@article {49865, title = {Density fractionation and 13C reveal changes in soil carbon following woody encroachment in a desert ecosystem}, journal = {Biogeochemistry}, volume = {112}, year = {2013}, month = {03/2013}, pages = {409-422}, chapter = {409}, abstract = {Woody encroachment has dramatically changed land cover patterns in arid and semiarid systems (drylands) worldwide over the past 150 years. This change is known to influence bulk soil carbon (C) pools, but the implications for dynamics and stability of these pools are not well understood. Working in a Chihuahuan Desert C4 grassland encroached by C3 creosote bush (Larrea tridentata), we used two density fractionation techniques (2 and 7 pool density fractionations) and isotopic analysis to quantify changes in C pools and dynamics among vegetation microsites typical of an encroachment scenario (remnant intact grassland, shrub subcanopies, and in shrub intercanopy spaces within a shrubencroached area). The C concentration of bulk soils varied with microsite, with almost twice the C in shrub subcanopies as in intercanopy spaces or remnant grasslands. Estimated SOC accumulation rates from Larrea encroachment (4.79 g C m-2 year-1 under canopies and 1.75 g C m-2 year-1 when intercanopy losses were taken into account) were lower than reported for higher productivity Prosopis systems, but still represent a potentially large regional C sink. The composition of soil C varied among microsites, with the shrub subcanopy C composed of proportionally more light fraction C (\1.85 g cm-3) and C that was soluble in sodium polytungstate. Grassland soils contained very little carbonate C compared to shrub subcanopies or shrub intercanopy spaces. Stable isotope analyses indicate that inputs from C3 shrubs were incorporated into all density fractions, even in heavy fractions in which shrub inputs did not change overall C concentration. The seven density fractionation yielded unexpected \ð13C patterns, where the two heaviest fractions were strongly depleted in 13C, indicating strong fractionation following organic matter inputs. These results suggest that the utility of isotope mixing models for determining input sources may be limited in systems with similar fractionation patterns. We propose a five pool model for dryland soil C that includes a relatively dynamic light fraction, aggregate and heavy fractions that are stable in size but that reflect dynamic inputs and outputs, a potentially large and seasonally dynamic pool of soluble C, and a large pool of carbonate C. Combined, these results suggest that dryland soil C pools may be more dynamic than previously recognized.}, keywords = {black grama grass, Bouteloua, carbonates, creosotebush, dryland, Jornada, Larrea, soil carbon stabilization, soil organic matter, woody encroachment}, doi = {10.1007/s10533-012-9735-y}, url = {/files/bibliography/13-020.pdf}, author = {Heather L. Throop and Kate Lajtha and Kramer, Marc} } @article {49909, title = {Response of dominant grass and shrub species to water manipulation: An ecophysiological basis for shrub invasion in a Chihuahuan Desert grassland}, journal = {Oecologia}, volume = {169}, year = {2012}, month = {01/15/2012}, pages = {373-383}, chapter = {373}, abstract = {Increases in woody vegetation and declines in grasses in arid and semi-arid ecosystems have occurred globally since the 1800s, but the mechanisms driving this major land-cover change remain uncertain and controversial. Working in a shrub-encroached grassland in the northern Chihuahuan Desert where grasses and shrubs typically differ in leaf-level nitrogen allocation, photosynthetic pathway, and root distribution, we asked if differences in leaflevel ecophysiology could help explain shrub proliferation. We predicted that the relative performance of grasses and shrubs would vary with soil moisture due to the diVerent morphological and physiological characteristics of the two life-forms. In a 2-year experiment with ambient, reduced, and enhanced precipitation during the monsoon season, respectively, the encroaching C3 shrub (honey mesquite Prosopis glandulosa) consistently and substantially outperformed the historically dominant C4 grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates while also maintaining a more favorable leaf water status. These differences persisted across a wide range of soil moisture conditions, across which