The impact of long-term elevated CO2 on C and N retention in stable SOM pools

Abstract

Elevated atmospheric CO 2 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 CO 2 can be sustained in the long-term is unclear. One approach to study C-N interactions under elevated CO 2 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 CO 2 . 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 CO 2 over long periods of time. The impact of elevated CO 2 and 15 N 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 CO 2 affects the accumulation of N and C into more stable SOM pools. Elevated CO 2 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 CO 2 did not significantly increase stable soil C formation. The increase in N retention under elevated CO 2 provides direct evidence that elevated CO 2 increases stable N formation as proposed by the PNL concept. In the Swiss FACE experiment, however, plant production increased under elevated CO 2 , indicating that the additional N supply through fertilization prohibited PNL for plant production at this site. Therefore, it remains unresolved why elevated CO 2 did not increase labile and stable C accumulation in these systems.

The rate of progression and stability of progressive nitrogen limitation at elevated atmospheric CO2 in a grazed grassland over 11 years of Free Air CO2 enrichment

“…In addition, there was significantly less N taken up from the soil pool in the soil that had been taken from the elevated CO 2 rings after 4 years of enrichment. In another long running FACE experiment on grassland in Switzerland, in this case fertilised with two rates of N (14 or 56 gN m -2 ); the N harvested at the lower fertiliser rate was reduced under elevated CO 2 ( Schneider et al 2004) and in a labelling experiment de Graaff et al (2008) saw that more of the fertiliser N was stabilised in soil organic matter pools at elevated CO 2 i.e. less was available to the plants; a result that parallels the Allard et al (2006) findings for our experiment.…”

Effects of climate change and elevated atmospheric CO2 on soil organic carbon: a response equation

“…Soil erosion reduces the soil nutrient level, whereas soybean can fix atmospheric N and mitigate the nutrient limit; effects of soil erosion on reducing SOC content and changing soil water holding capacity are stronger in soils with soybean and corn covers than in soil covered by grass, which contains a larger amount of SOC. Compared with soybean and grass, there was a more severe soil N limit for corn with a relatively severe soil erosion; the negative effect of precipitation on SOC for corn with severe soil erosion (Table 2) may be attributable to the fact that soil N leaching by precipitation aggravates soil N limit, and decreases plant production and SOC concentration in that case (Graaff et al 2008). These mechanisms have been installed in CENTURY.…”

Applicability of soil column incubation experiments to measure CO2 efflux

“…In the present study, moisture and temperature, important factors affecting soil respiration, were controlled within a relatively stable range, and thus the effects of tillage on soil CO 2 efflux could be accurately estimated. Rather than temperature and moisture conditions, other experimental conditions (eg climate factors, agricultural managements, soil gas state) can be created or precisely controlled in column incubation experiments (Case et al, 2014;De Graaff et al, 2008). In the investigation of the influence of climate factors or human activity on CO 2 release, incubation experiments are preferred if special or precise experimental conditions are required.…”