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Organic carbon content of soil in topsoil (0–10 cm, top panels) and deep soil (20–160 cm, bottom panels) at different times since fire as reported by Nagy et al. (2021) (recreated in panels a, e). The original figure identity was Figure 3d. The same data are represented in the 6 panels on the right, now differentiated by R&R class. Note: Axes start at a value of 2, which is less than the minimum value across all data. Not all variable combinations were represented in the dataset, thus some areas are left blank on the plot. Significant differences are reported for LSD tests within each depth class and across R&R classes in our reanalysis.
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Annual‐grass invasions are transforming desert ecosystems in ways that affect ecosystem carbon (C) balance, but previous studies do not agree on the pattern, magnitude and direction of changes. A recent meta‐analysis of 41 articles and 386 sites concludes that invasion by annual grasses such as cheatgrass (Bromus tectorum L) reduces C in biomass ac...
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Disturbances drive large changes in plant composition and ecosystem functioning in drylands, but current understanding of how recovery following disturbance depends on the environment is limited due to challenges in analysing effects of disparate disturbances across abiotic gradients.
We combined remote sensing and field observations across 5600+ k...
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... Studies of cheatgrass effects on SOC have varied in their conclusions on its impacts, with some suggesting that that both exotic annual grass invasion and wildfire can independently reduce SOC by almost 50%, yet other research has shown possible increased SOC stocks with cheatgrass invasion (reviewed in Germino et al 2016, Nagy et al 2021, Maxwell et al 2024. The variability in findings about cheatgrass effects on soil C could relate to (1) confusion of cause-and-effect relationships (Germino et al 2016, Maxwell andGermino 2022), (2) background climate and edaphic factors that modulate cheatgrass effects (Belnap et al 2016), (3) dissimilarity in soil depth and microsite type of sampling (e.g. shrub, grass, or bare-soil microsites; Maxwell and Germino 2022), or (4) type and form of C sampled, along with precision of the requisite soil bulk density measures needed to translate soil C content to ecosystem C stocks. ...
... Note the varying scale between the three C fractions. effects on SOC in past studies (Germino et al 2016, Maxwell andGermino 2022). Our study suggests that factors such as plant-soil microsite, soil depth, SOC fraction, and ecoregional location in which samples are collected all contribute to contrasting results in past studies. ...
Soil organic carbon (‘SOC’) in drylands comprises nearly a third of the global SOC pool and has relatively rapid turnover and thus is a key driver of variability in the global carbon cycle. SOC is also a sensitive indicator of longer-term directional change and disturbance-responses of ecosystem C storage. Biome-scale disruption of the dryland carbon cycle by exotic annual grass invasions (mainly Bromus tectorum, ‘Cheatgrass’) threatens carbon storage and corresponding benefits to soil hydrology and nutrient retention. Past studies on cheatgrass impacts mainly focused on total C, and of the few that evaluated SOC, none compared the very different fractions of SOC, such as relatively unstable particulate organic carbon (POC) or relatively stable, mineral-associated organic carbon (MAOC). We measured SOC and its POC and MAOC constituents in the surface soils of sites that had sagebrush canopies but differed in whether their understories had been invaded by cheatgrass or not, in both warm and relatively colder ecoregions of the western USA. MAOC stocks were 36.1% less in the 0–10 cm depth and 46.1% less in the 10–20 cm depth in the cheatgrass-invaded stands compared to the uninvaded stands of the warmer Colorado Plateau, but not in the cooler and more carbon-rich Wyoming Basin ecoregion. In plots where cheatgrass increased SOC, it was via unstable POC. These findings indicate that cheatgrass effects on the distribution of soil carbon among POC and MAOC fractions may vary among ecoregions, and that cheatgrass can reduce forms of carbon that are otherwise considered stable and ‘secure’, i.e. sequestered.
... The findings surrounding soil carbon have also been contradictory, and a recent review by Maxwell and Germino (2022) found that the carbon dynamics of sites across the Great Basin that have been invaded by cheatgrass vary from site to site and have high heterogeneity in the magnitude and direction of carbon change (Maxwell and Germino, 2022). Our study adds to the complex story of soil nutrient dynamics as cheatgrass invades and is removed, and native plants revegetate. ...
