@article {JRN54666, title = {Abiotic processes are insufficient for fertile island development: A 10-year artificial shrub experiment in a desert grassland}, journal = {Geophysical Research Letters}, volume = {44}, year = {2017}, month = {03/2017}, pages = {2245-2253}, chapter = {2245}, abstract = {

The relative importance of biotic and abiotic processes in the development of {\textquotedblleft}fertile islands{\textquotedblright} in dryland systems has rarely been investigated. Here we approached this question by using artificial shrubs, which exclude plant litter production and soil nutrient uptake, but retain the functions of trapping windblown material, funneling of stemflow, and differential rain splash. We conducted a vegetation manipulation study more than a decade ago in the desert grassland of southern New Mexico and subsequently revisited the site in 2012 and 2015. The results show that no notable soil mounds were observed under the artificial shrubs; however, soil texture under the artificial shrubs has gradually changed to resemble the patterns of soil particle-size distribution under natural shrubs. Our results highlight that with the exclusion of direct biotic additions, soils captured by shrub canopies are not necessarily fertile and thus do not themselves contribute to the development of fertile islands.

}, keywords = {abiotic processes, aeolian process, artificial shrubs, biotic process, desert grassland, fertile islands}, doi = {10.1002/2016GL072068}, url = {/files/bibliography/17-005.pdf}, author = {Junran Li and William P. Gilhooly III and Gregory S. Okin and John Blackwell III} } @article {JRN54140, title = {Consistency of wind erosion assessments across land use and land cover types: a critical analysis}, journal = {Aeolian Research}, volume = {15}, year = {2014}, month = {11/2014}, pages = {153-260}, chapter = {253}, abstract = {

In recent decades, large areas of rangeland have been converted to cropland or vice versa in the western United States and elsewhere in the world, driven largely by increased crop prices, loss of access to irrigation water, and agricultural expansion / contraction. Wind erosion and dust emissions are key processes that have not been well studied during land use and associated land cover changes. This assessment is challenging because currently no model is available that can provide field- to landscape-scale estimates of wind erosion on both rangeland and cropland, and account for soil, vegetation and management changes. In this paper, we compare aeolian sediment transport estimates from available cropland models and a number of mass flux equations developed for rangelands, for a bare soil surface with different levels of crust and surface roughness under different wind speeds. Our results show that the simulated horizontal sediment mass fluxes are similar for cropland and rangeland models at large surface crust coverage and aerodynamic roughness. In situations of small to moderate crust cover and soil roughness, horizontal mass fluxes varied by over three orders of magnitude among the tested models. A correlation analysis shows that horizontal mass fluxes simulated by cropland and rangeland models are correlated, with correlation R2 of 0.37-0.99 across different models. Finally, we propose an approach to estimate changes in aeolian transport with changes in land use. Although this approach may be limited to situations of unvegetated surfaces, it provides a preliminary method for land managers and policymakers to estimate potential wind erosion changes in response to land use change.

}, keywords = {horizontal mass flux, model comparison, RWEQ, SWEEP, WEPS, wind erosion modeling}, doi = {10.1016/j.aeolia.2014.04.007}, url = {/files/bibliography/14-038.pdf}, author = {Junran Li and Gregory S. Okin and John Tatarko and Nicholas Webb and Jeffrey E. Herrick} } @article {49854, title = {Evaluation of a new model of aeolian transport in the presence of vegetation}, journal = {Journal of Geophysical Research: Earth Surface}, volume = {118}, year = {2013}, month = {04/2013}, pages = {288-306}, chapter = {288}, abstract = {

