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 Br eton, Canada, on a Typic Cryoboralf: ii) wheat (Triticum aestivum L.)-fallo w (WF) and (ii) wheat-oat (Avena sativa L.)-barley. (Hordeum vulgare L,)-ha y (primarily alfalfa, Medicago sativa L.)-hay (WOBHH), in factorial combina tion with three Fertility levels: no added fertilizer [Nil], N-P-K-S fertil izers [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 WOB HH-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 thr ee-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 m aintained SOC over 51 yr would lead to a steady state of 2.9 times more C t han in 1939, and 26% higher than the native SOC content, Return of 30% of t he 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 > 10 00 kg ha(-1) yr(-1).