... The findings surrounding soil carbon have also been contradictory, and a recent review by Maxwell and Germino (2022) found that the carbon dynamics of sites across the Great Basin that have been invaded by cheatgrass vary from site to site and have high heterogeneity in the magnitude and direction of carbon change (Maxwell and Germino, 2022). Our study adds to the complex story of soil nutrient dynamics as cheatgrass invades and is removed, and native plants revegetate. ...
Land stewards in dryland ecosystems across the western U.S. face challenges to manage the exotic grass Bromus tectorum (cheatgrass), which is a poor forage, is difficult to remove, and increases risk of catastrophic fire. Managers may consider using indaziflam (Rejuvra™), a relatively new pre-emergent herbicide, which may reduce cheatgrass cover within drylands. However, few studies have explored the effects of indaziflam on non-target organisms. We tested how indaziflam application impacted cover and biomass of native and exotics within the plant community and composition and diversity of the soil microbiome by comparing untreated and treated arid shrubland sites in Boulder County, Colorado, USA. We found that indaziflam application decreased cheatgrass cover by as much as 80% and increased native plant cover by the same amount. Indaziflam application also was associated with increased soil nitrate (NO3⁻), decreased soil organic matter, and had a significant effect on the composition of the soil microbiome. Microbial community composition was significantly related to soil NO3⁻, soil organic matter, soil pH, and native species and cheatgrass biomass. An indicator species analysis suggested that indaziflam application shifted microbial communities. In untreated sites, ammonia-oxidizing bacteria Nitrosomonadaceae and nitrogen-digesting Opitutaceae and the fungi Articulospora proliferata were found. While in treated sites, ammonia-oxidizing archaea which are associated with intact drylands, Nitrososphaeraceae and toxin digesters and acidic-soil species Sphingomonas and Acidimicrobiia were significantly associated. Overall, these results demonstrate that indaziflam application can increase native plant recruitment, while also affecting soil properties and the soil microbiome. The findings from this study can be used to inform decision-making during dryland restoration planning process as indaziflam use may have benefits and unknown long-term consequences for the biogeochemistry and microbial ecology of the system.
... Cold desert-shrublands, and specifically sagebrush steppe, are thought to have the greatest relative potential to gain carbon stock among ecoregions in the Intermountain Western U.S 5 . Carbon-loss threats to sagebrush steppe are substantial and result specifically from exotic annual grass (EAG) invasions and the increased wildfire, which they benefit from and promote (i.e., the "grass-fire cycle") 1, 4,7,8 . The net impact of the feedback is the conversion of diverse, deep-rooted perennial plant communities to shallowrooted annual grasslands, which has impacted >50,000,000 ha already and is occurring at a rate of~400,000 ha annually 9 . ...
... Decreases in above-and belowground productivity and biomass resulting from plant-community shifts from perennial woody and herbaceous communities to annual grasslands should impact soil carbon stocks [11][12][13] . Wildfire, which is promoted by EAG invasion, is also expected to locally degrade soil carbon stocks both directly by volatilizing biomass carbon and indirectly by eliminating deep-rooted woody perennials and through the loss of carbon-rich topsoil by erosion 4,8,14 . Detecting changes in soil carbon in response to disturbance or land management actions is challenging because there is substantial vertical, horizontal, and temporal heterogeneity in dry shrubland plant community structure, and soil water and biogeochemical cycling that must be considered 12,[15][16][17] . ...
... Further, the effects of disturbance or management on soil carbon are not likely to be uniform, whether due to heterogeneity in disturbance severity or because of variation in the capacity of soils to store and stabilize carbon [18][19][20][21][22][23][24] . Conservation, management, or sequestration of soil carbon in dry shrublands can be improved with an understanding of how carbon and its sensitivity to disturbance are distributed across e-mail: mgermino@usgs.gov the mosaic of plant communities and soil types that dominate these ecosystems 4,10,23,[25][26][27] . A recent review revealed that neither the direct nor the synergistic effects of wildfire and EAG invasions on soil carbon stocks are well understood because the two disturbances often co-occur, and their separate effects are not trivial to identify 4 . ...