Aeolian transport is an important characteristic of many arid and semiarid regions worldwide that affects dust emission and ecosystem processes. The purpose of this paper is to evaluate a recent model of aeolian transport in the presence of vegetation [Okin, 2008]. This approach differs from previous models by accounting for how vegetation affects the distribution of shear velocity on the surface rather than merely calculating the average effect of vegetation on surface shear velocity or simply using empirical relationships. Vegetation, soil, and meteorological data at 65 field sites with measurements of horizontal aeolian flux were collected from the Western United States. Measured fluxes were tested against modeled values to evaluate model performance, to obtain a set of optimum model parameters, and to estimate the uncertainty in these parameters. The same field data were used to model horizontal aeolian flux using three other schemes. Our results show that the Okin [2008] model can predict horizontal aeolian flux with an approximate relative error of 2.1 and that further empirical corrections can reduce the approximate relative error to 1.0. The level of error is within what would be expected given uncertainties in threshold shear velocity and windspeed at our sites. The model outperforms the alternative schemes both in terms of approximate relative error as well as the number of sites at which threshold shear velocity was exceeded. These results lend support to an understanding of the physics of aeolian transport in which 1) vegetation{\textquoteright}s impact on transport is dependent upon the distribution of vegetation rather than merely its average lateral cover, and 2) vegetation impacts surface shear stress locally by depressing it in the immediate lee of plants rather than by changing the bulk surface{\textquoteright}s threshold shear velocity. Our results also suggest that threshold shear velocity is exceeded more than might be estimated by single measurements of threshold shear stress and roughness length commonly associated with vegetated surfaces, highlighting the variation of threshold shear velocity with space and time in real landscapes.

}, doi = {10.1002/jgrf.20040 }, url = {/files/bibliography/13-013.pdf}, author = {Junran Li and Gregory S. Okin and Jeffrey E. Herrick and Jayne Belnap and M. Miller and Vest, Kimberley and Draut, Amy E.} } @article {50046, title = {A simple method to estimate threshold friction velocity in the field}, journal = {Geophysical Research Letters}, volume = {37}, number = {L10402}, year = {2010}, pages = {1-5}, abstract = {This study provides a fast and easy-to-apply method to estimate threshold friction velocity (TFV) of wind erosion in the field. Wind tunnel experiments and a variety of ground measurements including air gun, pocket penetrometer, torvane, and roughness chain were conducted in Moab, Utah and cross-validated in the Mojave Desert, California. Patterns between TFV and ground measurements were examined to identify the optimum method for estimating TFV. The results show that TFVs were best predicted using the air gun and penetrometer measurements in the Moab sites. This empirical method, however, systematically underestimated TFVs in the Mojave Desert sites. Further analysis showed that TFVs in the Mojave sites can be satisfactorily estimated with a correction for rock cover, which is presumably the main cause of the underestimation of TFVs. The proposed method may be also applied to estimate TFVs in environments where other non-erodible elements such as postharvest residuals are found.}, url = {/files/bibliography/10-09.pdf}, author = {Junran Li and Gregory S. Okin and Jeffrey E. Herrick and Seth M. Munson and M. Miller and Jayne Belnap} } @article {50116, title = {Effects of enhanced wind erosion on surface soil texture and characteristics of windblown sediments}, journal = {Journal of Geophysical Research - Biogeosciences}, volume = {114, G02003}, year = {2009}, chapter = {8 pp}, abstract = {

It is well documented that wind redistributes and transports soil resources in semiarid ecosystems. However, fewer studies have quantitatively linked wind erosion to detailed grain-size fractions and associated nutrient content in surface soil and windblown sediment. In this study, we examined (1) the effects of enhanced wind erosion on surface soil particle-size distribution and (2) carbon (C) and nitrogen (N) characteristics of windblown sediments in a typical desert grassland of southern New Mexico. Our results show that surface soil has become noticeably coarser over a 2-year period. In particular, content of soil particles in the category of 250{\textendash}500 ?m increased significantly, but soil particles in the fractions of 50{\textendash}125 {\textmu}m and \<50 {\textmu}m were significantly depleted. In addition to the enrichment of C and N in the windblown sediments, our results reveal that fine particles (e.g., D \< 50 {\textmu}m) were enriched to a much higher degree at height than C and N. Significantly, our results reveal that nearly 12\% of total organic carbon (TOC) and 9\% of total nitrogen (TN) were found in the particles with diameter \<50 {\textmu}m, which account for only 1{\textendash}2\% of the mass of windblown sediments. In this wind susceptible environment, our findings suggest that (1) significant soil texture change (e.g., the loss of soil fines) driven by wind erosion could happen rapidly and soil fine particles (e.g., silt and clay) may be depleted within a few years and (2) the loss of even a small fraction of fine particles may indicate a substantial depletion of soil C and N.

