration was positively correlated with the soil temperature at 5 cm (P < 0.01), which was in accordance with the conclusions of previous researches (Franck et al. 2011; Lai et al. 2015). However, there was no significant correlation between the root respiration and soil water content (P > 0.05), which indicated that the seasonal variation of soil water content did not show significant effects on root respiration in our study area. The results of Yan’s study on seasonal changes in poplar plantation forests were consistent with our study (Yan et al. 2010). Therefore, the sensitivity of the root respiration to soil water content was relatively lower and soil temperature was the main controlling factor in the reclaimed soil during the non-growth season. Although the topsoil temperature explained 61.2–86.9% of the root respiration, it did not completely control the dynamic change of root respiration. The root respiration is controlled by soil temperature, water content and other environmental factors. In addition, when the temperature was low, temperature was the limiting factor for the root respiration and plant root microbial respiration. As temperature rising, root respiration and root microbial respiration increasing rely on other limiting factors, including soil water content and other environmental factors (Liu et al. 2009). Furthermore, the soil water content and the physical and chemical properties did not change significantly within a day. Therefore, the diurnal changes of the root respiration were mainly affected by soil temperature in reclaimed soil. In addition, the difference of the soil temperature was only about 1 °C between four sites in December 2017, but the maximum root respiration was about three times of the minimum value (Fig. 1, Fig. 2). Therefore, the effects of covering soil thickness on the root respiration were greater than soil temperature and humidity when the covering soil thickness changed.
4.2 The contribution of root respiration to total soil respiration
Root respiration does not depend on carbon pool in the soil. Thus, to quantify carbon emissions from soil respiration, it is necessary to understand the Rr/Rt ratio (Atarashi-Andoh et al. 2012). At present, most researches on root respiration concentrate on ecological systems such as forest, grassland, and farmland. Zeng et al. (2016)) found that annual mean Rr/Rt ratios were 17.46% and 24.44% in Armeniaca sibirica Lam. and Vitex negundo Linn. var. Heterophylla forests in semi-arid region of North China. Li et al. (2018)) reported that the average Rr/Rt ratios were 41.7% and 41.9% for the growing season and the annual mean value in 2008 and 2009 in the semi-arid grassland of northern China. Hao and Jiang (2014)) found the Rr/Rt ratio averaged 44.2% in a rape (Brassica campestris L.) field in Southwest China. However, there were few reports about the Rr/Rt ratio in mine reclaimed ecological system (Jørgensen et al. 2012). With the mine reclaimed soil acreage increasing, the Rr/Rt ratio become more and more importance. During the non-growing season, the Rr/Rt ratio increased with topsoil temperature increasing. The average Rr/Rt ratio was 51.13%, which indicated that the root respiration was the primary source of soil respiration in mine reclaimed soil. It also showed that the soil microbial respiration was relatively weak and soil respiration mainly came from root respiration, because the mine reclaimed soil affected by the harmful effects of coal gangue. In addition, previous studies showed that the proportion of root respiration to soil respiration was mostly between 10% and 90% in different terrestrial soil types (Lee et al. 2003a). In our study, the proportion of root respiration to soil respiration was between 16.16% and 83.02% (Table 2), which was within the reasonable range.