Sequestering Carbon in Cropping and Pasture Systems

Transcription

1 Sequestering Carbon in Cropping and Pasture Systems Alan J. Franzluebbers Ecologist Raleigh NC

2 Soil functions mediated by conservation cropping and pasture management 1. Sustaining viable plant cover 2. Cycling and retaining globally important nutrients a. Storing N in soil and releasing it for later root uptake b. Storing C in soil and reducing atmospheric CO 2 3. Supporting efficient cycling of water and nutrients 4. Protecting water quality 5. Providing physical stability to the landscape 6. Enabling animal habitat and promoting biodiversity 7. Buffering against toxic element accumulation and transport

3 Carbon cycling / sequestration Atmospheric CO 2 Photosynthesis Processes Photosynthesis CO 2 Dissolved CO 2 in water Carbonate minerals Animal respiration Plant respiration Soil respiration Soil organisms Soil organic C Biochemical transformations Harvest Respiration Plant Animal Soil microbe Fossil fuels Soil erosion Leaching

4 Conservation agricultural systems Guiding principles: Minimizing soil disturbance, consistent with sustainable production practices Maximizing soil surface cover by managing crops, pastures, and crop residues Stimulating biological activity through crop rotations, cover crops, and integrated nutrient and pest management Minimizing soil disturbance Maximizing permanent surface cover Stimulating biological activity

5 Soil organic carbon by land use Study Depth (cm) Forest Grass Crop Mg ha Significance Pr > F Eastern Texas Laws and Evans (1949), Potter et al. (1999) AL-AR-FL-GA-LA-MS- NC-SC-TX-VA McCracken (1959) Maryland Islam and Weil (2000) > > > < Alabama Fesha et al. (2002), Torbert et al. (2004) Mississippi, Georgia Rhoton and Tyler (1990), Franzluebbers et al. (2000) > > > > Mean a 47.4 a 31.1 b Franzluebbers (2005) Soil Till. Res. 83:

6 Soil organic carbon by land use Soil Organic Carbon (g. kg -1 ) Sequestration of SOC (Mg ha -1 yr -1 ) ** Soil Depth (cm) Conventional Tillage Cropping Pasture ( years) 0.17 ** 0.05 ns -20 Survey of 29 farm locations in AL, GA, SC, NC, VA 0-20 cm 0.74 ** Causarano et al. (2008) Soil Sci. Soc. Am. J. 72:

7 Cropping intensity Crop Residue (Mg. ha -1 ) On Surface Soil Depth (cm) Colorado 8-yr study NT wheat-fallow NT wheat-corn-millet-fallow More intensive (productive) systems have greater potential for C input Data from Ortega et al. (2002) Agron. J. 94:

8 Cropping intensity 10 8 Estimated Carbon Input (Mg. ha -1. yr -1 ) Texas 10-yr study Rotations and tillage F.M. Hons (P.I.) Soybean Conventional Tillage Sorghum Wheat No Tillage Sorghum - wheat / soybean Wheat / soybean Cropping Intensity (fraction of year that a crop is in the field) Franzluebbers et al. (1998) Soil Till. Res. 47:

9 Cropping intensity Soil Microbial Biomass Carbon (kg. ha -1 ) No Tillage Adjacent long-term pasture = 3360 Conventional Tillage Texas 10-yr study Rotations and tillage F.M. Hons (P.I.) 500 Soybean Sorghum Wheat Sorghum - wheat / soybean Wheat / soybean Cropping Intensity (fraction of year that a crop is in the field) Franzluebbers et al. (1998) Soil Till. Res. 47:

10 Soil organic carbon response to inputs 0.25 No Tillage 0.20 Soil Organic Carbon Sequestration (g SOC. g -1 C input) Conventional Tillage Texas Weswood SiCL Ustic Udifluvent 10-yr study Different rotations SOY SOR WHT ROT W/S Cropping Intensity (fraction of year) Franzluebbers et al. (1998) Soil Till. Res. 47:

11 Soil organic carbon sequestration in the southeastern USA Conservation tillage compared with conventional tillage cropland Greenhouse Gas Contributions and Mitigation Potential in Agricultural Regions of North America Special issue (Vol 83, August 2005) Property AL GA MD MS NC SC TX VA Mean Number of comparisons Duration of comparison (years) Soil depth (cm) Soil organic C with conventional tillage (Mg ha -1 ) Soil organic C with no tillage (Mg ha -1 )

12 Distribution of observations Conservation tillage Y = 0.93 / (1 + e -((x ) / 0.176) ) Probability Density Median Rate 0.32 Mg C ha -1 yr -1 32% chance 0.5of achieving at least 0.50 Mg C ha 0.0 yr Soil Organic Carbon Sequestration (Mg C. ha -1. yr -1 ) 1.0

