Search

Aim: Ammonia‐oxidizing archaea (AOA) and bacteria (AOB) are the primary agents for nitrification, converting ammonia (NH4+) into nitrate (NO3−) and modulating plant nitrogen (N) utilization and terrestrial N retention. However, there is still lack of a unifying framework describing the patterns of global AOA and AOB distribution. In particular, biotic interactions are rarely integrated into any of the conceptual models. Location: World‐wide. Time period: 2005–2016. Major taxa studied: Ammonia‐oxidizing archaea and ammonia‐oxidizing bacteria. Methods: A meta‐analysis and synthesis were conducted to obtain a general picture of global AOA and AOB distribution and identify the primary driving factors. A microcosm experiment was then conducted to assess effects of relative carbon to nitrogen availability for heterotrophic microbes on AOA and AOB in two distinct soils. A mesocosm experiment was further carried out to characterize the effects of plant roots and their arbuscular mycorrhizal fungi (AMF) on AOA and AOB abundances using hyphae‐ or root‐ingrowth techniques. Results: Our meta‐analysis showed that soil carbon to nitrogen (C/N) ratios explained the most variance in AOA and AOB abundances, although soil pH had a significant effect. Experimental results demonstrated that high cellulose and mineral N inputs increased total microbial biomass and microbial activities, but inhibited AOA and AOB, suggesting microbial inhibition of AOA and AOB. Also, AMF and roots suppressed AOA and AOB, respectively. Main conclusions: Our study provides convincing evidence illustrating that relative carbon to nitrogen availability can predominantly affect the abundances of AOA and AOB. Our experimental results further validate that biotic competition among plants, heterotrophic microbes and ammonia oxidizers for substrate N is the predominant control upon AOA and AOB abundances. Together, these findings provide new insights into the role of abiotic and biotic factors in modulating terrestrial AOA and AOB abundances and their potential applications for management of nitrification in an increasing reactive N world. [ABSTRACT FROM AUTHOR]

• Responses of alpine soil N transformations to warming and N input were examined. • Warming alone did not significantly affect soil N transformations. • N input significantly increased AOB and N 2 O emissions in the late growing season. • Warming amplified the effects of N input on nitrification and N 2 O emissions. • Ammonia-oxidizing bacteria dominate alpine soil N transformations and N 2 O emission. Alpine ecosystems worldwide are characterized with high soil organic carbon (C) and low mineral nitrogen (N). Climate warming has been predicted to stimulate microbial decomposition and N mineralization in these systems. However, experimental results are highly variable, and the underlying mechanisms remain unclear. We examined the effects of warming, N input, and their combination on soil N pools and N-cycling microbes in a field manipulation experiment. Special attention was directed to the ammonia-oxidizing bacteria and archaea, and their mediated N-cycling processes (transformation rates and N 2 O emissions) in the third plant growing season after the treatments were initiated. Nitrogen input (12 g m−2 y−1) alone significantly increased soil mineral N pools and plant N uptake, and stimulated the growth of AOB and N 2 O emissions in the late growing season. While warming (by 1.4 °C air temperature) alone did not have significant effects on most parameters, it amplified the effects of N input on soil N concentrations and AOB abundance, eliciting a chain reaction that increased nitrification potential (+83%), soil NO 3 −-N (+200%), and N 2 O emissions (+412%) across the whole season. Also, N input reduced AOB diversity but increased the dominance of genus Nitrosospira within the AOB community, corresponding to the increased N 2 O emissions. These results showed that a small temperature increase in soil may significantly enhance N losses through NO 3 − leaching and N 2 O emissions when mineral N becomes available. These findings suggest that interactions among global change factors may predominantly affect ammonia-oxidizing microbes and their mediated N-cycling processes in alpine ecosystems under future climate change scenarios. [ABSTRACT FROM AUTHOR]

Many high-elevation alpine ecosystems have been experiencing significant increases in air temperature and, to a lesser extent, nitrogen (N) deposition. These changes may affect N-cycling microbes and enhance emissions of nitrous oxide (N 2 O, a potent greenhouse gas) from soil. However, few studies have investigated whether and how the resulting changes in N-cycling microbes may affect the temperature sensitivity (Q 10 ) of N 2 O emission and in turn feed back to N 2 O emissions. We conducted two incubation experiments to examine N 2 O emissions and their temperature sensitivities in soils that had experienced 3-yr field treatments of warming, N inputs and their combination in a Tibetan alpine meadow. Our results showed that neither N inputs nor warming alone affected the rate or Q 10 of soil N 2 O emission, but combining the two significantly increased both parameters. Also, combined N and warming significantly increased the abundance of ammonia-oxidizing bacteria (AOB), corresponding with high soil N 2 O emission. In addition, N 2 O emission from nitrification accounted for 60-80% of total emissions in all soils, indicating that nitrifying microbes dominated the N 2 O production and its temperature sensitivity. Using random forest (RF) and structural equation model (SEM) analyses, we further evaluated the effects of various soil characteristics on soil N 2 O emissions and Q 10 . We identified soil moisture, pH, N mineralization and AOB abundance as the main predictors of the Q 10 of N 2 O emissions. Together, these findings suggest that alterations in soil moisture, pH and ammonia-oxidizing bacteria induced by long-term N inputs and warming may increase temperature sensitivity of soil N 2 O emissions, leading to a positive climate feedback in this high-altitude alpine ecosystem., (Copyright © 2019 Elsevier B.V. All rights reserved.)