1. Daily net carbon exchange as a mediator of heterotrophic soil respiration across two forest chronosequences Jared L. DeForest, Asko Noormets, and Jiquan Chen Earth, Ecological, & Environmental Sciences University of Toledo

2. GPP Soils 1500 Gt C Land Plants 560 Gt C Atmosphere 750 Gt C 120 Gt C yr -1 38 Gt C yr -1 82 Gt C yr -1 Ecosystem Respiration

3. Root (~50%) 82 Gt C yr -1 Heterotrophic (~50%) Belowground Respiration

4. 82 Gt C yr -1 38 Gt C yr -1 Belowground Respiration

5. 38 Gt C yr -1 Heterotrophic Respiration Root Exudates Soil Organic Matter Plant Litter

6. Sources of Heterotrophic Respiration

7. Plant productivity has the potential to explain daily variation in R h Increased photosynthesis Faster fine root activity More root exudates More C available for heterotrophic metabolism Increased R h

8. Plant productivity has the potential to stimulate soil organic matter decomposition More C available for heterotrophic metabolism More decomposition of recalcitrant SOM More energy available for enzyme production Because the decomposition of most SOM is an energy demanding process, an influx of labile C can stimulate SOM decay. The decay of lignin requires nitrogen starvation and a supply of labile carbon.

9. Hypothesis: Daily NEP will have a greater influence on R h in faster growing ecosystems. Objectives: To quantify the influence daily NEP has on R h Provide evidence that increases in daily NEP can stimulate soil organic carbon decomposition.

10. Study Site Chequamegon Nation Forest (northwestern Wisconsin) Two Forest Chronosequences Hardwoods Red Pine Three Age Groups Young Intermediate Mature

11. R E = Ecosystem Respiration R h = Heterotrophic Respiration R a = Autotrophic Respiration (0.67R E ) NCE = Net Carbon Exchange GEP = Gross Ecosystem Productivity (NEP - R E ) NPP = Net Primary Productivity (GEP – R a )

12. NCE Influence on R h : Hardwoods

13. NCE Influence on R h : Red Pine

15. Caveats R E = Often underestimated from nocturnal fluxes R a = Annually estimates at 67% of R E Can we apply for both forests and on a daily timeframe? NCE = Strong influence, almost guarantees a good correlation. NPP/GEP = Fixed or variable?

16. Respiration Partitions Red PineHardwoods

17. NPP/GEP fixed at: 0.5OK Fit Site Monthly MeansWorst Fit Site Daily MeansBest Fit Linear Rh = m * NEP + b Highest r 2 Logarithmic Rh = m * ln(NEP) + b Smaller r 2 Alternative Analysis

18. Carbon Use Efficiency

19. Summary NCE influence on R h decreases with forest age No evidence that suggests significant increases in NCE can stimulate SOM decomposition Estimated SR Ecosystem correlates well with SR Field rates R a is likely higher than 67% in red pine forests

20. Daily fluctuation in NEC can help explain variations in soil respiration rates Results support the idea of an ecosystem- scale link between autotrophic and heterotrophic activity Conclusions