The foliar incorporation of various reactive forms of nitrogen (N) has been identified and studied for nearly 30 years. However, the ecosystem-level ramifications of this uptake pathway have only ...recently been considered by the scientific community. In this review, I present our current understanding of the foliar uptake process and then discuss why this pathway of N addition to ecosystems should be considered separately from the bulk deposition of N to the soil surface. Direct foliar uptake is a direct addition of N to plant metabolism and could potentially more readily influence plant growth compared to soil-deposited N. Current ecosystem process models do not partition reactive N between foliar and soil entry pathways and the influence of N deposition on ecosystem C sequestration is likely inadequately represented in most models. I also outline several research priorities for the future understanding of the ecological consequences of foliar uptake of reactive N.
The potential of climate change to substantially alter human history is a pressing concern, but the specific effects of different types of climate change remain unknown. This question can be ...addressed using palaeoclimatic and archaeological data. For instance, a 300-year, low-frequency shift to drier, cooler climate conditions around 1200 BC is frequently associated with the collapse of several ancient civilizations in the Eastern Mediterranean and Near East
. However, the precise details of synchronized climate and human-history-scale associations are lacking. The archaeological-historical record contains multiple instances of human societies successfully adapting to low-frequency climate change
. It is likely that consecutive multi-year occurrences of rare, unexpected extreme climatic events may push a population beyond adaptation and centuries-old resilience practices
. Here we examine the collapse of the Hittite Empire around 1200 BC. The Hittites were one of the great powers in the ancient world across five centuries
, with an empire centred in a semi-arid region in Anatolia with political and socioeconomic interconnections throughout the ancient Near East and Eastern Mediterranean, which for a long time proved resilient despite facing regular and intersecting sociopolitical, economic and environmental challenges. Examination of ring width and stable isotope records obtained from contemporary juniper trees in central Anatolia provides a high-resolution dryness record. This analysis identifies an unusually severe continuous dry period from around 1198 to 1196 (±3) BC, potentially indicating a tipping point, and signals the type of episode that can overwhelm contemporary risk-buffering practices.
Isolating soil organic carbon (SOC) from soil inorganic carbon (SIC) is necessary to quantify SOC stocks and understanding SOC dynamics. Inorganic acids are commonly used to remove SIC and several ...methods have been developed to minimize the impacts these acid treatments have on the residual SOC. Negative impacts on the SOC pool, such as underestimating SOC stocks, are caused in part due to differences in the amount and composition of the organic matter pool. The effects of SIC removal on SOC are often ignored within experimental studies based on the assumption that soils from the same site do not differ enough to impact results. However, some experimental treatments, such as elevated atmospheric CO
2
, change SOC pools in both concentration and composition. Therefore, SIC removal can introduce different biases in control and treatment soils that may differ by method. In this work, we compare two commonly used methods of SIC removal on a set of soil samples from the same elevated CO
2
experiment. We use soils from the Nevada Desert Free Air Carbon dioxide Enrichment Facility to quantify how SIC removal with either acid washing or acid fumigation affect SOC in control and elevated CO
2
plots. We then use the difference in SOC (%C and δ
13
C) between methods to infer changes in the SOC pool driven by the elevated CO
2
treatment. Our results show that acid washing underestimates SOC relative to fumigation and that this difference is larger in soils from control CO
2
plots than elevated CO
2
plots. This may suggest that stabilization mechanisms sensitive to acidification, such as calcium bridging, are disrupted under elevated CO
2
treatment and therefore are less susceptible to SOC loss during acid washing. Our results present future research avenues for exploring the effects of acidic organic compounds, such as root exudates, on SOC stability in alkaline soils.
The identification and quantification of methane emissions from natural gas production has become increasingly important owing to the increase in the natural gas component of the energy sector. An ...instrumented aircraft platform was used to identify large sources of methane and quantify emission rates in southwestern PA in June 2012. A large regional flux, 2.0–14 g CH ₄ s ⁻¹ km ⁻², was quantified for a ∼2,800-km ² area, which did not differ statistically from a bottom-up inventory, 2.3–4.6 g CH ₄ s ⁻¹ km ⁻². Large emissions averaging 34 g CH ₄/s per well were observed from seven well pads determined to be in the drilling phase, 2 to 3 orders of magnitude greater than US Environmental Protection Agency estimates for this operational phase. The emissions from these well pads, representing ∼1% of the total number of wells, account for 4–30% of the observed regional flux. More work is needed to determine all of the sources of methane emissions from natural gas production, to ascertain why these emissions occur and to evaluate their climate and atmospheric chemistry impacts.
