1 Introduction

Coal mining has made a substantial contribution to the development of the international economy, but coal mining can lead to complex ground deformation processes, resulting in environmental problems and serious geological disasters (Hu et al. 2018; Sokolov et al. 2015; Wang et al. 2017b). To improve the soil ecology in coal mining areas, land reclamation technology is often utilized (Chugh 2018; Cheng et al. 2019). Backfilling with coal gangue is an economical and effective way to reclaim a damaged soil for restoring and improving the ecological environment. It can rehabilitate subsiding land and partly resolve the problem of excess coal gangue piled up on the land surface (Hu et al. 2009; **ao et al. 2014).

However, the hydraulic properties of coal gangue are significantly different from those of soil. Zhou et al. (2010) found that the soil infiltration rate and saturated hydraulic conductivity decreased with increasing gangue content. Chen et al. (2018a) indicated that there was a lower water-holding capacity, and higher water and air permeability in coal gangue compared with soil. In addition, the presence of coal gangue can have a significant impact on the soil temperature and water content in mine reclaimed soil. The temperature amplitude of topsoil has been found to decrease with an increase in the covering-soil thicknesses (Chen et al. 2017) and the surface volumetric water content of a soil column filled with coal gangue has been reported to be significantly lower than that of a column solely consisting of soil (Wang et al. 2017a). Soil temperature and water content have been identified as the most important environmental factors that influence soil respiration and root respiration (Fu et al. 2019; Thurgood et al. 2014; Bi et al. 2019). The soil respiration of reclaimed soil is therefore different from that of forest, grassland, and agricultural ecosystems.

Soil root respiration constitutes an important part of overall soil respiration, and is important for evaluating the effects of ecological restoration, and calculating the turnover rates of soil organic matter pools and the carbon budgets of vegetation (Lee et al. 2003; Li et al. 2011, 2010, 2016). The contribution of root respiration to total soil respiration (Rr/Rt ratio) is therefore important for understanding the mechanism of soil respiration and the carbon cycle (Dyukarev 2017). However, previous studies have paid relatively little attention to the Rr/Rt ratio during the non-growing season. Although the soil temperature, microbial activity, and root biomass during the non-growing season are obviously lower than growing season (Hao and Jiang 2014; Li et al. 2018), and even the microbial activity is negligible in frozen soils (Wang et al. 2009), the contribution of soil respiration to the total annual soil respiration has been reported to be approximately 12% and the Rr/Rt ratio varies from 13% to 50% (Schindlbacher et al. 2007). Root respiration during the non-growing season is therefore a significant component of the annual carbon budget. Most studies of root respiration have concentrated 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 a semi-arid region of north China. Li et al. (2018) reported average Rr/Rt ratios of 41.7% and 41.9% for the growing season in 2008 and 2009 in the semi-arid grassland of northern China. Hao and Jiang (2014) found that the Rr/Rt ratio averaged 44.2% in a rape (Brassica campestris L.) field in southwest China. However, the Rr/Rt ratio in mine reclaimed ecological systems has rarely been reported (Jørgensen et al. 2012).

Therefore, this study investigated the dynamic changes of root respiration and analyzed the Rr/Rt ratio in mine reclaimed soil during the non-growing season. The specific aims were to: (1) obtain the dynamic changes of root respiration in mine reclaimed soil; (2) analyze the soil temperature and water content sensitivity of root respiration in mine reclaimed soil; and (3) investigate the Rr/Rt ratio in mine reclaimed soil.

2 Materials and methods

2.1 Site description

The experiment was conducted in the ecological restoration area of Panyi Coal Mine, Huainan City, Anhui Province (E116.83°, N32.78°). The site has a total subsided area of about 10.8 ha, but the size of the actual study area was about 5.4 ha. The study area experiences a warm temperate semi-humid continental monsoon climate, with an annual mean temperature of 15.3ºC and average annual precipitation of 937 mm. There is a total of 2279.2 h of sunshine per year and an average of 141 d of frost. Relative humidity in the area is 76%. The ecological restoration area is part of a national mine geological environment control project, which aims to restore areas affected by coal mining subsidence. The ecological restoration area has been restored by the backfilling of coal gangue ten years ago. The vegetation is uniformly distributed and mainly consists of trees, shrubs, and weeds (Fig. S1). The topsoil is a loam clay, with a pH value of 7.76–8.02 and a density of 1.74–1.97 g/cm3. The soil structure and texture were similar, but the covering-soil thickness varied throughout the area.

2.2 Experimental design

An extensive investigation revealed that the covering-soil thickness was mainly between 10 and 70 cm in the study area. On the basis of the overall characterization of differences in the covering-soil thickness and vegetation type, four sites were chosen as a study area within the ecological restoration area (Table 1). The covering-soil thicknesses were 10–25 cm (site A), 25–45 cm (site B), 45–55 cm (site C), and 55–65 cm (site D), respectively. A trenching method was used to measure root respiration (Hao and Jiang 2014). Four trench plots with an area of 0.4 m × 0.4 m and depth of 0.4 m were randomly established as root-free plots, and four other plots (0.4 m × 0.4 m) were established as control plots in each experimental site (20 m × 20 m). All plant roots in trenches were cut and carefully removed. Polyethylene sheets were then placed around the trenches to prevent the inward growth of the surrounding plant roots. Finally, the soil was refilled to match the original soil profile (Fig. S2). The root-free plots were kept free of vegetation by periodic manual removal. Monitoring began after three months. The soil temperature, water content, root biomass, soil respiration, and root respiration were continuously monitored in the plots for 4 d in the middle of every month from November 2017 to April 2018. Monitoring occurred every 2 h between 8 a.m. and 6 p.m..

