Spatial optimization of manure and fertilizer application strategies to minimize nutrient surpluses and acidification rates in croplands of a typical Chinese county

Elevated inputs of mineral nitrogen (N) and phosphorus (P) fertilizer applications increased crop yield but enhanced eutrophication and soil acidification due to nitrate-induced base cation (BC) leaching in China. Increasing inputs of manure counteract soil acidification since it contains BC apart from N and P, but its use may enhance eutrophication by unwanted P accumulation in soils with a high P status. In addition, however, N and P fertilizer inputs can go down by improving management practices, increasing the N and P use efficiency (NUE and PUE). In this study, we assessed spatially explicit optimal manure and N and P fertilizer application rates, by balancing crop N and P demand with their inputs while using attainable NUE values for manure and fertilizer and enhancing PUE values by accounting for the legacy soil P pools, at 151 sites in Qiyang, a typical Chinese agricultural county. We evaluated the impacts on N and P surpluses and soil acidification rates (lime requirements) using the acidification model VSD+ and extrapolated these impacts to county-level. Historical acidification in paddy soils was mainly caused by natural acidification and crop removal, while in upland soils mineral fertilizers dominated the factors controlling acidification. Balancing N and P input with crop demand and recycling all available manure can nearly fulfil the crop P requirements, and reduced N and P fertilizer use by 74% and 99%, respectively. The N and P surpluses subsequently declined by 70 to 90%, respectively, whereas the organic carbon (C) input increased by 48% and the soil acidification rates for non-calcareous soils declined by 50%. Recommendation systems for fertilizer and manure use, combining the flows of C, N, P and base cations are key to counteract soil acidification and substantially reduce environmental impacts while sustaining crop production.