Carbon Balance of the Breton Classical Plots over Half a Century

We related C input and management to soil organic C (SOC) dynamics over 51 yr (1939–1990). We used two rotations from the Breton Classical Plots at Breton, Canada, on a Typic Cryoboralf: (i) wheat ( Triticum aestivum L.)–fallow (WF) and (ii) wheat–oat ( Avena sativa L.)–barley ( Hordeum vulgare L.)–hay (primarily alfalfa, Medicago sativa L.)–hay (WOBHH), in factorial combination with three fertility levels: no added fertilizer [Nil], N‐P‐K‐S fertilizers [F], and farmyard manure [M]. Net aboveground C productivity (NAGCP, kg ha −1 yr −1 ) averaged 576 in WF–Nil and 1078 in WF–F and SOC decreased in both, but NAGCP averaged 1208 in WF–M, where SOC increased. A NAGCP of 853 in WOBHH–Nil maintained SOC, while both 1831 in WOBHH–F and 1714 in WOBHH–M increased SOC. After 51 yr, WOBHH–M had 25 Mg ha −1 more SOC than did WF–Nil. Because of contrasting decay rates and root/shoot ratios, C input needed to maintain the original SOC was twofold greater in WF than in WOBHH, which required a fourfold increase in NAGCP to attain these inputs. A three‐compartment model fitted to the data suggested loss of C from the active compartments and gain of C by the passive compartments. Inputs of C that maintained SOC over 51 yr would lead to a steady state of 2.9 times more C than in 1939, and 26% higher than the native SOC content. Return of 30% of the crop C as manure would sustain SOC sequestration in all WOBHH rotations with NAGCP > 400 kg ha −1 yr −1 and in those WF rotations with NAGCP > 1000 kg ha −1 yr −1