Carbon and Nitrous Oxide in LCA

Transcription

1 Carbon and Nitrous Oxide in LCA Life Cycle Analysis for Bioenergy University Park, PA July, 2011 Armen R. Kemanian Dept. Crop & Soil Sciences Penn State University Introduction Why is this important? In grain, forage, and biomass production systems both the net C balance and the emission of N 2 O are the main factors affecting the farm-gate LCA outcome Agriculture is responsible for approximately 75% of the total GHG attributable to N 2 O emissions in the US EPA 2010: to qualify as renewable, advanced biofuels GHG emissions must be 50% of those from petroleum based fuel over the fuel lifecycle Management for improving the C balance or to reduce N 2 O emissions may involve optimizing the outcome for multiple criteria These are two pieces of a complicated puzzle! The rest of the analyses focuses on these two pieces 1

2 Introduction The C and N cycles Inputs of organic Carbon through photosynthesis GRAIN FORAGE RESIDUES Exported from farm for animal or human consumption. Most C respired or fermented, a fraction returned as manure or biosolid. Exported or not, ~50% respired or fermented (CH 4 ), the rest returned to the soil as manure Most returned to the soil, with a large fraction (>80%) respired as CO 2 Losses of CO 2 or CH 4 through respiration or fermentation The net balance is usually accounted for in the soil organic carbon pool, but as we can see, the overall LCA is more evolved. I will focus on the crop and soil aspects Introduction The C and N cycles Emission during Nitrification N 2 O Emission during Denitrification, N 2 and N 2 O N 2 O Emission during Nitrification 2

3 Carbon The carbon balance equation Sc = soil organic carbon Rc = residue input hx = humification coefficient k = soil organic carbon decomposition (apparent respiration) coeff. = Inputs - Outputs This equation states that the change in storage is equal the gains of C minus the losses of C for a given time interval At equilibrium: Carbon The carbon balance equation Soil carbon increases through higher inputs: Increase residue inputs! Limitation is soil C saturation, unlikely in most soils in temperate conditions Soil carbon increases through reduction in losses: Reduce k, the decomposition rate, by maintaining the soil drier (with crops that use the water, possible) or cooler (more difficult) or flooded (not the point obviously), or with less mechanical disturbance, and with minimal erosion 3

4 4l 8/1/2011 Carbon Carbon balance and bioenergy crops Increase residue inputs Simplistic proposition: Biofuel production entails removing biomass, not returning it to the soil But This biofuel offsets emissions from fossil fuel, therefore a neutral C balance is possibly a net gain More sophisticated proposition: Capture more radiation and water by intensifying the cropping sequence (cover crops?) Increase inputs through the roots of perennials (depth) Reduction in C losses Tillage: reduce tillage type or directly the frequency of tillage by using perennial crops Soil moisture: more cropping or perennial crops minimize the period of wet soils (e.g. after harvest) Erosion: perennial crops reduce erosion, and so does the use of notill in most circumstances Carbon Carbon balance and bioenergy crops There are too many factors to consider, how do we evaluate them? (1) Experimentally (long term, limited number of scenarios) (2) Using simulation models Carbon Input DPM SPM RPM K (1-E ) 1 CO K 1 2 (1-E ) 2 2 K 1 E 1 CO 2 K2 E 2 K (1-E ) 5 5 Biomass K (1-E ) 7 7 K 7E77 K 6 E 6 K5 E 5 K (1-E ) 6 6 CO 2 CO 2 Labile K 4m E 4 K4l 4l E 44 Metastable K CO (1-E ) K 8 E 8 K 3 E 3 K 9 E 9 Stable Soil Carbon 4

5 Nitrous Oxide The sources of nitrous oxide Nitrous oxide is produced by several processes in the soil, the most important of which are microbial denitrification and nitrification Denitrification consists on the sequential reduction of nitrate (NO 3 ) to NO, N 2 O, and N 2 Nitrification is the process by which NH 4 is oxidized to NO 3 ; N 2 O is a byproduct (~0.3%). The process is fast in aerobic conditions. In terms of the N mass balance, the N 2 O losses are low. In GHG terms, however, a loss of 1 kg of N 2 O-N equates to ~54 kg of C So, a small flux that in GHG terms is too important. Quantifying it is as challenging as it gets for LCA Nitrous Oxide Factors that promote the losses The whole process is somewhat perverse: Fertilizer Mineralization Deposition Urine / Manure NH 3 N 2 O Fertilizer Mineralization Deposition N 2, N 2 O NH 4 NO 3 nitrification denitrification N entering as NH 4 has two chances to be emitted as N 2 O When residues decompose, a fraction of the N is recycled back through NH 4! To uptake sufficient N a non-legume crop needs available NO 3 (perennials somewhat bypass it by internal recycling; the extent to which it can be coupled to high biomass removal is unknown) 5

6 Nitrous Oxide Factors that promote the losses Fertilizer Mineralization Deposition Urine / Manure NH 3 N 2 O Fertilizer Mineralization Deposition N 2, N 2 O NH 4 NO 3 nitrification denitrification Available C for heterotrophic respiration (residues, roots, organic matter) Low oxygen (< 10% of the absolute porosity filled with air) Fully anoxic conditions drive almost all of the denitrified N to N 2 ; maximum N 2 O rates are shifted with respect to maximum denitrification rates Typical rates: Natural environments < 1 kg N 2 O-N ha -1 yr -1 Ag systems ~ 2 to 4 kg N 2 O-N ha -1 yr -1 Higher losses reported > 40 kg N 2 O-N ha -1 yr -1 Nitrous Oxide Management By keeping NO 3 low Control of fertilization rates Use of non-nitrate sources Use of nitrification-inhibitors Use of perennial crops? Use of switchgrass / poplar or willow as buffer strips? By controlling factors affecting the rate other than the NO 3 level Place fertilizer away from C source Manage soils to provide good drainage, e.g. avoiding compaction Minimize N inputs when soil is moist and prone to higher moisture (snowmelt; marginal lands) 6

