Reduced tillage and organic fertilizer application usually result in an increase in soil aggregate stability (AS). However, the magnitude of the effects can vary with soil properties and season. The aim of this study was to investigate AS dynamics over three seasons in a soil under various tillage and fertilization practices. The study was performed under three tillage practices (moldboard plowing (MP), surface tillage (ST) and no-tillage (NT)) and two types of fertilizer (poultry manure and mineral) seven and eight years after their establishment in Northwest France. AS was measured in three different seasons: spring, summer and winter. Soil properties that potentially influence AS such as organic carbon (OC), hot-water extractable carbohydrates (HWEC), water content (WC) and water repellency (WR) were also studied. On average, for all sampling dates, AS was 34% higher under NT than MP. Conversely, the effect of ST on AS varied with sampling date with values close to NT in mid-spring and summer, and values close to MP in early spring and winter. Poultry manure increased AS by an average of 12% regardless of sampling date or tillage practice. Variations in AS due to management practices were related to OC (r = 0.92) and HWEC (r = 0.88). Differences in AS between sampling dates were slightly greater than the effects of management practices. On average across management practices, AS increased by 47% from early spring to summer and decreased by 59% in winter. These variations were related to soil WC (r = −0.67) and WR (r = 0.72) at time of sampling. We suggest that seasonal variations in AS were at least partly due to variations in WC which acted physically by modifying the water entry rate into the aggregates and slaking effects. In contrast, the long-term AS dynamics were related to the organic matter dynamics, which are controlled by management practices. Because of the predominant effect of climate on AS, we suggest measuring AS in winter and summer to better estimate the effects of management practices on soil erodibility in this region.