Geomorphic controls over Nitrogen and Carbon Stocks in Desert Playas

Background/Question/Methods

Playas are ephemeral desert wetlands situated at the bottom of closed catchments. Desert playas in the Southwestern US have not been intensively studied despite their potential importance for the functioning of desert ecosystems. The objective of this study is to assess geomorphic and ecological variables that control carbon and nitrogen stocks in playas. We hypothesize that the magnitude of carbon and nitrogen stocks depend on: (a) catchment size, (b) catchment slope, (c) catchment biomass, (d) catchment plant cover and (e) spatial arrangement (e.g. patch size distribution), and (f) catchment soil texture. We chose thirty playas from across the Jornada Basin (Las Cruces, NM) ranging from 0.005-0.60 km² in area and with varying catchment characteristics. We used a 5m digital elevation map (DEM) to calculate the catchment size and catchment slope for these thirty playas. We measured biomass, percent cover, and patch size using the point-intercept method with three 10m transects in each catchment. We used the Bouyoucos-hydrometer soil particle analysis to determine catchment soil texture. Stocks of organic carbon and nitrogen were measured from soil samples at four depths (0-10 cm, 10-30 cm, 30-60 cm, 60-100 cm) using C/N combustion analysis. 

Results/Conclusions

Using a 5m DEM we calculated the catchment sizes of the thirty playas, these ranged from 0.10-26.99 km². We used the same data to calculate the slope of each catchment, these range from 0.015-0.21 m elevation/ 5m distance. In terms of nitrogen and carbon storage, we found soil nitrogen values ranging from 0.007-0.114 %, and soil carbon values ranging from 0.002-1.36 %. The results of a regression analysis show both nitrogen and carbon decrease linearly as a function of soil depth (nitrogen: y= -0.015x+0.071, R²=0.30; carbon: y= -0.19x+0.89, R²=0.24). The results of an additional regression analysis show a positive relationship between catchment size and nutrient stock size (% of soil sample) for both nitrogen and carbon (nitrogen: y= 7.0*10^-5x +0.027, R²=0.14; carbon: y= 0.0014x + 0.29, R²= 0.25). These data support our hypothesis that catchment size is one of factors controlling carbon and nitrogen stock in desert playas.