Carbon Sequestration Potential of Silvopastoral and Other Land Use Systems in the Chilean Patagonia

Abstract

This study was undertaken to quantify the carbon (C) sequestration potentials in three predominant ecosystems on the volcanic soils in Patagonia, Chile. The systems were: Pinus ponderosa Dougl. ex P. Laws. – based silvopastoral systems arranged in strips (silvopasture), 18-year-old managed exotic stands (plantation), and natural prairie (prairie), in Patagonia, Chile. Most of the data used in the construction of C models were derived from experimental plots, where litterfall, decomposition, soil respiration, and soil C were measured. The values for greenhouse gas (GHG) emissions by cattle and fertilizer application were obtained from the literature. In the silvopasture and the plantation, total above- and belowground tree C stock accounted for 69% and 64% of the total system C, respectively. Total above- and belowground C pools were 224, 199, and 177 Mg C ha⁻¹, with the aboveground: belowground C pool ratios of 1:10, 1:5, and 1:177, respectively, for silvopasture, plantation, and prairie. Soil respiration decreased in the order prairie >silvopasture >plantation. The C leached beyond the root zone (in leachate collected at 80 cm soil depth) decreased in the order plantation >prairie >silvopasture. Estimated system net C flux was +1.8, +2.5, and −2.3 Mg C ha⁻¹ year⁻¹ for the silvopasture, plantation, and prairie, respectively. Based on this study it is estimated that establishing silvopastoral systems with cattle over a land area of approximately 481 km² or 0.33% of the Chilean Patagonia territory would be adequate to offset all C losses from cattle-based livestock systems.

Appendix

The following assumptions and calculations were used in order to build the models of carbon pools and fluxes within the pine-based silvopastoral system arranged in strip, the 18-year old Pinus ponderosa plantation, and the managed natural prairie:

• 1. The silvopasture and plantation have a tree density of 400 and 800 stems ha⁻¹, respectively. Pine strips in the silvopasture occupy 22% of the area available for pasture and have never been thinned. All trees were pruned to heights of 2.8 and 4 m in 2006 and 2009, respectively.

• 2. Using inventory data obtained since the establishment of the pine plantations, the mean above and belowground C sequestration rates were calculated for the 1991–2003 period at the initial tree density, and then for the 2003–2009 periods after thinning to a density of 800 trees ha⁻¹ in the Plantation and 400 trees ha⁻¹ in the silvopasture.

• 3. Aboveground tree C pools include trunks, branches, twigs, needles and cones. Belowground C pools include thick roots superior to 5 mm diameter.

• 4. It was assumed that cattle will consume most of the aboveground pasture biomass produced during the year in the silvopasture and prairie (part of it returning to the system as faeces, methane, nitrous oxide, and respiration) and that only belowground biomass C will be added to the soil C pool. The aboveground grass biomass present in the plantation remains in the system as no grazing occurs therein. The C content of aboveground net primary productivity (ANPP) was determined after manually harvesting grazing material three times a year over a 2-year period. The C content of belowground net primary productivity and net annual C sequestration by pasture alone could then be calculated, considering a 40% addition to the recalcitrant soil C pool (Falk 1976).

• 5. Knowing the stocking rates and the amount of faeces produced annually and C content, the cattle respiration (kg CO₂ ha⁻¹ year⁻¹), methane emissions from enteric fermentation, and nitrous oxide emissions from dung and urine patches, as well as their CO₂-equivalents (IPCC 2001) were calculated using data published by Flessa et al. (2002), Yang et al. (2003), and Byrne et al. (2007). Carbon dioxide, methane, and nitrous oxide emissions from a single animal are estimated to be 996, 56, and 1.29 kg head⁻¹ year⁻¹, respectively, and depend on the amount and kind of feed that is consumed. The reference weight per head unit is 500 kg. CO₂-equivalents were calculated using the Global Warming Potentials (GWP), which determine the relative contribution of a given gas to the greenhouse effect. The GWP values represent how many times more deleterious than CO₂ in a 100 year period are CH₄ (21) and N₂O (310) in terms of global warming. In addition, the number of cows per hectare in order to attain C neutrality was calculated as follows, using the net C sequestration values of 3.80 and 1.09 Mg C ha⁻¹ year⁻¹ for the silvopasture and prairie. For the silvopasture, 3.80 = [(56*#cows/1,000*21)/3.67) + (996*#cows/1,000/3.67) + (1.29*#cows/1,000*310)/3.67)]. For the Prairie, 1.09 = [(56*#cows/1,000*21)/3.67) + (996*#cows/1,000/3.67) + (1.29*#cows/1,000*310)/3.67)].

