Effects of nutrient fertilization and soil tillage on soil CO2 emissions in a long-term grassland experiment

A better understanding of how land management might affect soil respiration can greatly help enhancing the long-term sustainability of agricultural soils. This study investigated the effects of two key management practices, nutrient fertilization and soil tillage, on soil respiration in an intensive grassland system. Data were collected from a long-term grassland experiment established in 1970 in Northern Ireland, UK. The experiment commenced with eight nutrient treatments: an unfertilised control, inorganic fertiliser (NPK), two types of slurry: cattle and pig slurry at three application rates. In September 2019 half of the experimental plots were tilled and reseeded with a multi-species sward mixture. Static automated chambers were used to measure the fluxes of CO2 between 2018 and 2021. Surprisingly, we did not find any significant nutrient treatment effects on mean hourly or cumulative CO2 emissions. Likewise, there were no significant differences in CO2 emissions between the type of slurry, slurry application rate or the interaction of these treatment factors. However, seasonal changes in CO2 emissions were highly significant while ploughing had a significant decreasing effect on the cumulative daily CO2 emissions but only during the establishment period (first nine weeks). Significant relationships with CO2 flux were found for soil temperature and soil water content. We also found no relationship between short-term soil CO2 emissions and long-term soil organic carbon storage data, which highlights the complexity of soil C source-sink dynamics and C balance. Our study brings evidence that regarding CO2 emissions the impact of soil tillage is short-lived, and the direct effect of nutrient additions is small.