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LM3 is a new model of terrestrial water, energy, and carbon, intended for use in global hydrologic analyses and as a component of earth-system and physical-climate models. It is designed to improve upon the performance and to extend the scope of the predecessor Land Dynamics (LaD) and LM3V models by better quantifying the physical controls of climate and biogeochemistry and by relating more directly to components of the globalwater systemthat touch human concerns. LM3 includes multilayer representations of temperature, liquid water content, and ice content of both snowpack and macroporous soil–bedrock; topography-based description of saturated area and groundwater discharge; and transport of runoff to the ocean via a global river and lake network. Sensible heat transport by watermass is accounted throughout for a complete energy balance. Carbon and vegetation dynamics and biophysics are represented as in LM3V. In numerical experiments, LM3 avoids some of the limitations of the LaD model and provides qualitatively (though not always quantitatively) reasonable estimates, from a global perspective, of observed spatial and/or temporal variations of vegetation density, albedo, streamflow, water-table depth, permafrost, and lake levels. Amplitude and phase of annual cycle of total water storage are simulated well. Realism of modeled lake levels varies widely. The water table tends to be consistently too shallow in humid regions. Biophysical properties have an artificial stepwise spatial structure, and equilibrium vegetation is sensitive to initial conditions. Explicit resolution of thick (>100 m) unsaturated zones and permafrost is possible, but only at the cost of long (»300 yr) model spinup times.

Litter decomposition is a keystone ecosystem process impacting nutrient cycling and productivity, soil properties, and the terrestrial carbon (C) balance, but the factors regulating decomposition rate are still poorly understood. Traditional models assume that the rate is controlled by litter quality, relying on parameters such as lignin content as predictors. However, a strong correlation has been observed between the manganese (Mn) content of litter and decomposition rates across a variety of forest ecosystems. Here, we show that long-term litter decomposition in forest ecosystems is tightly coupled to Mn redox cycling. Over 7 years of litter decomposition, microbial transformation of litter was paralleled by variations in Mn oxidation state and concentration. A detailed chemical imaging analysis of the litter revealed that fungi recruit and redistribute unreactive Mn2+provided by fresh plant litter to produce oxidative Mn3+species at sites of active decay, with Mn eventually accumulating as insoluble Mn3+/4+oxides. Formation of reactive Mn3+species coincided with the generation of aromatic oxidation products, providing direct proof of the previously posited role of Mn3+-based oxidizers in the breakdown of litter. Our results suggest that the litter-decomposing machinery at our coniferous forest site depends on the ability of plants and microbes to supply, accumulate, and regenerate short-lived Mn3+species in the litter layer. This observation indicates that biogeochemical constraints on bioavailability, mobility, and reactivity of Mn in the plant–soil system may have a profound impact on litter decomposition rates.

A number of recent intensive and extended field campaigns have been devoted to the collection of land‐surface fluxes from a variety of platforms, with the purpose of inferring the long‐term C, water, and energy budgets across large areas (watershed, continental, or global scales). One approach to flux upscaling is to use land–atmosphere transfer schemes (LATS) linked to remotely sensed boundary conditions as an intermediary between the sensor footprint and regional scales. In this capacity, we examined the utility of a multiscale LATS framework that uses thermal, visible and near infrared remote sensing imagery from multiple satellites to partition surface temperature and fluxes between the soil and canopy. We conducted exercises using tower and aircraft flux data collected at three experiment sites in Oklahoma and Iowa, each with a different configuration of instrumentation. Combined, the two flux‐monitoring systems were found to be complementary: the towers provided high‐spatial‐resolution, time‐continuous validation at discrete points within the modeling domain, while with the aircraft data it could be confirmed that the model was reproducing broad spatial patterns observed at specific moments in time. High‐resolution flux maps created with the LATS allowed evaluation of differences in footprint associated with turbulent, radiative, and conductive flux sensors, which may be contributing to energy budget closure problems observed with eddy correlation systems. The ability to map fluxes at multiple resolutions (1 m–10 km) with a common model framework is beneficial in providing spatial context to an experiment by bracketing the scale of interest. Multiscale flux maps can also assist in the experimental design stage, in a priori assessments of sensor representativeness in complex landscapes.

