Whendee L. Silver

Nitrous oxide (N ₂ O) is a potent greenhouse gas (GHG) whose atmospheric concentration continues to rise, largely driven by nitrogen (N) inputs to agricultural soils. Over the past three decades, research on soil N ₂ O emissions has advanced substantially, yet key uncertainties still constrain mitigation efforts. Here, we synthesize developments in measurement techniques, process understanding, microbial ecology, and modelling from the 1990s to the present, and identify critical gaps for future research. Advances in high‐frequency measurements, laser spectroscopy, and isotopic approaches have revealed the importance of temporal “hot moments” and spatial “hotspots,” challenging earlier assumptions based on sparse sampling. Concurrently, molecular and multi‐omic tools have transformed our understanding of the microbial drivers of N ₂ O production and consumption, highlighting the role of community composition, truncated pathways, and previously overlooked N ₂ O‐producing and reducing organisms. Process‐based models have evolved from research tools into policy‐relevant frameworks underpinning GHG inventories, with emerging integration of data assimilation, ensemble modelling, and artificial intelligence. However, despite these advances, persistent challenges remain in linking scales, reducing uncertainties, and translating mechanistic insights into scalable mitigation strategies. Closing these gaps offers a unique opportunity to translate decades of scientific progress into next‐generation mitigation strategies that align agricultural productivity with climate stabilization goals.

Many wet tropical soils alternate frequently between fully oxygenated and anaerobic conditions, constraining the terminal electron acceptors available for microbial metabolism, and the mineral-organic matter interactions that regulate many aspects of soil carbon (C) cycling. However, it is still unclear how fluctuating soil redox conditions influence the microbial community composition and if microbially mediated C flux is sensitive to extended oxic or anoxic periods like those observed during drought and flooding respectively. Using a 44-day redox manipulation of tropical soils that experience daily-to-weekly oxygen (O ₂ ) fluctuations in the field, we measured how different redox regimes shape soil biogeochemistry and microbial and metabolite composition. Replicate microcosms were exposed to four treatments (static oxic, static anoxic, high frequency fluctuation (4 day oxic/4 day anoxic), or low frequency fluctuation (8 day oxic/4 day anoxic)) regimes, and harvested for microbial, metabolite, carbon dioxide (CO ₂ ) flux, and biogeochemical assays at multiple timepoints. Oxic and fluctuating redox conditions caused the microbial community to shift in a manner correlated with soil iron content and directly orthogonal to communities from anoxic soils. The identity of both iron oxidizers and iron reducers was distinct in static anoxic soils but was resilient to redox fluctuation and prolonged O ₂ exposure. The total amount of CO ₂ respired was similar across all four redox regimes. Water-extractable organic matter composition was distinct across redox treatments, with anoxic soils accumulating higher levels of carbohydrate-, proteins-, amino sugar-, and lignin-like compounds consistent with reduced enzymatic decomposition and release of mineral-associated organic matter via iron reduction, while oxic soils showed elevated lipid- and unsaturated hydrocarbon-like compounds indicative of greater microbial biomass turnover. The microbial community adapted to dynamic redox conditions and the results substantiate cycling of distinct C compounds under varying redox conditions resulting from varying bioavailability (driven by mineral-OM dynamics) and/or shifted microbial metabolism.

Continuous automated measurements are essential for quantifying the spatial and temporal variability in soil greenhouse gas fluxes and for resolving “hot spot” and “hot moments” that contribute disproportionately to ecosystem-scale emissions. This is especially important for methane (CH4) and nitrous oxide (N2O), where short-lived emission pulses can account for a substantial fraction of the annual mean and are readily missed by low-frequency sampling.We found that hot moments accounted for 700% of seasonal means. Similarly, in peatland maize system, hot spots and hot moments accounted for

Enhanced weathering in agriculture is a potential gigatonne-scale carbon dioxide removal (CDR) pathway, but its potential remains difficult to constrain. We used a formal expert elicitation process to estimate CDR potential and efficiency, uncertainties, and key data needs for six feedstocks. Expert opinion of global potential varied by feedstock, with estimates averaging 0.2-0.7 Gt CO2e/yr, but with a wide range (from a source to greater than 5 Gt CO2e/yr removal). When focusing on the American Midwest (pH 5.5-6), carbon dioxide removal efficiency, meaning the fraction of potential ultimately realized, ranged from 27-39%. Key uncertainties included feedstock availability, calcite saturation, and deep soil/freshwater emission pathways. There is a need for empirical data in key stages, with potential to leverage liming data where appropriate. Overall, there appears to be strong potential CDR at broad scales. However, continued research is necessary to build confidence when quantifying that potential and actual removals.

