Wetting-induced soil CO2 emission pulses are driven by interactions among soil temperature, carbon, and nitrogen limitation in the Colorado Desert

Warming-induced changes in precipitation regimes, coupled with anthropogenically enhanced nitrogen (N) deposition, are likely to increase the prevalence, duration, and magnitude of soil respiration pulses following wetting via interactions among temperature and carbon (C) and N availability. Quantifying the importance of these interactive controls on soil respiration is a key challenge as pulses can be large terrestrial sources of atmospheric carbon dioxide (CO₂ ) over comparatively short timescales. Using an automated sensor system, we measured soil CO₂ flux dynamics in the Colorado Desert-a system characterized by pronounced transitions from dry-to-wet soil conditions-through a multi-year series of experimental wetting campaigns. Experimental manipulations included combinations of C and N additions across a range of ambient temperatures and across five sites varying in atmospheric N deposition. We found soil CO₂ pulses following wetting were highly predictable from peak instantaneous CO₂ flux measurements. CO₂ pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO₂ pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO₂ pulses in low N deposition sites, whereas adding N decreased CO₂ pulses in high N deposition sites. At a low N deposition site, simultaneous additions of C and N during wetting led to the highest observed soil CO₂ fluxes reported globally at 299.5 μmol CO₂  m^-2  s^-1 . Our results suggest that soils have the capacity to emit high amounts of CO₂ within small timeframes following infrequent wetting, and pulse sizes reflect a non-linear combination of soil resource and temperature interactions. Importantly, the largest soil CO₂ emissions occurred when multiple resources were amended simultaneously in historically resource-limited desert soils, pointing to regions experiencing simultaneous effects of desertification and urbanization as key locations in future global C balance.

Linkages between nutrient ratio and the microbial community in rhizosphere soil following fertilizer management

To unravel the linkages between ecological ratios (C:N:P) and the microbial community in rhizosphere soil in response to fertilizer management, soil samples were collected from a proso millet (Panicum miliaceum L.) field under different fertilizer management systems, including nitrogen fertilizer (NF), phosphorus fertilizer (PF), combined N and P (NP) fertilizer, and organic fertilizer (OF); no fertilizer (CK) was used as a control. Furthermore, 16S rRNA and ITS gene sequencing were applied to represent the bacterial and fungal diversity in the soil. Moreover, the elemental properties, including the carbon (C), nitrogen (N), and phosphorus (P) contents, in the microbial biomass and rhizosphere soil were evaluated. The results showed that the C, N, and P contents and microbial biomass (MBC, MBN and MBP, respectively) in the rhizosphere soil were augmented following fertilizer management. Increases in the alpha diversity indices (Shannon and Chao 1) of soil bacteria and fungi were observed in response to the fertilizers, and the responses were more closely related to the soil C:N and N:P ratios than to the C:P ratio. Additionally, with high relative abundances (>1%) across all soil samples, the composition of soil microbial phyla levels revealed different trends following fertilizer management. The abundances of Actinobacteria and Gemmatimonadetes increased, while the abundances of Acidobacteria and Nitrospirae decreased (P < 0.05) following fertilizer management. Among the fungal taxa, the abundances of Ascomycota and Mortierellomycota responded positively to fertilizer. These results were largely influenced by changes in the C:N and N:P ratios in both the soil and microbial biomass. Overall, significantly increased C:N and decreased N:P ratios in the soil reflected the N deficiency that would limit increased microbial biomass and diversity. Together, all of these results indicated that interactions between ecological ratios (C:N:P) and microbial community composition play vital roles in resource imbalance in dynamic environments. Thus, N status should be an important factor for sustainable agricultural management. Moreover, the synergistic effects were better with the combination of C, N, and P or with organic fertilizer than with C, N and P separately.

Contribution of root respiration to total soil respiration in a semi-arid grassland on the Loess Plateau, China

Using the trenching method, a study was conducted in a grassland on the Loess Plateau of northern China in 2008 and 2009 to partition total soil respiration (Rt) into microbial respiration (Rm) and root respiration (Rr). Using the measurements of soil CO₂ diffusivity and soil CO₂ production, an analytical model was applied to correct the data, aiming to quantify the method-induced error. The results showed that Rm and Rr responded differently to biotic and abiotic factors and exhibited different diurnal and seasonal variations. The diurnal variation of Rm was strongly controlled by soil temperature, while Rr might be mainly controlled by photosynthesis. The combination of soil temperature and moisture could better explain the seasonal variation in Rm (r²=0.76, P<0.001). The seasonal variation of Rr was influenced mainly by the plant activity. The contribution of root respiration to total soil respiration (Rr/Rt ratio) also exhibited substantial diurnal and seasonal variations, being higher at nighttime and lower at daytime. In the different growing stages, the Rr/Rt ratios ranged from 15.0% to 62.0% in 2008 and 14.5% to 63.6% in 2009. The mean values of the Rr/Rt ratio in the growing season and the annual mean Rr/Rt ratio were 41.7% and 41.9%, respectively, during the experiment period. Different precipitation distributions in the two years did not change the yearly Rr/Rt ratio. Corrected with the analytical model, the trenching method in small root-free plots led to an underestimation of Rr and Rr/Rt ratio by 4.2% and 1.8%.

