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Land use and altered carbon dynamics are two of the primary components of global change, and the effect of land use on carbon cycling is a crucial issue in regional scale biogeochemistry. Previous studies have shown that climate and soil conditions control net primary production (NPP) at regional scales, and that agricultural land use can influence NPP at local scales through altered water availability and carbon allocation patterns. However, few studies have attempted to quantify the effect of cultivation on NPP at regional scales, and no studies have examined this relationship for the most heavily cultivated region of the United States, the Great Plains. We quantified current regional aboveground and belowground productivity (including cultivation) for nine years on a county basis from (1) USDA agricultural census data, and (2) STATSGO range site production values. By comparing these data with values of native vegetation NPP (precultivation) derived from STATSGO, we estimated that cultivation is increasing regional NPP by ~10%, or 0.046 Pg C/yr. In addition, we examined the relationship between cultivation of particular crops and NPP change and characterized the influence of individual crops on primary productivity. Key words: agriculture; carbon; cropping; grassland; land use; primary production; regional scale.

Warmer regions generally exhibit greater rates of soil respiration and organic matter decomposition than colder regions. In the Great Plains of the United States, soil organic matter declines from the northern part of the region to the south, suggesting greater decomposition rates in areas with warmer temperatures, our study used a regional data set of aboveground net primary production, soil organic carbon, soil texture, and climate to evaluate the environmental controls over areal patterns in decomposition rates, (k; expressed as grams per year per gram of initial mass), throughout the U.S. Great Plains. We conducted multiple regression analyses of steady-state k with respect to mean annual temperature, mean annual precipitation, and percentage soil clay content to examine both the combined and individual effects of these independent variables on regional decomposition rates. Our results indicated that precipitation contributes more than either temperature or soil texture to areal patterns of decomposition rates in the U.S. Great Plains, explaining >30% of the areal variability in k. Decomposition rates increased with increasing precipitation and with decreasing soil clay content. Temperature explained Key words: aboveground net primary production; areal patterns; carbon dynamics; decomposition; grasslands; Great Plains (USA); precipitation; regional scale; soil organic matter; soil texture; temperature.

Spatial variability that occurs at large scales has long been used by ecologists as a tool to examine the controls over ecosystem structure and function. Correlations of control variables such as climatic factors and response variables such as vegetation and soil carbon storage across broad regions have played a crucial role in predicting the response of ecosystems to global climate change. Despite the importance of these large-scale space-for-time substitutions, there are substantial limitations. One of these limitations is that many of the possible control factors covary with one another, and only some of the important control factors actually exist in large-scale databases. Thus, the true proximal controls may be difficult to identify. A second limitation is that models of spatial variability may not be appropriately applied to temporal variability. In this paper, we utilize a new approach to determine the extent to which N availability may constrain aboveground primary productivity in the Central Grassland region of the U.S. The strong relationship between average annual primary production and average annual precipitation found in spatial patterns in ecosystems globally has often been interpreted as evidence of a fundamental water limitation. However, temporal variation in annual aboveground net primary production (ANPP) indicates that other factors constrain production. We generated a spatial and temporal database for annual aboveground net primary production and annual net N mineralization by linking a database of input variables (precipitation, temperature, and soils) with predictive models. We generated independent data sets of aboveground net primary production and net N mineralization by using regression models to predict aboveground net primary production, and the Century model to simulate net N mineralization. Our analyses indicate that net primary production and net N mineralization both increase with mean annual precipitation; thus, it is not possible to separate the extent to which ANPP is controlled by water or N availability. Nitrogen use efficiency (NUE) increased with increasing precipitation across the region. Aboveground net primary production decreased with increasing temperature across the region, while N mineralization increased slightly, leading to decreasing (NUE) with increasing temperature. At high precipitation levels, aboveground net primary production increased and N mineralization decreased slightly with increasing soil fineness. Nitrogen use efficiency generally increased with increasing pools of soil organic matter, likely because in grasslands, the proportion of recalcitrant organic matter increases with the total organic matter pools. A comparison of interannual variation in net N mineralization with average spatial variation indicated a high degree of inertia in the response of N availability to precipitation levels. Our simulation results as well as field results of Lauenroth and Sala (1992) raise important questions about the applicability of space-for-time substitutions when dealing with ecosystem function. The structure of the systems appears to provide important constraints on the temporal variability that are not evident in an analysis of spatial variability. Key words: grasslands; net nitrogen mineralization; net primary productivity; nitrogen limitation: regional analysis., INTRODUCTION Spatial variability in ecosystem structure and function is profoundly evident at landscape to regional and global scales. Ecosystem ecologists have long used this spatial variability to explain the controls [...]