}, keywords = {aeolian processes, surface soil texture, wind erosion, windblown sediment}, doi = {10.1029/2008JG000903}, url = {files/bibliography/JRN00526.pdf}, author = {Junran Li and Gregory S. Okin and Howard Epstein} } @article {50117, title = {Sediment deposition and soil nutrient heterogeneity in two desert grassland ecosystems, southern New Mexico}, journal = {Plant and Soil}, volume = {319}, year = {2009}, pages = {67-84}, abstract = {

The role of wind in changing the spatial heterogeneity of soil resources in erosion-dominated semiarid ecosystems is well known. Yet the effect of windblown sediment deposition on soil nutrient distribution and ecosystem dynamics at local and landscape scales has received little attention. We examined the effects of enhanced sediment deposition on the spatial distribution of soil nutrients at the Jornada Experimental Range, southern New Mexico. Enhanced sediment deposition was obtained as a result of grass cover reduction in the upwind portion of the experiment in two sites co-dominated by mesquite and one of two grass species with different morphologies. The spatial characteristics of soil available nitrogen (including ammonium, nitrite, and nitrate), phosphate, potassium, and calcium were quantified using a variety of traditional and geostatistical analyses. Our results showed that enhanced deposition led to considerable reduction in both mean soil nutrient concentrations and coefficients of variation over a two-year period (2004{\textendash}2006). Given the observed increase in the scale of spatial dependence for available nitrogen, but not for potassium, phosphate, and calcium following enhanced sediment deposition, we suggest that soil available nitrogen may be particularly responsive to increased aeolian activities due to livestock grazing and other anthropogenic activities that remove vegetation. Our study further suggests that soil particles deposited in the downwind area may be {\textquotedblleft}nutrient-imbalanced.{\textquotedblright} Specifically, the lower-than-normal available nitrogen concentrations in the wind-deposited soils may inhibit the growth of grasses and the germination of seeds. For wind-erodible ecosystems found in southern New Mexico, structures of Bouteloua-dominated communities may be particularly susceptible to change under enhanced soil erosion conditions.

}, keywords = {aeolian processes, available nitrogen, ecosystem dynamics, geostatistics, sediment deposition, spatial heterogeneity}, doi = {10.1007/s11104-008-9850-7}, url = {files/bibliography/JRN00528.pdf}, author = {Junran Li and Gregory S. Okin and Lorelei Alvarez and Howard Epstein} } @article {50236, title = {Effects of wind erosion on the spatial heterogeneity of soil nutrients in two desert grassland communities}, journal = {Biogeochemistry}, volume = {88}, number = {1}, year = {2008}, note = {//USDA//LTER-V//entered 06/06/2008; PDF on file}, month = {2008}, pages = {73}, abstract = {

