Microscale imaging of phosphate mobility under unsaturated flow as affected by a fertilizer enhancing polymer

Abstract

Phosphorus (P) fixation in soil following fertilizer application is a major issue that may be mitigated through a number of methods. Our main objective was to utilize an innovative unsaturated flow system to directly image the mobility of P in soil material as affected by AVAIL, a commercial copolymer phosphate fertilizer enhancer. Air-dried clayey soil material, passed through a 0.25-mm sieve and adjusted to pH 6, was packed in three side-by-side 1.4-cm wide, 20-cm long, and 0.6-cm deep tracks. Water was applied to the tracks under approximately 1 cm tension and moved downstream by sorptivity. Five microliters of aqueous solutions containing 400 mmol P/L and varying levels of AVAIL were applied at a point near the upstream end of each track. Once water passed approximately 6 cm from the point of P application, each track was sectioned into small blocks and air-dried. Synchrotron micro X-ray fluorescence (μ-XRF) images of P, silicon (Si), and aluminum (Al) were collected on block samples to measure relative movement of phosphorus along the soil tracks. Our results show that, despite the high affinity of soil minerals for P, it was relatively mobile with no enhancer under unsaturated flow conditions. Images showed diminished rate of P movement and enhanced clay dispersion with increasing proportion of AVAIL. We postulate that clay dispersion, corroborated by X-ray computed tomography imaging, affected P transport due to interactions with Si and Al minerals. Our results demonstrate the potential utility of our unsaturated flow system combined with μ-XRF imaging to measure convective mobility of plant nutrients or contaminants through soils.