Understanding fertility management and the potential role of legumes in smallholder agroecosystems assists in promoting sustainable intensification of these systems. We used field-level nutrient mass balances in Bolivian mountain croplivestock systems to understand drivers of nutrient cycling and gradients in soil fertility created by these drivers. Experiments with legumes and phosphorus (P) fertilization tested the response of legume attributes like nitrogen (N) fixation and residue quality to these gradients, with and without P fertilization. Legumes were fertilized with Bolivian Capinota rock phosphate and soluble P to assess soil conditions where added P would improve legume impacts. Field nutrient balances showed that manuring rates, rangeland productivity, and soil erosion were dominant drivers of soil nutrient trends. Fields near to communities received more manure and were less steep than far fields, resulting in more positive near field balances. Mean potassium (K) trends were negative due to tuber crop harvests and export of crop residues as forage. Across 17 experiment fields, four principal components encompassed 87% of site soil variation: P fertility and pH; organic matter; texture; and calcium phosphate (Ca-P) levels/ elevation. Phosphorus fertility was higher in near than far fields, mirroring near/far contrasts in nutrient balances. In the experiment, legumes differed in adaptation to elevation and soil type. However P fertility was the strongest driver of legume attributes. Legumes at P-fertile sites were more likely to improve soil nutrient cycling via attributes such as N fixed, soil cover, and microbial symbioses. Legume attributes can thus mediate degrading and restoring feedbacks to soil fertility. For N, P, biomass carbon stocks, and residue quality, legumes and forage oats (a benchmark non-N fixer) had complementary attributes for soil nutrient cycling. Soil texture and Ca-P levels determined impacts of P fertilization: the largest increases in N fixed were 67% for RP and 150% for TSP in soils with low levels of clay and Ca-P. Results suggest that erosion prevention, legume-grass mixes, and P fertilization giving attention to differences in soil type and elevation, would dramatically improve sustainability of nutrient management in extensive mountain agroecosystems.