Changes in bacterial community composition and soil respiration indicate rapid successions of protist grazers during mineralization of maize crop residues

Abstract

Decomposition of organic matter is crucial for ecosystem functioning. Microorganisms, which are responsible for the mineralization of organic matter, are usually treated as a homogeneous functional guild, despite mineralization capacity can differ profoundly between taxa. In addition, a significant part of the microbial community is top-down controlled by microbial grazers, such as protist. Since protist grazing is selective, and selectivity differs among species, we hypothesized that protist taxa complement each other in grazing intensity and thereby affect bacterial community structure and mineralization rate. In a laboratory experiment the species richness of protist communities was manipulated in an arable field soil and the mineralization rate of maize litter residues followed during the decomposition of the labile (4 days) and recalcitrant (3 weeks) carbon (C) fractions. Mineralization rate overall increased in the presence of protists. Changes in microbial function could be related to changes in microbial community composition (measured by phospholipid fatty acids pattern). During microbial decomposition, different protist grazers gained influence on mineralization rates over consecutive time intervals, indicating that a succession of protists caused an enhanced bacterial C-mineralization of plant detritus. Protist identity and species richness affected the microbial community composition, but not the magnitude of its mineralization function. In general, protist identity appeared to be more relevant for the composition of the microbial communities at the beginning of decomposition while the protist species richness appeared to be more critical in the later, slow phase of decomposition. This study provides an example that the overall outcome of ecosystem processes, such as mineralization rate is regulated by the sum of positive and negative effects of complex species interactions operating at a very fine spatial and temporal scales.