Year-round warming and autumnal clipping lead to downward transport of root biomass, carbon and total nitrogen in soil of an alpine meadow
Introduction
Climate warming due to rising concentrations of greenhouse gases is an established fact (Oreskes, 2004, IPCC, 2007, Xia et al., 2014). The global temperature is predicted to be from 1.4 to 3.0 °C higher in 2050 than it was over the last century (Rowlands et al., 2012), and according to predictions might continue to rise by 1.8–4.8 °C until the end of this century (Shi et al., 2012). The increasing amplitudes of temperature are greater in high-latitude and high-altitude regions where ecosystems are found more sensitive to elevated temperatures (Grabherr et al., 1994, Thomas et al., 2004). In such ecosystems plant growth is often adapted to low temperature and there soil respiration is found more sensitive to warming (Li et al., 2011a). Low temperature in alpine ecosystems is one of the most important limiting factors for the performance of alpine plants, whereas temperature enhancement reinforces photosynthetic capacity and growth rates of these alpine plants (Li et al., 2011a). These alpine ecosystems store the greatest fraction of carbon stocks in soils (Li et al., 2011a). Compared to soils from temperate ecosystems, cold soils comprise more labile soil organic matter due to slower decomposition and humification processes (Sjögersten et al., 2003).
Climate warming and human activity have been considered as two factors responsible for grassland degradation, and the influences of human activity on vegetation are greater than any other environmental factors. The dominant anthropogenic factor seems therefore to be controlling the plant community characteristics (Brown et al., 1997, Wang et al., 2006). In terms of the human activity, land-use change seems to be one important factor that could fundamentally change ecosystem carbon cycling and its response to climate warming (Chapin et al., 2008). Livestock grazing, as one of the most prevalent land uses in grasslands, has the potential to substantially alter carbon cycling in those ecosystems by: (1) altering microclimate and the availability of light, water, and nutrients (Niu et al., 2013, Zhou et al., 2007); (2) changing photosynthetic activity and stimulating compensatory growth (Anten and Ackerly, 2001, Zhao et al., 2008); and (3) modifying the species composition (Derner et al., 2006, Niu et al., 2010). Moderate livestock grazing intensity not only can prevent the grassland from being destroyed by herbivores, but also can promote the growth of grassland primary productivity and enhance the utilization rate of grassland (Cao et al., 2004, Gu et al., 2011, Niu et al., 2010). However, heavy livestock grazing intensity trends toward a decline in grassland productivity, which slows down the recovery of vegetation (Belsky, 1993, Trilica and Rittenhouse, 1993).
Increased soil temperature typically affects soil properties. However, the effects may be counterbalanced by declining soil moisture (Saleska et al., 1999). The response of soil CO2 effluxes to rising temperature also depends on how plants and their carbon allocation to belowground sinks respond to warming (Schindlbacher et al., 2009). For example, the decrease in soil moisture after warming was reported to decrease plant net primary productivity (Melillo et al., 1993). De Boeck et al. (2007) showed that the aboveground biomass and belowground biomass decreased by 18% and 23%, respectively, as a result of the reduction in moisture. Due to the restriction of moisture after large amplitude warming, belowground biomass can migrate to deep root zones (Li et al., 2011b). Numerous studies have looked into the effects of experimental warming on carbon dynamics between ecosystems and the atmosphere and were carried out in various ecosystems such as boreal forests (Niinistö et al., 2004, Bronson et al., 2008), high latitudes (Oechel et al., 1993, Oberbauer et al., 2007), or in lichen-rich dwarf shrub tundra (Biasi et al., 2008). These studies reported ecosystem-dependent responses in carbon fluxes with initial carbon losses in dry tundra and boreal forests, but dampened effects under anoxic conditions. One study conducted at high altitude was in a dry alpine meadow in Colorado, where soil heating had stronger indirect effects on soil carbon cycling by changing plant species composition and inducing moisture limitations for soil respiration (Saleska et al., 1999).
To our knowledge, there is a lack of studies on experimental warming and simulated livestock grazing and on plant and soil nutrient dynamics in high-altitude areas particularly in the alpine ecosystems. We conducted a manipulation experiment to investigate the individual and combined effects of experimental warming and livestock grazing by clipping on plant and soil properties in an alpine meadow of the Qinghai-Tibetan Plateau (QTP). We hypothesize that the reduction of soil moisture due to warming in upper layers may induce the allocation of plant belowground biomass to deeper layers by growing new roots, and thus have effects on the distribution of soil properties at different soil depths. The objective of this research was to explore the single and combined effects of year-round warming and autumn clipping on root biomass, carbon, and nitrogen in the soil column of an alpine meadow in the QTP.
Section snippets
Site description
The QTP, regarded sometimes as the Earth's third pole, belongs to a sensitive and fragile area to climate warming and is an ideal region for studying the responses of terrestrial ecosystems to climate warming (Zhao and Zhou, 1999, Li et al., 2011b, Shi et al., 2012). The carbon content has been shown to be higher in soils of QTP than in soils of other areas (Fang et al., 2010). Climate warming stimulates the release of a substantial portion of this reservoir, turning alpine ecosystems from a
Effects of experimental warming on moisture and temperature
Soil moisture was lower in CK than W at 60 cm and 100 cm depths (P < 0.05) (Table 1). The changes of W–CK with respect to soil moisture were smaller at 10 and 20 cm depths, while greater at 60 and 100 cm depths (P < 0.05). Soil temperature increased at the depths of 0, 5, 15, and 30 cm (P < 0.05), while no changes were observed for air temperature at 20 cm height and soil temperature at the depths of 60, 100, and 150 cm (P > 0.05). In addition, the changes of W–CK with respect to soil temperature were greater
Response of root biomass to warming and clipping
Warming not only directly affects plants and ecosystems by raising temperature, but it also indirectly influences them by modifying soil moisture (Shaver et al., 2000, Wan et al., 2005). Warming may reduce root biomass by decreasing soil moisture. De Boeck et al. (2007) found that belowground biomass decreased by 23% due to the reduction of soil moisture induced by warming. However, warming may also increase root biomass by increasing plant photosynthetic capacity. For example, elevated
Conclusions
We report for the first time the results from experimental warming and simulated grazing in an alpine meadow of the Qinghai-Tibetan Plateau. Soil microbial biomass was not sensitive to the initial effects of warming and clipping probably due to the low temperature environment in the alpine meadow. Our study complements earlier reports that climate warming and livestock grazing can affect the distribution of biomass and nutrients in the soil. The downward transport of moisture, driven by
Acknowledgments
This study was financially supported by the Hundreds of Talents Project of Chinese Academy of Sciences and the National Natural Science Funds (41301211 and 41,201,195). The authors thank many lab members for their help in measuring and collecting the environment and vegetation data. The authors also greatly thank the language editor and anonymous reviewers for their kind help with refining this manuscript.
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