The shortgrass steppe of northern Colorado is characterized by patchy plant cover and associated spatial heterogeneity of soil resources. Zones of relatively high soil organic matter (SOM) under plants are maintained by direct litter inputs and physical stabilization of aeolian material. We studied the duration of plant-associated soil enrichment following plant death. We sampled soils between plants, under live plants, and under plants dead for 1, 9, and 36 mo. Live- and dead-plant soils were more enriched in total C and N than bare soils. We found a general pattern following plant death of initial active and total SOM increase due to greater litterfall than decomposition. Next, SOM decreased as substrate supplies declined, and decomposition continued. The temporal pattern depended upon SOM turnover. Though decomposition initially provided resources to maintain enriched plant-associated zones, our results suggest that enriched nutrient-supply zones under dead plants do not persist beyond several months. Key words: litterfall vs. decomposition; Northern Colorado (USA) shortgrass steppe; plant microsite; resource gap; semiarid ecosystems; shortgrass steppe soils; soil heterogeneity; soil organic matter., INTRODUCTION Arid and semiarid systems are characterized by discontinuous aboveground plant cover due to intense competition for water (Noy-Meir 1973). The shortgrass steppe of northern Colorado follows this pattern, with [...]

Several studies have suggested the existence of a positive relationship between the Normalized Difference Vegetation Index (NDVI) derived from AVHRR/NOAA satellite data and either biomass or annual aboveground net primary production (ANPP) for different geographic areas and ecosystems. We calibrated a 4-yr average of the ingegral of the NDVI (NDVI-I) using spatially aggregated values of ANPP. We also provided an estimate of the energy conversion efficiency coefficient ([Epsilon]) of Monteith's equation. This is the first attempt to calibrate a standard NDVI product for temperate perennial grasslands. We found a positive and statistically significant relationship between NDVI-I and ANPP for grassland areas with mean annual precipitation between 280 and 1150 mm, and mean annual temperature between 4 ° and 20 ° C. Depending on the method used to estimate the fraction of photosynthetic active radiation, the energy conversion officency coefficient was constant (0.24 g C/MJ), or varied across the precipitation gradient, from 0.10 g C/MJ for the least productive to 0.20 g C/MJ for the most productive sites. Key words: absorbed photosynthetic active radiation; energy conversion efficiency coefficient; grasslands; Normalized Difference Vegetation Index; net primary production; remote sensing., The ability to translate spectral data to biologically meaningful variables is a key step in increasing the use and value of satellite information. Theoretical work provides strong support for the [...]