Ecological disturbance can affect carbon storage and stability and is a key consideration for managing lands to preserve or increase ecosystem carbon to ameliorate the global greenhouse gas problem. Dryland soils are massive carbon reservoirs that are increasingly impacted by species invasions and altered fire regimes, including the exotic-grass-fire cycle in the extensive sagebrush steppe of North America. Direct measurement of total carbon in 1174 samples from landscapes of this region that differed in invasion and wildfire history revealed that their impacts depleted soil carbon by 42–49%, primarily in deep horizons, which could amount to 17.1–20.0 Tg carbon lost across the ~400,000 ha affected annually. Disturbance effects on soil carbon stocks were not synergistic, suggesting that soil carbon was lowered to a floor—i.e. a resistant base-level—beneath which further loss was unlikely. Restoration and maintenance of resilient dryland shrublands/rangelands could stabilize soil carbon at magnitudes relevant to the global carbon cycle.
... Rangelands comprise ∼40% of the terrestrial Earth surface ( White et al. 20 0 0 ) and store 10-30% of the global soil organic carbon, carbon which is secure as long as it remains undisturbed ( Anderson 1991 ;Derner and Schuman 2007 ;Schlesinger and Bernhardt 2013 ). However, aboveground disturbances from invasive grasses, woody plant expansion, increased prevalence of wildfires, and land conversions render stored soil organic carbon insecure and potentially irrecoverable over time ( Gill and Jackson 20 0 0 ;Mcculley and Jackson 2012 ;Rau et al. 2012 ;Nagy et al. 2021 ;Zhang et al. 2021 ;Lohse et al. 2022 ;Maxwell and Germino 2022 ). Thus, we are faced with vegetation management and carbon management problems that ultimately distill down to how vegetation management influences carbon dynamics and security. ...
... Thus, we need to approach rangeland carbon management differently from agriculture. Currently, carbon management is focused on the amount of carbon sequestered or stored, a feature that is difficult to manage in heterogeneous rangeland soil systems ( Jackson and Caldwell 1993 ;Schlesinger et al. 1996 ;Six et al. 2002 ;Maxwell and Germino 2022 ). Terms like irrecoverable carbon are important for the discussion, but more importantly, we must consider how to secure carbon in disturbance prone rangelands once it is sequestered and stored. ...
... These groups comprise the largest fractional cover categories of Core Sagebrush Areas in the Great Basin ( Doherty et al. 2022 ), and are resilient to disturbances while providing consistent inputs to soil organic carbon in the upper 1-m of soil ( Rau et al. 2011a ;Austreng 2012 ;McAbee et al. 2017 ;Germino et al. 2019 ;Miller et al. 2019 ;Johnson et al. 2022 ). The annual forb and grass cover fraction (AFG) as well as the tree cover fraction (TRE) are penalized in our model because of positive feedbacks from the annual grass-fire cycle and the increased fire severity risks associated with 1 0 0 0-h and 10 0 0 0-h fuels (i.e., greatest potential carbon loss) that impact ecosystem health ( Rau et al. 2011a( Rau et al. , 2011bNagy et al. 2021 ;Mahood et al. 2022 ;Maxwell and Germino 2022 ). The denominator aggregates all the fractional cover values including bare ground (BGR). ...
Rangeland carbon is often conceptualized similarly to intensively managed agricultural lands, in that we need to sequester and store more carbon. Unlike intensively managed agricultural lands, rangeland soils cannot sequester more carbon due to pedogenic and climatic limitations that influence plant community and microbial community dynamics. This requires a new paradigm for rangeland carbon that focuses on maintaining carbon security following disturbances like fire and plant community conversions (e.g., annual grasslands and conifer woodlands). To attain this, we propose the creation of a Carbon Security Index (CSI). CSI is a unitless, scalable value that can be used to compare carbon security across range-land sites and over time and incorporates a plant fractional cover ratio, resistance and resilience, and wildfire probability. Using the Great Basin as a case study, we found that CSI decreased by 53% basin wide from 1989 to 2020. Using the Sagebrush Conservation Design's sagebrush ecological integrity categories across the Great Basin, we found that CSI in "core" areas remained relatively unchanged between 1998 and 2020 (decreased by 1%), whereas "growth opportunity" areas CSI began to change (decreased by 13%) and "other rangeland" areas CSI decreased by 67%. We found that CSI was able to act as an indicator for determining when carbon security would decrease several years prior to a wildfire disturbance, which then rapidly reduced CSI. Finally, we created a carbon security management map to help prioritize potential management for achieving greatest carbon security and locations for restoration. These results show that CSI provides landowners and land managers an opportunity to assess how secure their carbon is on the land and help them prioritize areas for restoration.
