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1. Microbes alter substrate from mineral‐associated carbon to litterfall with nitrogen additions and warming

   https://doi.org/10.1111/ejss.13487  

   Min, Kyungjin ; Billings, Sharon A. ( April 2024 , European Journal of Soil Science)

   Nitrogen (N) additions often decrease soil respiration and increase soil organic carbon (C) stock. However, it is unclear how microbial substrates may shift with N additions and increasing temperature. Leveraging 12 years of N fertilization experiments and the associated shift in the dominant vegetation from C₄to C₃, we explored the δ¹³C‐CO₂and temperature sensitivities of respired CO₂and extracellular enzyme activities in control and fertilized soils. N additions increased cellulose‐decaying extracellular enzyme activity while respiration remained similar between the control and fertilized soils. Temperature sensitivity of cellulose‐decaying extracellular enzyme activity decreased with the N additions. The δ¹³C‐CO₂data reveal that, as temperature increased, microbes in fertilized soils changed their dominant substrate from bulk soil organic C to plant litterfall. Our results suggest that long‐term N fertilization imposed C limitation on microbes, leading to enhanced microbial efforts to acquire C. This study highlights how long‐term N additions can promote the relative preservation of organic C in mineral soil while litterfall, the precursor to mineral‐associated C, is increasingly decayed as temperatures increase.

    

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2. Temperature sensitivity of biomass‐specific microbial exo‐enzyme activities and CO 2 efflux is resistant to change across short‐ and long‐term timescales

   https://doi.org/10.1111/gcb.14605  

   Min, Kyungjin ; Buckeridge, Kate ; Ziegler, Susan E. ; Edwards, Kate A. ; Bagchi, Samik ; Billings, Sharon A. ( March 2019 , Global Change Biology)

   Accurate representation of temperature sensitivity (Q₁₀) of soil microbial activity across time is critical for projecting soil CO₂efflux. As microorganisms mediate soil carbon (C) loss via exo‐enzyme activity and respiration, we explore temperature sensitivities of microbial exo‐enzyme activity and respiratory CO₂loss across time and assess mechanisms associated with these potential changes in microbial temperature responses. We collected soils along a latitudinal boreal forest transect with different temperature regimes (long‐term timescale) and exposed these soils to laboratory temperature manipulations at 5, 15, and 25°C for 84 days (short‐term timescale). We quantified temperature sensitivity of microbial activity per g soil and per g microbial biomass at days 9, 34, 55, and 84, and determined bacterial and fungal community structure before the incubation and at days 9 and 84. All biomass‐specific rates exhibited temperature sensitivities resistant to change across short‐ and long‐term timescales (meanQ₁₀ = 2.77 ± 0.25, 2.63 ± 0.26, 1.78 ± 0.26, 2.27 ± 0.25, 3.28 ± 0.44, 2.89 ± 0.55 for β‐glucosidase,N‐acetyl‐β‐d‐glucosaminidase, leucine amino peptidase, acid phosphatase, cellobiohydrolase, and CO₂efflux, respectively). In contrast, temperature sensitivity of soil mass‐specific rates exhibited either resilience (theQ₁₀value changed and returned to the original value over time) or resistance to change. Regardless of the microbial flux responses, bacterial and fungal community structure was susceptible to change with temperature, significantly differing with short‐ and long‐term exposure to different temperature regimes. Our results highlight that temperature responses of microbial resource allocation to exo‐enzyme production and associated respiratory CO₂loss per unit biomass can remain invariant across time, and thus, that vulnerability of soil organic C stocks to rising temperatures may persist in the long term. Furthermore, resistant temperature sensitivities of biomass‐specific rates in spite of different community structures imply decoupling of community constituents and the temperature responses of soil microbial activities.

    

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3. Soil carbon and nitrogen cycling at the atmosphere–soil interface: Quantifying the responses of biocrust–soil interactions to global change

   https://doi.org/10.1111/gcb.17519  

   Witzgall, K. ; Hesse, B_D ; Pacay‐Barrientos, N_L ; Jansa, J. ; Seguel, O. ; Oses, R. ; Buegger, F. ; Guigue, J. ; Rojas, C. ; Rousk, K. ; et al ( October 2024 , Global Change Biology)

