It has been frequently suggested that root exudates play a role in trace metal mobilization and uptake by plants, but there is little in vivo evidence. We studied root exudation of dicotyledonous plants in relation to mobilization and uptake of Cu and Zn in nutrient solutions and in a calcareous soil at varying Cu and Zn supply. Spinach (Spinacia oleracea L.) and tomato (Lycopersicon esculentum L.) were grown on resin-buffered nutrient solutions at varying free ion activities of Cu (pCu 13.0-10.4) and Zn (pZn 10.1-6.6). The Cu and Zn concentrations in the nutrient solution increased with time, except in plant-free controls, indicating that the plant roots released organic ligands that mobilized Cu and Zn from the resin. At same pCu, soluble Cu increased more at low Zn supply, as long as Zn deficiency effects on growth were small. Zinc deficiency was observed in most treatment solutions with pZn ≥ 9.3, but not in nutrient solutions of a smaller volume/plant ratio in which higher Zn concentrations were observed at same pZn. Root exudates of Zn-deficient plants showed higher specific UV absorbance (SUVA, an indicator of aromaticity and metal affinity) than those of non-deficient plants. Measurement of the metal diffusion flux with the DGT technique showed that the Cu and Zn complexes in the nutrient solutions were highly labile. Diffusive transport (through the unstirred layer surrounding the roots) of the free ion only could not explain the observed plant uptake of Cu and of Zn at low Zn 2+ activity. The Cu and Zn uptake by the plants was well explained if it was assumed that the complexes with root exudates contributed 0.4% (Cu) or 20% (Zn) relative to the free ion. In the soil experiment, metal concentrations and organic C concentrations were larger in the solution of planted soils than in unplanted controls. The SUVA of the soil solution after plant growth was higher for unamended soils, on which the plants were Zn-deficient, than for Znamended soils. In conclusion, root exudates of dicotyledonous plants are able to mobilize Cu and Zn, and plants appear to respond to Zn deficiency by exuding root exudates with higher metal affinity.

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Land use changes, especially the conversion of native forest vegetation to cropland and plantations in tropical regions, can potentially alter soil C dynamics. A study was conducted to assess the effects of various land uses and soil managements (agro-forestry plantation, vegetable field, tube-well irrigated rice–wheat, sewage-irrigated rice–wheat, and uncultivated soils) on soil pH, bulk density, soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), C mineralisation (Cmin), microbial quotient, and microbial metabolic quotient (qCO2) in 0−0.05, 0.05−0.10, and 0.10−0.20 m soil depths. At 0−0.05 m, the bulk density was lowest (1.29 Mg/m3) in agro-forestry soil, whereas the uncultivated soil (jointly with vegetable field soil) showed highest bulk density (1.48 Mg/m3). Sewage-irrigated rice–wheat soil showed lowest pH particularly in the 0−0.05 and 0.10−0.20 m soil layer. Irrespective of soil depths, agro-forestry plantation showed greater SOC followed by sewage-irrigated rice–wheat soil. Nevertheless, agro-forestry soil also showed highest stock of SOC (33.7 Mg/ha), POC (3.58 Mg/ha), and MBC (0.81 Mg/ha) in the 0−0.20 m soil layer. Sewage-irrigated rice–wheat jointly with agro-forestry soil showed greatest Cmin in the 0−0.20 m soil layer, although the former supported lower SOC stock. The decrease in SOC (SOC0−0.05 m/SOC0.10−0.20 m) and Cmin (Cmin 0−0.05 m/Cmin 0.10−0.20 m) along soil depth was significantly higher in the agro-forestry system than in most of the other land use and soil management systems. Microbial quotient was highest in sewage-irrigated rice–wheat soil, particularly in the 0−0.05 m soil depth, whereas qCO2 was greater in uncultivated soil. In general, microbial quotients decreased, whereas qCO2 increased down the soil profile.