Disentangling the effects of long-term fertilisation on soil carbon dynamics

Abstract

The application of fertilisers has greatly increased agricultural yields. However, it has also had wide ranging consequences for ecosystem properties and processes, generating concern over the long-term sustainability of fertiliser management. Despite this, there is considerable uncertainty regarding the long-term impacts of fertilisation, especially on soil carbon, and this is largely due to the paucity of published findings from long-term experiments. In this project very long-term field experiments and a short-term microcosm trial were used to investigate the effects of fertilisation on agroecosystem properties with a particular focus on soil carbon dynamics. The effects of long-term fertiliser addition on key ecosystem properties in the Palace Leas Hay Meadow Experiment was determined and compared with those observed at other long-term and with short-term experiments. Farmyard manure addition was found to have many benefits over the use of inorganic fertilisers for improving agricultural production including higher nutrient availability, hay yields and the prevention of soil acidification. The magnitude of short- and long-term effects of fertilisation differed markedly, underlining the value of using long-term experiments to realise the true ecosystem response. The relative importance of three major classes of fertiliser-mediated mechanism was also evaluated using structural equation modelling and a complementary ley-arable rotation experiment. These studies found that soil C dynamics were altered by both direct (nutrient) and indirect (plant community- and soil pH-change) effects of fertilisation. Soil pH was the primary regulator of the microbial response to fertilisation, with soil acidification effects strongly negatively affecting microbial functioning when it reduced soil pH below 5. A microcosm study indicated that these effects of acidification on the microbial community cannot easily be reversed. As microorganisms regulate biogeochemical cycles which are critical for food production, fertiliser-induced acidification could therefore hinder long-term agricultural sustainability. Accordingly, soil pH in fertilised systems must be managed vigilantly.