   In drylands, where water scarcity limits vascular plant growth, much of the primary production occurs at the soil surface. This is where complex macro‐ and microbial communities, in an intricate bond with soil particles, form biological soil crusts (biocrusts). Despite their critical role in regulating C and N cycling in dryland ecosystems, there is limited understanding of the fate of biologically fixed C and N from biocrusts into the mineral soil, or how climate change will affect C and N fluxes between the atmosphere, biocrusts, and subsurface soils. To address these gaps, we subjected biocrust–soil systems to experimental warming and drought under controlled laboratory conditions, monitored CO₂fluxes, and applied dual isotopic labeling pulses (¹³CO₂and¹⁵N₂). This allowed detailed quantification of elemental pathways into specific organic matter (OM) pools and microbial biomass via density fractionation and phospholipid fatty acid analyses. While biocrusts modulated CO₂fluxes regardless of the temperature regime, drought severely limited their photosynthetic C uptake to the extent that the systems no longer sustained net C uptake. Furthermore, the effect of biocrusts extended into the underlying 1 cm of mineral soil, where C and N accumulated as mineral‐associated OM (MAOM_<63μm). This was strongly associated with increased relative dominance of fungi, suggesting that fungal hyphae facilitate the downward C and N translocation and subsequent MAOM formation. Most strikingly, however, these pathways were disrupted in systems exposed to warming, where no effects of biocrusts on the elemental composition of the underlying soil nor on MAOM were determined. This was further associated with reduced net biological N fixation under combined warming and drought, highlighting how changing climatic conditions diminish some of the most fundamental ecosystem functions of biocrusts, with detrimental repercussions for C and N cycling and the persistence of soil organic matter pools in dryland ecosystems.

    

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4. Multiple carbon cycle mechanisms associated with the glaciation of Marine Isotope Stage 4

   https://doi.org/10.1038/s41467-022-33166-3  

   Menking, James A. ; Shackleton, Sarah A. ; Bauska, Thomas K. ; Buffen, Aron M. ; Brook, Edward J. ; Barker, Stephen ; Severinghaus, Jeffrey P. ; Dyonisius, Michael N. ; Petrenko, Vasilii V. ( September 2022 , Nature Communications)

   Here we use high-precision carbon isotope data (δ¹³C-CO₂) to show atmospheric CO₂during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ¹³C-CO₂during peak glaciation suggests increased ocean carbon storage. Variations in δ¹³C-CO₂in early MIS 4 suggest multiple processes were active during CO₂drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO₂remained low during MIS 4 while δ¹³C-CO₂fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO₂at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO₂during late MIS 4 (Heinrich Stadial 6, ~63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage.

    

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5. Long‐term nitrogen deposition inhibits soil priming effects by enhancing phosphorus limitation in a subtropical forest

   https://doi.org/10.1111/gcb.16718  

   Wang, Xiaohong ; Li, Shiyining ; Zhu, Biao ; Homyak, Peter M. ; Chen, Guangshui ; Yao, Xiaodong ; Wu, Dongmei ; Yang, Zhijie ; Lyu, Maokui ; Yang, Yusheng ( April 2023 , Global Change Biology)

   It is widely accepted that phosphorus (P) limits microbial metabolic processes and thus soil organic carbon (SOC) decomposition in tropical forests. Global change factors like elevated atmospheric nitrogen (N) deposition can enhance P limitation, raising concerns about the fate of SOC. However, how elevated N deposition affects the soil priming effect (PE) (i.e., fresh C inputs induced changes in SOC decomposition) in tropical forests remains unclear. We incubated soils exposed to 9 years of experimental N deposition in a subtropical evergreen broadleaved forest with two types of¹³C‐labeled substrates of contrasting bioavailability (glucose and cellulose) with and without P amendments. We found that N deposition decreased soil total P and microbial biomass P, suggesting enhanced P limitation. In P unamended soils, N deposition significantly inhibited the PE. In contrast, adding P significantly increased the PE under N deposition and by a larger extent for the PE of cellulose (PE_cellu) than the PE of glucose (PE_glu). Relative to adding glucose or cellulose solely, adding P with glucose alleviated the suppression of soil microbial biomass and C‐acquiring enzymes induced by N deposition, whereas adding P with cellulose attenuated the stimulation of acid phosphatase (AP) induced by N deposition. Across treatments, the PE_gluincreased as C‐acquiring enzyme activity increased, whereas the PE_celluincreased as AP activity decreased. This suggests that P limitation, enhanced by N deposition, inhibits the soil PE through varying mechanisms depending on substrate bioavailability; that is, P limitation regulates the PE_gluby affecting soil microbial growth and investment in C acquisition, whereas regulates the PE_celluby affecting microbial investment in P acquisition. These findings provide new insights for tropical forests impacted by N loading, suggesting that expected changes in C quality and P limitation can affect the long‐term regulation of the soil PE.

    

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