Using the DGT to investigate the soil chemistry of P and Cd in sandy loam soil after the application of phosphorus fertilizers

Abstract

A recently developed DGT with a mixed binding layer of Chelex and Metsorb (TiO2) was successfully deployed in fresh and seawater to simultaneously measure available cations and anions. But, to date, no studies have used similar DGTs in soils. This study used the Metsorb Gel in two types of DGTs the Metsorb Chelex Mixed Binding layer (MBL-DGT) and the Metsorb Chelex Double Binding layer (DBL-DGT) or sandwich DGT to determine the concentration of available P and Cd (CDGT) in soil. Three experiments were conducted to assess the accuracy, sensitivity, and capacity of the new DGTs to measure the availability of P and Cd (CDGT) in a sandy loam soil. The study also investigated the relationship between the CDGT of P and Cd in the soil and concentrations of available P and Cd in plants and leachate water. Three scenarios were prepared for the assessment of the DGTs. First, the MBL-DGT was deployed in a large-scale flume filled with sandy loam soil, which was fertilized with Single Super Phosphate (SSP) of three different doses (0.01, 0.1, and 0.5 mg P L-1). Second, a biological experiment was conducted in a greenhouse using the tomato plant (Lycopersicon esculentus) as an indicator to assess the reliability of the CDGT of the available P and Cd measured in soil solution and that absorbed by the plants. This experiment used the same type of sandy loam soil, with three types of P fertilizers: SSP, Egyptian Reactive Phosphate (ER) and phosphorus soft rock from Nutri-tech Solution (NTS) applied at four dosages (0, 100, 200 and 250 kg P ha-1). Third, three consecutive doses of the three fertilizers were added to the sandy loam soil to investigate the capacity of the MBL-DGT. The repeated fertilizer application simulated the repeating application of P fertilizer in the field cycle. The mobility of available P and Cd to the leachate water was monitored by deploying the DGTs at different depths in the soil. This technique was used to evaluate the sensitivity of the DGTs when deployed in the soil for 4, 7 and 14 days. In addition, the new DGT measurements were compared with conventional measurements of P and Cd, using the Olsen and NH4Cl extraction techniques, respectively. Results of all three experiments confirmed that the MBL-DGT and the DBL-DGT were versatile measures of P and Cd availability and mobility in soil. The Metsorb mixed binding layer uptake of iii P and Cd rose with increasing P fertilizer dose. The new DGTs were sensitive to fertilizer type due to the differences in the solubility properties of the three fertilizers. The new DGTs showed that the excessive use of P fertilizers in the sandy loam soil increased the mobility of available P and Cd to the leachate water. PDGT and CdDGT had stronger correlations with P and Cd content in the plants than the conventional P and Cd extraction methods. These findings indicated that the DBL-DGT mimicked the process of plant P uptake. The MBL-DGT was also able to measure the most plant accessible form of Cd from the soil solution. MBL-DGT measurements of P and Cd were also better correlated with total P and Cd than those of conventional extraction methods. The capacity of the MBL-DGT for Cd uptake did not exceed the effective capacity of the MBL-DGT previously recorded by Panther et al. (2010). On the other hand, the MBL-DGT capacity for available P uptake was 42,000 ng per disc which was slightly higher than that recorded by Panther et al. (2014). Quantitative assessment of available P in the soil which has recently received high doses of P fertilizer will require MBL-DGTs with higher capacity. Further research is needed on different soil types and different crops to obtain a more comprehensive understanding of when the MBL-DGT can be practically applied in the field.