Carbon and Irrigation Dynamics on Metal Mobilization in Semi-Arid Urban Agricultural Soils.

Altered irrigation patterns on semi-arid land and increased carbon inputs, such as the addition of compost or manure, can lead to the mobilization of geogenic trace metals within a soil profile. The mobilization of geogenic trace metals due to increased carbon loading and water input has not fully been investigated and is likely to play a significant role in contaminant release. Furthermore, fate and transport of metals during redox fluctuations caused by flood irrigation in semi-arid climate zones is still poorly characterized. To understand the influence of irrigation type and carbon amendments on trace metal mobilization, we examined two orange grove soils within a semi-arid metropolitan of Southern California. One grove applied low carbon additions with furrow irrigation while the other had higher carbon additions with micro-sprinkler irrigation. Soil cores (1 m depth) were taken at both sites and subsampled at 10 cm resolution. Soils were analyzed for total carbon including organic and inorganic carbon fractions, total nitrogen, bulk metal and trace metal concentrations (including As, Mn, Cr, Pb, Cd, Zn, and Cu), particle size distribution, permanganate oxidizable carbon (POXC), and sequential extractions. To simulate the mobility of the trace elements under redox fluctuating conditions, grove soils were incubated in irrigation water and alternatingly purged with N₂ and air. Incubations were amended with organic carbon of varying molecular complexity to examine their impact on metal release. Aqueous concentrations and speciation of trace metals released, redox potential, and pH were then monitored overtime.  Results from this investigation can provide insight on the role that carbon and irrigation type play on metal biogeochemistry in semi-arid agricultural soils.