Nitrogen (N) fertilizer supports global food production, but its use and overuse drive emissions of nitrous oxide (N ₂ O), a potent and long-lived greenhouse gas. Understanding the drivers of N ₂ O fluxes remains elusive, making it difficult to predict emissions in time and space and to develop and evaluate ways to lower emissions through management. Major scientific uncertainties underlying the understanding of the drivers of N ₂ O fluxes identified in a workshop of N ₂ O emissions experts include poor process-based understanding of controls on soil N ₂ O emissions in the field; insufficient data to reduce uncertainty in N ₂ O budgets from the field to regional scales, including N ₂ O emission measurements and importantly, field-scale N balances; and high uncertainty in model predictions of soil N ₂ O emissions across environmental and management conditions. To reduce these uncertainties, we present the concept of N ₂ Onet, a global collaborative initiative to accelerate advances in N ₂ O measurement, analyses, and mitigation. N ₂ Onet will serve as an observational network of supersites with multi-scale measurements; a database hub for N ₂ O flux and ancillary data; and a catalyst for community building, information sharing, and training. By coalescing and coordinating the global community of researchers, N ₂ Onet will provide a roadmap for reducing N ₂ O emissions from agriculture worldwide.

Root distributions are typically based on root mass per soil volume. This plant‐focused approach masks the biogeochemical influence of fine roots, which weigh little. We assert that centimeter‐scale root presence‐absence data from soil profiles provide a more soil‐focused approach for probing depth distributions of root‐regolith interfaces, where microsite‐scale processes drive whole‐ecosystem functioning. In 75 soil pits across the continental USA, Puerto Rico, and the Alps, we quantified fine and coarse root presence as deep as 2 m. In 70 of these pits we estimated root mass and created standardized metrics of both data sets to compare their depth distributions. We addressed whether: (a) depth distributions of root presence‐absence data differ from root mass data, thus implying different degrees of root‐regolith interactions with depth; and (b) if root presence or any depth‐dependent differences between these data sets vary predictably with environmental conditions. Presence of fine roots exhibited diverse depth‐dependent patterns; root mass generally declined with depth. In B and C horizons, standardized root presence was greater than standardized root mass; random forest analyses suggest these discrepancies are greater in B horizons with increasing mean annual precipitation and in C horizons with increasing mean annual temperature. Our work suggests that deep in the subsurface, biogeochemical and reactive transport processes result from more numerous root‐regolith interfaces than mass data suggest. We present a new paradigm for discerning patterns in depth distributions of root‐regolith interfaces across multiple biomes and land uses that promotes understanding of the roles of those interfaces in driving key critical zone processes.

Composted amendments provide an opportunity to sequester organic carbon in soil, contribute to a circular carbon and nutrient economy, reduce landfill waste, and enhance soil health. In some cases, carbon sequestered in soils through compost land applications may be creditable in carbon markets, bringing in revenue to producers as well. One key challenge in the push to incentivize composting through carbon markets is the difficulty in assessing long-term compost carbon permanence accurately, which is critical for pricing compost credits. In this study, we tested a range of commercially available compost samples from Texas and California using standard organic geochemical metrics for organic matter permanence derived from 13C solid-state NMR spectroscopy. The results show variability in compost carbon permanence as assessed by this tool, suggesting that NMR may provide markets with finer resolution information for price setting. One potential uncertainty in NMR assessments of compost carbon permanence is the use of bulking agents in the composting process. Overall, however, 13C solid state NMR offers advantages over traditional California compost maturity tests by enabling standardized maturity indices and more robust quantification of long-term carbon sequestration.

