Northern Europe has wide distribution of silty agricultural soils with a fragile and unstable aggregated structure. Water is the main agent of aggregate breakdown through the mechanisms of slaking, differential swelling and mechanical rupture. Of the factors influencing aggregate stability, organic matter has a predominant role, particularly by the stimulation of microbial activity. Microbial exudates are know to stabilize soil by bonding soil particles, but another mechanism that has received less attention is an increase to hydrophobicity. Stabilization by organic matter additions is highly temporal and difficult to predict. This is primarily due to the complexity of the processes and large number of variables involved (quantity and quality of OM, internal soil characteristics, climate and management). Straw incorporation, for instance, will help stabilize fragile soils, but the long-term impacts are not understood. The objective of this work was to establish, in controlled conditions, quantitative relationships between rate of straw additions to soil and the temporal evolution of aggregate stability and the main microbial agents. Here, we focused on the evolution of hydrophobicity. Different doses of maize straw (from 0 to 20 gC kg^-1 soil) were added to 3-5 mm silty soil aggregates. Soil was incubated in jars at 20°C during 8 months at -10 kPa. At different incubation times we measured total respiration, microbial biomass carbon, ergosterol content (as a biomarker of fungi), soil water repellency, and aggregate stability. We used the slaking test from the aggregate stability method of Le Bissonnais (1996). We determined the water repellency of aggregates using two methods: water drop penetration time (WDPT) (Chenu et al., 2000) and microinfiltration with two liquids, water and ethanol (Repellency Index – R ) (Hallett and Young, 1999). Over the very wide range of doses of C input used in this study, aggregate stability towards slaking, microbiological variables and water repellency responded positively and proportionally to the C added after one week of incubation. After adding 20 gC kg-1 soil the aggregates developed a truly hydrophobic character with WDPT of ~70s and R of ~13, whereas the reference sample (no straw addition) exhibited wettable behavior with WDPT< 2s and R of ~2. The repellency index correlated well with ergosterol content, which suggests that fungi play a major role in hydrophobicity. After one month, aggregate stability and ergosterol content were still proportional to the doses of C added, but hydrophobicity decreased suggesting that its contribution to aggregate stability at this stage is less important. Both repellency methods were very well correlated (R² = 0.953). We conclude that the decomposition of plant residue in soil induced aggregate stabilization at least partly due to fungal induced repellency. Furthermore, fungal mediated hydrophobicity and fungal mediated physical entanglement of aggregates do not have the same temporal dynamics after organic matter additions.