We examined soil organic matter (SOM) dynamics in a shortgrass steppe ecosystem along spatial gradients in plant inputs and temporal gradients in disturbance age because a better understanding of decay characteristics of SOM pools may improve our ability to predict ecosystem responses to perturbation. We assessed measurable pools of SOM that are thought to correspond to active, intermediate, and passive SOM based upon turnover characteristics in three separate experiments. In a first, we evaluated SOM pools along a spatial gradient in plant inputs, from locations under individual bunch grasses to natural areas of plant removal (ant mounds). In a second experiment we assessed SOM pools across a temporal gradient in ant mounds ranging from ≅5 to 60 yr old. Finally, we compared our results from the second experiment to a single human-induced plant-removal experiment. We found that reduced root biomass accounted for up to 90% of the variation in SOM across our spatial gradients. In our examination of temporal dynamics of SOM, we found that locations with little or no plant inputs for ≅45 yr had 28% less total soil C than in native areas with average plant cover. We measured the greatest proportional losses (4878%) of SOM in response to reduced plant inputs (over 30-60 yr) in the active SOM as indexed by C and N mineralization and microbial biomass C and N. The range of losses from intermediate organic matter as indexed by particulate organic matter (POM) ranged from 38 to 73%, and losses from the passive pool were proportionately lower (57%). Though this general pattern follows the course predicted by theory, and our measurements of turnover of intermediate SOM agree closely with models, results indicate a considerably slower turnover rate for the active pool and a considerably faster rate for the passive pool than expected. We compared our field estimates of plant-removal-induced SOM losses to simulation modeling results and cultivation studies. Our comparison of field results to Century model simulations indicated a possible model overestimation of the impact of plant removal on SOM loss. By comparing our plant-removal results to studies of cultivation-induced losses of SOM on the shortgrass steppe, we found that plant-removal is not as severe a disturbance as cropping, likely as a result of physical disturbances associated with tillage such as surface erosion and disturbance of soil aggregate structure. Key words: ant mounds; Central Plains Experimental Range, Colorado, USA; kinetic characterization of soil organic matter; plant removal; shortgrass steppe; soil organic matter., INTRODUCTION Soil organic matter (SOM) contributes to the fertility and structure of grassland soils (Anderson and Coleman 1985), provides ecosystem resilience (Tisdall and Oades 1982), and is a substantial sink [...]

The effect of plant community structure on nutrient cycling is fundamental to our understanding of ecosystem function. We examined the importance of plant species and plant cover (i.e., plant covered microsites vs. bare soil) on nutrient cycling in shortgrass steppe of northeastern Colorado. We tested the effects of both plant species and cover on soils in an area of undisturbed shortgrass steppe and an area that had undergone nitrogen and water additions from 1971 to 1974, resulting in significant shifts in plant species composition. Soils under plants had consistently higher C and N mineralization rates and, in some cases, higher total and microbial C and N levels than soils without plant cover. Four native grasses, one sedge, and one shrub differed from one another in the quantity and quality of above- and belowground biomass but differences among the six species in soil nutrient cycling under their canopies were slight. However, soils under bunchgrasses tended to have higher C mineralization and microbial biomass C than soil under the rhizomatous grass, Agropyron smithii. Also, the one introduced annual in the study, Kochia scoparia, had soils with less plant-induced heterogeneity and higher rates of C and N mineralization as well as higher levels of microbial biomass C than soils associated with the other species. This species was abundant only on plots that had received water and nitrogen for a 4-yr period that ended 20 yr ago, where it has persisted in the absence of resource additions for 20 yr, suggesting a positive feedback between plant persistence and soil nutrient status. Plant cover patterns had larger effects on ecosystem scale estimates of soil properties than the attributes of a particular plant species. This result may be due to the semiarid nature of this grassland in which plant cover is discontinuous and decomposition and nutrient availability are primarily limited by water, not by plant species-mediated characteristics such as litter quality. That local plant-induced patterns in soil properties significantly affected ecosystem scale estimates of these properties indicates that consideration of structural attributes, particularly plant cover patterns, is critical to estimates of ecosystem function in shortgrass steppe. Key words: carbon; disturbance; functional groups; island of fertility; nitrogen; nutrient cycling; plant-soil feedback; shortgrass steppe; soil heterogeneity., INTRODUCTION Individual plant characteristics, such as life-span, biomass allocation, and tissue chemical composition have been shown to have significant effects on ecosystem processes such as soil organic matter and nutrient [...]

