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The molecular composition of soil organic carbon remains contentious. Microbial-, plant- and fire-derived compounds may each contribute, but whether they vary predictably among ecosystems remains unclear. Here we present carbon functional groups and molecules from a diverse spectrum of North American surface mineral soils, collected primarily from the National Ecological Observatory Network and quantified by nuclear magnetic resonance spectroscopy and a molecular mixing model. We find that soils vary widely in relative contributions of carbohydrate, lipid, protein, lignin and char-like carbon, but each compound class has similar overall abundance. Ninety percent of the variance in carbon composition can be explained by three principal component axes representing a trade-off between lignin and protein, a trade-off between carbohydrate and char, and lipids. Reactive aluminium, crystalline iron oxides and pH plus overlying organic horizon thickness—predictors that are all related to climate—best explain variation along each respective axis. Together, our data point to continental-scale trade-offs in soil carbon molecular composition that are linked to environmental and geochemical variables known to predict carbon mass concentrations. Controversies regarding the genesis of soil carbon and its potential responses to global change can be partially reconciled by considering diverse ecosystem properties that drive complementary persistence mechanisms.Environmental factors influence the molecular composition of carbon in soils across continental gradients, according to analyses of North American mineral soils.

Geomorphic position often correlates with nutrient cycling across landscapes. In tropical forests, topography is known to influence phosphorus (P) availability, but its effect on nitrogen (N) cycling has received less exploration, especially in lowland forests where widespread N richness is frequently assumed. Here, we report significant effects of topographic slope and landscape position on multiple aspects of the N cycle across a highly dissected lowland tropical forest on the Osa Peninsula, Costa Rica. A suite of N cycle metrics measured along a topographic sequence revealed a distinct gradient in N availability. Values of soil δ 15 N, inorganic N pools, net nitrification rates, and nitrification potentials were all substantially lower on a flanking steep hillslope (~28°) compared to a relatively flat ridge top (~6°), indicating lower N availability and a less open N cycle in steep parts of the landscape. Slope soils also hosted smaller total carbon and nitrogen stocks and notably less weathered soil minerals than did ridge soils. These latter findings suggest that elevated N loss resulting from high rates of soil and particulate organic matter erosion could underpin the spatial variation in N cycling and availability. Expanding our analysis to the larger study landscape, a strong negative linear relationship between soil δ 15 N values and surface slope angles was observed. N isotope mass balance models suggest that this pattern is most plausibly explained by an increase in N loss via erosive, non-fractioning pathways from steep zones, as most other variables commonly assumed to affect soil δ 15 N values (such as temperature, precipitation, and vegetation type) did not vary across the sampled region. Together, these results reveal notable hillslope-scale variation in N richness and suggest an important role for non-fractionating N loss in the maintenance of this pattern. Such findings highlight the importance of geomorphology and the significant capacity of erosion to influence N availability in steepland ecosystems.

Lignin is an abundant and complex plant polymer that may limit litter decomposition, yet lignin is sometimes a minor constituent of soil organic carbon (SOC). Accounting for diversity in soil characteristics might reconcile this apparent contradiction. Tracking decomposition of a lignin/litter mixture and SOC across different North American mineral soils using lab and field incubations, here we show that cumulative lignin decomposition varies 18-fold among soils and is strongly correlated with bulk litter decomposition, but not SOC decomposition. Climate legacy predicts decomposition in the lab, and impacts of nitrogen availability are minor compared with geochemical and microbial properties. Lignin decomposition increases with some metals and fungal taxa, whereas SOC decomposition decreases with metals and is weakly related with fungi. Decoupling of lignin and SOC decomposition and their contrasting biogeochemical drivers indicate that lignin is not necessarily a bottleneck for SOC decomposition and can explain variable contributions of lignin to SOC among ecosystems. Lignin’s contribution to soil organic carbon (SOC) is contentious. The authors find a decoupling of lignin and SOC decomposition and their contrasting relationships with geochemical and microbial factors, addressing a long-standing controversy.

Determining the fate of deposited nitrogen (N) in natural ecosystems remains a challenge. Heterogeneity of vegetation types and resulting plant–soil feedbacks interact with topo-hydrologic gradients to mediate spatial patterns of N availability and loss, yet net effects of variation in these two factors together across complex terrain remain unclear. Here we measured a suite of N-cycle pools and fluxes in sites that differed factorially in vegetation type (mixed forest vs. herbaceous) and topographic position (upslope vs. downslope) in a protected montane watershed near Salt Lake City, UT. Vegetation type was associated with large variation in N availability—herbaceous sites had larger NO₃⁻ pools, higher NO₃⁻:NH₄⁺ ratios, higher nitrification potentials, lower soil C:N values, enriched δ¹⁵N values, and lower microbial biomass compared to forests, especially those upslope. Downslope sites tended to exhibit higher N availability and indicators of N-cycle openness, but patterns were moderated by vegetation type. In downslope forest, soil NO₃⁻ depth profiles and higher foliar N content suggested trees were accessing deep soil N and transferring it to the surface via litterfall, while more deep soil NO₃⁻ but no change in surface or foliar N suggested herbaceous cover was not N limited or deeper N pools were not accessible. Soil NO₃⁻ leaching from below the rooting zone closely tracked N availability, revealing a link between N status and hydrologic loss as well as an important role for roots in N retention. NO₃⁻ isotopes did not reveal a similar link for gaseous losses (that is, denitrification), instead reflecting nitrification and/or transport dynamics. Together, these results suggest a coupled ecological, topo-hydrologic perspective can help assess the fate of N in complex landscapes.

