Molybdenum (Mo) is an essential plant nutrient, present in only small concentrations in soils. Also, Mo is an essential micronutrient for plants. The most important role of Mo in higher plants is the one of cofactor for nitrogenase and nitrate reductase, both crucial enzymes in the N-metabolism. Mo is required by plants in a smaller amount than any other essential nutrient, yet the importance of Mo for plants may not be underestimated. Mo deficiency in plants (<1 mg kg-1 d.m.) is often associated with N deficiency and can thus have far-reaching consequences in agricultural systems. To overcome Mo deficiency, additional Mo needs to be applied, typically with fertilizer products that also contain macronutrients (e.g. N). Legumes (e.g. chickpea, soybean, lentil, peanut) are very dependent on symbiotic N fixation, and, therefore, often require Mo fertilization. The most used Mo fertilizer forms are highly water-soluble forms of Mo, such as Na2MoO4 and (NH4)2MoO4. Fertilizer Mo is obtained from mined Mo porphyries, which is recognized by the European Commission as one of the scarcest mineral resources. A strong foreign dependency for Mo pushes for efficient internal use of this resource in a range of sectors, including agriculture. The availability of many micronutrients in soils decreases with an increasing soil pH. For Mo, however, the general trend is that plant availability increases with increasing soil pH. In acid Fe/Al-oxide rich soils, on the one hand, MoO42- is strongly adsorbed on amorphous oxide surfaces via inner-sphere complexes, giving rise to Mo deficient conditions. Readily available Mo from soluble fertilizers (Na2MoO4) is quickly sorbed on oxide surfaces, limiting Mo availability for plants. In sandy soils, on the other hand, MoO42- is bound very weakly and released MoO42- is exposed to leaching through the soil column, after which it is lost for plant uptake. Both cases (strong Mo sorption or Mo leaching) are undesired situations because i) high losses though sorption or leaching of Mo result in a low Mo use efficiency of soluble Mo fertilizers, and thus a poor use of resources, ii) in these conditions, high Mo application rates are needed to maintain plant-available Mo on grassland, which increases the risk of Mo toxicity in grazing animals (molybdenosis) and iii) Mo fertilizers need to be applied more frequently, making it labor intensive. In this project, it is investigated how careful control of Mo release from fertilizers using slow-release fertilizer formulations can increase the Mo use efficiency and limit Mo leaching losses.
The innovation in this project results from developing new Mo fertilizer compounds and products, and the knowledge of how to efficiently use these Mo compounds in fertilizer products and fertilizer application management. This can result in better Mo fertilizer products, from which fertilizer companies can benefit, and ultimately also the farmers using these products. On a societal scale, this project can improve the efficient use of scarce Mo resources and reduce the need for N fertilization (which is very energy demanding and can contribute to environmental degradation).
The overall objectives of the project include both the research objectives and the training objectives:
(i) Research objectives:
- Develop new Mo fertilizer forms, including SRF based on LDH materials or synthetic inorganic compounds embedded in macronutrient carriers, and characterize the obtained materials using state-of-the-art techniques (WP1);
- Fully characterize and understand the behavior of the new Mo fertilizers and the macronutrient carrier in both solution and soil systems, and compare the release properties with those of Na2MoO4-based fertilizers (WP2);
- Assess the performance of the proposed alternative Mo fertilizers relative to that of Na2MoO4, and visualize the interaction of plant roots with the varying fertilizer types (WP3).
(ii) Training objectives: Improve skills in grant writing, teaching, science communication, and project management
