The cycling of carbon within the earth system is intrinsically linked with major nutrients, notably nitrogen and phosphorus, due to the tendency of these elements to limit the productivity of terrestrial ecosystems. To understand the response of the carbon cycle to global change pressures, models must integrate Carbon-Nitrogen-Phosphorus cycles. Whilst such models exist, to-date these have focused on natural and semi-natural ecosystems. Agriculture results in significant modification to natural biogeochemical cycling, and currently represents approximately 37% of land-use. With the projected increase in global food demand over the 21st century, this area is expected to increase. It is therefore critical to understand and simulate biogeochemical cycling in both natural and agricultural systems, and the transition between these, to estimate ecosystem response to environmental change. In this study we present an integrated C-N-P model including both natural and agricultural temperate ecosystems. The N14CP model has been developed to include representation of both arable and grassland systems, with the inclusion of agricultural management practices such as fertilizer application, crop removal, grazing and yield estimation. The model has been tested both spatially and temporally using a range of long-term experimental sites across Northern-Europe, and applied at both local and national scales. We use the model to assess impacts of land-use change and management on long-term nutrient cycling, and discuss the implications of this for sustainable agriculture and ecosystem functioning.