Soil carbon saturation controls labile and stable carbon pool dynamics

Recently, it has been suggested that soil organic C (SOC) does not always respond linearly to increasing C input, thereby limiting the rate and efficiency of C stabilization in soils. Therefore, we postulated that when a soil is exposed to a broad range of C inputs through a range of manure treatments, it will show C saturation behavior and different SOC pools will saturate at different rates. To test this, different SOC pools were isolated by physical fractionation techniques from a long-term agricultural experiment in Lethbridge, Canada. In this experiment, manure has been applied since 1973 at rates of 0, 60, 120, and 180 Mg ha(-1) yr(-1) (wet weight). In the total mineral soil as well as the small macroaggregates (250-2000 mu m), microaggregates (53-250 mu m), and the silt plus clay fraction (<53 mu m), an increase was observed in SOC contents with an increase in manure application rate to 120 Mg ha(-1)yr(-1). However, no additional C was sequestered when the manure application rate was augmented to 180 Mg ha(-1) yr(-1), indicating C saturation in these SOC pools. Large macroaggregates (>2000 mu m) were the only water-stable aggregate fraction that increased in C content across all manure input levels. Further physical separation of macroaggregates into subpools by microaggregate isolation showed that coarse (>250 mu m) particulate organic matter (POM) was the fraction that accounted most for the increase in C content of the large macroaggregates. Furthermore, the turnover of large macroaggregates increased with increasing manure applications, as indicated by decreased formation and stabilization of intramicroaggregate POM within the large macroaggregates. We conclude that as C input increases, the mineral fraction of a soil saturates and consequently additional C input will only accumulate in labile soil C pools that have a relatively faster turnover.