... Machmuller et al. 2015). There is little agreement on the longer-term effects of EAG invasion on soil C and SOM; these may be contingent upon site properties such as soil, climate, fire history, and plant-community context, and overall are not well understood (Nagy et al. 2021;Nichols et al. 2021;Maxwell & Germino 2022). Longertime observations may or may not reveal more effects of chemical-or bio-herbicides on carbon sequestration than observed here. ...
Community‐type conversions, such as replacement of perennials by exotic annual grasses in semiarid desert communities, are occurring due to plant invasions that often create positive plant–soil feedbacks, which favor invaders and make restoration of native perennials difficult. Exotic annual grass control measures, such as pre‐emergent herbicides, can also alter soil ecosystems directly or indirectly (i.e. via the plant community), yet there are few studies on the topic in natural, non‐cropped landscapes. We asked how spray treatments applied to soil post‐fire with the intention of inhibiting invasive annual grasses (such as Bromus tectorum L.) and releasing existing native perennial grasses affected soil resources, a microbial process, and invertebrates in three climatically varied sagebrush steppe sites. Spray treatments included chemical herbicides (imazapic and rimsulfuron) that strongly affected plant communities and a bioherbicide ( Pseudomonas fluorescens strain D7) that did not. Chemical herbicides increased soil mineral nitrogen in proportion to their negative effects on plant cover for 2 years after treatments in all sites and increased soil water and net N mineralization (measured at one site) but did not affect total carbon, nitrogen, or organic matter. Invertebrate responses to herbicides varied by site, and invertebrates increased with chemical herbicides at the highest, wettest site. We show that herbicide treatments can exacerbate pulses of mineral nutrients, which previous studies have shown can weaken ecosystem resistance to invasion. Thus, restoration strategies that increase the likelihood that desired plants can capture mineralized nutrients after herbicide application will likely be more successful.
... Over recent decades, the extent and dominance of exotic annual grass species such as cheatgrass (Bromus tectorum), medusahead (Taeniatherium caput-medusae), and ventenata (Ventenata dubia) have continued to increase, displacing native perennial vegetation and causing numerous problems for land management (Smith et al., 2022). Annual grasses decrease native plant biodiversity (Davies, 2011), reduce forage reliability for wildlife and livestock (Young & Allen, 1997), negatively impact ecosystem functions such as carbon storage (Maxwell & Germino, 2022;Nagy et al., 2021), and transform the fire cycle by producing continuous carpets of fine fuels that make these ecosystems more flammable than ever before (Balch et al., 2013;Bradley et al., 2018;Davies & Nafus, 2013). ...
