Long-term nutrient fertilization and the carbon balance of permanent grassland: Any evidence for sustainable intensification?

Abstract

Sustainable grassland intensification aims to increase plant yields while maintaining the ability of soil to act as a sink rather than sources of atmospheric CO2. High biomass yields from managed grasslands, however, can be only maintained through long-term nutrient fertilization, which can significantly affect soil carbon (C) storage and cycling. Key questions remain about (1) how long-term inorganic vs. organic fertilization influences soil C stocks, and (2) how soil C gains (or losses) contribute to the long-term C balance of managed grasslands. Using 43 years of data from a permanent grassland experiment, we show that soils not only act as significant C sinks but have not yet reached C saturation. Even unfertilized control soils showed C sequestration rates of 0.35MgCha'1yr'1 (i.e. 35gCm'2yr'1; 0-15cm depth) between 1970 and 2013. High application rates of liquid manure (i.e. cattle slurry) further increased soil C sequestration to 0.86MgCha'1yr'1 (i.e. 86gCm'2yr'1) and a key cause of this C accrual was greater C inputs from cattle slurry. However, average coefficients of slurry-C retention in soils suggest that 85% of C added yearly through liquid manure is lost possibly via CO2 fluxes and organic C leaching. Inorganically fertilized soils (i.e. NPK) had the lowest C-gain efficiency (i.e. unit of C gained per unit of N added) and lowest C sequestration (similar to control soils). Soils receiving cattle slurry showed higher C-gain and N-retention efficiencies compared to soils receiving NPK or pig slurry. We estimate that net rates of CO2-sequestration in the top 15cm of the soil can offset 9-25% of GHG (greenhouse gas) emissions from intensive management. However, because of multiple GHG sources associated with livestock farming, the net C balance of these grasslands remains positive (9-12MgCO2-equivalentha'1yr'1), thus contributing to climate change. Further C-gain efficiencies (e.g. reduced enteric fermentation and use of feed concentrates, better nutrient management) are required to make grassland intensification more sustainable.