13 Soil organic carbon response to no tillage and cover cropping Soil Organic Carbon Sequestration in the Southeastern USA Mg C ha -1 yr -1 ( without cover cropping) n = 60 Photos of 2 no-tillage systems in Virginia USA Mg C ha -1 yr -1 ( with cover cropping) n = 87

14 Time needed for conservation systems to mature Data from multiple sources Reported in Franzluebbers (2005) Soil Till. Res. 83:

15 An investment for the future Total Soil Nitrogen Accumulation (kg. ha -1. yr -1 ) TSN = (SOC) r 2 = Soil Organic Carbon Sequestration (kg. ha -1. yr -1 ) Franzluebbers and Stuedemann (2010) Soil Sci. Soc. Am. J. 74:

16 Impacts of animals on soil organic carbon and compaction

17 Distribution of observations in the southeastern USA Y = 0.96 / (1 + e -((x ) / 0.313) ) Pasture establishment n = 35 Probability Density Median Rate 0.65 Mg C ha -1 yr % chance of 0.5 achieving >0.50 Mg C ha -1 yr Soil Organic Carbon Sequestration (Mg C. ha -1. yr -1 )

18 Impact of animals on soil Soil compaction Soil compaction reduces porosity, thereby limiting air and water storage and transport which alters nutrient cycling and exploration potential of plant roots. Compaction responses are often determined with: 1. Bulk density 2. Penetration resistance

19 Impact of animals on soil How extensive is compaction in grazed pastures? Poaching of soil with heavy animal traffic can damage forage and cause soil compaction leading to reduced infiltration, greater water runoff, and contamination of receiving water bodies with nutrients and fecal-borne pathogens In a review of grazing effects on bulk density [Greenwood and McKenzie (2001) Aust. J. Exp. Agric. 41: ], an increase in bulk density was observed with animal treading in most studies: Mg m -3 (n = 46) This situation represents an extreme treading condition, not what would be typical for judiciously managed pastures

20 Impact of animals on soil Do cattle always compact soil? End of 12 years of bermudagrass / tall fescue management in Georgia Soil Bulk Density (Mg m -3 ) 0-20-cm depth Unharvested 1.42 Low Grazing Pressure 1.40 High Grazing Pressure 1.41 Hayed 1.44 Soil Depth (cm) Soil Bulk Density (Mg. m -3 ) *** ** -5 Unharvested Low Grazing Pressure High Grazing Pressure Hayed Franzluebbers and Stuedemann (2010) Soil Sci. Soc. Am. J. 74:

21 Impact of animals on soil Short-term grazing of cover crops On silt loam and silty clay loam soils (Mollisols) in Iowa, soil bulk density was not affected by monthly rotational grazing of corn stalks during the winter, irrespective of whether soil was frozen or not [Clark et al. (2004) Agron. J. 96: ]. On Mollisols in Argentina, soil bulk density increased with winter grazing of corn and soybean residues, but it depended on tillage system:. Ungrazed Grazed Mg m -3 CT 1.17 < 1.34 NT Diaz-Zorita et al. (2002) Soil Till. Res. 65:1-18

22 Impact of animals on soil Soil aggregation Stabilizes soil surface against the energy input of rainfall and traffic (equipment and animals) Creates sufficient porosity for retention and transport of water and air Protects soil organisms from predation and rapid decomposition of organic matter

23 Impact of animals on soil Temporal dynamics of soil aggregation Mean-weight diameter of soil increased with establishment of a forage legume (alfalfa) that left soil undisturbed and enriched in soil organic matter. A relatively short time period (3-4 yr) was required to enrich the soil. Angers (1992) Soil Sci. Soc. Am. J. 56:

24 At the end of 3 years on a sandy clay loam soil (Ultisol) in Georgia, stability of aggregation was similar whether cover crops (winter and summer cover crops) were grazed by cowcalf pairs for 1 ½ months each year [Franzluebbers and Stuedemann (2008) Soil Till. Res. 100: ]. Impact of animals on soil Grazing of crop residues and cover crops On silt loam and silty clay loam soils (Mollisols) in Iowa, soil aggregate stability was unaffected by monthly rotational grazing of cow-calf pairs on corn stalks in the winter [Clark et al. (2004) Agron. J. 96: ].

25 Impact of animals on soil Summary of soil aggregation responses Grazing cattle under moderate stocking conditions will have little impact on stability of soil aggregation. Presence of grass roots and surface residue appears to be more important for aggregation than the presence of grazing animals. A recent literature review on stocking rate effects on aggregate size and stability [Greenwood and McKenzie (2001) Aust. J. Exp. Agric. 41: ] suggests a generally negative response to animal grazing But most responses were weak or related to intense treading.

