Over the last five decades global ecosystem services (ES) have continued to diminish, mainlydue to anthropogenic activities. Agro-ecosystems cover almost 40% of the terrestrial surface onEarth, and have been considered as one of the most significant ecological experiments with apotential to both contribute to and mitigate global ES loss. Organic farming has been suggested asone possible solution to alleviate the loss of ES in agro-ecosystems due to its environmental benefitscompared with conventional farming. However, only a few studies have accounted for the economicvalue of ES in different organic crop production systems and little is known about howanthropogenic activities affect the supply of ES in such organic crop production systems. Ecologicalstoichiometry, which is the study of the fluxes of chemical elements and the ratio between them, hasbeen considered as a new approach to provide fundamental insights into ecological processes thatmay link individual organisms, communities, and ecosystems at different levels from the gene to thebiosphere, and contribute to the understanding of the effects of anthropogenic activities on ES. Theoverall objective of this project is to explore the relationship between stoichiometry and ES valuesin organic crop production systems and explain the mechanisms behind this relationship. In orderto achieve this objective, different organic crop production systems characterized by differentstoichiometric ratios and ES values need to be identified and investigated.An important aspect of organic farming is the maintenance and enhancement of soil organicmatter. The study reported in Paper I was designed to investigate the economic values of ES inorganic cereal crop production systems with different field management practices in relation toorganic matter input. The results demonstrated that agricultural management practices significantlyaffected the total ES values of organic production systems, with high organic matter inputs resultingin high crop yields and non-marketable ES values of organic cereal production systems. A strongpositive relationship between organic matter input level and non-marketable ES value was found.These results can be used by land managers and decision makers to adjust management practices inagro-ecosystems in order to address the environmental challenges associated with different types ofsensitive soil in Denmark. Moreover, monetizing the non-marketable ES by integrating payment forES in agricultural policy support schemes could be a way to preserve ES in the long term.In the second paper we investigated the long-term effects of conversion to organic farmingon a selection of ES in order to determine whether converting back to conventional farming ispotentially more detrimental on farms with a long history of organic farming. A model simulationcase study and an on-farm case study both showed that non-marketable ES values increased withincreasing number of years under organic farming, and that trade-offs between non-marketable andmarketable ES were only present when the price difference between organic and conventionalproducts in the model simulation case study was the smallest, and when an alfalfa pre-crop boostedthe yield and increased the associated marketable ES in the on-farm case study. These results couldbe used to argue for agricultural policy interventions to offset further reduction in the number oforganic farms or the land area under organic farming.Finally in Paper III, the relationship between stoichiometry and ES was explored. For thispurpose, all results from paper I and paper II were analyzed together with data from ten organicgrass production systems. A significant positive linear correlation between the soil C:Nstoichiometric ratio and non-marketable ES values was found in this study, but there was norelationship between the C:O and O:H ratios and the ES values. The organic farming systems with ahigh soil C:N stoichiometric ratio had a potential to produce more food, sequester more carbon fromthe atmosphere, store more water in the soil, attract more aphid predators, and regulate morenitrogen compared with the organic farming systems with a low soil C:N stoichiometric ratio. Basedon these results we suggest that the soil C:N stoichiometric ratio can be used as a new tool fordetermining the ES values in organic farming systems in Denmark.In conclusion, the overall results of Paper I and Paper II demonstrate that the ES of organiccrop production systems have been significantly affected by organic matter input levels and timesince conversion to organic farming. Paper III demonstrates a clear relationship between soil C:Nstoichiometric ratio and ES values in organic crop production systems.