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Herbivores and fire are important consumers of plant biomass that influence vegetation structure, nutrient cycling, and biodiversity globally. Departures from historic biomass consumption patterns due to wild herbivore losses, livestock proliferation, and altered fire regimes can have critical ecological consequences. We set out to (i) understand how consumer dominance and prevalence responded to spatial and temporal moisture gradients in Holocene North America and (ii) examine how past and present consumer dominance patterns in North America compare to less altered consumer regimes of modern Sub-Saharan Africa. We developed long-term records of bison abundance and biomass burning in Holocene midcontinent North America and compared these records to reconstructions of moisture availability and vegetation structure. We used these reconstructions to characterize bison and fire prevalence across associated moisture and vegetation gradients. We found that bison herbivory dominated biomass consumption in dry settings whereas fire dominated in wetter environments. Historical distributions of herbivory and burning in midcontinent North America resemble those of contemporary Sub-Saharan Africa, suggesting disturbance feedbacks and interactions regulate long-term consumer dynamics. Comparisons of consumer dynamics in contemporary North America with Holocene North America and Sub-Saharan Africa also reveal that fire is functionally absent from regions where it was once common, with profound ecological implications.

 

The determinants of fire-driven changes in soil organic carbon (SOC) across broad environmental gradients remains unclear, especially in global drylands. Here we combined datasets and field sampling of fire-manipulation experiments to evaluate where and why fire changes SOC and compared our statistical model to simulations from ecosystem models. Drier ecosystems experienced larger relative changes in SOC than humid ecosystems—in some cases exceeding losses from plant biomass pools—primarily explained by high fire-driven declines in tree biomass inputs in dry ecosystems. Many ecosystem models underestimated the SOC changes in drier ecosystems. Upscaling our statistical model predicted that soils in savannah–grassland regions may have gained 0.64 PgC due to net-declines in burned area over the past approximately two decades. Consequently, ongoing declines in fire frequencies have probably created an extensive carbon sink in the soils of global drylands that may have been underestimated by ecosystem models.

 

Abstract Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO 3 − ) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N 2 ) or the greenhouse gas nitrous oxide (N 2 O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO 3 − available for downstream denitrification. Here we use patterns in the isotopic composition of NO 3 − observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ 15 N and δ 18 O values of NO 3 − , as well as increasing δ 15 N values with decreasing NO 3 − concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO 3 − from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO 3 − mass balance with δ 15 N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N 2 O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase. 

Abstract. American bison (Bison bison L.) have recovered from the brink ofextinction over the past century. Bison reintroduction creates multipleenvironmental benefits, but impacts on greenhouse gas emissions are poorlyunderstood. Bison are thought to have produced some 2 Tg yr−1 of theestimated 9–15 Tg yr−1 of pre-industrial enteric methane emissions,but few measurements have been made due to their mobile grazing habits andsafety issues associated with measuring non-domesticated animals. Here, wemeasure methane and carbon dioxide fluxes from a bison herd on an enclosedpasture during daytime periods in winter using eddy covariance. Methaneemissions from the study area were negligible in the absence of bison(mean ± standard deviation = −0.0009 ± 0.008 µmol m−2 s−1) and were significantly greater than zero,0.048 ± 0.082 µmol m−2 s−1, with a positively skeweddistribution, when bison were present. We coupled bison location estimatesfrom automated camera images with two independent flux footprint models tocalculate a mean per-animal methane efflux of 58.5 µmol s−1 per bison, similar to eddy covariance measurements ofmethane efflux from a cattle feedlot during winter. When we sum theobservations over time with conservative uncertainty estimates we arrive at81 g CH4 per bison d−1 with 95 % confidence intervalsbetween 54 and 109 g CH4 per bison d−1. Uncertainty wasdominated by bison location estimates (46 % of the total uncertainty),then the flux footprint model (33 %) and the eddy covariance measurements(21 %), suggesting that making higher-resolution animal location estimatesis a logical starting point for decreasing total uncertainty. Annualmeasurements are ultimately necessary to determine the full greenhouse gasburden of bison grazing systems. Our observations highlight the need tocompare greenhouse gas emissions from different ruminant grazing systems anddemonstrate the potential for using eddy covariance to measure methaneefflux from non-domesticated animals. 

Plant functional traits are thought to drive biomass production and biogeochemical cycling in tropical forests, but it remains unclear how nitrogen (N)‐fixing legumes influence the functional traits of neighbouring trees and forest‐wide biomass dynamics. Further, the degree to which effects of N‐fixers are density‐dependent and may depend on stem size and spatial scale remains largely unknown.

Here, we examine 30 years of stem demography data for ~20,000 trees in a lowland tropical forest in Trinidad that span a wide range of functional traits thought to drive above‐ground biomass (AGB) dynamics.

These forests show positive but decreasing long‐term net AGB accumulation resulting from constant average productivity but increasing mortality of non‐fixing trees over time. We find that high abundance of N‐fixing trees is associated with compositional shifts in non‐fixer functional traits that confer lower competitive performance and biomass accumulation. Across tree size classes, most interactions between N‐fixers and non‐fixers were negative, density‐dependent, and strongest at smaller spatial scales.

Synthesis. Overall, our findings suggest that local trait‐based interactions between N‐fixing and non‐fixing trees can influence long‐term carbon accumulation in tropical forests.

 

The need for sustainable agricultural practices to meet the food, feed, and fuel demands of a growing global population while reducing detrimental environmental impacts has driven research in multi‐faceted approaches to agricultural sustainability. Perennial cropping systems and microbial biofertilizer supplements are two emerging strategies to increase agricultural sustainability that are studied in tandem for the first time in this study. During the establishment phase of a perennial switchgrass stand in SW Montana, USA, we supplemented synthetic fertilization with a nitrogen‐fixing cyanobacterial biofertilizer (CBF) and were able to maintain aboveground crop productivity in comparison to a synthetic only (urea) fertilizer treatment. Soil chemical analysis conducted at the end of the growing season revealed that late‐season nitrogen availability in CBF‐supplemented field plots increased relative to urea‐only plots. High‐throughput sequencing of bacterial/archaeal and fungal communities suggested fine‐scale responses of the microbial community and sensitivity to fertilization among arbuscular mycorrhizal fungi, Planctomycetes, Proteobacteria, and Actinobacteria. Given their critical role in plant productivity and soil nutrient cycling, soil microbiome monitoring is vital to understand the impacts of implementation of alternative agricultural practices on soil health.