Megan L. Mobley

Monitoring Earth's Critical Zone

Earths rapidly changing near-surface environment needs systematic observation to better manage future crop production, climates, and water quality. Geologists tell us that we live in the Anthropocene, the period marked by humanitys global transformation of the environment (1). More than half of Earths terrestrial surface is now plowed, pastured, fertilized, irrigated, drained, fumigated, bulldozed, compacted, eroded, reconstructed, manured, mined, logged, or converted to new uses. These activities have long-lasting effects on life-sustaining processes of the near-surface environment, recently termed Earths “critical zone” (2). The full range of Anthropocene changes in Earths critical zone is not well quantified, especially belowground (see the figure) (3–6), where observed changes justify a major expansion in monitoring to better ensure the sustainability of crop and soil productivity, and the functioning of the global atmosphere and hydrosphere (3).

Surficial gains and subsoil losses of soil carbon and nitrogen during secondary forest development

Reforestation of formerly cultivated land is widely understood to accumulate above- and belowground detrital organic matter pools, including soil organic matter. However, during 40 years of study of reforestation in the subtropical southeastern USA, repeated observations of above- and belowground carbon documented that significant gains in soil organic matter (SOM) in surface soils (0-7.5 cm) were offset by significant SOM losses in subsoils (35-60 cm). Here, we extended the observation period in this long-term experiment by an additional decade, and used soil fractionation and stable isotopes and radioisotopes to explore changes in soil organic carbon and soil nitrogen that accompanied nearly 50 years of loblolly pine secondary forest development. We observed that accumulations of mineral soil C and N from 0 to 7.5 cm were almost entirely due to accumulations of light-fraction SOM. Meanwhile, losses of soil C and N from mineral soils at 35 to 60 cm were from SOM associated with silt and clay-sized particles. Isotopic signatures showed relatively large accumulations of forest-derived carbon in surface soils, and little to no accumulation of forest-derived carbon in subsoils. We argue that the land use change from old field to secondary forest drove biogeochemical and hydrological changes throughout the soil profile that enhanced microbial activity and SOM decomposition in subsoils. However, when the pine stands aged and began to transition to mixed pines and hardwoods, demands on soil organic matter for nutrients to support aboveground growth eased due to pine mortality, and subsoil organic matter levels stabilized. This study emphasizes the importance of long-term experiments and deep measurements when characterizing soil C and N responses to land use change and the remarkable paucity of such long-term soil data deeper than 30 cm.

Soil in the Anthropocene

With scholars deliberating a new name for our geologic epoch, i.e., the Anthropocene, soil scientists whether biologists, chemists, or physicists are documenting significant changes accruing in a majority of Earths soils. Such global soil changes interact with the atmosphere, biosphere, hydrosphere, and lithosphere (i.e., Earths Critical Zone), and these developments are significantly impacting the Earths stratigraphic record as well. In effect, soil scientists study such global soil changes in a science of anthropedology, which leads directly to the need to transform pedostratigraphyinto an anthro-pedostratigraphy, a science that explores how global soil change alters Earths litho-, bio-, and chemostratigraphy. These developments reinforce perspectives that the planet is indeed crossing into the Anthropocene.

Natural recovery of soil organic matter in 30–90‐year‐old abandoned oil and gas wells in sagebrush steppe

We addressed the rarely studied issue of how different soil organic matter pools respond to disturbances from historical oil and gas well development in semi-arid Intermountain sagebrush steppe. We selected twenty-nine study well sites in south-central Wyoming that were plugged and abandoned 33–90 years ago. We designed our study to understand the long term impact of oil and gas development for soil organic matter pools on non-reclaimed sites, and evaluate the importance of this disturbance type relative to other major influences on soil organic matter and the fine-scale, shrub-induced heterogeneity of soil organic matter. We compared total, labile, and recalcitrant pools of soil C and N in disturbed sites to adjacent, un-disturbed sites. We found that that natural site-specific conditions such as soil texture and fine-scale heterogeneity associated with shrubs are the most important controls over soil C and N, particulate organic matter C and N, and potential C and N mineralization, and that these older well-pad disturbances did not have a significant effect on any soil organic matter pools. Fine-scale, shrub-induced heterogeneity was higher for those pools that have a fast turnover rate than those with a slow turnover rate. Moreover, fine-scale heterogeneity of total soil C was higher for well sites that were located in loamy sand soils compared to well sites on sandy soils. In addition, heterogeneity of total soil C recovered through time on the finer textured soils. Overall our study suggests that soil organic matter pools were not affected by old oil well development, and that recovery of shrub cover is a key component of soil organic matter recovery in these semi-arid shrublands. A comparison of our work to other work on recent and reclaimed sites suggests that some reclamation procedures may decrease soil organic matter more than the absence of reclamation.

