Is there an optimal frequency of temporary grasslands in crop rotation to maintain SOM at a level sustaining agricultural soil quality?

Land-use type has a strong impact on soil organic matter (SOM) content which shows higher level in permanent grasslands than in arable lands. The inclusion of temporary grasslands in crop rotation is an efficient practice to limit SOM loss in arable lands. Nonetheless, temporary grasslands can be seen in competition with food-production for human consumption as they produce forage to feed livestock.

In a context of agricultural specialization and reduction of animal-based protein consumption, it is imperative to evaluate the impact of a decrease of temporary grasslands and their frequency in crop rotation on SOM content. Moreover, it is crucial to assess if the decrease of temporary grasslands frequency, SOM content and soil quality is a linear process or if an optimum exists that maximize soil quality while minimizing temporary grasslands in the rotation.

Here, we used a 40-year-old soil monitoring network in South-West Switzerland (FRIBO Network) to assess the relationship between temporary grasslands frequency, SOM loss and soil quality. Soil organic carbon (SOC) dynamics was estimated by sampling the monitoring sites every 5 years. SOC stocks were measured down to 50 cm depth. Permanent grasslands were used as reference for estimating SOM loss, controlling for variations in soil characteristics (e.g., clay, pH) and site characteristics (e.g., altitude, precipitation, crops type) among sites. The SOC-to-clay ratio was used as indicator of soil quality.

Our results showed that SOM loss is proportional to the decrease of temporary grasslands frequency so that any change of frequency has the same consequence on SOM loss independently from the absolute frequency of the temporary grasslands. This suggests that there is not an optimal frequency of temporary grasslands in a soil C sequestration perspective. This contribution, however, will also show that soil quality indictors related to SOM such as the SOC-to-clay ratio exhibits thresholds that enable to define an optimal frequency of temporary grasslands in order to maintain a level of soil quality that minimizes the negative impacts on soil functioning. Hence, providing a decision framework to target agricultural fields where temporary grasslands should be promoted or could be reduced with less impact.