26 Soil Organic Carbon (g. kg -1 ) Transition from perennial pasture to cropping -20 Conventional tillage Initially high surface C Soil Depth (cm) *** *** *** *** *** *** No tillage *** *** Initiation End of 1 year End of 2 years Following moldboardplow tillage, soil organic C became relatively uniformly distributed with depth Soil organic C with NT was greater than with CT in the surface 6 cm, but lower than with CT below 12 cm

27 Environmental responses Soil organic C by depth and time cm 3-6 cm 6-12 cm cm cm 12 Soil Organic Carbon 8 (Mg. ha -1 ) 4 LSD 0.05 *** *** *** No tillage ungrazed grazed Conventional tillage ungrazed grazed *** Years of Management End of 2 years Franzluebbers and Stuedemann (2008) Soil Sci. Soc. Am. J.

28 Environmental responses Soil C stock change with tillage and cover crop management 50 Soil Organic Carbon (Mg. ha -1 ) [0-30 cm] SOC (NT CT) = 0.5 Mg C ha -1 yr Years of Management (NT-G) 0.22 Mg/ha/yr (NT-U) 0.18 Mg/ha/yr (CT-G) Mg/ha/yr (CT-U) Mg/ha/yr No tillage ungrazed grazed Conventional tillage ungrazed grazed Franzluebbers and Stuedemann (2008) Soil Sci. Soc. Am. J.

29 Time effects on soil organic carbon Soil Organic 8 Carbon Accumulation (Mg. ha -1 6 ) Soil 0.8 Organic Carbon 0.6 Accumulation (Mg. ha -1. yr -1 ) Years of Management Impact Saturation to a theoretical threshold is likely (determined by climate, soil type, etc.) Years of Management

30 Fertilizer and time effects on soil organic carbon Grazed Coastal bermudagrass Soil Organic Carbon (Mg. ha -1 ) Mean yearly change (Mg ha -1 yr -1 ) Inorganic Clover + inorganic Broiler litter 0-6 cm soil 16 Impact Fertilizer sources were equally effective in sequestering soil organic C Years of Management Franzluebbers et al. (2001) Soil Sci. Soc. Am. J. 65:

31 Manure effect on soil organic carbon From a compilation of available literature around the world (Conant et al., 2001, Ecol. Appl. 11: ), SOC sequestration was compared between inorganic and organic fertilization. Rate of SOC Sequestration Management (Mg ha -1 yr -1 ) Inorganic fertilizer 0.29 Organic fertilizer 0.28

32 Pasture management effect on soil organic carbon Long-term pasture survey (15- to 19-year old fields, 3 each) Soil Organic Carbon (g. kg -1 ) Soil Depth (cm) Carbon Stock (Mg. ha -1 ) Grazed bermudagrass Soil (0-20 cm) Hayed bermudagrass *** b a Surface residue Surface residue Soil (0-20 cm) Difference 7.5 Mg ha -1 Franzluebbers et al. (2000) Soil Biol. Biochem. 32:

33 Pasture management and time effects on soil organic carbon Grazed Jesup tall fescue Soil Organic Carbon Mean yearly change (Mg ha -1 yr -1 ) Hayed 1.29 Grazed 1.52 (Mg ha -1 ) 0-20 cm Impact Grazing returns carbon to soil to build fertility Years of Management Franzluebbers et al. (in press)

34 Pasture management and time effects on soil organic carbon Soil Organic Carbon (Mg. ha -1 ) [0-6 cm] High Grazing Pressure (Low Forage Mass) LSD 0.05 Low Grazing Pressure (High Forage Mass) Unharvested Cut for hay Yearly Accumulation Rate (Mg ha -1 yr -1 ) Years of Management Franzluebbers and Stuedemann (2010) Soil Sci. Soc. Am. J. 74:

35 Spatial effects on soil organic carbon At the end of 5 years of grazing Coastal bermudagrass in the summer cm 6-12 cm Surface residue Standing Stock of C (Mg. ha -1 ) a b 3-6 cm b a b cm a a b a a a a a Total C a Franzluebbers and Stuedemann (2010) Soil Sci. Soc. Am. J. 74: b b Distance from Shade (m) b 44 b

36 Summary Establishment of perennial grass pastures can sequester soil organic C at rates of 0.25 to 1.25 Mg C ha -1 yr -1 Soil organic C sequestration rate can be affected by: Forage type Fertilization Forage utilization Animal behavior Soil sampling depth

37 Summary We can expect positive changes in soil aggregation, nutrients, and organic matter under grass-based systems Extent dependent on environment and previous conditions of land Negative and positive changes in soil porosity, infiltration, and organic matter can occur with animal grazing Dependent on the balance between carrying capacity and stocking density

38 Conclusions Sequestration of SOC under grassland management systems is significant Balance needed with yet-to-be-defined CH 4 and N 2 O emissions Rate of SOC sequestration under the wide diversity of pasture conditions in the USA still largely unknown Variations in management, climate, soils, etc. Greater collaboration is needed to efficiently utilize limited resources and better understand the impacts of diverse conditions on SOC sequestration Among plant, animal, soil, and water science disciplines Long-term field studies need conceptual and financial support