In arid environments such as deserts, nitrogen is often the most limiting nutrient for biological activity. The majority of the ecosystem nitrogen flux is typically thought to be driven by production ...and loss of reactive nitrogen species by microorganisms in the soil. We found that high soil-surface temperatures (greater than 50°C), driven by solar radiation, are the primary cause of nitrogen loss in Mojave Desert soils. This abiotic pathway not only enables the balancing of arid ecosystem nitrogen budgets, but also changes our view of global nitrogen cycling and the predicted impact of climate change and increased temperatures on nitrogen bioavailability.
The impacts of drought are expanding worldwide as a consequence of climate change. However, there is still little knowledge of how species respond to long‐term selection in seasonally dry ecosystems. ...In this study, we used QST‐FST comparisons to investigate (i) the role of natural selection on population genetic differentiation for a set of functional traits related to drought resistance in the seasonally dry tropical oak Quercus oleoides and (ii) the influence of water availability at the site of population origin and in experimental treatments on patterns of trait divergence. We conducted a thorough phenotypic characterization of 1912 seedlings from ten populations growing in field and greenhouse common gardens under replicated watering treatments. We also genotyped 218 individuals from the same set of populations using eleven nuclear microsatellites. QST distributions for leaf lamina area, specific leaf area, leaf thickness and stomatal pore index were higher than FST distribution. Results were consistent across growth environments. Genetic differentiation among populations for these functional traits was associated with the index of moisture at the origin of the populations. Together, our results suggest that drought is an important selective agent for Q. oleoides and that differences in length and severity of the dry season have driven the evolution of genetic differences in functional traits.
The increasing concentration of CO
in the atmosphere is perturbing the global carbon (C) cycle, altering stocks of organic C, including soil organic matter (SOM). The effect of this disturbance on ...soils in arid ecosystems may differ from other ecosystems due to water limitation. In this study, we conducted a density fractionation on soils previously harvested from the Nevada Desert FACE Facility (NDFF) to understand how elevated atmospheric CO
(eCO
) affects SOM stability. Soils from beneath the perennial shrub, Larrea tridentata, and from unvegetated interspace were subjected to a sodium polytungstate density fractionation to separate light, particulate organic matter (POM, <1.85 g/cm
) from heavier, mineral associated organic matter (MAOM, >1.85 g/cm
). These fractions were analyzed for organic C, total N, δ
C and δ
N, to understand the mechanisms behind changes. The heavy fraction was further analyzed by pyrolysis GC/MS to assess changes in organic compound composition. Elevated CO
decreased POM-C and MAOM-C in soils beneath L. tridentata while interspace soils exhibited only a small increase in MAOM-N. Analysis of δ
C revealed incorporation of new C into both POM and MAOM pools indicating eCO
stimulated rapid turnover of both POM and MAOM. The largest losses of POM-C and MAOM-C observed under eCO
occurred in soils 20-40 cm in depth, highlighting that belowground C inputs may be a significant driver of SOM decomposition in this ecosystem. Pyrolysis GC/MS analysis revealed a decrease in organic compound diversity in the MAOM fraction of L. tridentata soils, becoming more similar to interspace soils under eCO
. These results provide further evidence that MAOM stability may be compromised under disturbance and that SOC stocks in arid ecosystems are vulnerable under continued climate change.
The increasing concentration of CO2 in the atmosphere is perturbing the global carbon (C) cycle, altering stocks of organic C, including soil organic matter (SOM). The effect of this disturbance on ...soils in arid ecosystems may differ from other ecosystems due to water limitation. In this study, we conducted a density fractionation on soils previously harvested from the Nevada Desert FACE Facility (NDFF) to understand how elevated atmospheric CO2 (eCO2) affects SOM stability. Soils from beneath the perennial shrub, Larrea tridentata, and from unvegetated interspace were subjected to a sodium polytungstate density fractionation to separate light, particulate organic matter (POM, <1.85 g/cm3) from heavier, mineral associated organic matter (MAOM, >1.85 g/cm3). These fractions were analyzed for organic C, total N, δ13C and δ15N, to understand the mechanisms behind changes. The heavy fraction was further analyzed by pyrolysis GC/MS to assess changes in organic compound composition. Elevated CO2 decreased POM‐C and MAOM‐C in soils beneath L. tridentata while interspace soils exhibited only a small increase in MAOM‐N. Analysis of δ13C revealed incorporation of new C into both POM and MAOM pools indicating eCO2 stimulated rapid turnover of both POM and MAOM. The largest losses of POM‐C and MAOM‐C observed under eCO2 occurred in soils 20–40 cm in depth, highlighting that belowground C inputs may be a significant driver of SOM decomposition in this ecosystem. Pyrolysis GC/MS analysis revealed a decrease in organic compound diversity in the MAOM fraction of L. tridentata soils, becoming more similar to interspace soils under eCO2. These results provide further evidence that MAOM stability may be compromised under disturbance and that SOC stocks in arid ecosystems are vulnerable under continued climate change.