Understanding how different physical processes can shape the probability distribution function (PDF) of surface temperature, in particular the tails of the distribution, is essential for the attribution and projection of future extreme temperature events. In this study, the contribution of soil moisture–atmosphere interactions to surface temperature PDFs is investigated. Soil moisture represents a key variable in the coupling of the land and atmosphere, since it controls the partitioning of available energy between sensible and latent heat flux at the surface. Consequently, soil moisture variability driven by the atmosphere may feed back onto the near-surface climate—in particular, temperature. In this study, two simulations of the current-generation Geophysical Fluid Dynamics Laboratory (GFDL) Earth System Model, with and without interactive soil moisture, are analyzed in order to assess how soil moisture dynamics impact the simulated climate. Comparison of these simulations shows that soil moisture dynamics enhance both temperature mean and variance over regional “hotspots” of land–atmosphere coupling. Moreover, higher-order distribution moments, such as skewness and kurtosis, are also significantly impacted, suggesting an asymmetric impact on the positive and negative extremes of the temperature PDF. Such changes are interpreted in the context of altered distributions of the surface turbulent and radiative fluxes. That the moments of the temperature distribution may respond differentially to soil moisture dynamics underscores the importance of analyzing moments beyond the mean and variance to characterize fully the interplay of soil moisture and near-surface temperature. In addition, it is shown that soil moisture dynamics impacts daily temperature variability at different time scales over different regions in the model.

Widespread negative correlations between summertime-mean temperatures and precipitation over land regions are a well-known feature of terrestrial climate. This behavior has generally been interpreted in the context of soil moisture–atmosphere coupling, with soil moisture deficits associated with reduced rainfall leading to enhanced surface sensible heating and higher surface temperature. The present study revisits the genesis of these negative temperature–precipitation correlations using simulations from the Global Land–Atmosphere Coupling Experiment–phase 5 of the Coupled Model Intercomparison Project (GLACE-CMIP5) multimodel experiment. The analyses are based on simulations with five climate models, which were integrated with prescribed (noninteractive) and with interactive soil moisture over the period 1950–2100. While the results presented here generally confirm the interpretation that negative correlations between seasonal temperature and precipitation arise through the direct control of soil moisture on surface heat flux partitioning, the presence of widespread negative correlations when soil moisture–atmosphere interactions are artificially removed in at least two out of five models suggests that atmospheric processes, in addition to land surface processes, contribute to the observed negative temperature–precipitation correlation. On longer time scales, the negative correlation between precipitation and temperature is shown to have implications for the projection of climate change impacts on near-surface climate: in all models, in the regions of strongest temperature–precipitation anticorrelation on interannual time scales, long-term regional warming is modulated to a large extent by the regional response of precipitation to climate change, with precipitation increases (decreases) being associated with minimum (maximum) warming. This correspondence appears to arise largely as the result of soil moisture–atmosphere interactions.

Land-use change is a prominent feature of the Anthropocene. Transitions between natural and human-managed ecosystems affect biogeochemical cycles in many ways, but soil processes are among the least understood. We used a global meta-analysis (62 studies, 1670 paired comparisons) to examine effects of land conversion on soil–atmosphere fluxes of methane (CH₄) and nitrous oxide (N₂O) from upland soils, and determine soil and environmental factors driving these effects. Conversion from a natural ecosystem to any anthropogenic land use increased soil CH₄ and N₂O fluxes by 234 kg CO₂-equivalents ha⁻¹ y⁻¹, on average. Reversion of managed ecosystems to that resembling natural ecosystems did not fully reverse those effects, even after 80 years. In general, neither the type of ecosystem converted, nor the type of subsequent anthropogenic land use, affected the magnitude of increase in soil emissions. Land-use changes in wetter ecosystems resulted in greater increases in CH₄ fluxes, but reduced N₂O fluxes. An interacting suite of soil variables influenced CH 4 and N₂O fluxes, with availability of inorganic nitrogen (that is, extractable ammonium and nitrate), pH, total carbon, and microclimate being strong mediators of effects of land-use change. In addition, time after a change in land use emerged as a critical factor explaining the effects of land-use change—with increased emissions of both greenhouse gases diminishing rapidly after conversion. Further research is needed to elucidate complex biotic and abiotic mechanisms that drive land-use change effects on soil greenhouse gas emissions, but particularly during this initial disturbance when emissions are greatest relative to native vegetation. Efforts to mitigate emissions will be severely hampered by this gap in knowledge.

Microbial C use efficiency (CUE) and turnover are critical to driving the global C cycle because they regulate C flux between soil and atmosphere. However, the elevational patterns of microbial CUE and turnover are still unclear. This study investigated microbial growth, respiration, biomass turnover time, and CUE at two soil depths (0–20 cm and 20–40 cm) across nine sites along an elevational gradient from 864 to 2856 m in a subtropical forest using an 18O-H2O-DNA approach. Microbial CUE increased with elevation in subsoil (20–40 cm), while it was not affected by elevation in topsoil (0–20 cm), and the increasing CUE with elevation was more related to microbial growth rather than respiration. Microbial growth normalized to MBC (qGrowth) increased with elevation, while microbial biomass turnover time decreased, and microbial respiration normalized to MBC (qCO2) was stable with elevation. Soil pH was negatively related to microbial CUE in topsoil and explained the most elevational variation of CUE. Differently, C:N stoichiometric imbalance between soil and microorganisms determined microbial CUE in subsoil. Altogether, our study provides strong evidence that the elevational pattern and driving factors of microbial CUE vary with soil depth, which may improve our understanding of C cycling and sequestration in the subtropical forest soil.

