Nutrient effects on long-term primary productivity and C sequestration at the Breton Plots: mathematical modelling with ecosys under current and future climates

Including perennial legumes in crop rotations has been found consistently to increase soil C sequestration. This increase may be attributed to greater litterfall from greater primary productivity sustained by symbiotic N₂ fixation during longer growing seasons. In this study we corroborated increases in SOC and harvest removals modelled from 1929 to 2018 with those measured in 2-year wheat-fallow (WF) and 5-year wheat-oats-barley-alfalfa/brome-alfalfa/brome (5Y) rotations amended with fertilizer and manure. Gains in SOC and harvests were modelled and measured in the 5Y vs. WF rotation with or without amendments. These gains were closely related to rates of N₂ fixation by alfalfa modelled during the two years of alfalfa/brome forage. SOC, harvests and N₂ fixation were greater in treatments amended with N and P than in those without, but were substantially smaller in treatments without P amendments relative to those with P. When model runs were extended from 2019 to 2118 under RCP 8.5 climate change projected for the Breton region, changes in SOC modelled with rotations and amendments remained similar to those modelled without climate change. Harvests modelled in WF with climate change rose little from those without because N and P inputs remained unchanged, and so limited the response of primary productivity to rising CO₂ concentrations (C_a). Harvests modelled in 5Y with N and P amendments rose more with climate change than did those in WF because N inputs in 5Y rose with increased N₂ fixation driven by rising C_a. However increases in harvests modelled with climate change in 5Y were limited in treatments without P amendments, indicating the importance of P availability in determining responses of productivity and C sequestration to climate change. In the model, rises in N₂ fixation with climate change also drove rises in N₂O emissions which partially offset the greater productivity that this fixation enabled when estimating ecosystem GHG balances.