Nitrogen (N) is a key limiting nutrient in terrestrial ecosystems, but there remain critical gaps in our ability to predict and model controls on soil N cycling. This may be in part due to lack of standardized sampling across broad spatial–temporal scales. Here, we introduce a continentally distributed, publicly available data set collected by the National Ecological Observatory Network (NEON) that can help fill these gaps. First, we detail the sampling design and methods used to collect and analyze soil inorganic N pool and net flux rate data from 47 terrestrial sites. We address methodological challenges in generating a standardized data set, even for a network using uniform protocols. Then, we evaluate sources of variation within the sampling design and compare measured net N mineralization to simulated fluxes from the Community Earth System Model 2 (CESM2). We observed wide spatiotemporal variation in inorganic N pool sizes and net transformation rates. Site explained the most variation in NEON’s stratified sampling design, followed by plots within sites. Organic horizons had larger pools and net N transformation rates than mineral horizons on a sample weight basis. The majority of sites showed some degree of seasonality in N dynamics, but overall these temporal patterns were not matched by CESM2, leading to poor correspondence between observed and modeled data. Looking forward, these data can reveal new insights into controls on soil N cycling, especially in the context of other environmental data sets provided by NEON, and should be leveraged to improve predictive modeling of the soil N cycle. Plain Language Summary Nitrogen (N) is not only a key limiting nutrient that often constrains plant growth but also a major pollutant in places where supply exceeds demand. However, we lack the ability to accurately predict and model rates of soil N cycling. This paper introduces a first of its kind, standardized and publicly available data set of soil inorganic N pools and net transformation rates spanning the United States. We describe the data set in detail, then examine spatiotemporal trends in N pools and fluxes and how those measurements compare to predictions from an Earth system model. Key Points The National Ecological Observatory Network (NEON) measures soil inorganic nitrogen (N) pools and net flux rates at continental scale The most N cycle variation is observed at the site level, followed by plots within sites. Organic horizons have high N pools and flux rates The Community Earth System Model 2 poorly predicts temporal trends in net N mineralization. NEON data can be used to improve predictive power

Macrosystem‐scale research is supported by many ecological networks of people, infrastructure, and data. However, no network is sufficient to address all macrosystems ecology research questions, and there is much to be gained by conducting research and sharing resources across multiple networks. Unfortunately, conducting macrosystem research across networks is challenging due to the diversity of expertise and skills required, as well as issues related to data discoverability, veracity, and interoperability. The ecological and environmental science community could substantially benefit from networking existing networks to leverage past research investments and spur new collaborations. Here, we describe the need for a “network of networks” (NoN) approach to macrosystems ecological research and articulate both the challenges and potential benefits associated with such an effort. We describe the challenges brought by rapid increases in the volume, velocity, and variety of “big data” ecology and highlight how a NoN could build on the successes and creativity within component networks, while also recognizing and improving upon past failures. We argue that a NoN approach requires careful planning to ensure that it is accessible and inclusive, incorporates multimodal communications and ways to interact, supports the creation, testing, and promulgation of community standards, and ensures individuals and groups receive appropriate credit for their contributions. Additionally, a NoN must recognize important trade‐offs in network architecture, including how the degree of centralization of people, infrastructure, and data influence network scalability and creativity. If implemented carefully and thoughtfully, a NoN has the potential to substantially advance our understanding of ecological processes, characteristics, and trajectories across broad spatial and temporal scales in an efficient, inclusive, and equitable manner.

Soil extracellular enzymes mediate organic matter turnover and nutrient cycling yet remain little studied in one of Earth’s most rapidly changing, productive biomes: tropical forests. Using a long-term leaf litter and throughfall manipulation, we explored relationships between organic matter (OM) inputs, soil chemical properties and enzyme activities in a lowland tropical forest. We assayed six hydrolytic soil enzymes responsible for liberating carbon (C), nitrogen (N) and phosphorus (P), calculated enzyme activities and ratios in control plots versus treatments, and related these to soil biogeochemical variables. While leaf litter addition and removal tended to increase and decrease enzyme activities per gram soil, respectively, shifts in enzyme allocation patterns implied changes in relative nutrient constraints with altered OM inputs. Enzyme activity ratios in control plots suggested strong belowground P constraints; this was exacerbated when litter inputs were curtailed. Conversely, with double litter inputs, increased enzymatic investment in N acquisition indicated elevated N demand. Across all treatments, total soil C correlated more strongly with enzyme activities than soluble C fluxes, and enzyme ratios were sensitive to resource stoichiometry (soil C:N) and N availability (net N mineralization). Despite high annual precipitation in this site (MAP ~5 m), soil moisture positively correlated with five of six enzymes. Our results suggest resource availability regulates tropical soil enzyme activities, soil moisture plays an additional role even in very wet forests, and relative investment in C, N and P degrading enzymes in tropical soils will often be distinct from higher latitude ecosystems yet is sensitive to OM inputs.

