ELEVATED NITRATE LEVELS IN SOILS OF THE EASTERN MOJAVE DESERT

Deep cores taken on an uncultivated desert alluvial fan revealed two large volumes of coarse soil with high NO3 levels (20-208 mg L-1 NO3-N in water saturation extracts). Nitrate levels were unpredictable both laterally and vertically and unrelated to alluvial strata. The large soil volumes with elevated NO3 levels were found 2.7 to 7.3 m deep in four cores at one 3 by 3 m site, and 8.8 to 21 m deep in two cores at another site. Isolated samples with elevated NO3 were found in three other cores and six cores contained no elevated NO3. Overall, nitrate was best correlated with EC, Na, SO4, and Cl (r = 0.68, 0.66, 0.67, and 0.73, respectively; P ≤ 0.05). Stronger correlations were found within the four individual sites and 15 cores, but the ions best correlated with NO3 varied, even between cores from the same site. Saturation extract NO3 was not correlated with bulk soil total organic C or total N. The NO3 distribution patterns found appear related to inorganic processes (salinization and leaching), but specific processes of N redistribution and primary N sources remain unclear. Elevated NO3 in deep soils appears to be a natural phenomena that reflects Pleistocene site conditions and possible geologic NO3 sources. Local volcanic rocks contain up to 73 mg NO3-N kg-1 rock (water-extractable). Naturally elevated NO3 levels in rocks and coarse alluvial fan soils are poorly understood and easily overlooked potential sources of groundwater NO3.