1. Fig. S1 Soil microcosms set up. Microcosms consisted of two compartments: a organic matter patch compartment (A) and a plant compartment (B). Plantago lanceolata growing in the root compartment was colonized by the arbuscular mycorrhizal fungus Glomus intraradices. The two compartments were either separated by a mesh avoiding or allowing access of hyphae to the patch compartment. Plant roots were confined to the root compartment. The red lines in the compartments indicate arbuscular mycorrhizal hyphal mycelium; brown lines indicate roots; the green rectangle represents the organic matter patch consisting of 15 N labelled plant matter mixed with mineral soil. A B 3.5 cm 7.5 cm 9.5 cm 2.5 cm Supporting Information Figs S1-S3

2. δ 13 C of soluble carbon in the patch -28.5 -28.0 -27.5 -27.0 -26.5 -26.0 -25.5 CTRLMMPMPP * Fig. S2 Plant derived carbon (δ 13 C) in the organic matter patch after Plantago lanceolata was colonized by the arbuscular mycorrhizal fungus (Glomus intraradices) and fungal hyphae had no access into the OM patch (CTRL), with access of fungal hyphae into the OM patch (M), and in additional presence of protozoa (Acanthamoeba castellanii) in OM patch compartment (MP) and presence of protozoa in both the OM patch and root compartment (MPP). Differences between treatments were analyzed by GLM using a priori contrast analysis: (0) Control vs. mycorrhiza only (M) (i) Mycorrhiza only (M) vs. mycorrhiza + protozoa in the organic matter patch (MP), (ii) mycorrhiza + protozoa in the patch compartment (MP) vs. mycorrhiza + protozoa in both the patch and root compartment (MPP) and (iii) mycorrhiza only (M) vs. mycorrhiza + protozoa in both patch and root compartment (MPP). Error bars represent 1 sd of the mean (Ctrl, M and MP, n = 6; MPP, n = 7 ); * P < 0.05.

3. Fig. S3 Specific microbial respiration (qO2) in the organic matter (OM) patch after Plantago lanceolata was colonized by the arbuscular mycorrhizal fungus (Glomus intraradices) and fungal hyphae had no access into the OM patch (CTRL), with access of fungal hyphae into the OM patch (M), and in additional presence of protozoa (Acanthamoeba castellanii) in OM patch compartment (MP) and presence of protozoa in both the OM patch and root compartment (MPP). Differences between treatments were analyzed by GLM using a priori contrast analysis: (0) Control vs. mycorrhiza only (M) (i) Mycorrhiza only (M) vs. mycorrhiza + protozoa in the OM patch (MP), (ii) mycorrhiza + protozoa in the patch compartment (MP) vs. mycorrhiza + protozoa in both the patch and root compartment (MPP) and (iii) mycorrhiza only (M) vs. mycorrhiza + protozoa in both patch and root compartment (MPP). Error bars represent 1 sd of the mean (Ctrl, M and MP, n = 6; MPP, n = 7 ); * P < 0.05. qO 2 [ml O 2 mg -1 C mic h -1 ] CTRLMMPMPP 0.00 0.02 0.04 0.06 *