We separated soils from a long‐term elevated atmospheric CO2 experiment into less‐stable and more‐stable pools of organic matter. We found a decrease in organic carbon from both pools in soils beneath the dominant perennial shrubs, Larrea tridentata, indicating that elevated CO2 stimulated decomposition. Using stable isotope analysis, we also show that new organic carbon is incorporated into the more‐stable pool of organic matter. These results demonstrate that elevated CO2 disrupts and possibly accelerates soil carbon cycling in arid ecosystems, leading to a loss of stable organic carbon in soils under some perennial vegetation cover types.
Despite long‐standing theory for classifying plant ecological strategies, limited data directly link organismal traits to whole‐plant growth rates (GRs). We compared trait‐growth relationships based ...on three prominent theories: growth analysis, Grime's competitive–stress tolerant–ruderal (CSR) triangle, and the leaf economics spectrum (LES). Under these schemes, growth is hypothesized to be predicted by traits related to relative biomass investment, leaf structure, or gas exchange, respectively. We also considered traits not included in these theories but that might provide potential alternative best predictors of growth. In phylogenetic analyses of 30 diverse milkweeds (Asclepias spp.) and 21 morphological and physiological traits, GR (total biomass produced per day) varied 50‐fold and was best predicted by biomass allocation to leaves (as predicted by growth analysis) and the CSR traits of leaf size and leaf dry matter content. Total leaf area (LA) and plant height were also excellent predictors of whole‐plant GRs. Despite two LES traits correlating with growth (mass‐based leaf nitrogen and area‐based leaf phosphorus contents), these were in the opposite direction of that predicted by LES, such that higher N and P contents corresponded to slower growth. The remaining LES traits (e.g., leaf gas exchange) were not predictive of plant GRs. Overall, differences in GR were driven more by whole‐plant characteristics such as biomass fractions and total LA than individual leaf‐level traits such as photosynthetic rate or specific leaf area. Our results are most consistent with classical growth analysis—combining leaf traits with whole‐plant allocation to best predict growth. However, given that destructive biomass measures are often not feasible, applying easy‐to‐measure leaf traits associated with the CSR classification appear more predictive of whole‐plant growth than LES traits. Testing the generality of this result across additional taxa would further improve our ability to predict whole‐plant growth from functional traits across scales.
Biological soil crusts (biocrusts) are an integral part of the soil system in arid regions worldwide, stabilizing soil surfaces, aiding vascular plant establishment, and are significant sources of ...ecosystem nitrogen and carbon. Hydration and temperature primarily control ecosystem CO₂ flux in these systems. Using constructed mesocosms for incubations under controlled laboratory conditions, we examined the effect of temperature (5-35 °C) and water content (WC, 20-100%) on CO₂ exchange in light (cyanobacterially dominated) and dark (cyanobacteria/lichen and moss dominated) biocrusts of the cool Colorado Plateau Desert in Utah and the hot Chihuahuan Desert in New Mexico. In light crusts from both Utah and New Mexico, net photosynthesis was highest at temperatures >30 °C. Net photosynthesis in light crusts from Utah was relatively insensitive to changes in soil moisture. In contrast, light crusts from New Mexico tended to exhibit higher rates of net photosynthesis at higher soil moisture. Dark crusts originating from both sites exhibited the greatest net photosynthesis at intermediate soil water content (40-60%). Declines in net photosynthesis were observed in dark crusts with crusts from Utah showing declines at temperatures >25 °C and those originating from New Mexico showing declines at temperatures >35 °C. Maximum net photosynthesis in all crust types from all locations were strongly influenced by offsets in the optimal temperature and water content for gross photosynthesis compared with dark respiration. Gross photosynthesis tended to be maximized at some intermediate value of temperature and water content and dark respiration tended to increase linearly. The results of this study suggest biocrusts are capable of CO₂ exchange under a wide range of conditions. However, significant changes in the magnitude of this exchange should be expected for the temperature and precipitation changes suggested by current climate models.
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