Solution chemistry influence on metal mobility in biosolids-amended soils.

Many studies have implicated dissolved organic carbon (DOC) as an important contributor to the elevated mobility of trace metals in soils amended with biosolids. Few of these studies, however, have quantified both DOC and metal concentrations. We completed laboratory leaching column studies on a dryland Platner loam (fine, smectitic, mesic Aridic Paleustoll) and an irrigated Osgood sand (loamy, mixed, mesic Arenic Ustollic Haplargid), both with a history of biosolids application. The soils were neutral to slightly alkaline in pH prior to amendment. We performed an additional application of biosolids to one set of columns in the laboratory at a rate of 28 Mg ha(-1) to investigate the effect of time following application on metal mobility. The effect of electrolyte concentration was studied by using both distilled water and simulated irrigation water. Biosolids application increased both DOC and Cu in the column effluents resulting in a positive correlation between Cu and DOC across application treatments for both soils. Both Cu and Pb were mobilized under conditions of low electrical conductivity (EC). This may be the result of the release of a strong metal-binding component of DOC under these conditions. Conversely, Zn mobility was positively correlated with EC, suggesting that either cation exchange or the formation of inorganic complexes influences Zn mobility. Anodic stripping voltammetry measurements indicated that only a small percentage of the total dissolved metals existed as free ions or inorganic complexes; the remainder appears to be complexed to DOC.

Nitrate Leaching in Dryland Agroecosystems as Influenced by Soil and Climate Gradients.

Farmers in dryland agriculture areas of the Central Great Plains have characteristically practiced alternate crop-fallow to stabilize yields. Large amounts of N released soon after sodbreaking and more recent fertilizer additions may have contributed to N movement below crop root zones. Improved water conservation techniques during fallow periods increases the possibility of NO3 leaching below root zones in modern-day crop-fallow systems. Soil topography and water supply may affect leaching potential. This study was conducted to test hypotheses regarding landscape position effects and potential evapotranspiration on depth of water and NO3 penetration. Three sites in eastern Colorado with equal annual precipitation, but with mean potential evapotranspiration varying from 1000 to 1900 mm yr-1 were evaluated. A soil catena was sampled at each site to test landscape effects. All sites were previously managed under tilled wheat-fallow systems for at least 50 yr. All summit (upland) sites had higher water and NO3 contents below their crop root zones than nearby native prairie sites. Soil water and NO3 content of soil profiles to a depth of 9 m were not related to slope position, but there was an inverse relationship between water and NO3 content of soil profiles and potential evapotranspiration. The total NO3 -N in cultivated profiles below the normal root zone of crops varied from 126 kg ha-1 at the northern site to 47 kg ha-1 at the southern site. Although some NO3 leaching had occurred, it did not appear to be a major problem for any particular landscape position or climatic zone on cultivated soils.