Wind is known to affect the spatial heterogeneity of soil resources in arid and semiarid systems, but multi-year, quantified observations are largely absent. We studied the effects of wind erosion on the spatial distribution of soil organic carbon (SOC) and other soil nutrients at the Jornada Experimental Range, in southern New Mexico. Enhanced wind erosion was encouraged by grass cover reduction in a Sporobolus-mesquite dominated site (SM) and a Bouteloua-mesquite dominated site (BM). The scale and magnitude of spatial dependence for the soil analytes were quantified using geostatistical analyses. Results of this study show that soil organic matter related analytes such as SOC, TN, Navail, and SO4 2 are among the first to be eroded and redistributed; cations such as Ca2+ and Mg2+ may not be removed and redistributed significantly; and other ions such as K+, Na+ and Cl- showed no discernible pattern of change. Geostatistics show that wind appeared to increase the scale of spatial autocorrelation, but decrease the scale of spatial dependence of most soil analytes over 2{\textendash}3 windy seasons. In the wind enhanced plot of the SM site, up to 99\% of the spatial dependence of SOC was autocorrelated at the distance of 1.45 m before the initiation of wind erosion, but the spatial dependence dropped significantly to only 60\% at a larger autocorrelation distance of 2.76 m after three windy seasons. Similar but less significant changes were observed for SOC in the BM site. Despite the differential effects of wind on the soil analytes, we conclude that the overall results of wind on the grass cover reduction plots are the disappearance of small, strong fertile islands, which may be related to grasses; and the reinforcement of large fertile islands, which are likely related to mesquite shrubs. In addition, the change of the spatial patterns of SOC and other soil nutrients induced by enhanced wind erosion may persist and reinforce soil islands associated with shrubs, thus allowing a positive feedback for further desertification in this arid grassland.

}, keywords = {article, geostatistics, journal, soil carbon, soil nitrogen, soil nutrients, spatial heterogeneity, wind erosion, soil carbon, wind erosion, soil nitrogen, wind erosion, soil nutrients}, doi = {10.1007/s10533-008-9195-6}, url = {files/bibliography/JRN00499.pdf}, author = {Junran Li and Gregory S. Okin and Lorelei Alvarez and Howard Epstein} } @article {50356, title = {Quantitative effects of vegetation cover on wind erosion and soil nutrient loss in a desert grassland of southern New Mexico, USA}, journal = {Biogeochemistry}, volume = {85}, year = {2007}, note = {//USDA//LTER-V//entered 08/14/2007}, month = {2007}, pages = {317-332}, abstract = {

Wind is a key abiotic factor that influences the dynamics of arid and semiarid systems. We investigated two basic relationships on vegetation manipulation (grass cover reduction) plots at the Jornada Experimental Range in southern New Mexico: (1) wind erosion rates (horizontal mass flux and dust emission) versus vegetative cover, and (2) nutrient loss versus vegetative cover. The results indicate that wind erosion rates and nutrient loss by dust emission are strongly affected by plant cover; however, the importance of shrubs and grasses in reducing dust flux may not be equal. The dramatic increase of wind erosion between 75\% grass cover reduction and 100\% grass cover reduction suggests that sparsely distributed mesquites are relatively ineffective at reducing wind erosion and nutrient loss compared to grasses. Comparisons of nutrients between surface soils and wind blown dust indicate that aeolian transport is a major cause for the loss of soil nutrients in susceptible environments. We found that increased aeolian flux over three windy seasons (March 2004{\textendash}July 2006) removed up to 25\% of total organic carbon (TOC) and total nitrogen (TN) from the top 5 cm of soil, and about 60\% of TOC and TN loss occurred in the first windy season (March{\textendash}July 2004). The balance between net loss of nutrients by aeolian processes and the addition of nutrients by biotic processes changed from negative (net loss) to positive (net accumulation) between 50\% grass cover reduction and 25\% grass cover reduction. The estimated lifetime of surface soil TOC and TN of about 10 years on the plot with 100\% grass cover reduction indicates that impacts of wind erosion on soil resources can occur on very short timescales.

}, keywords = {aeolian processes, aeolian processes, desertification, article, deserts, journal, soil carbon, soil nitrogen, soil nutrients, carbon, soil nutrients, nitrogen, vegetation change, nutrient loss, vegetation change, wind erosion, vegetation cover, wind erosion, windblown sediment}, doi = {10.1007/s10533-007-9142-y}, url = {files/bibliography/JRN00480.pdf}, author = {Junran Li and Gregory S. Okin and Lorelei Alvarez and Howard Epstein} }