Natural abundance nitrate (NO₃⁻) isotopes represent a powerful tool for assessing denitrification, yet the scale and context dependence of relationships between isotopes and denitrification have received little attention, especially in surface soils. We measured the NO₃⁻ isotope compositions in soil extractions and lysimeter water from a semi-arid meadow and lawn during snowmelt, along with the denitrification potential, bulk O₂, and a proxy for anaerobic microsites. Denitrification potential varied by three orders of magnitude and the slope of δ¹⁸O/δ¹⁵N in soil-extracted NO₃⁻ from all samples measured 1.04 ± 0.12 (R² = 0.64, p < 0.0001), consistent with fractionation from denitrification. However, δ¹⁵ N of extracted NO₃⁻ was often lower than bulk soil δ¹⁵N (by up to 24 ‰), indicative of fractionation during nitrification that was partially overprinted by denitrification. Mean NO₃- isotopes in lysimeter water differed from soil extractions by up to 19 ‰ in δ¹⁸O and 12 ‰ in δ¹⁵N, indicating distinct biogeochemical processing in relatively mobile water versus soil microsites. This implies that NO₃⁻ isotopes in streams, which are predominantly fed by mobile water, do not fully reflect terrestrial soil N cycling. Relationships between potential denitrification and δ¹⁵N of extracted NO₃⁻ showed a strong threshold effect culminating in a null relationship at high denitrification rates. Our observations of (1) competing fractionation from nitrification and denitrification in redox-heterogeneous surface soils, (2) large NO₃⁻ isotopic differences between relatively immobile and mobile water pools, (3) and the spatial dependence of δ¹³O/δ¹⁵N relationships suggest caution in using NO₃⁻ isotopes to infer site or watershed-scale patterns in denitrification.

Secondary and managed plantation forests comprise a rapidly increasing portion of the humid tropical forest biome, a region that, in turn, is a major source of nitrous oxide (N 2 O) emissions to the atmosphere. Previous work has demonstrated reduced N 2 O emissions in regenerating secondary stands compared to mature forests, yet the importance of species composition in regulating N 2 O production in young forests remains unclear. We measured N 2 O fluxes beneath four native tree species planted in replicated, 21-yr-old monodominant stands in the Caribbean lowlands of Costa Rica in comparison with nearby mature forest and abandoned pasture sites at two time points (wetter and drier seasons). We found that species differed eight-fold in their production of N 2 O, with slower growing, late-successional species (including one legume) promoting high N 2 O fluxes similar to mature forest, and faster growing, early successional species maintaining low N 2 O fluxes similar to abandoned pasture. Across all species, N 2 O flux was positively correlated with soil nitrate concentration in the wetter season and with soil water-filled pore space (WFPS) in the drier season. However, the strongest predictor of N 2 O fluxes was fine-root growth rate, which was negatively correlated with N 2 O emissions at both time points. We suggest that tree-specific variation in growth habits creates differences in both N demand and soil water conditions that may exert significant control on N 2 O fluxes from tropical forests. With the advent of REDD+ and related strategies for fostering climate mitigation via tropical forest regrowth and plantations, we note that species-specific traits as they relate to N 2 O fluxes may be an important consideration in estimating overall climate benefits.

Environmental perturbations such as changes in land use, climate, and atmospheric carbon dioxide concentrations may alter organic matter inputs to surface soils. While the carbon (C) cycle response to such perturbations has received considerable attention, potential responses of the soil nitrogen (N) cycle to changing organic matter inputs have been less well characterized. Changing litter inputs to surface to soils may alter the soil N cycle directly, by controlling N substrate availability, or indirectly, via interactions with soil C biogeochemistry. We investigated soil N-cycling responses to a leaf litter manipulation in a lowland tropical forest using isotopic and molecular techniques. Both removing and doubling leaf litter inputs decreased the size of the soil nitrate pool, gross nitrification rates, and the relative abundance of ammonia-oxidizing microorganisms. Gross nitrification rates were correlated with the relative abundance of ammonia-oxidizing archaea, and shifts in the N-cycling microbial community composition correlated with concurrent changes in edaphic properties, notably pH and C:N ratios. These results highlight the importance of understanding coupled biogeochemical cycles in global change scenarios and suggest that environmental perturbations that alter organic matter inputs in tropical forests could reduce inorganic N losses to surface waters and the atmosphere by limiting nitrate production.