Proteins are the main N compounds and are important sources of both C and N for plants and microbes. Triple-labeled dissolved organic N obtained from white clover shoots was separated into five different molecular weight (Mw) size fractions (> 1 kDa). In this study, we investigated the mineralization of and the presence of C and N remaining in soil solution over 14 days across different soil pH values. The evolved CO2 decreased by increasing Mw, suggesting an apparent bottleneck in the decomposition of organic N above Mw 30 kDa. The initial parallel loss of 13C and 15 N from soil was followed by an increase in dissolved 15 N describing a two-phase process of organic N turnover. The first-phase concerns the removal of organic N whereas the second-phase concerns the release of inorganic N. Microbial N and C use efficiency was increased with increasing Mw size suggesting the importance of organic N quality in predicting the impact on soil C and N cycling.