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The article "Climate change mitigation through soil carbon sequestration in working lands: A reality check" questions the validity and realism of claims made by Almaraz et al. regarding the potential magnitude of soil carbon sequestration (SCS) in agricultural soils. The authors argue that Almaraz et al. overestimate the availability of organic matter for compost and biochar, inaccurately interpret carbon sequestration rates, and make premature assumptions about the applicability of enhanced rock weathering. They emphasize the need for realistic evaluations of the limitations and trade-offs of SCS practices and caution against overly optimistic estimates that can mislead policymakers. The authors declare no conflicts of interest. [Extracted from the article]

The stable N and O isotope composition of soil and soil‐respired N2O is increasingly measured, yet a solid theoretical framework for interpreting the data remains to be developed. Here, the physical processes that affect soil N2O and its isotopes are embedded in a diffusion/reaction model. Numerical experiments are compared to data to demonstrate how various soil processes influence depth profiles and surface fluxes of soil N2O, δ15NN2O, and δ18ON2O. Model predictions and data suggest that the isotope composition of the net N2O soil flux, in soils that have N2O consumption, is a function of the net flux rate, and the isotope differences between the atmosphere and the biological source. Asymptotically large negative or positive δ15Nflux and δ18Oflux values occur as the net soil N2O flux approaches zero from positive or negative flux rates, respectively. This implies that the isotopic imprint of soil fluxes on the global atmospheric N2O pool is more variable than previously suggested. Additionally, the observed isotope values in static flux chambers are possibly complicated by the fact that consumption fluxes increase as the concentration in the chambers increases. This work reveals that even simple chamber flux measurements may possess isotope effects imparted by consumption during the chamber measurement and suggests ways to experimentally test this possibility. Additionally, simple methods to estimate depth‐dependent net production/consumption and its isotope effects are suggested. However, understanding the gross rates of the production and consumption of soil N2O remains an elusive goal. Plain Language Summary: Soils are one of the largest global emitters of the greenhouse gas nitrous oxide (N2O). Yet, unlike greenhouse gases such as carbon dioxide, the emission patterns bear weak linkages to climatic patterns. One reason for this may be that the N2O that finally escapes soil is the residue of much larger rates of microbial production that are counterbalanced by large rates of consumption. Here, we use hundreds of previously published data on the concentration and stable nitrogen and oxygen isotope ratio of N2O and re‐interpret them through a physics‐based model. We find unexpected evidence that the isotope composition of the emitted N2O, which is sometimes used as a signal to estimate the total global soil emission rate, is a complex function of the balance between production and consumption, and the net flux rates. A better understanding of the conditions that may increase these consumption rates will be of value to potentially reduce soil emissions and their impact on the climate system. Key Points: Currently, there is no unified theoretical framework to interpret soil N2O isotope ratiosModel and limited empirical measurements indicate that the N and O isotope values of N2O fluxes from soils may be dependent on flux rates, and may be much more variable than previously assumedSimple calculations of observed soil N2O profiles (concentration and isotope values) can provide insights into spatial variability in the net result of N2O production and consumptive processes [ABSTRACT FROM AUTHOR]

The genus Celtis includes widespread trees that produce drupes with aragonite endocarps, or "hackberries." These carbonate endocarps are preserved in the fossil record, often in cave deposits or packrat middens, and thus are targets for paleoclimate reconstructions. Stable oxygen isotopes in Celtis endocarps have been used as proxies for oxygen isotopic composition of past stream water and for paleothermometry. Here, we explore the suitability of hackberry carbonates for paleoclimate reconstructions based on carbonate clumped‐isotope thermometry. We sampled modern hackberries grown at sites across North America (n = 37) for stable and clumped isotope analyses. Measured clumped‐isotope temperatures are found to be within the range of measured local modern growing season surface temperatures and typically in dual clumped‐isotope equilibrium. As such, we propose that hackberry clumped‐isotope measurements can be used to reconstruct past Earth‐surface air temperatures. Plain Language Summary: Hackberries are tree‐growing fruits that create carbonate mineral structures surrounding their seeds. Geochemical properties of these minerals can give us insight into growing conditions, particularly the growing season temperature of the hackberry plants. We tested the promise and limitations of clumped isotope as a paleothermometer for carbonate growth temperatures using modern hackberries collected across North America. We find recorded clumped‐isotope temperatures are consistent with mean growing season temperatures. We propose the use of hackberry carbonate clumped isotope measurements to reconstruct past environmental temperatures using ancient hackberry deposits found in caves (formed by humans) and middens (formed by animals such as packrats) throughout the past 10,000+ years. Key Points: Hackberry plants create an aragonitic mineral endocarp within their fruit which reflects environmental conditionsClumped‐isotope analyses reconstruct temperatures comparable to mean growing season temperatures in modern hackberry endocarpsClumped isotopes in hackberries provide a potential paleothermometer for Quaternary‐aged cave deposits and packrat middens [ABSTRACT FROM AUTHOR]