• 6. The annual mass loss values of cattle faeces obtained after a 12 month litterbag decomposition experiment made possible the calculation of net C additions to soil C pools, considering that 2.2 and 2.5 years are required to get a maximum decomposition in the silvopasture and prairie, respectively. The net addition to soil C pool in each treatment represents therefore the sum of annual C incorporations over these periods. Hirata et al. (2009) reported similar results, where cattle dung reached an average decomposition of 79.1% after 2.2 years.

• 7. Annual litterfall and needle decomposition in the plantation and the silvopasture were obtained from field measurements over a 2-year period to illustrate the importance of annual C inputs and net additions to soil C pools. It was assumed that annual fine root C turnover in pines is 30% of litterfall (Abohassan 2004).

• 8. Since time constraints did not permit to undertake a long term experiment for the decomposition of the ponderosa pine needles, a maximum mass loss of 84.5% for Scots pine needles in Scandinavia was assumed to be representative of the situation, considering the similar climatic conditions encountered and values of initial N and lignin contents found in green litter (Berg and Laskowski 2006; Dube 2010). Theoretically, the C contribution to soil from litterfall and fine root turnover for the last 18 years was 25 and 32 Mg C ha⁻¹ in silvopasture and plantation, respectively. However, an average of 14.2% in the silvopasture and 14.5% in the plantation of the C added annually via litterfall and fine roots was released back into the atmosphere through microbial decomposition. Based on the results of the decomposition experiment, approximately 6 years (84.5%/14.2% year⁻¹ in the silvopasture and 84.5%/14.5% year⁻¹ in the plantation) would be required to obtain maximum needle decomposition. This represents 0.20 Mg C ha⁻¹ year⁻¹ (25 Mg C ha⁻¹/18 years * 14.2% year⁻¹) in the silvopasture and 0.26 Mg C ha⁻¹ year⁻¹ (32 Mg C ha⁻¹/ 18 years * 14.5% year⁻¹) in the plantation that are lost due to decomposition. Considering a maximum mass loss after a period of 6 years, the amount of C incorporated into the stable soil C pools will be 0.04 Mg C ha⁻¹ year⁻¹ (25 Mg C ha⁻¹/18 years * 15.5%/6 years) in the silvopasture and 0.05 (32 Mg C ha⁻¹/18 years * 15.5%/6 years) in the plantation. However, these values represent what is lost and gained from the annual litterfall, and do not consider accumulated litterfalls from the previous years that are gradually decomposing and also being added to the soil C pool. Taking this process into account, the sum of annual losses as decomposition during the 6 year period in the silvopasture and plantation reached 1.2 and 1.6 Mg C ha⁻¹ year⁻¹, respectively, whereas the net annual additions to stable soil C pools were 0.24 and 0.29 Mg C ha⁻¹ year⁻¹ for the silvopasture and plantation, respectively.

• 9. With respect to decomposition of necromass and coarse woody debris (CWD), the annual mass loss was determined using published k values (year⁻¹) for decomposition of dead branches of Pinus ponderosa (Hart et al. 1992; Yin 1999; Hall et al. 2006) and boles/stumps of N. pumilio (Frangi et al. 1997). Knowing the dry weight of dead branches after 2 years of decomposition and CWD, and assuming a 95% loss of initial weights, it was possible to calculate their mass losses and net addition to soil C pools. It should be noted that decomposition of the duff needle layer is not considered here as it has already been accounted for in the calculation of annual litterfall decomposition.

• 10. Soil C sequestration for the upper 0–40 cm layer was determined using weighted averages of C contents at three measured depths and a bulk density of 0.9 g cm⁻³ (Dube et al. 2009). In the silvopastoral system, an average value was calculated from the C contents obtained within the tree strips and at 2.5 m intervals on either side of strips (up to 10.5 m, corresponding to the middle of the 21 m wide pasture strip).

