Plant Litterfall Dynamics and Soil N Pools in the Negev and Chihuahuan Desert
Investigators (most current known information)
Plant community dynamics in aridlands have a direct effect on spatial and temporal distributions of soil C and N. Trees and shrubs, such as mesquite (Prosopis sp.) in desert grasslands of the southwestern U.S. and Sarcopoterium sp. in the semiarid rangelands of the Northern Negev desert of Israel, have been expanding their ranges in recent decades. This is largely through human activities such as livestock grazing, which affects vegetation cover. These activities have indirectly caused increased heterogeneity in soil N and microbial activity. The mechanisms remain poorly understood, and have a direct impact on restoration and preservation of semiarid and aridland plant species composition.
In this study we investigated the temporal and spatial distribution of soil C and N pools and soil enzyme activity, associated with Prosopis velutina and Sarcopoterium spinosum, in locations with a well-documented land-use history and long-term ecological research. The specific objectives of this study were to evaluate: 1) spatial effects of Prosopis velutina and Sarcopoterium spinosum litterfall on soil N concentrations, including soil organic C and N, plant-available N (NH4+ and NO3-), and labile organic N forms (amino acids and amino sugars); 2) seasonal changes in litterfall quantity and nutrient concentrations and how these changes correspond to seasonal changes in soil nutrient concentrations, and 3) soil enzyme activity with respect to the locations of Prosopis velutina and Sarcopoterium spinosum plants.
Two sites were selected, one in the Chihuahuan desert, on the U.S. Army base at Ft. Huachuca, Arizona, USA. The other site was located in the Negev desert, at the Lehavim LTER research station in Israel. Soil and litterfall sampling were conducted during an 18-month period. Spatial distribution of plant litterfall was determined during a 12-month period. Soil C and N pools, C and N isotopes, and enzyme activity, were analyzed in soils collected quarterly.
Litterfall amounts at Ft. Huachuca followed a distinct pattern, with litterfall during June through November. The spatial pattern of litterfall deposition reflected the prevailing SW winds, with most of the litterfall occurring to the N, NE, and E of the trunk, and litterfall deposition was more than five-fold higher at the mid-point of the canopy than at the canopy edge. At Lehavim, a significant increase of litterfall was found in December, March and April. The amount of litterfall was higher in the grazed plot in comparison to the ungrazed plot in most of the sampling dates. Wind direction had little effect on litterfall accumulation.
The asymmetry of litterfall inputs was not generally reflected in soil C and N pools at Ft. Huachuca, where soil organic C, carbohydrates, organic N, inorganic N, amino acids , and enzyme activity were generally not significantly affected by distance from the shrubs. This fact suggests either redistribution of litter or litter-derived nutrients after deposition, or significant amounts of C and N loss from litter or soils. However, at one-half the canopy radius, the average *13C was -19.44 %o, compared to -17.97 at 1.5 times the canopy radius. These differences were significant and represent a decreasing influence of mesquite and an increasing influence of C4 grasses on soil C with increasing distance from the shrub.
At Lehavim, significant differences were found for organic C and N and inorganic N between soils within macrophytic patches (under shrubs) and those from microphytic patches (between shrubs). Soil organic C and N and inorganic N were 60% higher within the macrophytic patches than within the microphytic patches. Furthermore, soil microbial activity, as indicated by enzyme analysis, was two to three times higher in soils from the shrub patches. These shrub patches are important sites for C and N sequestration, and sites of high microbial activity, compared to the microphytic patches.
The “fertility island” effect was more dramatic at Lehavim than at Ft. Huachuca. Despite the clear evidence that the shrubs influence soil fertility, we found little evidence of temporal differences in soil nutrients, suggesting that litter deposition and decomposition are significantly separated in time. Finally, we found, at Lehavim, a two- to three-fold increase in enzyme activity in soil within shrub patches compared to soils outside the shrub patches. We did not find such differences at Ft. Huachuca. Overall, our data suggest that “fertility islands” were centered under the shrubs, and featured sequestered nutrients and higher microbial activity on both sites, but the effect was greatest at the Negev desert site.
Articles in Journals
Wilson, T.B. and T.L. Thompson. 2005. Soil nutrient distributions of mesquite-dominated desert grasslands: Changes in time and space. Geoderma (in press).
Thompson, T.L., E. Zaady, T.B. Wilson, P. Huancheng and D.A. Martens. 2005. "Soil C and N in patchy shrublands of the Negev and Chihuahuan deserts: A comparative study." Soil Biology and Biochemistry (in preparation).