Exotic annual grass invasion is a widespread threat to the integrity of sagebrush ecosystems in Western North America. Although many predictors of annual grass prevalence and native perennial vegetation have been identified, there remains substantial uncertainty about how regional‐scale and local‐scale predictors interact to determine vegetation heterogeneity, and how associations between vegetation and cattle grazing vary with environmental context. Here, we conducted a regionally extensive, one‐season field survey across burned and unburned, grazed, public lands in Oregon and Idaho, with plots stratified by aspect and distance to water within pastures to capture variation in environmental context and grazing intensity. We analyzed regional‐scale and local‐scale patterns of annual grass, perennial grass, and shrub cover, and examined to what extent plot‐level variation was contingent on pasture‐level predictions of site favorability. Annual grasses were widespread at burned and unburned sites alike, contrary to assumptions of annual grasses depending on fire, and more common at lower elevations and higher temperatures regionally, as well as on warmer slopes locally. Pasture‐level grazing pressure interacted with temperature such that annual grass cover was associated positively with grazing pressure at higher temperatures but associated negatively with grazing pressure at lower temperatures. This suggests that pasture‐level temperature and grazing relationships with annual grass abundance are complex and context dependent, although the causality of this relationship deserves further examination. At the plot‐level within pastures, annual grass cover did not vary with grazing metrics, but perennial cover did; perennial grasses, for example, had lower cover closer to water sources, but higher cover at higher dung counts within a pasture, suggesting contrasting interpretations of these two grazing proxies. Importantly for predictions of ecosystem response to temperature change, we found that pasture‐level and plot‐level favorability interacted: perennial grasses had a higher plot‐level cover on cooler slopes, and this difference across topography was starkest in pastures that were less favorable for perennial grasses regionally. Understanding the mechanisms behind cross‐scale interactions and contingent responses of vegetation to grazing in these increasingly invaded ecosystems will be critical to land management in a changing world.
... rubens), ventenata (Ventenata dubia), and others (Mack, 1981;Young and Evans, 1970). Compared to intact sagebrush and perennial grass and forb communities, annual grass-dominated communities support lower native plant diversity (Davies, 2011;Davies and Svejcar, 2008) and diminished habitat value for sensitive sagebrush-dependent wildlife (Coates et al., 2016;Knick et al., 2003;O'Neil et al., 2020), store less carbon (Maxwell and Germino, 2022;Nagy et al., 2020), and exhibit altered inter-and intra-annual variation in primary production (Bradley and Mustard, 2005;Clinton et al., 2010). ...
Sagebrush ecosystems of western North America are experiencing widespread loss and degradation by invasive annual grasses. Positive feedbacks between fire and annual grasses are often invoked to explain the rapid pace of these changes, yet annual grasses also appear capable of achieving dominance among vegetation communities that have not burned for many decades. Using a dynamic, remotely-sensed vegetation dataset in tandem with remotely-sensed fire perimeter and burn severity datasets, we examine the role of fire in transitions to and persistence of annual grass dominance in the U.S. Great Basin over the past 3 decades. Although annual grasses and wildfire are so tightly associated that one is rarely mentioned without the other, our findings reveal surprisingly widespread transformation of sagebrush ecosystems by invasive annual grasses in the absence of fire. These findings are discussed in the context of strategic management; we argue a pivot from predominantly reactive management (e.g., aggressive fire suppression and post-fire restoration in heavily-infested areas) to more proactive management (e.g., enhancing resistance and managing propagule pressure in minimally-invaded areas) is urgently needed to halt the loss of Great Basin sagebrush ecosystems.
The term carbon (C) sequestration has not just become a buzzword but is something of a siren's call to scientific communicators and media outlets. Carbon sequestration is the removal of C from the atmosphere and the storage, for example, in soil. It has the potential to partially compensate for anthropogenic greenhouse gas emissions and is, therefore, an important piece in the global climate change mitigation puzzle. However, the term C sequestration is often used misleadingly and, while likely unintentional, can lead to the perpetuation of biased conclusions and exaggerated expectations about its contribution to climate change mitigation efforts. Soils have considerable potential to take up C but many are also in a state of continuous loss. In such soils, measures to build up soil C may only lead to a reduction in C losses (C loss mitigation) rather than result in real C sequestration and negative emissions. In an examination of 100 recent peer‐reviewed papers on topics surrounding soil C, only 4% were found to have used the term C sequestration correctly. Furthermore, 13% of the papers equated C sequestration with C stocks. The review, further, revealed that measures leading to C sequestration will not always result in climate change mitigation when non‐CO2 greenhouse gases and leakage are taken into consideration. This paper highlights potential pitfalls when using the term C sequestration incorrectly and calls for accurate usage of this term going forward. Revised and new terms are suggested to distinguish clearly between C sequestration in soils, SOC loss mitigation, negative emissions, climate change mitigation, SOC storage, and SOC accrual to avoid miscommunication among scientists and stakeholder groups in future.