mesquite photosynthesis was decoupled from leaf water status and moisture in the upper 50 cm of the soil proWle. Mesquite\’s ability to maintain physiologically active leaves for a greater fraction of the growing season than black grama potentially ampliWes and extends the importance of physiological differences. These physiological and phenological differences may help account for grass displacement by shrubs in drylands. Furthermore, the greater sensitivity of the grass to low soil moisture suggests that grasslands may be increasingly susceptible to shrub encroachment in the face of the predicted increases in drought intensity and frequency in the desert of the southwestern USA.}, keywords = {drought, grassland community, Jornada, photosynthesis, precipitation manipulation, shrub invasion, water manipulation, water potential, woody encroachment}, doi = {10.1007/s00442-011-2217-4}, url = {/files/bibliography/12-015.pdf}, author = {Heather L. Throop and Lara Reichmann and O. E. Sala and Steven R. Archer} } @article {49908, title = {When bulk density methods matter: implications for estimating soil organic carbon pools in coarse soils}, journal = {Journal of Arid Environments}, volume = {77}, year = {2012}, month = {02/2012}, pages = {66-71}, chapter = {66}, abstract = {

Resolving uncertainty in the carbon cycle is paramount to refining climate predictions. Soil organic carbon (SOC) is a major component of terrestrial C pools, and accuracy of SOC estimates are only as good as the measurements and assumptions used to obtain them. Dryland soils account for a substantial portion of global SOC, but the pool dynamics are highly uncertain. One crucial component of accurate estimates of SOC on an areal basis is bulk density (ρb), the mass of soil per unit volume. Here, we review methods used for calculating ρb and assess their prevalence. We show how treatment of coarse fragments (particles \>2 mm diameter) influences ρb values and discuss the implications for SOC estimates in drylands. In four dryland examples, methods that varied in their treatment of coarse fragments led to substantial (up to 26\%) differences in ρb. Calculated SOC pools responded proportionally, with SOC differing by up to 518 g C m-2. We suggest a revised method for accounting for coarse fractions in ρb calculations. A large portion of the world{\textquoteright}s soils, particularly in drylands, are fine enough to allow ρb determination with cores, but contain coarse fragments that substantially impact SOC mass estimates if not explicitly considered.

}, keywords = {bulk density, carbon pool, SOC, soil organic carbon}, doi = {10.1016/j.jaridenv.2011.08.020}, url = {/files/bibliography/12-001.pdf}, author = {Heather L. Throop and Steven R. Archer and Curtis H. Monger and S. Waltman} } @article {49998, title = {When bulk density methods matter: Implications for estimating soil organic carbon pools in rocky soils}, journal = {Journal of Arid Environments}, volume = {77}, year = {2011}, pages = {66-71}, abstract = {Resolving uncertainty in the carbon cycle is paramount to refining climate predictions. Soil organic carbon (SOC) is a major component of terrestrial C pools, and accuracy of SOC estimates are only as good as the measurements and assumptions used to obtain them. Dryland soils account for a substantial portion of global SOC, but the pool dynamics are highly uncertain. One crucial component of accurate estimates of SOC on an areal basis is bulk density (pb), the mass of soil per unit volume. Here, we review methods used for calculating pb and assess their prevalence. We show how treatment of coarse fragments (particles \>2 mm diameter) influences pb values and discuss the implications for SOC estimates in drylands. In four dryland examples, methods that varied in their treatment of coarse fragments led to substantial (up to 26\%) differences in pb. Calculated SOC pools responded proportionally, with SOC differing by up to 518 g C m-2. We suggest a revised method for accounting for coarse fractions in pb calculations. A large portion of the world\’s soils, particularly in drylands, are fine enough to allow pb determination with cores, but contain coarse fragments that substantially impact SOC mass estimates if not explicitly considered.}, doi = {doi:10.1016/j.jaridenv.2011.08.020}, author = {Heather L. Throop and Steven R. Archer and Curtis H. Monger and S. Waltman} }