The impact of long-term elevated CO2 on C and N retention in stable SOM pools
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Elevated atmospheric CO2 frequently increases plant production and concomitant soil C inputs, which may cause additional soil C sequestration. However, whether the increase in plant production and additional soil C sequestration under elevated CO2 can be sustained in the long-term is unclear. One approach to study C–N interactions under elevated CO2 is provided by a theoretical framework that centers on the concept of progressive nitrogen limitation (PNL). The PNL concept hinges on the idea that N becomes less available with time under elevated CO2. One possible mechanism underlying this reduction in N availability is that N is retained in long-lived soil organic matter (SOM), thereby limiting plant production and the potential for soil C sequestration. The long-term nature of the PNL concept necessitates the testing of mechanisms in field experiments exposed to elevated CO2 over long periods of time. The impact of elevated CO2 and 15N fertilization on L. perenne and T. repens monocultures has been studied in the Swiss FACE experiment for ten consecutive years. We applied a biological fractionation technique using long-term incubations with repetitive leaching to determine how elevated CO2 affects the accumulation of N and C into more stable SOM pools. Elevated CO2 significantly stimulated retention of fertilizer-N in the stable pools of the soils covered with L. perenne receiving low and high N fertilization rates by 18 and 22%, respectively, and by 45% in the soils covered by T. repens receiving the low N fertilization rate. However, elevated CO2 did not significantly increase stable soil C formation. The increase in N retention under elevated CO2 provides direct evidence that elevated CO2 increases stable N formation as proposed by the PNL concept. In the Swiss FACE experiment, however, plant production increased under elevated CO2, indicating that the additional N supply through fertilization prohibited PNL for plant production at this site. Therefore, it remains unresolved why elevated CO2 did not increase labile and stable C accumulation in these systems.
KeywordsElevated CO2 N-15 Labile and stable SOM pools Soil N retention Soil C sequestration
Free air carbon dioxide enrichment
Soil organic matter
Progressive nitrogen limitation