Increasing the productivity of an upland pasture with the least environmental impacts

Abstract

The environmental impacts of increasing pasture productivity in the United Kingdom’s (UK) uplands is under-researched. A field trial on a representative upland farm investigated the effect of implementing pasture improvement options on unimproved land between 2017 and 2019. The options were cultivation (ploughing and rotovating) and reseeding in combination with lime and fertiliser inputs. Soil pH, grass growth, pasture quality and nitrous oxide (N2O) emissions from soil were measured throughout the growing seasons. Carbon dioxide (CO2) emissions were also measured from cultivation until pasture establishment in 2017. In the year of establishment, reseeding and increased nitrogen (N) fertiliser application led to greater daily grass growth, with the mean over two times greater from the ploughed and reseeded treatment (20.0 kg DM ha−1 day−1) as opposed to the control (8.2 kg DM ha−1 day−1) during the growing season (p < 0.05). Yield effects in subsequent years were dependent upon nutrient input regimes. Despite a significant difference in pasture energy values between the treatments in the year of establishment, there was no effect of reseeding and targeted lime and fertiliser applications on pasture quality in the two subsequent years. Rotovating and ploughing increased soil N2O emissions, with higher N fertiliser emission factors in 2017 (2.26% ± 0.79 from the rotovated treatment) compared with the IPCC default value of 1.6%. However, low N2O fluxes in 2018 meant that the emission factors during this growing season were also low (highest value of 0.25% ± 0.04 from the lime and fertiliser input only treatment). Rotovating and ploughing did not significantly impact CO2 emissions in the establishment period. From a purely production perspective, the results would favour reseeding similarly unproductive upland pastures typical in much of the UK. However, yield-scaled N2O emission calculations showed that reseeding was not the most effective method in reducing N2O emissions per unit of grass yield, rather, applying lime and fertiliser resulted in the lowest N2O emissions per unit of grass produced (0.07 g N2O kg−1 DM of grass). These findings demonstrate the importance of considering both pasture production and N2O emissions together when improving upland pasture for maximum nitrogen use efficiency and to reduce environmental impacts.