It has been frequently suggested that root exudates play a role in Zn mobilization. We studied root exudation of low-molecular weight organic anions (LMWOAs) in three aerobic rice genotypes in relation to their Zn uptake efficiency. Plants were grown in a rhizotron filled with a Zn deficient clay soil added with or without 5 mg Zn kg-1 soil as ZnSO4. Soil solution was sampled when symptoms of Zn deficiency for –Zn plants appeared. We measured LWMOAs in the soil solution and found that plants responded to Zn deficiency with increased (64%) root exudation of malate. The efficient genotype exuded higher amounts of malate than the inefficient genotype. The genotype with medium Zn uptake efficiency, however, exuded less malate than the inefficient genotype. No significant correlation between malate concentrations in rhizosphere and plant Zn uptake efficiency was found. These results suggest that Zn mobilization by soil-grown rice genotypes cannot be explained by increased malate exudation and the variation in plant Zn uptake is most likely not be explained by their capacity to exude LMWOAs as a response to Zn deficiency.

Strong relationships between Extractable Organic N (EON) and N mineralization suggest that they can be used to assess the N mineralization potential of soils. EON is often used to proxy Dissolved Organic Nitrogen (DON), which is assumed to play an intermediate role in N mineralization. We tested whether EON relates to N mineralization and how this relation is affected by the extraction methodology used. We synthesized results from 171 papers on correlation coefficients between EON and N mineralization using meta-analysis. EON was positively related to N mineralization, and its size explained 14-65% of the variation in N mineralization. Best results were obtained with hot CaCl2 and acid KMnO4 as extractants. EON extracted with alkali solutions explained less of the variation in N mineralization than the other soil tests. There was no direct relationship between the intensity of extraction and the performance of EON as predictor for N mineralization. When EON levels are used to make fertilizer-N recommendations, the extraction methodology used to develop the above described relationship is closely followed.

It is often suggested but so far not substantiated that soils with a higher fungal/bacterial (F/B) ratio retain nitrogen better. If this were true, it would be wise for farmers who want to reduce the environmental impact to increase this F/B ratio. We did field studies on experimental plots and on commercial farms and found that lower fertilizer rates led to higher fungal biomass in grassland soils. In incubation studies with intact soil columns we showed that soil with a higher fungal biomass had lower leaching and denitrification rates because of lower mineralization and nitrification rates while maintaining production levels.