Enhanced weathering (EW) in agriculture is a potential gigatonne-scale carbon dioxide removal (CDR) pathway. The true scale of potential CDR remains difficult to constrain due to its complexity across the entire field-to-ocean pathway and a paucity of system-level empirical data. We used a formal expert elicitation process to quantify the ranges of best CDR estimates, uncertainties, and key data needs for six EW feedstocks. Expert opinion of the CDR potential varied by feedstock, with estimates averaging 0.2-0.7 Gt CO2e/yr, but with a wide range (less than zero to greater than 5 Gt CO2e/yr). The efficiency of CDR, meaning the fraction of potential CDR ultimately realized from a given amount of material applied ranged from 27-39%. Key constraints included feedstock availability at scale (especially for wollastonite), calcite saturation, secondary clay formation, and deep soil/freshwater emission pathways. The results suggest a strong need for additional data collection (given deployments are already occurring), leveraging existing data on liming where appropriate, and continued study as applications occur at scale. Overall, there appears to be significant CDR potential for EW at broad scales, though quantification and underlying data uncertainties are significant and should be resolved.

Formation of mineral-associated organic matter (MAOM) is a key process in the global carbon cycle, stabilising organic carbon in soils. The relative importance of mineral composition and land use as potential controls of MAOM stability at regional scales and underlying microbial processes are still unresolved. Here, we assessed the stability of MAOM formed on goethite (iron oxide) and illite (phyllosilicate clay) exposed for five years in topsoils at 68 forest and grassland sites across Germany. We incubated the newly formed MAOM, determined its extractability, and analysed the composition and functioning of associated microbial communities. Decomposition of MAOM was always significantly lower for goethite than illite, highlighting that higher organic carbon accumulation on goethite was not exclusively due to its larger sorption capacity. Instead, reduced organic carbon extractability and higher phosphorus-acquiring enzyme activities indicated stronger substrate limitation of microbial growth on goethite than illite. Across the two minerals, MAOM decomposition was consistently lower for forests than grasslands, relating to greater nutrient constraints and a different microbial community composition in forests. Overall, mineral type and land use explained 34.6 and 23.2% of the variance in MAOM decomposition. The pronounced land use effect on MAOM stability underlines its potential responsiveness to environmental change.

Carbon dioxide (CO₂) removal (carbon dioxide removal (CDR)) that combines decreased greenhouse gas emissions with atmospheric CO₂ reduction is needed to limit climate change. Enhanced rock weathering (ERW) of ground silicate minerals is an emerging CDR technology with the potential to decrease atmospheric CO₂. However, there are few multi‐year field studies and considerable uncertainty in field‐rates of ERW. We explored combining finely ground metabasaltic rock with other soil CDR technologies (compost and biochar amendments) to stimulate carbon (C) sequestration. The combined ground rock (GR), compost, and biochar amendment had the greatest increases in soil C stocks over 3 years (15.3 ± 4.8 Mg C ha⁻¹). All other treatments slowed or reversed background C losses, with GR‐only treatments reducing rates of soil C loss relative to the control but still losing soil C over time. Ground rock amendments lowered nitrous oxide (N₂O) emissions by 11.0 ± 0.6 kg CO₂e ha⁻¹ yr⁻¹ and increased methane (CH₄) consumption by 9.5 ± 3.5 to 18.4 ± 4.4 kg CO₂e ha⁻¹ yr⁻¹; while noteworthy, emissions reductions were an order of magnitude smaller than organic C sequestration with compost amendments. The combined amendment yielded the greatest estimated net ecosystem benefit (3 year relative changes in soil C, estimated ERW rates, and greenhouse gas emissions) of −86.0 ± 24.7 Mg CO₂e ha⁻¹. Benefits were dominated by soil organic C gains, directly from organic amendments and indirectly from increased plant growth. Weathering rates were <10% of the theoretical potential. Combined ERW and organic amendments increased estimated weathering rates and stimulated soil organic C sequestration.