Nutrient losses in runoff from grassland and shrubland habitats in southern New Mexico: I. Rainfall simulation experiments --SUMMARY of SHRUB nitrogen and phosphorus chemistry

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This data set was collected to provide data for comparison of the losses of dissolved nitrogen and phosphorus in the runoff from grass- and shrub- dominated plots in the Jornada Basin of southern New Mexico. This summary dataset contains the following columns for the shrub plots: Plot: the identification number for the 6 grassland plots. Q: the mean rate of discharge during the entire experiment, obtained by dividing the total runoff by l800 sec. The units are cubic centimeters per second. Total N yield: the total yield of dissolved nitrogen discharged from the plot during the duration of the experiment. The units are grams per square meter per l800 sec. This yield has been adjusted, by subtracting the content of total dissolved N applied in the simulated rainfall. Organic N yield: the yield of dissolved organic N, as above. Inorganic N yield: the yield of NH4-N plus NO3-N discharged from the plot during the duration of the experiment, as above. Total P yield: the yield of total dissolved P discharged from the plot during the duration of the experiment. The units are grams per square meter, per l800 sec. Organic P yield: the yield of dissolved organic P, as above. Inorganic P yield: the yield of PO4-P discharged from the plot during the duration of the experiment. Fines: the percent of the plot surface covered by materials < 2.0 mm diameter Gravel: the percent of the plot surface covered by materials > 2.0 mm diameter Vegetation: the percent of the plot surface covered by vegetation. Litter: the percent of the plot surface covered by plant litter. Slope: it was not possible to measure slope in plots with a shrub in them, since the mound dominated the slope in the plot.

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Field data sheets for field data collection. Instrumental for analysis by autoanalyzer.

Rainfall simulations in the grassland were performed on six plots, each 1 x 2 m in dimension. Plant cover in each of these plots was about 50%, composed of individual clumps of Bouteloua eriopoda, which are typically each about 20 cm in diameter. In the shrubland, 1 x 1 m plots were centered on individual shrubs (n=8) or in bare areas between shrubs (n=10). The 1 x 1 m size was selected for these plots so that the shrub plots would not extend beyond the perimeter of the shrub canopy. A field-portable rainfall simulator, developed and described by Luk et al. (1986), was used to apply water at a nominal rate of 140 mm/hr to these plots. This simulator delivers rainfall with 90% of the kinetic energy of natural rainfall and a comparable drop-size distribution. All rainfall simulations were performed during the dry season, during June 1995 and 1996, on initially dry soils. Each simulation lasted 30 minutes, and the actual rainfall delivered to each plot was measured in an array of 6 wedge-shaped raingages located around the periphery of each plot. The mean rainfall intensity was 146, 118, and 136 mm/hr during the rainfall simulations on grassland, shrub, and intershrub plots, respectively. Runoff rates (discharge) were determined by taking timed volumetric samples of the water discharged from a trough placed along the lowest side of the plot. Once runoff commenced, it increased rapidly, so samples of the outflow were initially collected at 30-s intervals. Later in the experiment, when runoff stabilized, samples were collected at longer intervals ranging from 2 to 4 min. Samples were collected in polypropylene bottles. The duration of the sampling lasted 15 s early in the experiment and ranged from 20 to 30 s later in the experiment, depending on the fill time of the sample bottle. Following each simulation, the surface cover of fines (i.e., materials <2 mm diameter), gravel (material >= 2 mm), plant litter, and vegetation was estimated for each plot using a grid of 200 points. Runoff samples were filtered through pre-rinsed 0.45 micron Millipore HA filters, and analyzed for NH4, NO3, and PO4 using standard methods on a Traacs 800 Autoanalyzer. Inorganic N is taken as the sum of NH4-N + NO3-N. Each sample was then subjected to a persulfate digestion (D'Elia et al. 1977) and reanalyzed. The difference between the digested and undigested concentrations is assumed to represent dissolved organic forms of N and P. Contents of N and P in the applied water were subtracted from the contents in runoff to estimate the net losses in runoff.

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The shrublands selected for study were located in two areas dominated by creosotebush (Larrea tridentata). One area was approximately 600 m downslope from the grassland site. (The grassland studied was located on the alluvial piedmont of Mount Summerford near the powerline road running along the base of Mt. Summerford. The location of the simulations is south of the LTER-I Alternate Control Transect and immediately north of the LTER-I Hydrology Natural Runoff Plots but downslope of the powerline road.) The other shrubland site was located 1200 m to the southeast.


June 1995 and 1996

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