Denitrification potential of riparian soils in relation to multiscale spatial environmental factors: a case study of a typical watershed, China

The objective of this study was to test the hypothesis that environmental regulators of riparian zone soil denitrification potential differ according to spatial scale within a watershed; consequently, a second objective was to provide spatial strategies for conserving and restoring the purification function of runoff in riparian ecosystems. The results show that soil denitrification in riparian zones was more heterogeneous at the profile scale than at the cross-section and landscape scales. At the profile scale, biogeochemical factors (including soil total organic carbon, total nitrogen, and nitrate-nitrogen) were the major direct regulators of the spatial distribution of soil denitrification enzyme activity (DEA). At the cross-section scale, factors included distance from river bank and vegetation density, while landscape-scale factors, including topographic index, elevation, and land use types, indirectly regulated the spatial distribution of DEA. At the profile scale, soil DEA was greatest in the upper soil layers. At the cross-section scale, maximum soil DEA occurred in the mid-part of the riparian zone. At the landscape scale, soil DEA showed an increasing trend towards downstream sites, except for those in urbanized areas.

Groundwater-surface water mixing shifts ecological assembly processes and stimulates organic carbon turnover

Environmental transitions often result in resource mixtures that overcome limitations to microbial metabolism, resulting in biogeochemical hotspots and moments. Riverine systems, where groundwater mixes with surface water (the hyporheic zone), are spatially complex and temporally dynamic, making development of predictive models challenging. Spatial and temporal variations in hyporheic zone microbial communities are a key, but understudied, component of riverine biogeochemical function. Here, to investigate the coupling among groundwater-surface water mixing, microbial communities and biogeochemistry, we apply ecological theory, aqueous biogeochemistry, DNA sequencing and ultra-high-resolution organic carbon profiling to field samples collected across times and locations representing a broad range of mixing conditions. Our results indicate that groundwater-surface water mixing in the hyporheic zone stimulates heterotrophic respiration, alters organic carbon composition, causes ecological processes to shift from stochastic to deterministic and is associated with elevated abundances of microbial taxa that may degrade a broad suite of organic compounds.

Multivariate regulation of soil CO2 and N2 O pulse emissions from agricultural soils

Climate and land-use models project increasing occurrence of high temperature and water deficit in both agricultural production systems and terrestrial ecosystems. Episodic soil wetting and subsequent drying may increase the occurrence and magnitude of pulsed biogeochemical activity, affecting carbon (C) and nitrogen (N) cycles and influencing greenhouse gas (GHG) emissions. In this study, we provide the first data to explore the responses of carbon dioxide (CO2 ) and nitrous oxide (N2 O) fluxes to (i) temperature, (ii) soil water content as percent water holding capacity (%WHC), (iii) substrate availability throughout, and (iv) multiple soil drying and rewetting (DW) events. Each of these factors and their interactions exerted effects on GHG emissions over a range of four (CO2 ) and six (N2 O) orders of magnitude. Maximal CO2 and N2 O fluxes were observed in environments combining intermediate %WHC, elevated temperature, and sufficient substrate availability. Amendments of C and N and their interactions significantly affected CO2 and N2 O fluxes and altered their temperature sensitivities (Q10 ) over successive DW cycles. C amendments significantly enhanced CO2 flux, reduced N2 O flux, and decreased the Q10 of both. N amendments had no effect on CO2 flux and increased N2 O flux, while significantly depressing the Q10 for CO2 , and having no effect on the Q10 for N2 O. The dynamics across DW cycles could be attributed to changes in soil microbial communities as the different responses to wetting events in specific group of microorganisms, to the altered substrate availabilities, or to both. The complex interactions among parameters influencing trace gas fluxes should be incorporated into next generation earth system models to improve estimation of GHG emissions.

Impacts of altered precipitation regimes on soil communities and biogeochemistry in arid and semi-arid ecosystems

Altered precipitation patterns resulting from climate change will have particularly significant consequences in water-limited ecosystems, such as arid to semi-arid ecosystems, where discontinuous inputs of water control biological processes. Given that these ecosystems cover more than a third of Earth's terrestrial surface, it is important to understand how they respond to such alterations. Altered water availability may impact both aboveground and belowground communities and the interactions between these, with potential impacts on ecosystem functioning; however, most studies to date have focused exclusively on vegetation responses to altered precipitation regimes. To synthesize our understanding of potential climate change impacts on dryland ecosystems, we present here a review of current literature that reports the effects of precipitation events and altered precipitation regimes on belowground biota and biogeochemical cycling. Increased precipitation generally increases microbial biomass and fungal:bacterial ratio. Few studies report responses to reduced precipitation but the effects likely counter those of increased precipitation. Altered precipitation regimes have also been found to alter microbial community composition but broader generalizations are difficult to make. Changes in event size and frequency influences invertebrate activity and density with cascading impacts on the soil food web, which will likely impact carbon and nutrient pools. The long-term implications for biogeochemical cycling are inconclusive but several studies suggest that increased aridity may cause decoupling of carbon and nutrient cycling. We propose a new conceptual framework that incorporates hierarchical biotic responses to individual precipitation events more explicitly, including moderation of microbial activity and biomass by invertebrate grazing, and use this framework to make some predictions on impacts of altered precipitation regimes in terms of event size and frequency as well as mean annual precipitation. While our understanding of dryland ecosystems is improving, there is still a great need for longer term in situ manipulations of precipitation regime to test our model.