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1. Phosphorus rather than nitrogen enhances CO 2 emissions in tropical forest soils: Evidence from a laboratory incubation study

   https://doi.org/10.1111/ejss.12885  

   Hui, Dafeng ; Porter, Wesley ; Phillips, Jana R. ; Aidar, Marcos P. M. ; Lebreux, Steven J. ; Schadt, Christopher W. ; Mayes, Melanie A. ( January 2020 , European Journal of Soil Science)

   Ecosystem functional responses such as soil CO₂emissions are constrained by microclimate, available carbon (C) substrates and their effects upon microbial activity. In tropical forests, phosphorus (P) is often considered as a limiting factor for plant growth, but it is still not clear whether P constrains microbial CO₂emissions from soils. In this study, we incubated seven tropical forest soils from Brazil and Puerto Rico with different nutrient addition treatments (no addition, Control; C, nitrogen (N) or P addition only; and combined C, N and P addition (CNP)). Cumulative soil CO₂emissions were fit with a Gompertz model to estimate potential maximum cumulative soil CO₂emission (C_m) and the rate of change of soil C decomposition (k). Quantitative polymerase chain reaction (qPCR) was conducted to quantify microbial biomass as bacteria and fungi. Results showed that P addition alone or in combination with C and N enhancedC_m, whereas N addition usually reducedC_m, and neither N nor P affected microbial biomass. Additions of CNP enhancedk, increased microbial abundances and altered fungal to bacterial ratios towards higher fungal abundance. Additions of CNP, however, tended to reduceC_mfor most soils when compared to C additions alone, suggesting that microbial growth associated with nutrient additions may have occurred at the expense of C decomposition. Overall, this study demonstrates that soil CO₂emission is more limited by P than N in tropical forest soils and those effects were stronger in soils low in P.

   A laboratory incubation study was conducted with nitrogen, phosphorus or carbon addition to tropical forest soils. Soil CO₂emission was fitted with a Gompertz model and soil microbial abundance was quantified using qPCR. Phosphorus addition increased model parametersC_mand soil CO₂emission, particularly in the Puerto Rico soils. Soil CO₂emission was more limited by phosphorus than nitrogen in tropical forest soils.

    

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2. Multi-year incubation experiments boost confidence in model projections of long-term soil carbon dynamics

   https://doi.org/10.1038/s41467-020-19428-y  

   Jian, Siyang ; Li, Jianwei ; Wang, Gangsheng ; Kluber, Laurel A. ; Schadt, Christopher W. ; Liang, Junyi ; Mayes, Melanie A. ( November 2020 , Nature Communications)

   Global soil organic carbon (SOC) stocks may decline with a warmer climate. However, model projections of changes in SOC due to climate warming depend on microbially-driven processes that are usually parameterized based on laboratory incubations. To assess how lab-scale incubation datasets inform model projections over decades, we optimized five microbially-relevant parameters in the Microbial-ENzyme Decomposition (MEND) model using 16 short-term glucose (6-day), 16 short-term cellulose (30-day) and 16 long-term cellulose (729-day) incubation datasets with soils from forests and grasslands across contrasting soil types. Our analysis identified consistently higher parameter estimates given the short-term versus long-term datasets. Implementing the short-term and long-term parameters, respectively, resulted in SOC loss (–8.2 ± 5.1% or –3.9 ± 2.8%), and minor SOC gain (1.8 ± 1.0%) in response to 5 °C warming, while only the latter is consistent with a meta-analysis of 149 field warming observations (1.6 ± 4.0%). Comparing multiple subsets of cellulose incubations (i.e., 6, 30, 90, 180, 360, 480 and 729-day) revealed comparable projections to the observed long-term SOC changes under warming only on 480- and 729-day. Integrating multi-year datasets of soil incubations (e.g., > 1.5 years) with microbial models can thus achieve more reasonable parameterization of key microbial processes and subsequently boost the accuracy and confidence of long-term SOC projections.

    

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

   https://doi.org/10.1111/gcb.16669  

   Andrews, Holly M. ; Krichels, Alexander H. ; Homyak, Peter M. ; Piper, Stephanie ; Aronson, Emma L. ; Botthoff, Jon ; Greene, Aral C. ; Jenerette, G. Darrel ( March 2023 , Global Change Biology)

   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 CO₂flux 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 CO₂pulses following wetting were highly predictable from peak instantaneous CO₂flux measurements. CO₂pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO₂pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO₂pulses in low N deposition sites, whereas adding N decreased CO₂pulses 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 CO₂fluxes reported globally at 299.5 μmol CO₂ m⁻² s⁻¹. Our results suggest that soils have the capacity to emit high amounts of CO₂within small timeframes following infrequent wetting, and pulse sizes reflect a non‐linear combination of soil resource and temperature interactions. Importantly, the largest soil CO₂emissions 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.

    

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4. 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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5. Disentangling the long‐term effects of disturbance on soil biogeochemistry in a wet tropical forest ecosystem

   https://doi.org/10.1111/gcb.14027  

   Gutiérrez del Arroyo, Omar ; Silver, Whendee L. ( January 2018 , Global Change Biology)

   Climate change is increasing the intensity of severe tropical storms and cyclones (also referred to as hurricanes or typhoons), with major implications for tropical forest structure and function. These changes in disturbance regime are likely to play an important role in regulating ecosystem carbon (C) and nutrient dynamics in tropical and subtropical forests. Canopy opening and debris deposition resulting from severe storms have complex and interacting effects on ecosystem biogeochemistry. Disentangling these complex effects will be critical to better understand the long‐term implications of climate change on ecosystem C and nutrient dynamics. In this study, we used a well‐replicated, long‐term (10 years) canopy and debris manipulation experiment in a wet tropical forest to determine the separate and combined effects of canopy opening and debris deposition on soil C and nutrients throughout the soil profile (1 m). Debris deposition alone resulted in higher soil C and N concentrations, both at the surface (0–10 cm) and at depth (50–80 cm). Concentrations of NaOH‐organic P also increased significantly in the debris deposition only treatment (20–90 cm depth), as did NaOH‐total P (20–50 cm depth). Canopy opening, both with and without debris deposition, significantly increased NaOH‐inorganic P concentrations from 70 to 90 cm depth. Soil iron concentrations were a strong predictor of both C and P patterns throughout the soil profile. Our results demonstrate that both surface‐ and subsoils have the potential to significantly increase C and nutrient storage a decade after the sudden deposition of disturbance‐related organic debris. Our results also show that these effects may be partially offset by rapid decomposition and decreases in litterfall associated with canopy opening. The significant effects of debris deposition on soil C and nutrient concentrations at depth (>50 cm), suggest that deep soils are more dynamic than previously believed, and can serve as sinks of C and nutrients derived from disturbance‐induced pulses of organic matter inputs.

    

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