Table 1 The covering-soil thickness of the experimental areas
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2.3 Soil respiration

Soil respiration was measured using a closed chamber system, with a pump suction infrared gas CO2 analyzer (Mu et al. 2008). Polyvinyl chloride (PVC) barrels (11 cm inner diameter, 25 cm height) were used to close the chamber. Before measurements, plants in the topsoil were removed with scissors to minimize the effects of plant respiration. The PVC barrels were inserted 10 cm into the soil to prevent gas leakage from the bottom of the barrels. The background CO2 concentrations in the PVC barrels were recorded before measurement. A pump suction infrared gas analyzer was then inserted into the PVC barrels to measure the CO2 concentration. The soil respiration was calculated by the following equation (Tomotsune et al. 2013):

$$F\left( {{\text{CO}}_2} \right) = \frac{\rho \times \Delta C \times V}{{S \times \Delta t}} \times \frac{273 + T}{{273}}$$
(1)

where, ρ is the CO2 density in the standard state, ΔC is the change in the CO2 volume fraction, Δt is the timing of specimen collection, S is the bottom surface area of a PVC barrel, V is the volume of a PVC barrel, and T is the soil temperature.

2.4 Root respiration and environmental factors

In the trenching method, the root respiration rate was calculated using the following equation (Li et al. 2018):

$${R_{\text{r}}} = {R_{\text{t}}} - {R_{\text{m}}}$$
(2)

where, Rr is root respiration, and Rt and Rm are total soil respiration and microbial respiration, which were measured in the control plots and the root-free plots, respectively.

In addition, the soil temperatures were measured simultaneously with a soil temperature probe, the soil water content was measured with a portable moisture probe meter at 5 cm depth and soil microbial biomass was determined by a chloroform fumigation-extraction method (Zhang et al. 2017; Li et al. 2013). Due to the root biomass and topsoil temperature being affected by covering-soil thickness in mine reclaimed soil (Chen et al. 2017), the covering-soil thickness affects root respiration.

Fig. 8
figure 8

Average root biomass at study sites with different covering-soil thicknesses (the covering-soil thicknesses of site A, B, C and D were 10–25, 25–45, 45–55 and 55–65 cm, respectively)

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In general, the physical and chemical properties of sites A, B, C, and D did not change significantly over the course of a day, and therefore the diurnal changes of root respiration were mainly affected by soil temperature and covering-soil thickness. In addition, the difference in the soil temperature was only about 1 °C between the four sites in December 2017, but the maximum root respiration was about three times the minimum value. Therefore, the effects of covering-soil thickness on root respiration were greater than the effects of soil temperature in mine reclaimed soil.

4.2 The contribution of root respiration to total soil respiration

The Rr/Rt ratio is important for understanding the mechanism of soil respiration and the carbon cycle. The Rr/Rt ratio therefore becomes increasingly important as the area of mine reclaimed soil increases, which will enable us to better understand the influences of reclamation on carbon emissions in mining areas. There were differences in root respiration between the four sites and the rank order of Rr/Rt ratios was B > D > A > C, which was consistent with the rank order of soil respiration. This indicated that the covering-soil thickness affected the root respiration and then affected the Rr/Rt ratio. Although the monthly variations of Rr/Rt ratio were irregular, the Rr/Rt ratio of the entire study area displayed a pronounced monthly variation. When plotted on a monthly basis, the slope of root respiration was lower than that of microbial respiration from November 2017 to December 2017, but the slope of root respiration was higher than that of microbial respiration from December 2017 to April 2018 (Fig. 7). Thus, the Rr/Rt ratio decreased from November 2017 to December 2017 and then increased significantly from December 2017 to April 2018. The microbial respiration was higher than root respiration before January 2018, which suggested that soil respiration was dominated by microbial respiration in the early non-growing season and then by root respiration in the late non-growing season in mine reclaimed soil. Lee et al. (2003) showed that the proportion of overall soil respiration accounted for by root respiration was largely between 10 and 90% in different terrestrial soil types. In our study, the proportion of root respiration to soil respiration was between 16.16% and 83.02% (Table 2), which was within a reasonable range. Additionally, the average Rr/Rt ratio was 51.13% during the non-growing season, indicating the importance of roots as the source of respired carbon in mine reclaimed soil.

5 Conclusions

The present study showed that the root respiration exhibited diurnal and monthly variations in mine reclaimed soil. Root respiration had an exponential and positive relationship with soil temperature (P < 0.01), but there was no significant correlation (P > 0.05) with soil water content. The covering-soil thickness and soil temperature were the main factors that affected root respiration and then affected the Rr/Rt ratios in mine reclaimed soil. The Rr/Rt ratios ranged from 16.16% to 83.02%, with an average value of 51.13% during the non-growing season, indicating the importance of roots as the source of respired carbon in mine reclaimed soil.