4.1. Scenario analysis on potential manure P applied to available arable land
Four rule-based scenarios were used to compare the change in soil P status in the available arable landbank of Beijing, as a result of different cropping structures and manure P application strategies. A digitized land use map (2004) was produced, demonstrating that in Beijing, typical crop rotations include winter wheat–maize and vegetable–vegetable, whilst G-749 trees belong to an annual cropping system.
In Scenario 1, a high proportion of cereal cropping was assumed (S1), with the proportions of land under cereals, vegetables and orchards being 67%, 5% and 28%, respectively. In Scenario 2, a adaptation high proportion of fields under vegetables was assumed (S2), with the proportions of land under cereals, vegetables and orchards being 34%, 36% and 28%, respectively (NDRC and MOA, 2012). Both cropping scenarios included two contrasting P management strategies: (1) Traditional manure P management (S1.1 and S2.1), where both chemical and manure P were applied at traditional rates following the farmer survey results of 2010; (2) Balanced manure P management (S1.2 and S2.2), where a balanced P supply (sustainable) was assumed, based on crop P removal and P replenishment by manure (only).