Soils are inherently heterogeneous and plant plasticity facilitates optimization of nutrient uptake within this environment. The negative feedback effect of soil nitrogen availability on biological nitrogen fixation (BNF) provides an interesting framework for investigating how plant phenology and species mixtures interact with heterogeneous soil nitrogen pools. Understanding plant responses to nutrient availability driven by heterogeneous organic matter pools is critical to developing an alternative paradigm of nutrient management in agroecosystems that optimizes nutrient cycling to maintain production while reducing nutrient losses to the surrounding environment. We compared BNF in an annual (Pisum sativum) and a perennial (Trifolium pratense) legume grown in monocultures and intercropped with non-N-fixing plants (Dactylis glomerata and Avena sativum) across 17 farm fields in central New York State. The fields were selected to represent a management-induced fertility gradient across soils of similar texture. Field averages for total soil nitrogen and carbon vary more than two-fold from 0.12% to 0.28% and from 1.3% to 2.9%, respectively. Perennial species were established in May 2006 and annual species were established in August 2006. Biomass and soils were sampled in October 2006. BNF was quantified using the ¹⁵N natural abundance approach and soil nitrogen availability was estimated using both biological and chemical methods, including microbial biomass, particulate organic matter, and extractable inorganic nitrogen. Preliminary results show that BNF was consistently higher for perennial legumes in mixtures or monocultures compared with annuals. The perennial legume monoculture BNF showed the strongest negative response to increasing soil nitrogen.