The spatial distribution of soil microbial necromass affects nutrient mobilization and beech nutrition on silicate and calcareous forest soils

Only a little information is available about how the spatial heterogeneity (homogenous vs. patchy distribution) of microbial necromass affects microbial and plant phosphorus (P) and nitrogen (N) nutrition in the rhizosphere of forest soils. Therefore, a rhizotron experiment using soil from two silicate and one calcareous forest site of contrasting nutrient scarcity and P forms was conducted to investigate N and P uptake strategies of microbes and Fagus sylvatica [L.] roots depending on (i) site-specific physico-chemical properties and (ii) availability of 33P and 15N-labeled microbial necromass. The microbial necromass 33P uptake into soil microbial biomass and beech leaves decreased with increasing heterogeneity of necromass. This indicates an improved mobilization of P with a homogeneous necromass distribution especially for P-deficient silicate and calcareous soil. This was in line with increasing effect sizes of alkaline phosphatase activity with rising heterogeneity of microbial necromass. Moreover, we demonstrate site-specific N and P mobilization patterns. Up to 2-fold higher enrichment of residue-derived 15N in leaves and stems for the calcareous than for the silicate soils which reflected a faster mobilization, farther transportation, and greater distribution of 15N into the above-ground biomass, indicating an enhanced beech N nutrition. The different mechanisms governing small-scale necromass N and P distribution are affecting nutrient acquisition strategies of plants and of soil microorganisms in temperate forest ecosystems.