Elevated CO₂ and Extreme Climatic Events Modify Nitrogen Content and Ruminal Protein Digestion of Temperate Grassland

Elevated CO₂ and Extreme Climatic Events Modify Nitrogen Content and Ruminal Protein Digestion of Temperate Grassland

This study was aimed at analyzing changes in nitrogen (N) content and in vitro protein rumen digestion of an upland grassland exposed to climate changes in controlled conditions. Monoliths of grassland were inserted in 12 macrocosms in which climatic conditions for the 2050s were simulated (i.e., +2.3°C and 33 mm less precipitation compared to the current climatic conditions). Six of them were subjected to ambient CO₂ (390 ppm) while the other six were subjected to elevated CO₂ (520 ppm). After four months, an extreme climatic event (ECE) consisting of four weeks of reducted precipitation (-50%) followed by two weeks without irrigation combined with a heat wave (+6°C) were applied in three macrocosms at ambient CO₂ and three macrocosms at elevated CO₂. Then, all the macrocosms were irrigated to allow the vegetation to recover. The N content and in vitro parameters of rumen protein digestion were measured on plant samples collected before the extreme event (two cuts) and after recovery. Our results indicate that, irrespective of the sampling date, elevated CO₂ results in a decrease in plant N content (P < 0.01). Inversely, the application of the extreme event resulted in a large increase in N content (P < 0.001) without a significant interaction with the CO₂ effect. These changes significantly impacted ruminal protein digestion as evidenced by changes in the production of the fermentation end-products indicators of the proteolysis, namely ammonia and iso-volatile fatty acids. We conclude that several components of climate change can impact the nitrogenous quality of the forage and its use by ruminants.