The Mojave Desert has warmed >2°C, and aridified, in the past 50 years, making it a strategic location to investigate climate change impacts on arid soil processes. We resampled a climosequence of soils in the Mojave first sampled in 1973 and compared current soil properties to those 45+ years earlier. Radiocarbon changes revealed that C is cycling rapidly through the soils, particularly near the surface, with a temperature‐sensitive decomposition rate (Arrhenius Ea = 66 kJ/mol). Significant decreases in soil C/N ratios and increases in δ15N values occurred, suggestive of enhanced rates of soil C and N cycling and their losses. Covariation between changes in soil radiocarbon, δ13C, δ15N, and C/N point toward emerging chemical impacts on the coupled C and N cycles in response to climate change. Plain Language Summary: Global warming and drying may causes soils to lose C as CO2, accelerating global climate change. However, evidence for this response is inconsistent in many environments, due to experimental challenges and natural soil heterogeneity. Here, we use archived soils samples (collected in 1973), and resample the same locations in 2018, to observe that in the Mojave Desert, the soil C and N cycles appears to be adjusting to the rapid warming and drying of this region over the past 50 years. These observations underscore the importance of ecosystem feedbacks to changes in the climate system. Key Points: The Mojave Desert has experienced about 2°C warming since 1973Radiocarbon analyses shows that Mojave soils are cycling significant amounts of C on decadal time scalesThe apparent temperature sensitivity of radiocarbon‐determined C cycling is consistent with observed increases in the soil C/N ratio and increases in 15N in soil organic N [ABSTRACT FROM AUTHOR]

Despite the wealth of information available on respiratory fluxes of C[O.sub.2] from the soil surface, relatively few studies have examined the depth-related flux and production of C[O.sub.2] within the soil. We developed a depth-integrated C[O.sub.2] flux model based on soil profile C[O.sub.2] measurements to explore both the temporal and spatial patterns of, and controls on, C[O.sub.2] production in two contrasting California coastal ecosystems: a redwood [Sequoia sempervirens (D. Don) Endl.] forest and a seasonal grassland ecosystem. In the forest, C[O.sub.2] production increased exponentially with temperature but the dominant control on C[O.sub.2] efflux was due to changes in soil diffusivity with water content. Here, ~67% of the total efflux was produced in the top 20 cm, with little seasonal variation. In the grassland, soil temperature and water content both exerted strong positive controls on C[O.sub.2] production, with C[O.sub.2] efflux highest in early spring (8 g C [m.sup.-2][d.sup.-1]) and lowest in late summer (0.4 g C [m.sup.2] d-I]. Here, the relative proportion of total respiration in the top 20 cm varied from 5% at the end of the dry season to 70% during the wet winter months. Soil respiration in the redwood soil appeared to be well buffered from climate extremes due to the stable microclimate created by the forest canopy. The grassland soil, however, lacking such buffering, was subject to large and rapid changes in C[O.sub.2] production throughout the year. These results indicate that despite some uncertainty in the calculation of diffusion rates, significantly more information on soil biological processes is gained from the C[O.sub.2] production-diffusion approach than from surface flux measurements alone. doi: 10.2136/sssaj2009.0290

Climate controls erosion and weathering on soil-mantled landscapes through diverse processes that have remained difficult to disentangle due to their complex interactions. We quantify denudation, soil and saprolite weathering, and soil transport near the base and crest of the western slope of the Sierra Nevada to examine how large differences in climate affect these processes. Depth profiles of fallout radionuclides and field observations show relative differences in erosion and weathering processes at these two climatically diverse sites, and our data suggest fundamentally different patterns of soil production and transport mechanisms: biotically driven soil transport at low elevation, and surface erosion driven by overland flow at high elevation. Soil production rates from cosmogenic [sup.10]Be decrease from 31.3 to 13.6 m/Ma with increasing soil depth at low elevation, but show uncertain depth dependence at the high elevation site. Our data also show a positive correlation between physical erosion and saprolite weathering at both sites. Highly weathered saprolites are overlain by weakly weathered and rapidly eroding soils, while chemically less depleted saprolites are overlain by slowly eroding, more weathered soils. Our data are among the first to quantify the critical role of saprolite weathering in the evolution of actively eroding upland landscapes, and our results provide quantitative constraints on how different climates can shape hillslopes by driving processes of erosion and weathering. doi: 10.1130/G30045A.1

We estimated the soil organic carbon (SOC) and soil inorganic carbon (SIC) inventory for the conterminous USA using the State Soil Geographic Database (STATSGO). The relative contribution of each soil order and Land Resource Region (LRR) to the national SOC and SIC inventory was determined. There are 302 to 1499 x [10.sup.8] Mg of SOC and 226 to 937 x [10.sup.8] Mg of SIC in the upper 2 m of soil in the conterminous USA. About 30 and 80% of the upper 2-m SOC is in the 0- to 0.20- and 0- to 1.0-m soil layers, respectively. For SIC, only about 8% of the upper 2-m SIC is in the upper 0.2 m, and about 50% is in the top 1.0-m layer. The relative spatial variability of SOC increases dramatically as soil depth increases while the largest relative variability of SIC is in the surface layer. Because of its large area (27% of the soil area in the conterminous USA), Mollisols are the largest contributors both to the SOC stock (about 31 to 39%) and to the SIC stock (about 43 to 44%) in the conterminous USA. The results of this study provide a view of soil C partitioning by taxonomic group and land resource area, information that may be useful for assessing the impact of land use and climatic change on SOC and SIC pools.

