Soil is a key component of the Earth system and contributes to services, goods, and resources to humankind (Brevik et al., 2015). Soil stores more carbon (C) than the atmosphere and vegetation (Köchy et al., 2015a; Keesstra et al., 2016). Soil organic carbon (SOC) is a key component of the global C cycle, and its potential to sink from atmosphere carbon dioxide (CO2) has been widely discussed in the scientific literature throughout the world (Guo and Gifford, 2002; Lal, 2004; De Deyn et al., 2008; Deng et al., 2014a; Parras-Alcántara et al., 2015). In terrestrial ecosystems, SOC pool dynamics can be affected by many factors, such as climate change (Lal, 2004; Field et al., 2007), management practices (Luo et al., 2010; Ono et al., 2015), land use, etc. (Post and Kwon, 2000; Don et al., 2011; Deng et al., 2014b; Muñoz-Rojas et al., 2015).

SOC plays a critical role in controlling soil fertility and cropping system productivity and sustainability (Hurisso et al., 2013; De Moraes Sá et al., 2015), particularly in low-productivity arid and semiarid agro-ecosystems (Behera and Shukla, 2015; Willaarts et al., 2016). To develop farming methods that conserve SOC is therefore of a great importance (Lal, 2004) in this area. Cultivated grasslands have many more advantages than natural grassland regeneration, such as accelerating vegetation restoration and improving grassland productivity. Establishing artificial grassland is therefore an effective method to restore vegetation and improve soil fertility by accumulation of SOC (Fu et al., 2010; Wu et al., 2010; Li et al., 2014). Vegetation degradation and exponential population growth have caused massive amounts of soil and water to be lost. The Chinese government has implemented the most ambitious ecological program titled “Grain-for-Green” Project (converting degraded, marginal land, and cropland into grassland, shrubland, and forest), with the objective of transforming the low-yield slope cropland into grassland, reducing soil erosion, maintaining land productivity, and improving environmental quality (Fu, 1989; Liu et al., 2008). The large scale of the project can enhance C sequestration capacity in China, especially in arid and semi-arid areas (Chang et al., 2011; Song et al., 2014).

Many prior studies about SOC have paid much attention to converting farmland to grassland, shrubland, or forest (Fu et al., 2010; Deng et al., 2014a). However, less attention has been devoted to the SOC among different species of grasslands. In this study, we focused on ascertaining the influence of leguminous and gramineous grasslands on SOC sequestration capacity and efficiency. Many studies have demonstrated that there is a significant and positive relationship on SOC and nitrogen (Deng et al., 2013; Zhu et al., 2014). Therefore, we hypothesize that leguminous grassland has higher SOC sequestration capacity than gramineous grassland. More specifically, our objectives are (i) to analyze the differences of storage efficiency under different grasslands and (ii) to determine which type of cultivated grassland might better improve SOC storage in arid and semi-arid areas.

Between 2008 and 2012, the five cultivated grasslands had in general greater total biomass values (mean by 189.36 %) than the uncultivated grassland and natural grassland. In addition, the three grasslands cultivated with the leguminous species had greater annual total biomass (mean by 72.6 %) than two gramineous grasslands. L-MS grassland consistently had the greatest total biomass throughout the study period (Fig. 2a).

Results from two-way ANOVA showed that the plots, year, and interactions all significantly affected total biomass, SOC content, and BD (p < 0.001; Table 3). The average SOC content during the study period followed leguminous grasslands (4.21 ± 0.31 g kg-1), natural grasslands (2.90 ± 0.14 g kg-1), uncultivated grasslands (2.58 ± 0.17 g kg-1), gramineous grasslands (2.46 ± 0.15 g kg-1), and it increased over time in all grasslands (Table 4). The effects of grassland types on BD followed uncultivated and natural grassland (1.44 ± 0.02 g cm-3), gramineous grasslands (1.43 ± 0.01 g cm-3), leguminous grasslands (1.40 ± 0.01 g cm-3, Table 5).

SOC storage under all grassland types increased significantly throughout the study period (Table 6). The three leguminous grasslands accumulated C with an average rate of 1.00 Mg C ha-1 yr-1, which is more than the 0.34 Mg C ha-1 yr-1 in gramineous grassland, and more than the average of uncultivated and natural grasslands (0.25 Mg C ha-1 yr-1; Fig. 2c). The mean SOC sequestration efficiency in the leguminous grassland was 0.26, which was significantly greater than other grassland types (0.13; p < 0.05; Fig. 2d).

