Measuring and Modeling Soil Carbon Changes on Dutch Dairy Farms

Soil carbon sequestration is one of the pathways for the dairy sector to mitigate climate change. Soil carbon measures have been reviewed extensively, including estimates of their impacts on regional or national scales. Eventually, these measures are to be implemented by the farmers themselves, justifying an assessment at farm and field level. Here, we used soil and management data from 96 fields on nine dairy farms to quantify annual stock changes under current management and the effect of several carbon measures on soil carbon sequestration in relation to farm configurations. The fields were in use as permanent grassland or grass-arable rotation with forage maize or other crops. We compared the observed changes in the soil layer of 0–25 cm with the RothC simulated changes, and we also simulated the effect of carbon measures on soil carbon stocks. We found a moderate (R2 = 0.30) relation between simulated and measured soil carbon changes. Factors that contribute to the uncertainties are the estimates of field-specific carbon inputs from crop residues and manures, especially for farms that temporarily exchange land with other farmers. The current standard agronomic soil sampling program is unable to reliably detect soil carbon changes at a farm or field level. The annual changes in simulated soil carbon were negatively related to the initials carbon stocks, which has important implications for the potential of additional carbon storage. Therefore, we propose an indicator that expresses the current soil carbon stock in relation to the location-specific maximal achievable carbon stock for permanent grassland that receives an equivalent of 170 kg nitrogen per ha per year from animal manure. This can be used to compare farms and indicate whether a farmer’s focus should be on additional carbon storage or the protection of existing stocks. The simulation of carbon measures showed that the proportion of grassland is key in soil carbon storage.