Biogeochemistry of terrestrial ecosystems is controlled by interactions among factors operating at several spatial and temporal scales. The purpose of this study was to evaluate the relative importance and interaction of relatively static landscape factors and more dynamic factors in a shortgrass steppe landscape. The landscape factors examined were topographic position, and soil texture. The dynamic factors studied were seasonal climate and the localized effects of individual plants on soils. Patterns were evaluated by sampling soil between and under individual Bouteloua gracilis plants in paired upland (erosional) and lowland (depositional) plots at eight locations at the Central Plains Experimental Range (CPER), Colorado. We quantified five organic C and N pools (total, fine and coarse particulate organic matter [POM], mineral-associated organic matter [MAOM], and potentially mineralizable C and N), and we estimated seasonal patterns of in situ N dynamics with three methods (extractable inorganic N, net N mineralization in uncovered cores, and N adsorbed on ion exchange resin [IER] bags). Topographic position and soil texture each explained much of the landscape-scale variation of C and N pools and vegetation structure. Most lowland plots were enriched in silt, clay, C, and N relative to adjacent upland plots, and topographic position affected most pools significantly. Most vegetation and biogeochemical variables were strongly correlated with soil sand content. Across the range of sand content encountered (40-83%), the fraction of area in bare soil openings [is greater than] 5 cm across increased sevenfold, and most C and N fractions increased by 2-4.5 times. Plant-induced, microscale heterogeneity of soil C and N was comparable in magnitude to landscape-scale heterogeneity for pools with more rapid turn-over (POM and mineralizable C and N), but presence or absence of plants did not affect more stable, mineral-associated organic matter. Plant-induced heterogeneity was significant in all locations, but its importance likely decreases with decreasing sand content as cover becomes more continuous, particularly in lowlands. Total extractable inorganic N and nitrate, N adsorbed on resin bags, and the proportion of mineralized N that was nitrified during incubations increased with increasing soil water content or precipitation, but net N mineralization did not vary systematically with precipitation. Inorganic N availability was greatest during relatively moist spring periods, and these were the only times when indices of in situ N availability followed the spatial patterns expected from laboratory assays of C and N pool distribution. The relatively weak spatial patterns for N dynamics contrast with the substantial landscape and individual-plant scale variation in C and N pools. Landscape patterns of N mineralization and availability may be tied to N pools, such as POM N, that are not strongly related to topography or texture. Particulate organic matter appears to be especially important to N retention and availability on sandy soils, which typify most sites at the CPER, the proportion of total N residing in POM was high and POM C:N ratios were low in sandy upland soils. We suggest that soil texture is a key proximal control over biogeochemical processes and is largely responsible for observed landscape-scale patterns, including topographic differences. Models that integrate effects of texture and topography have the potential to link landscape biogeochemical patterns to short-term processes that directly influence SOM dynamics and to long-term geomorphic processes that influence soil distribution, including eolian redistribution of soil materials, which is important in many dry regions. Key words: biogeochemistry; catena model; landscape ecology; microclimate; nitrogen mineralization; semiarid ecosystems; shortgrass steppe; soil carbon; soil nitrogen; soil organic matter; soil texture; topography.

Factors controlling N turnover in sagebrush ecosystems are separable into two groups. The first group consists of properties that exhibit strong spatial patterning at a landscape scale but are temporally static at time scales of years or tens of years. These static properties include plant species assemblages, total soil nutrient pools, and soil texture. A second group includes properties that vary across the landscape over shorter time scales, i.e., annually, seasonally, and diurnally. These dynamic properties include soil moisture, temperature, and amount of available nutrients. This paper evaluates the landscape variability of properties in both of these groups, and examines the extent to which these factors control N turnover. Static ecosystem properties were entered into a principal components analysis resulting in four axes of landscape variability. A statistical analysis of the relationship of net N mineralization with the principal components and with soil temperature and moisture suggested that soil microclimate and organic matter quality both control in situ N turnover. Soil microclimate limited N mineralization to a short season in early spring and summer; only during this time did soil organic matter exert control. In landscape positions where soil organic matter pools were low, improved soil microclimate conditions did not increase N mineralization rates. A similar approach may be useful in evaluating control over ecosystem processes in other systems that are characterized by strong seasonal and spatial variability.

