Gypsum and structure lime amendments in boreal agricultural clay soils: Do climate emissions compromise water quality benefits?

We examine cost-effectiveness and social net benefits of using soil amendments, gypsum and structure lime, in reducing phosphorus loading while accounting for the climate emissions from both amendments. Recent field experiments and large-scale pilots in Finland and Sweden suggest that both gypsum and structure lime improve soil structure and can reduce total P loading from clayey fields but differ as soil amendments. While gypsum does not change soil pH, structure lime helps to adjust it to a desired level. Drawing on literature, gypsum is postulated to reduce both dissolved (25%) and particulate losses (50%) of phosphorus, while structure lime is postulated to reduce only particulate phosphorus (40%). Life-cycle analysis is applied to determine greenhouse gas emissions from both soil amendments. We examine 5 and 10 years impacts on phosphorus loss by choosing doses and their timing accordingly. Both amendments provide the highest water quality benefits on erodible soils or soils with high soil phosphorus. Accounting for climate issues drastically changes the picture. Greenhouse gas emissions from gypsum production are 14.43 kgCO2e ha-1, and those from structure lime from pristine materials are 1837 kgCO2e ha-1. Cost-effectiveness of P load reduction including carbon price of GHG emissions is 59 € kg-1P for gypsum and 122 € kg-1P for structure lime. At the national level (application to 0.54 Mha), differences in greenhouse gas emissions without soil emissions are huge and in favour of gypsum (0.048 Mt and 1.04 Mt). Structure lime from recycled zero-emission materials performs well but its supply is very limited.