This project address the mechanisms of mineral-associated organic matter (MAOM) formation and persistence by nitric acid weathering under soil nitrogen (N) enrichment. Though biological N fixation introduces significant quantities of N in ecosystems across the globe, the effects of nitric acid on weathering and subsequent MAOM persistence have not been studied. We will quantify MAOM content in temperate forest soils, collected along one of the widest naturally occurring N gradients in the world. In the Pacific Northwest, the primary source of N from biological fixation is from the symbiotic N-fixing tree Alnus rubra (red alder). To address the short-term effects of alder growth on soil carbon, we will investigate potential differences in MAOM in soils planted under red alder or douglas fir along a latitudinal gradient in the Pacific Northwest coast range. This work offers an opportunity to improve the accuracy of MAOM predictions and the accuracy of terrestrial C cycling models. Many of the current terrestrial models partition soil organic matter (SOM) into particulate vs mineral-associated pools, because they respond to global change differently, where particulate SOM is often less protected from microbial decomposition. The mineral horizons contain most of the protected C pool, especially considering their depth. Because deep soils store a substantial amount of soil C, one of the aims of this proposal is to understand SOM protection by mineral-associations and destabilization with depth.