Tillage and reseeding effects on soil carbon stocks: evidence from 500 agricultural grasslands in the UK

Several studies across different agroecosystems worldwide have compared the effects of single soil tillage events to no-till on soil carbon (C) stocks. Yet, there is still significant uncertainty about how the frequency of 'tillage + reseeding' (e.g. number of reseeding events) might affect soil C stocks in agricultural grasslands in the long term. Here, we addressed this knowledge gap by sampling for the first time 500 grassland fields in Northern Ireland, UK, based on their management history including the number of tillage and reseeding events occurred in the previous 50 years. We show that soil C and N stocks within 0-30-cm depth were not significantly affected by the frequency of 'tillage + reseeding' events. We found, however, that soil C (%) and N (%) concentrations significantly decreased while soil bulk density increased with the frequency of soil till + grassland reseeding. Because soil C stocks are calculated by multiplying C (%) with bulk density, the opposite response of these two key factors has possibly contributed to 'homogenise' the soil C stock product partly explaining the lack of a significant relationship between soil C stocks and the frequency of soil till + reseeding events. Our evidence is that the most important variable affecting soil C (%) concentration was bulk density and thus soil compaction. We also show that grasslands mainly managed for grazing are significantly less tilled and reseeded and have soils with lower bulk density and higher C (%) than grasslands managed for silage production. Our findings suggest that while increases in the frequency of tillage and reseeding can negatively affect soil C (%), the soil C storage potential of agricultural grasslands can be more affected by increases in soil compaction associated with greater machinery traffic.