Rainer Schulin

Modified micro suction cup/rhizobox approach for the in-situ detection of organic acids in rhizosphere soil solution

Root–soil interactions can strongly influence the soil solution chemistry in the rhizosphere. In the present study we propose a modification of the classical rhizobox/micro suction cup system to make it suitable for the collection and analysis of organic acids in the rhizosphere. In order to show the potential of the method, we tested the modified system with Lupinus albus L. as a model plant known to exude large amounts of citrate. The suction cups were installed through the transparent front plate of the rhizoboxes just after the emergence of cluster roots in order to allow optimal localized collection of soil solution. A small dead-volume allowed almost immediate stabilisation with formaldehyde of the sampled soil solutions in the collection container to prevent microbial degradation. The concentrations of organic acids were significantly larger in the rhizosphere soil solution of active cluster roots of Lupinus albus L. than in the bulk soil solution (about 400xa0µM of citrate versus <0.05xa0µM). We were able to follow the exudation process in-situ, which occurred during 2–3xa0days. Also the concentrations of other organic acids and inorganic anions differed between the bulk soil and the rhizosphere of cluster roots, normal roots, and nodules.

Extractability and Plant Uptake of Cu, Zn, Pb and Cd from a Sludge-amended Haplargid in Central Iran

ABSTRACT Owing to the lack of regulations limiting pollutant concentrations in sludge and rates of sludge application on agricultural lands, high loads of heavy metals are introduced into agricultural soils by the use of sewage sludge as fertilizer in Iran. In order to examine the risk of metal transfer into vegetables arising from this practice under the farming conditions of central Iran, we performed a four-year field experiment in which we investigated the uptake of heavy metals from a sludge-amended soil (fine loamy, mixed thermic Typic Haplarigid) by lettuce (Lactuca sativa L.) and spinach (Spinacia oleracea L.). Sludge application rates were 0, 22.5, and 45 Mg ha−1. To determine the effect of repetitive sludge applications, applications were repeated on two-thirds of each plot in the second year and on one-half of these subplots again in the third year. Thus, total sludge amounts applied were 22.5 and 45 Mg ha−1 in plots that received one application, 45 and 90 Mg ha−1 in plots with two applications, and 67.5 and 135 Mg ha−1 in plots with three applications. Single applications increased the EDTA-extractable concentrations of copper (Cu), zinc (Zn), and lead (Pb). Cadmium (Cd) concentrations increased only after two or three consecutive applications. In subsequent years with no further sludge applications, EDTA-extractable metal concentrations in soil showed a decreasing trend but, even after four years EDTA extractable metal concentrations were still significantly higher in plots that had received more than 45 Mg ha−1 sludge than in control plots. Metal uptake by the plants also increased with sludge application and decreased again after applications stopped. Close correlation with the EDTA-extractable soil metal concentrations suggests that EDTA is an effective indicator of plant-available metal concentrations in the investigated soil. Lettuce and spinach yields both increased significantly with all sludge application rates and this fertilizer effect was still evident three years after a single sludge application, indicating that sewage sludge can improve fertility of arid soils in the region.