Deep fertilizer placement is a proposed strategy to increase crop yield and nitrogen (N) use efficiency while decreasing nitrous oxide (N 2 O) emissions from soil to atmosphere. Our objective was to test three fertilization depth orientations to compare overall N use efficiency, based on a 2-year field trial on a mineral soil cropped with cereals in Uppsala, Sweden. The field was fertilized with ammonium nitrate at a rate of 120 kg ha −1 (2016) and 105 kg ha −1 (2017) and a deep fertilizer placement (DP) at 0.20 m was compared to a shallow placement (SP) at 0.07 m and a mixed-depth placement (MP) where fertilizer was halved between the depths of 0.07 and 0.20 m, and a non-fertilized control (NF). In 2016, compared to SP, MP and DP increased N content in harvested grain by 3.6% and 2.5% respectively, and DP increased grain yield by 11% ( P  < 0.05). In both years, N 2 O emissions were similar in DP and NF, whereas SP and MP emissions were similar but generally higher than those in DP and NF. Fertilizer-induced emission factors (EF) for the growing season of 2017 decreased with fertilizer placement depth and were 0.77 ± 0.07, 0.58 ± 0.03, and 0.10 ± 0.02 for SP, MP, and DP, repectively. Although deep N placement benefits are likely dependent on weather conditions and soil type, this strategy has a clear potential for mitigating N 2 O emissions without adversely affecting yield.

The dissolution of sulfide minerals in subsurface porous media has important environmental implications. We investigate the oxidative dissolution of pyrite under evaporative conditions and advance a mechanistic understanding of the interactions between multiple physical processes and mineral/surface reactions. We performed a set of experiments in which initially water saturated and anoxic soil columns, containing a top layer of pyrite, are exposed to the atmosphere under no evaporation (single‐phase) and natural evaporative (two‐phase) conditions. The oxidative dissolution of pyrite was monitored by non‐invasive high‐resolution measurements of oxygen and pH. Additionally, we developed and applied a multiphase and multicomponent reactive transport model to quantitatively describe the experimental outcomes and elucidate the interplay between the physico‐chemical mechanisms controlling the extent of pyrite dissolution. The results confirm that the extent of pyrite dissolution under single‐phase conditions was constrained by the slow diffusive transport of oxygen in the liquid phase. In contrast, during evaporation, the evolution of fluid phases and interphase mass transfer processes imposed distinct physical constraints on the dynamics of pyrite oxidation. Initially, the invasion of the gaseous phase led to a fast delivery of high oxygen concentrations in the reactive zone and thus markedly increased pyrite oxidation and acidity/sulfate production. However, such enhanced release of reaction products was progressively limited over time as drying conditions prevailed in the reactive zone and inhibited pyrite oxidation. The transient phase displacement was also found to control the distribution of aqueous species and formation of secondary minerals by creating spatio‐temporally variable redox conditions. Key Points Oxidative dissolution of pyrite in chemically heterogeneous porous media was investigated under water saturated and evaporative conditions O2 and pH evolution shows spatially and temporally variable response of geochemical reactions to evaporation‐induced water dynamics A multiphase reactive transport model captures the data and reveals distinct mineral dissolution regimes under transient water dynamics

This paper investigates the influence of soil moisture on the development and triggering of convection in different early-morning atmospheric conditions. A one-dimensional model of the atmospheric boundary layer (BL) is initialized with atmospheric sounding data from Illinois and with the soil moisture set to either extremely wet (saturated) or extremely dry (20% of saturation) conditions. Two measures are developed to assess the lowlevel temperature and humidity structure of the early-morning atmosphere. These two measures are used to distinguish between four types of soundings, based on the likely outcome of the model: 1) those soundings favoring deep convection over dry soils, 2) those favoring deep convection over wet soils, 3) those unlikely to convect over any land surface, and 4) those likely to convect over any land surface. Examples of the first two cases are presented in detail. The early-morning atmosphere is characterized in this work by the newly developed convective triggering potential (CTP) and a low-level humidity index, HIlow. The CTP measures the departure from a moist adiabatic temperature lapse rate in the region between 100 and 300 mb (about 1–3 km) above the ground surface (AGS). This region is the critical interface between the near-surface region, which is almost always incorporated into the growing BL, and free atmospheric air, which is almost never incorporated into the BL. Together, these two measures form the CTP-HIlowframework for analyzing atmospheric controls on soil moisture–boundary layer interactions. Results show that in Illinois deep convection is trigged in the model 22% of the time over wet soils and only 13% of the time over dry soils. Additional testing varying the radiative conditions in Illinois and also using the 1D model with soundings from four additional stations confirm that the CTP-HIlowframework is valid for regions far removed from Illinois.