Maps of soil organic carbon (SOC) and inorganic carbon (SIC) were generated from the State Soil Geographical database (STATSGO) and were overlain with land-cover, topography (elevation and slope), mean annual precipitation (MAP), and mean annual temperature (MAT) databases to study the effects of environmental driving factors (or 'state factors') in the conterminous USA. In the USA, human disturbance has significantly reduced SOC and SIC in the surface layer. The SOC decreases as elevation increases. Level topography has twice the SOC content of other slope classes. Soil organic C increases as MAP increases up to values of 700-850 mm [yr.sup.-1]. There is no obvious pattern in SIC as MAP increases until MAP exceeds 1000 mm, at which point on SIC drops dramatically. For natural vegetation with MAP < 1000 mm, SOC decreases as MAT increases (within restricted ranges of elevation and topography). The relationship between SOC versus MAT varies with MAP, and SOC is most sensitive to temperature in low and moderate MAP regions and in the upper soil layers. This GIS-based analysis is a relatively coarse evaluation given the nature of the database, but does provide a first quantitative assessment of soil C to 'state factors' for the USA as a whole using commonly available digital databases.

Burrowing organisms assist in shaping earth surfaces and are simultaneously affected by the environment they inhabit; however, a conceptual framework is not yet available to describe this feedback. We introduce a model that connects the population density of soil-burrowing animals to sediment transport via energy. The model, combined with available data from California hillslopes where soil erosion is driven by pocket gophers (Thomomys bottae), suggests that a gopher annually expends ~9 kJ of energy, or ~1% of reported burrowing energy expenditure, in generating sediment transport. The model is used to evaluate the case that gophers prefer to populate thicker soils. The results suggest that this behavior may drastically dampen the spatial and temporal variations of soil thickness and gopher populations, implying that burrowing organisms may create landscapes distinct from those affected by abiotic processes. Keywords: sediment transport, bioturbation, pocket gopher, soil thickness, hillslope.

In this paper, we report the taxonomic structure, the spatial distribution, and relative abundance of soils in the USA. In the analysis, we used the STATSGO (1997 version) database, which contains information on 11 orders, 52 suborders, 232 great groups, 1175 subgroups, 6226 families, and 13 129 series. The analysis of taxonomic structure showed that the numbers of taxa in any category were distributed unevenly in relation to taxa present at the next higher category. This uneven distribution becomes more pronounced in the lower categories of the hierarchy. In addition, there is a trend for taxa to produce only one, or a very small number, of taxa in the next lower category at the lower categories of the system. The analysis of the spatial distribution of taxa showed that 10 662 (51.2%) out of 20 825 taxa exist only 1 Major Land Resource Area (MLRA), indicating that most soil taxa are not widely spread, and are specific to particular combinations of state factors. The area abundance of soil taxa is asymmetric with most taxa in a category having relative small area extent. Five (2.1%) great-groups, 37 (2.1%) subgroups, 417 (6.7) families, and 827 (6.3%) series had total areas less than 10 [km.sup.2], and were defined as rare taxa. Among the rare taxa, four (80%) rare great groups, 36 (97.3%) rare subgroups, 378 (90.6%) rare families, and 750 (90.7%) rare series were found in only 1 MLRA. The portion of rare soils might be much higher because not all the soils are included in STATSGO because of their too limited area. The spatial and area abundance analyses of the soils provides a perspective useful for discussions on the preservation of soil resources in the USA, a topic whose importance is likely to grow in conjunction with increasing interest in global biodiversity and more intense uses of the world's soil resources.

Comparison of [sup.14]C (carbon-14) in archived (pre-1963) and contemporary soils taken along an elevation gradient in the Sierra Nevada, California, demonstrates rapid (7 to 65 years) turnover for 50 to 90 percent of carbon in the upper 20 centimeters of soil (A horizon soil carbon). Carbon turnover times increased with elevation (decreasing temperature) along the Sierra transect. This trend was consistent with results from other locations, which indicates that temperature is a dominant control of soil carbon dynamics. When extrapolated to large regions, the observed relation between carbon turnover and temperature suggests that soils should act as significant sources or sinks of atmospheric carbon dioxide in response to global temperature changes., Two critical questions regarding the role of soils in the global carbon cycle are (i) what is the potential for soils to release carbon from increased decomposition rates in a [...]