• 11. Soil respiration values refer to total respiration, including tree root, mycorrhizae and microbial respiration, and annual decomposition losses of needles, fine roots, cattle faeces, necromass, and coarse woody debris. Annual soil respiration for the three ecosystems was calculated from the monthly respiration rates presented in this study. For the months that soil respiration was not measured, estimates were done as follows: A regression between soil respiration and air temperature (+5 cm) was adjusted for every treatment (R² = 0.94), using the values obtained in the field. Knowing the mean monthly superficial air temperatures, these equations were then used to estimate monthly soil respiration and check the values calculated initially, the differences being less than 5%. Within the silvopasture, it was assumed that soil respiration in the tree strip accounts for 22% from the spatial area, while respiration from 1 and 7.5 m from the tree strip accounts for 78%. Since respiration chambers were installed within the pine strips, at 1 m and at 7.5 m from the strips, tree roots growing into the grass band could be taken into consideration in the calculations.

• 12. In order to determine the annual amount of leaching C, it was assumed that 24% of the annual rainfall leaches to the ground water (Gisi 1997; Peichl et al. 2006). Annual rainfall at the research site is 1,206 mm out of which 290 mm ha⁻¹ year⁻¹ is lost as leaching. The mean annual total C concentrations of leached soil solution from the land uses were then used to estimate the annual leached C losses in conjunction with total annual leaching losses. As above, C leaching within the tree strip was assumed to account for 22% of the spatial area, whereas leaching from 1 and 7.5 m from the tree strip account for 78% of the area.

• 13. The annual atmospheric C deposition to the systems was determined as follows: knowing that the annual rainfall is 1,206 mm year⁻¹, the volume occupied by this amount over 1 ha was 1.2 × 10⁴ m³. Since the density of water is 1 g cm⁻³, 1% C of 1 l leaching soil solution is equivalent to 10 g C. Therefore, 0.12 Mg C ha⁻¹ year⁻¹ represents the amount of atmospheric deposition.

• 14. Since approximately 1.25% of N fertilizer applied to the soil is lost in the form of N₂O emissions (IPCC 1997), and knowing the amount of N fertilizer applied to the pasture every 3 years, annual emissions of N₂O and CO₂-equivalent were estimated.

• 15. Carbon storage in ecosystems pools was calculated using the following equation:

  $$ {\text{C}}_{\text{pools}}={\text{C}}_{\text{agt}}+{\text{C}}_{\text{bgt}}+{\text{C}}_{\text{agg}}+{\text{C}}_{\text{soil}},$$

  where C_pools = total carbon stored in ecosystem pools, C_agt = aboveground tree carbon, C_bgt = belowground tree carbon, C_agg = aboveground grass carbon in the Ponderosa pine plantation and C_soil = soil organic carbon pool.

• 16. Positive or negative carbon flux into or out of the ecosystems was calculated using the following equation:

  $$ {\text{C}}_{\text{flux}}={\text{C}}_{\text{TrU}}+{\text{C}}_{\text{GrU}}+{\text{C}}_{\text{AtD}}+{\text{C}}_{\text{FecS}}+{\text{C}}_{\text{CwdS}}-{\text{C}}_{\text{SRes}}-{\text{C}}_{\text{Lch}}-{\text{C}}_{\text{Fert}}-{\text{C}}_{\text{AnC}}$$

  where C_flux = net carbon flux in the ecosystem, C_TrU = carbon input via total tree uptake, C_GrU = carbon input via total grass uptake, C_AtD = carbon input through atmospheric depositions (rain and snow), C_FecS = net addition to soil carbon pool via faeces input, C_CwdS = net addition to soil carbon pool via coarse woody debris and necromass decomposition, C_SRes = carbon output via total soil respiration, C_Lch = carbon leachate output from the soil solution, C_Fert = volatile carbon-equivalent output from fertilizer application, and C_AnC = carbon output through pasture consumption by animals (divided between cattle fattening, faeces production and GHG emissions). Therefore, losses as animal respiration, CH₄ emissions from enteric fermentation and N₂O from faeces have already been accounted for in cattle consumption.