Sagebrush steppe recovery on 30–90-year-old abandoned oil and gas wells

Oil and natural gas extraction is rapidly expanding in semiarid intermountain big sagebrush ecosystems of the western USA. Formerly covering over 60 million ha, this ecosystem has lost almost half its area due to land use changes. In this study, we measured the natural recovery of the big sagebrush plant community across a chronosequence of 29 oil and gas well sites that were abandoned without reclamation between 1923 and 1980. We measured big sagebrush cover and density with strip transects inside and outside of the well sites, and we estimated big sagebrush biomass using both canopy measurements and allometric equations. Cover of other shrubs, grasses, and forbs was estimated with line transects inside and outside of well sites. We estimated that it takes at least 87 years for Wyoming big sagebrush cover to recover naturally, although big sagebrush density recovered in fewer than 70 years. Grasses and non-sagebrush shrubs recovered rapidly, shown by the high cover of those groups in the youngest sites. ...

Inorganic Nitrogen Supply and Dissolved Organic Nitrogen Abundance across the US Great Plains

Across US Great Plains grasslands, a gradient of increasing mean annual precipitation from west to east corresponds to increasing aboveground net primary productivity (ANPP) and increasing N-limitation. Previous work has shown that there is no increase in net N mineralization rates across this gradient, leading to the question of where eastern prairie grasses obtain the nitrogen to support production. One as-yet unexamined source is soil organic N, despite abundant literature from other ecosystems showing that plants take up dissolved soil organic N. This study measured KCl-extractable dissolved organic N (DON) in surface soils across the grassland productivity gradient. We found that KCl-extractable DON pools increased from west to east. If available to and used by plants, this DON may help explain the high ANPP in the eastern Great Plains. These results suggest a need for future research to determine whether, in what quantities, and in what forms prairie grasses use organic N to support primary production.

Grazing and No-Till Cropping Impacts on Nitrogen Retention in Dryland Agroecosystems

As the worlds population increases, marginal lands such as drylands are likely to become more important for food production. One proven strategy for improving crop production in drylands involves shifting from conventional tillage to no-till to increase water use efficiency, especially when this shift is coupled with more intensive crop rotations. Practices such as no-till that reduce soil disturbance and increase crop residues may promote C and N storage in soil organic matter, thus promoting N retention and reducing N losses. By sampling soils 15 yr after a N tracer addition, this study compared long-term soil N retention across several agricultural management strategies in current and converted shortgrass steppe ecosystems: grazed and ungrazed native grassland, occasionally mowed planted perennial grassland, and three cropping intensities of no-till dryland cropping. We also examined effects of the environmental variables site location and topography on N retention. Overall, the long-term soil N retention of >18% in these managed semiarid ecosystems was high compared with published values for other cropped or grassland ecosystems. Cropping practices strongly influenced long-term N retention, with planted perennial grass systems retaining >90% of N in soil compared with 30% for croplands. Grazing management, topography, and site location had smaller effects on long-term N retention. Estimated 15-yr N losses were low for intact and cropped systems. This work suggests that semiarid perennial grass ecosystems are highly N retentive and that increased intensity of semiarid land management can increase the amount of protein harvested without increasing N losses.

Evolution of Soil, Ecosystem, and Critical Zone Research at the USDA FS Calhoun Experimental Forest

The US Department of Agriculture (USDA) Forest Service Calhoun Experimental Forest was organized in 1947 on the southern Piedmont to engage in research that today is called restoration ecology, to improve soils, forests, and watersheds in a region that had been severely degraded by nearly 150 years farming. Today, this 2,050-ha research forest is managed by the Sumter National Forest and Southern Research Station. In the early 1960s, the Calhoun Experimental Forest was closed as a base of scientific operations making way for a new laboratory in Research Triangle Park, NC. Many papers were written during the Calhoun’s 15 years of existence, papers that document how land-use history creates a complex of environmental forcings that are hard to unwind. One Calhoun field experiment remains active, however, and over nearly six decades has become a model for the study of soil and ecosystem change on timescales of decades. The experiment contributes greatly to our understanding of the effects of acid atmospheric deposition on soils, forests, and waters and of decadel changes in carbon and nutrient cycling in soils and forests. Perhaps the long-term experiment’s major contribution is its clear demonstration that soils are highly dynamic systems on timescales of decades and that this dynamism involves both surface and deep subsoils. The on-going experiment’s success is attributed to relatively simple experimental design, ample plot replication, rigorous (but not too arduous) protocol for resampling and archiving, and to its ability to address changing scientific and management priorities that are important to society and the environment. In the last decade, the experiment has become a platform for research and education that explore basic and applied science. As this manuscript goes to press, the Calhoun Experimental Forest has been designated to become one of the National Science Foundation’s national Critical Zone (CZ) Observatories, a development that will allow researchers to, return to the questions that originated the Calhoun Experimental Forest in the first place: how and why severely disturbed landscapes evolve through time.