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.

Phosphorus is a critical nutrient in sustaining food production. In agricultural systems, application of P fertilizers has significantly increased since the green revolution to become common practice globally, contributing to increased productivity. However, excess use of P fertilizer does not only pose a cost to farmers, but costs for society in the form of water quality problems and environmental degradation. Furthermore, rock phosphates from which these fertilizers are derived are a finite resource, which brings into question the long-term sustainability of this resource and the food production it supports. Soils play a critical role in hosting the P cycle, and organic forms of P (monoesters, diesters) often represent a significant portion of soil P stocks, that are so often overlooked. The mineralization of organic P by phosphatase enzymes is recognized as a key mechanism for converting organic to inorganic forms, which can then be potentially used for P uptake by plants. However, quantification of their contribution still remains a significant challenge. In order to sustainably meet growing food production demands over the next century and reduce the impacts of P fertilizers on waterways, there is a need to understand the extent that soil organic P is available, or can be made available for plants. Here, we present recent findings from a soil-plant biogeochemical model of integrated carbon-nitrogen-phosphorus cycling in agricultural environments. Comparison of observational yield data taken from various long-term experimental sites with model simulations indicate a gap in current scientific understanding of P sources. Whilst yields in the experiments are maintained under low P addition conditions, the model indicates yield declines due to exhaustion of available P resources. We use the model to explore the missing links: potential P inputs, processes and pathways. Finally, we discuss the need for additional empirical evidence to support understanding of organic P cycling, and development of models to include these processes to inform future land management and ensure long-term food security and sustainable water resources.