Greenhouse gas budgets for two carbon farming options show small or negative climate benefit

Considerable quantities of soil organic carbon (SOC) can be sequestered in agricultural land by adopting practices broadly known as carbon farming. Two main options for SOC sequestration in arable land are to (1) replace annual crops for perennial crops such as grass or forage legumes, and (2) grow cover crops. Experimental evidence shows that additional perennials and cover crops in temperate agriculture can sequester ca. 300-600 kg C/ha/year. The total climate benefit however depends on other changes to the greenhouse gas (GHG) budget in the cropping system and in the wider system where crops are used. Moreover, SOC sequestration tapers off over time whereas other GHG budget terms remain constant.

To study the potential total GHG budget of carbon farming options in a Swedish context, we are working on a model-based evaluation of two scenarios: (1) shifting a dairy production system to more grass-based feed, thereby replacing annual feed crops with perennial grass-clover, and (2) introducing cover crops in a crop rotation with only annual crops. Both scenarios were evaluated using mathematical models with a life-cycle perspective from cradle to farm gate. Since GHG emissions from the nitrogen cycle (importantly nitrous oxide, N2O) are strongly shaped by crop and livestock management, we made a detailed model of nitrogen flows throughout the system. We also accounted for energy-related GHG emissions and methane (CH4) emissions from enteric fermentation and manure management.

In this talk, I will present some of the results of this work. In both scenarios, the climate benefit of SOC sequestration is strongly counteracted by other GHG emissions. In the cover crop scenario, increased emissions of mainly N2O reverse more than half of the climate benefit (GWP100). In the dairy system, increased emissions of mainly CH4 lead to increased total GHG emissions both per unit milk and per hectare. A main positive side effect in both scenarios is decreased leaching of nitrate. Over time, climate benefit in both scenarios diminishes further as C sequestration tapers off while N2O and CH4 emissions remain largely unchanged.