Original article Nematodes and protozoa affect plants differently, depending on soil nutrient status Lisa Bjørnlund a , Manqiang Liu a, b , Regin Rønn a , Søren Christensen a , Flemming Ekelund a, * a University of Copenhagen, Department of Biology, Øster Farimagsgade 2D, DK-1353 Copenhagen, Denmark b Soil Ecology Lab, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, People’s Republic of China article info Article history: Received 29 September 2011 Received in revised form 23 November 2011 Accepted 28 November 2011 Available online 11 December 2011 Handling editor: Bryan Griffiths Keywords: Bacterial feeders Plant performance Protozoa Nematodes Microbial loop selective feeding abstract We grew plants in soil amended with either finely ground (<0.2 mm) or larger pieces (4  5 mm 2 ) of maize leaves. Finely ground material will be mineralized faster; hence nitrogen is released faster in the soil. We inoculated the soil with the plant growth inhibiting bacterium Arthrobacter BEM463, a relatively poor food source for bacterial feeders, and the plant growth promoter Sphingopyxis BEM760 which is a good food source. Protozoa and nematodes were introduced separately or in combination. In soil with larger maize leaf pieces (low turnover, slow nitrogen release), bacterial feeders generally increased plant nitrogen uptake and growth. In systems with finely ground material (high turnover, fast nitrogen release), plants hardly benefited from presence of grazers; bacterial mineralization activity alone prob- ably provided enough N for the plants. The harmful Arthrobacter thrived better when grazed by protozoa than when grazed by nematodes, probably because the selective protozoa could avoid it, which resulted in reduced plant performance. We suggest that bacterial feeders generally increase plant performance when nutrients are limited. At high nutrient availability, however, bacterial feeders promote grazer resistant bacteria, and, if these bacteria are harmful to plants, the net effect of bacterial feeders on plant growth can be negative. Ó 2011 Elsevier Masson SAS. All rights reserved. 1. Introduction Optimal plant performance depends on sufficient nutrient supply but various bacterial metabolites also affect plants [1e5]. Bacterial feeders generally improve plant growth by stimulating mineralization of plant nutrients [6,7]. Recently it has been sug- gested that protozoa also affect root growth through hormonal effects mediated via effects on the bacterial communities [8,9] but this mechanism is not fully understood, and the relative impor- tance of these two effects is still under debate [7]. Bacteria vary in their quality as food for protozoa and not all bacteria are preyed upon to the same extent [10e13], thus bacterial feeders affect the composition of bacterial communities [14e17]. Protozoa ingest their food by phagocytosis and can discriminate and select between individual food items [18,19]. Due to their much larger size, nematodes cannot discriminate among individual bacterial cells but use their chemosensory apparatus to search for bacterial micro-colonies and seek out the most suitable food at the mmemm scale based on a ‘trial and error’ learning approach [20]. Defence mechanisms that protect bacteria from grazing can thus have quite different effects on protozoa and nematodes [21]. Furthermore, protozoa and nematodes may interact directly. For example, we recently demonstrated that protozoa (Cercomonas sp.) are able to kill juvenile nematodes while adult nematodes feed on the protozoa [22]. Liberation of nitrogen (and other mineral plant nutrients) during decomposition of dead organic matter depends on particle size of the material [23]. Generally, smaller particles provide a larger surface area and better opportunity for microbial decom- posers for attack. We utilized this to construct systems with different rates of nutrient mineralization by amending soils with either larger pieces of dried maize leaves (4  5 mm 2 ) or finely ground maize leaf material (<0.2 mm). In these systems, we examined the separate and combined effects of nematodes and protozoa on mineral nutrient turnover, bacterial community structure, and plant growth. We hypothesized that 1) the ability of grazers to directly facilitate the provision of plant nutrients is more important in systems with larger pieces where mineralization rate is lower, and, 2) that the ability of grazers to shape bacterial communities by selective feeding is most evident in systems with finely ground material with higher nutrient release. * Corresponding author. Tel.: þ 45 35 32 21 59; fax: þ45 35 32 23 21. E-mail address: fekelund@bio.ku.dk (F. Ekelund). Contents lists available at SciVerse ScienceDirect European Journal of Soil Biology journal homepage: http://www.elsevier.com/locate/ejsobi 1164-5563/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2011.11.012 European Journal of Soil Biology 50 (2012) 28e31