Introduction

1. Soil Organic Carbon and Nitrogen Accumulation of Rhizoma Perennial Peanut and Bahiagrass Grown under Elevated CO2 and Temperature Leon H. Allen,ARS-FL Stephan L. Albrecht,ARS-OR Kenneth J. Boote,UF Jean M.G. Thomas,UF and Katherine Skirvin ARS-OR USDA-ARS and University of Florida

2. Introduction More work has been done on carbon accumulation in forests and natural grasslands than in managed grasslands, especially in the Southeastern USA

3. Hypotheses • 1. Shift from cultivated land to forage crops will increase soil organic carbon (SOC) and nitrogen (SON). • 2. Accumulation of SOC and SON will be enhanced by elevated CO2 and diminished by elevated temperatures. • 3. Forage species will affect SOC and SON responses.

4. Objectives • Measure SOC and SON accumulation of two contrasting perennial forage species, rhizoma perennial peanut (PP), C3 legume, and bahiagrass (BG), C4 grass to test hypotheses.

5. Materials and Methods-1 • Two forage crops • Rhizoma perennial peanut (Arachis glabrata) • Bahiagrass (Paspalum notatum) • Four temperatures tracking ambient • Baseline, +1.5, +3.0, and +4.5°C • Approx +1.5, +3.0,+4.5, +6.0 °C above ambient • Two CO2 concentrations, 360 and 700 ppm

6. Materials and Methods-2 • In April 1995, plants established in field soil in Temperature-Gradient Greenhouse (TGG) • Fertilized and irrigated well

7. Materials and Methods-3 • Temperature gradients of 4.5 Celsius were maintained with variable speed ventilation fans and on-off heaters. • CO2 was controlled with injection of gas and measurement of concentrations down wind in the TGGs for feedback control.

8. CONTROLLED VENTILLATION FAN BG PP CELL #4 WARM B + 4.5°C BG PP CELL #3 B + 3.0°C BG PP PLOTS ARE 5 m x 2 m CELL #2 B + 1.5°C BG PP AIR FLOW DIRECTION CELL #1 BASELINE AMBIENT Baseline, B AIR INTAKE

10. Materials and Methods-4 • Herbage was harvested four times each year (Boote et al., 1999; Fritschi et al., 1999a, 1999b; Newman et al., 2001, 2005). • In 1996 and 1997, measurements of biomass of belowground components were made.

11. Materials and Methods-5 • Four replicated soil samples were collected from the top 20 cm of each plot in Feb. 1995 and each year thereafter. • Soil samples were dried and plant fragments were separated using a 2.2-mm sieve.

12. Materials and Methods-6 • Total C and N were determined at Pendleton Oregon with a Thermo-Finnigan Flash EA 1112 CNS analyzer at 1800 Celsius

13. Materials and Methods-7 • Data from the beginning and the end of the experiment analyzed by SAS ANOVA to determine overall effects of conversion from cropped land to forages on SOC and SON. • Differences of SOC and SON between final and initial years were analyzed by SAS ANOVA to determine the effects of CO2, temperature, and forage species on 6-year increments of SOC and SON

14. Results and Conclusions

15. 1. Overall Effect of Forage on SOC and SON

18. Overall Effect of Forage on SOC and SONAcross the whole 6-year period: • Overall SOC increased 1.08 g/kg (26%) • Overall SON increased 0.095 g/kg (34%) • Hypothesis that conversion from cultivated land to forages will enhance SOC and SON is supported

19. 2. Species Effect on Increase of SOC and SON

22. Species Effect on Increase of SOC and SON • SOC increased by 0.75 g/kg for PP • SOC increased by 1.40 g/kg for BG • BG/PP ratio = 1.87 for SOC • BG/PP ratio = 1.46 for SON • Conclusion: Growth of BG promotes more SOC accumulation than PP, but with relatively less SON accumulation <&>

23. 3. CO2 Effect on Increase of SOC and SON

26. CO2 Effect on Increase of SOC and SON • SOC increase = 0.94 g/kg for 360 ppm • SOC increase = 1.20 g/kg for 700 ppm • SON increase = 0.084 g/kg for 360 ppm • SON increase = 0.112 g/kg for 700 ppm

27. CO2 Effect on Increase of SOC and SON • 700/330 ratio = 1.27 for SOC • 700/330 ratio = 1.13 for SON • Conclusion: Elevated CO2 promotes relatively more SOC accumulation than SON

