Soil Microbial Responses To Global Change In A Semi-arid Steppe In Northern China

The impacts of climate warming, changing precipitation regimes and atmospheric nitrogen(N)deposition on terrestrial ecosystems are crucial for the sustainability of social-economics and human well-beings. Responses and feedbacks of terrestrial ecosystems to global change pose great uncertainties and challenges for convincing projection of climate-biosphere feedback. Soil microbes play key roles in the biogeochemical cycles of terrestrial ecosystems. However, underlying mechanisms that mediate soil microbial responses and feedbacks to global change are still illusive.Our study explored the responses of soil microbes to the driving factors of global change through conducting three experiments of asymmetrically diurnal warming, warming and elevated precipitation, and nitrogen addition and increased precipitation in a semi-arid steppe in Duolun County, Northern China.In the experiment with asymmetrically diurnal warming, there were four treatments including control,daytime warming, nighttime warming, and diurnal warming with four replicates for each treatment. The experiment simulating climate warming and changing precipitation pattern included four treatments:control, warming, elevated precipitation, and warming plus elevated precipitation with six replicates for each treatment. There were four treatments including control, N addition, elevated precipitation, and N addition plus elevated precipitation in the experiment manipulating N and precipitation with four replicates for each treatment. Microbial biomass, phospholipid fatty acids, basal respiration, soil organic carbon(C),total N, ammonium, nitrate, p H, moisture, soil respiration, and plant community variables were measured.The results showed that:1. Soil microbes had differential responses to short- and long-term asymmetrically diurnal warming.Soil microbial structure showed significant differences under short-term(2006, 2007) warming compared with long-term(2011, 2013, 2015) warming. Shannon-Wiener diversity index of soil microbial communities, the abundance of gram-negative bacteria, bacteria, and the ratios of gram-negative to positive bacteria under long-term warming increased by 0.20 mol%, 8.95 mol%, 10.69 mol%, 0.36, respectively comparing to those under short-term warming(absolute change, all P < 0.01). The abundance ofgram-positive bacteria, fungi, arbuscular mycorrhizal fungi, the ratios of fungi to bacteria, and the ratios of cy17:0 to 16:1 ?7 under long-term warming were 6.03 mol%, 0.96 mol%, 0.08 mol%, 0.05, and 0.06 lower than those under short-term warming(absolute change, all P < 0.01). Long-term daytime warming decreased the ratios of fungi to bacteria, whereas nighttime warming did not affect it. Redundancy analysis showed that soil microbial communities under long-term warming were mainly mediated by plant communities and soil water content but by soil moisture under short-term warming. Reduced ratios of fungal to bacterial abundance under long-term daytime warming could lead to the unstable soil C pool in the steppe ecosystem.2. Warming and elevated precipitation could alter soil microbial community structure by changing soil water content and dissolved organic C. Warming and elevated precipitation had no interactions on soil microbial community structure. Microbial biomass C decreased by 5.4%(P < 0.05) under warming and increased by 27%(P < 0.001) under elevated precipitation. Microbial basal respiration was enhanced by58%(P < 0.001) under elevated precipitation, but did not change under warming. Gram-positive bacteria increased by 0.51 mol%(absolute change, P < 0.05) under warming and reduced by 1.35 mol%(absolute change, P < 0.001) under elevated precipitation. Elevated precipitation enhanced gram-negative bacteria,arbuscular mycorrhizal fungi, the ratios of gram-negative to positive bacteria by 1.41 mol%(absolute change, P < 0.01), 0.45 mol%(absolute change, P < 0.001), and 0.08(absolute change, P < 0.001),respectively.Warming did not change soil C and N contents. However, elevated precipitation increased soil dissolved organic C, organic C, nitrate, and total N by 5.7%(P < 0.001), 11.8%(P < 0.001), 32%(P <0.01), and 9.9%(P < 0.001), respectively.3. Plant communities, soil ammonium, nitrate, and dissolved organic C regulated soil microbial structure under N addition and elevated precipitation. Nitrogen addition reduced microbial biomass C and basal respiration by 11.9%(P < 0.01) and 14.5%(P < 0.05), respectively. Nitrogen addition significantly shifted soil microbial structure. Nitrogen addition decreased the abundance of fungi, arbuscular mycorrhizal fungi, and the ratios of fungi to bacteria by 1.34 mol%, 1.04 mol%, and 0.02, respectively(absolute change, all P < 0.001). Nitrogen addition reduced soil dissolved organic C by 8.8%(P < 0.05),but enhanced soil ammonium and nitrate by 235% and 242%, respectively(both P < 0.001). Nitrogenaddition decreased soil p H by 0.67(absolute change, P < 0.001) and led to soil acidification.Elevated precipitation increased microbial biomass C and N, and basal respiration by 80%, 148%, and68%, respectively(all P < 0.001). Elevated precipitation decreased gram-positive bacteria by 1.47 mol%(absolute change, P < 0.001), whereas increased gram-negative bacteria, arbuscular mycorrhizal fungi, and the ratios of gram-negative to positive bacteria by 1.78 mol%(absolute change, P < 0.01), 0.73 mol%(absolute change, P < 0.001), and 0.09(absolute change, P < 0.001), respectively. Elevated precipitation stimulated soil dissolved organic C by 17.2%(P < 0.01), but reduced ammonium by 47%(P < 0.01).Elevated precipitation enhanced soil p H by 0.31(absolute change, P < 0.01) and relieved soil acidification.Nitrogen addition and elevated precipitation interactively affected microbial biomass C, the abundance of fungi and gram-negative bacteria, the evenness index of microbial communities, and soil ammonium. As two of major limiting resources, N and water could influence soil microbial communities through interactions of microbes-soil-plants in a semi-arid steppe in Northern China.Our experiments explored microbial responses to the driving factors of global change in a semi-arid steppe in Northern China. The results suggest that the microbial responses to global change are regulated by plant communities and soil properties. The findings are critical for mechanistic understanding of global C cycles and adaptive management strategies for mitigating the negative impacts of global change on terrestrial ecosystems.