Grasslands can have a significant effect in arid and semi-arid areas' C cycle through changing soil C accumulation rates and turnover, soil erosion, and vegetation biomass (Deng et al., 2014a; Liu et al., 2017). Plants regulate SOC stock by controlling, assimilating, and accumulating C in the plant root system, and then through plant respiration and leaching release from soil to atmosphere (De Deyn et al., 2008; Garcia-Diaz et al., 2016). SOC inputs mostly originate from decaying aboveground and belowground plant tissue, so greater soil C accumulation can be mainly ascribed to increasing soil C input from higher biomass production (Deng et al., 2014c; Wu et al., 2016). Mutualistic symbionts (N-fixing bacteria and mycorrhizal fungi) are also an important source of C input to soil, especially in actively growing plants (Bardgett et al., 2005). In grasslands, atmospheric C (CO2) is sequestrated through photosynthesis and respiration. Then, C fixes in a stable SOC pool or releases back into the atmosphere (Post and Kwon, 2000). Therefore, studying the C sequestration in grassland ecosystems can help to identify the magnitude of global C sinks and sources (Li et al., 2014). Our results showed that leguminous grassland had greater SOC contents and storage efficiency than gramineous grassland. Different species may incorporate more or less C according to their specific metabolism. Legumes have been identified as a key driver of C sequestration in many studies (Fornara and Tilman, 2008; Wu et al., 2016). These species live in symbiosis with Rhizobium bacteria, which fix atmospheric N. Moreover, many previous studies have demonstrated that soil C and total nitrogen are significantly and positively correlated (Deng et al., 2013; De Oliveira et al., 2015). Since mycorrhizal fungi can immobilize C in their mycelium and improve C sequestration in soil aggregates (Rillig and Mummey, 2006), it might be expected that the cultivated leguminous grasslands significantly improved soil N content that led to larger C sequestration ability than the non-leguminous grasslands.

The biomass fraction resulting in SOC build-up (plant residuals) was possibly strongly affected by management practices including the selection of plant species (Don et al., 2011). Species composition may have had a critical role in determining the aboveground productivity (Liu et al., 2016). Over a relatively long time, the proportion of the aboveground biomass enters soil as organic matter and incorporates into soil through physical and biological processes. Some leachates from plant material in the litter layer, root exudates, solid decomposed litter, and fragmented plant structure materials could have been the main sources of soil organic matter (Jones and Donnelly, 2004; Novara et al., 2015). The amount of plant residuals returned to the soil may have affected the SOC (Musinguzi et al., 2015; Wasak and Drewnik, 2015). Mostly perennial plants were managed with high planting densities to produce greater biomass exports (Hobbie et al., 2007; Köchy et al., 2015b). Deng et al. (2014c) found that plant biomass is the key driver in soil C sequestration. In this study, SOC increased dramatically in leguminous grassland most likely due to the greater total biomasses of the leguminous grasses, which result in the increasing soil C inputs from the litter layer and root biomass (De Deyn et al., 2008; Wu et al., 2010; Novara et al., 2015). Moreover, symbiosis might have increased plant productivity through enhancing the acquisition of limited resources. Our results demonstrate that a key variable associated with higher SOC content in leguminous grasslands compared to gramineous grasslands is the greater total biomass accumulation. The leguminous grasslands had both higher above- and belowground biomasses than gramineous grasslands. Total biomass was 16.35 kg m-2 in leguminous grasslands, which was 9.47 kg m-2 higher than in gramineous grasslands from 2008 to 2012. The grasslands in our study harvested the aboveground biomass once annually, so all the stubble and plant litters input to soil as a C supply.

SOC sequestration rates in the leguminous grasslands were significantly higher than those found in the gramineous grasslands (Fig. 1c). This maybe resulted from the different decomposition rates in soils, because the leguminous and gramineous grass species result in multifarious nutrient conditions. Litter and fragmented plant parts at the soil surface are decomposed by micro-organisms and are gradually incorporated into the soil through some complex processes, such as humification and mineralization (Novara et al., 2015). Legumes have the ability to develop root nodules and to fix nitrogen in symbiosis with compatible rhizobia, which can improve the soil nutrient status and microbial community (Hurisso et al., 2013). Root nodules promote the symbiosis with micro-organisms, which are responsible for the decomposition of the plants and therefore constitute the key of the transmission of the stored C into the soil. Furthermore, the increasing fertility of the soils in the leguminous grasses should facilitate the increasing productivity of the plants (Wu et al., 2016). Our results showed that SOC sequestration efficiency under leguminous grassland was evidently greater than that in the gramineous grassland (Fig. 1d). It is noteworthy that L-MS grassland had the highest total biomass of 22.59 kg m-2, which is 2.38 times as much as the average of gramineous grasslands (Fig. 2a). Moreover, SOC sequestration in L-MS grassland was 3 times higher than the average of gramineous grasslands (Fig. 2b).

Despite the indications from this study of higher SOC sequestration rate and efficiency in leguminous grassland, specific research is still needed to determine the potential mechanisms of each species in sequestrating C. Many studies have demonstrated that legumes are high water-consuming plants compared to gramineous ones in arid and semi-arid areas, so it is necessary to balance the ecological effect of grassland for rational utilization of resources.

Leguminous grassland had greater SOC storage, sequestration rate, and efficiency than gramineous grassland. The greater soil C accumulation of leguminous grasslands was mainly ascribed to the capacity to incorporate C and the higher biomass production. Leguminous grasslands accumulated an average rate of 0.64 Mg C ha-1 yr-1 more than gramineous grasslands. The average SOC sequestration efficiency in leguminous grasslands was 2 times greater than that in the gramineous grasslands. Our results provide a reference for ecological management in arid and semi-arid areas.

The data are not publicly available due to copyright issues.