This synthetic data set contains plant species relative abundance measures from 35 nitrogen (N) fertilization experiments conducted at 10 sites across North America. The data set encompasses the fertilization responses of 575 taxa from 1159 experimental plots. The methodology varied among experiments, in particular with regard to the type and amount of N added, plot size, species composition measure (biomass harvest, pin count, or percent cover), additional experimental manipulations, and experimental duration. At each site, each species has been classified according to a number of easily identified categorical functional traits, including life history, life form, the number of cotyledons, height relative to the canopy, potential for clonal growth, and nativity to the United States. Additional data are available for many sites, indicated by references to publications and web sites. Analyses of these data have shown that N enrichment significantly alters community composition in ways that are predictable on the basis of plant functional traits as well as environmental context. This data set could be used to answer a variety of questions about how plant community composition and structure respond to environmental changes. Key words: database; fertilization; functional trait; nitrogen; plant community; synthesis.

Livestock grazing is a globally important land use and has the potential to significantly influence plant community structure and ecosystem function, yet several critical knowledge gaps remain on the direction and magnitude of grazing impacts. Furthermore, much of our understanding of the long-term effects on plant community composition and structure are based on grazer exclusion experiments, which explicitly avoid characterizing effects along grazing intensity gradients. We sampled big sagebrush plant communities using 68 plots located along grazing intensity gradients to determine how grazing intensity influences multiple aspects of plant community structure over time. This was accomplished by sampling plant communities at different distances from 17 artificial watering sources, using distance from water and cow dung density as proxies for grazing intensity at individual plots. Total vegetation cover and total grass cover were negatively related to grazing intensity, and cover of annual forbs, exotic cover, and exotic richness were positively related to grazing intensity. In contrast, species richness and composition, bunchgrass biomass, shrub density and size, percentage cover of bare ground, litter, and biological soil crusts did not vary along our grazing intensity gradients, in spite of our expectations to the contrary. Our results suggest that the effects of livestock grazing over multiple decades (mean = 46 years) in our sites are relatively small, especially for native perennial species, and that the big sagebrush plant communities we sampled are somewhat resistant to livestock grazing. Collectively, our findings are consistent with existing evidence that indicates the stability of the big sagebrush plant functional type composition under current grazing management regimes., (© 2022 The Ecological Society of America.)

The probability of extreme weather events is increasing, with the potential for widespread impacts to plants, plant communities, and ecosystems. Reports of drought-related tree mortality are becoming more frequent, and there is increasing evidence that drought accompanied by high temperatures is especially detrimental. Simultaneously, extreme large precipitation events have become more frequent over the past century. Water-limited ecosystems may be more vulnerable to these extreme events than other ecosystems, especially when pushed outside of their historical range of variability. However, drought-related mortality of shrubs-an important component of dryland vegetation-remains understudied relative to tree mortality. In 2014, a landscape-scale die-off of the widespread shrub, big sagebrush (Artemisia tridentata Nutt.), was reported in southwest Wyoming, following extreme hot and dry conditions in 2012 and extremely high precipitation in September of 2013. Here we examine how severe drought, extreme precipitation, soil texture and salinity, and shrub-stand characteristics contributed to this die-off event. At 98 plots within and around the die-off, we quantified big sagebrush mortality, characterized soil texture and salinity, and simulated soil-water conditions from 1916 to 2016 using an ecosystem water-balance model. We found that the extreme weather conditions alone did not explain patterns of big sagebrush mortality and did not result in extreme (historically unprecedented) soil-water conditions during the drought. Instead, plots with chronically dry soil conditions experienced greatest mortality following the global change-type (hot) drought in 2012. Furthermore, mortality was greater in locations with high potential run-on and low potential run-off where saturated soil conditions were simulated in September 2013, suggesting that extreme precipitation also played an important role in the die-off in these locations. In locations where drought alone contributed to mortality, stem density negatively impacted big sagebrush. In locations that may have been affected by both drought and saturation, however, mortality was greatest where stem density was lowest, suggesting that these locations may have already been less favorable to big sagebrush. Paradoxically, vulnerability to both extreme events (drought and saturation) was associated with finer-textured soils, and our results highlight the importance of soils in determining local variation of the vulnerability of dryland plants to extreme events., (© 2019 by the Ecological Society of America. This article has been contributed to by US Government employees and their work is in the public domain in the USA.)

