No abstract

The increasing input of anthropogenically derived nitrogen (N) to ecosystems raises a crucial question: how does available N modify the decomposer community and thus affects the mineralization of soil organic matter (SOM). Moreover, N input modifies the priming effect (PE), that is, the effect of fresh organics on the microbial decomposition of SOM. We studied the interactive effects of C and N on SOM mineralization (by natural (13) C labelling adding C4 -sucrose or C4 -maize straw to C3 -soil) in relation to microbial growth kinetics and to the activities of five hydrolytic enzymes. This encompasses the groups of parameters governing two mechanisms of priming effects - microbial N mining and stoichiometric decomposition theories. In sole C treatments, positive PE was accompanied by a decrease in specific microbial growth rates, confirming a greater contribution of K-strategists to the decomposition of native SOM. Sucrose addition with N significantly accelerated mineralization of native SOM, whereas mineral N added with plant residues accelerated decomposition of plant residues. This supports the microbial mining theory in terms of N limitation. Sucrose addition with N was accompanied by accelerated microbial growth, increased activities of β-glucosidase and cellobiohydrolase, and decreased activities of xylanase and leucine amino peptidase. This indicated an increased contribution of r-strategists to the PE and to decomposition of cellulose but the decreased hemicellulolytic and proteolytic activities. Thus, the acceleration of the C cycle was primed by exogenous organic C and was controlled by N. This confirms the stoichiometric decomposition theory. Both K- and r-strategists were beneficial for priming effects, with an increasing contribution of K-selected species under N limitation. Thus, the priming phenomenon described in 'microbial N mining' theory can be ascribed to K-strategists. In contrast, 'stoichiometric decomposition' theory, that is, accelerated OM mineralization due to balanced microbial growth, is explained by domination of r-strategists.

No abstract

Abstract. In this review, we summarise factors contributing to plant availability of magnesium (Mg) in soils, the role of Mg in plant physiological processes related to yield formation and abiotic stress tolerance, and soil and fertiliser parameters related to Mg leaching in fertilised soils. Mg is a common constituent in many minerals, comprising 2% of Earth's crust; however, most soil Mg (90-98%) is incorporated in the crystal lattice structure of minerals and thus not directly available for plant uptake. Plants absorb Mg from the soil solution, which is slowly replenished by soil reserves. Duration and intensity of weathering, soil moisture, soil pH, and root-microbial activity in soil are key factors that determine plant-available Mg release from soils. On the other hand, the amount of Mg released from soil minerals is generally small compared with the amounts needed to sustain high crop yield and quality. Thus, in many agro-ecosystems, application of Mg fertilisers is crucial. Magnesium is involved in many physiological and biochemical processes; it is an essential element for plant growth and development and plays a key role in plant defence mechanisms in abiotic stress situations. An early effect of Mg deficiency in plants is the disturbed partitioning of assimilates between roots and shoots because the supply of sink organs with photosynthetic products is impaired, and sugars accumulate in source leaves. Thus, optimal supply of Mg is required to improve crop tolerance to various stresses and to increase yield and quality parameters of harvested products. Unlike other cations, Mg is very mobile in soils because it is less bound to the soil charges. Therefore, Mg losses by leaching might occur in sandy soils with high water conductivity. Leaching of Mg in soils when applied with various water-soluble fertilisers may also vary depending on the fertiliser's chemical composition, granule size, and effect on soil pH and cation balance, as we discuss in detail.