28. 4. Temperature Effect on Increase of SOC and SON

31. Temperature Effect on Increase of SOC and SON • SOC increased 1.12, 1.21, 0.97, and 0.92 g/kg at the increasing temperatures • SON increased 0.104, 0.106, 0.087, and 0.079 g/kg at the increasing temperatures • Conclusion: Accumulation of SOC and SON decreases with increasing temperature only at 1.5 to 3 Celsius above Gainesville ambient <&>

32. 5. Species X CO2 Interaction on Increase of SOC and SON

34. Species X CO2 Interaction on Increase of SOC and SON • SOC increase = 0.54 g/kg for PP at 360 • SOC increase = 0.95 g/kg for PP at 700 • SOC increase = 1.34 g/kg for BG at 360 • SOC increase = 1.45 g/kg for BG at 700 • Conclusion #1: Increase of SOC was greater for BG than PP

35. Species X CO2 Interaction on Increase of SOC and SON • SOC ratio of PP: 700/360 = 1.74 • SOC ratio of BG: 700/360 = 1.10 • Conclusion #2: Elevated CO2 caused much greater increase of SOC for PP than BG

37. Species X CO2 Interaction on Increase of SOC and SON • SON increase = 0.0655 g/kg for PP at 360 • SON increase = 0.0875 g/kg for PP at 700 • SON increase = 0.112 g/kg for BG at 360 • SON increase = 0.112 g/kg for BG at 700 • Conclusion #1: Increase of SON was somewhat greater for BG than PP

38. Species X CO2 Interaction on Increase of SOC and SON • SON ratio of PP: 700/360 = 1.34 • SON ratio of BG: 700/360 = 1.00 • Conclusion #2: Elevated CO2 caused no increase of SON for BG • Conclusion #3: Elevated CO2 caused less increase of SON than of SOC for PP

39. Comparisons of Belowground Biomass with SOC Accumulation

40. BELOWGROUND BIOMASS of PP and BG vs. CO2 --------------------------------------------------------------------------------------------------------- VARIABLE PERENNIAL PEANUT BAHIAGRASS 360 ppm 700 ppm 360 ppm 700 ppm --------------------------------------------------------------------------------------------------------- - - - - - - - - - - - - - - Biomass, g m-2 - - - - - - - - - - - - - Rhizome or Stolon 1996 697 893 1066 1178 1997 1097 1326 1537 1727 Root 1996 73 71 622 593 1997 100 86 692 674 Total belowground 1996 770 964 (1.25) 1688 1771 (1.05) 1997 1197 1412 (1.18) 2229 2401 (1.08) Belowground ratio, BG/PP, at 360 and 700 ppm 1996 2.19 1.84 1997 1.86 1.70 --------------------------------------------------------------------------------------------------------- Adapted from Boote et al. (1999). Data in parenthesis are 700/360 ratios.

41. ANNUAL HERBAGE YIELD of PP and BG vs. CO2 ------------------------------------------------------------------------------------------------------------- VARIABLE PERENNIAL PEANUT BAHIAGRASS 360 PPM 700 PPM 360 PPM 700 PPM ------------------------------------------------------------------------------------------------------------- - - - - - - - - - - - - - - Biomass, g m-2 - - - - - - - - - - - - - Total herbage biomass 1996 1320 1680 (1.27) 880 1020 (1.16) 1997 1460 1870 (1.28) 740 910 (1.23) 1998 1850 2280 (1.23) 710 780 (1.10) Ratio, BG/PP, at 360 and 700 ppm 1996 0.67 0.61 1997 0.51 0.50 1998 0.38 0.34 ------------------------------------------------------------------------------------------------ Adapted from Boote et al. (1999) and Newman et al. (2001). Data in parenthesis are 700/360 ratios.

42. Comparisons of Belowground Biomass with SOC Accumulation • Herbage Yields were greater for PP than for BG. • However, both belowground biomass and SOC accumulation were greater for BG than for PP.

43. Conclusions • Conversion of cultivated land to forage crops could sequester more SOC. • BG has the potential to sequester more carbon than PP. • C/N ratio appears to be higher in BG than PP • PP, a C3 legume, responds more to CO2 than BG in SOC accumulation and herbage yield.

44. Comparisons with other data • SOC accumulation = 540 kg/ha per year • Without CO2 effect = 425 kg/ha per year • Albrecht (1938) = 380 kg/ha per year • Potter et al. (1999) = 450 kg/ha year • Allen & Nelson = 370 kg/ha per year for PP, which is lower than for grasslands.

45. END