In drylands, the coexistence of grasses and woody plants has been attributed to soil-water resource partitioning. Soil texture and precipitation seasonality can influence the amount and distribution of water in the soil, and their interaction may play an important role in determining the relative importance of grasses and woody plants. We investigated the influence of this interaction on plant functional types across a broad range of precipitation regimes and soil textures in western North America by analyzing plant-cover data collected at 2,084 plots that included the widespread shrub big sagebrush (Artemisia tridentata Nutt.). We characterized how the significance of the inverse-texture effect varies across soil conditions by quantifying relationships between precipitation and foliar cover on finer- vs. coarser-textured soils across a range of potential texture divisions represented by sand content. We found evidence of the inverse-texture effect for every plant functional type (except for cheatgrass) that we examined with at least one component of precipitation (annual, warm, or cold season), and provide the first evidence for this effect in locations with cold-season-dominated precipitation regimes. The texture and precipitation combinations that exhibited the inverse-texture effect varied with plant functional type, presumably because of effects of soil texture on water availability at different soil depths with season. Furthermore, we found an inverse-texture effect that was remarkably similar for shrub cover with cold-season precipitation and grass cover with warm-season precipitation. These results provide new insight into how the inverse-texture effect interacts with precipitation seasonality to influence plant functional type composition in drylands, and further suggest that quantifying the soil-texture division at which the inverse-texture effect is relevant under a given set of environmental conditions may provide support for the effect across dryland plant communities., (© 2019 by the Ecological Society of America.)

Both biogeographical and rainfall manipulation studies show that soil water content can be a strong driver of microbial community composition. However, we do not yet know if these patterns emerge because certain bacterial taxa are better able to survive at dry soil moisture regimes or if they are due to other drought-sensitive ecosystem properties indirectly affecting microbial community composition. In this study, we evaluated (1) whether bacterial community composition changed under an 11-year drought manipulation and (2) whether shifts under drought could be explained by variation in the moisture sensitivity of growth among bacterial taxa (moisture niche partitioning). Using 454 pyrosequencing of 16S rRNA, we observed shifts in bacterial community composition under drought, coincident with changes in other soil properties. We wet-up dry soils from drought plots to five moisture levels, and measured respiration and the composition of actively growing communities using bromodeoxyuridine (BrdU) labeling of DNA. The field drought experiment affected the composition of the active community when incubated at different moisture levels in the laboratory, as well as short-term (36-hour) respiration rates. Independent of history, bacterial communities also displayed strong niche partitioning across the wet-up moisture gradient. Although this indicates that moisture has the potential to drive bacterial community composition under long-term drought, species distributions predicted by response to moisture did not reflect the community composition of plots that were subjected to long-term drought. Bacterial community structure was likely more strongly driven by other environmental factors that changed under long-term drought, or not shaped by response to water level upon wet-up. The approach that we present here for linking niches to community composition could be adapted for other environmental variables to aid in predicting microbial species distributions and community responses to environmental change.