7 4l 8/1/2011 Nitrous oxide Nitrous oxide and bioenergy crops There are too many factors to consider, how do we evaluate them? (1) Experimentally (long term, limited number of scenarios) (2) Using simulation models (3) Yes, the same as slide 8 for C! Carbon Input DPM SPM RPM K (1-E ) 1 CO K 1 2 (1-E ) 2 2 K 1 E 1 CO 2 K2 E 2 Biomass K (1-E ) 7 7 K 7E77 K 6 E 6 Stable K (1-E ) 5 5 K5 E 5 K (1-E ) 6 6 CO 2 CO 2 Labile K 4m E 4 K4l 4l E 44 Metastable K CO (1-E ) K 8 E 8 K 3 E 3 K 9 E 9 Soil Carbon Models A commentary on models why do this? The answer I d give is that models are an enormously important tool for clarifying your thought. You don t have to literally believe your model in fact, you re a fool if you do to believe that putting together a simplified but complete account of how things work helps you gain a much more sophisticated understanding of the real situation. People who don t use models end up relying on slogans that are much more simplistic than the models [fill in with your favorite slogan] all of which are just wrong some of the time. Paul Krugman November 18,

8 Example A simple model for a simple system Disclaimer: This is a hypothetical situation. The crop yield and soil properties are fictional and any similarity with a real situation is mere coincidence Soil: 50 Mg C ha -1 in topsoil (0.3 m) Crop: Maize producing 3 Mg ha -1 of root (~1.3 Mg of C) 8 Mg ha -1 of residue (~3.5 Mg of C) 8 Mg ha -1 grain, removed from the field Fertilization: 150 kg N as ammonium nitrate Example A simple model for a simple system Assume: The k or soil apparent respiration is 1.5% per year The humification is about 16% (i.e. stabilization of residue inputs) About 0.75% of the fertilizer is lost as N 2 O Then: Soil C respired: x 50 = 0.75 Mg ha -1 yr -1 Residue C humified: 0.16 x ( ) = 0.77 Mg ha -1 yr -1 Therefore soil C is approximately in steady state 8

9 Example A simple model for a simple system What about nitrous oxide? N 2 O-N lost x 150 = 1.1 kg ha -1 yr -1 This is, approximately, equivalent to 0.06 Mg ha -1 yr -1 of C lost. Therefore, the GHG balance is slightly negative. It is worth noting that nitrous oxide losses can be much larger Example A simple model for a simple system We can conclude that: Further removal of C by harvesting the residue may tilt the balance towards soil C losses (and erosion). However, removal coupled with the incorporation of a cover crop may restore the equilibrium, effectively intensifying the system. And if that cover crop includes a legume, it may reduce the need of external N inputs. Once again, models become extremely important to help think through the impact of different management options 9

10 There are limitations! Quantitatively, most controls of soil carbon dynamics have been incorporated in simulation models, yet we are still unable to use these models without much supervision Soil carbon is rarely uniform across the landscape 1 m 40 m 600 m The Palouse as study case 10

11 Frequency distribution of C s Profile Topsoil Subsoil Frequency distribution of soil organic carbon in the profile (left panel), the top 0.3-m of the profile (middle panel) and between 0.3 and 1.5 m in the Cook Agronomy Farm in eastern Washington (n = 177). Huggins et al., unpublished Soil Carbon and Carbon Inputs Huggins et al., unpublished 11

12 CART Topsoil C in the landscape Huggins et al., unpublished CART for soil organic carbon in the topsoil (A.depth = thickness of the A horizon, curv.pln = plan curvature, ems00 = electromagnetic conductivity in spring of 2000, Bw.depth = depth of the Bw horizon, flod = flow direction). Soil carbon in the Palouse region Fraction of cases in the upper, middle or lower third of soil productivity and topsoil organic carbon Soil Carbon Low Medium High Productivity Low ^ 0.03^ Medium ^ High If productivity is stable: 25% of area could gain soil carbon 47% of area is likely at equilibrium with inputs 28% of area could lose soil carbon 12

13 Soil carbon in Texas, modeling study Change in SOC (Mg ha -1 ) Change from CT to RT Change from CT to NT Change from NT to RT Change from NT to CT SD=0.5 Mg ha Years Change in soil carbon when moving a system from till (CT) to reduced till (RT) or no-till (NT), and viceversa. Meki et al., unpublished Soil carbon in Texas, modeling study Change in soil carbon when moving a system from till (CT) to reduced till (RT) or no-till (NT), and viceversa. Meki et al., unpublished 13

14 Nitrous oxide, hypothetical landscape Concluding Remarks Tools are available that compute the carbon balance and nitrous oxide emission of a system Variation in the landscape is known but difficult to quantify and manage. Advances in this area are rapid. To provide useful outputs, simulation models need adequate inputs Pairing biofuel production with landscape management appears as a strategy that can greatly enhance the appeal of bioenergy crops and have a favorable impact in the LCA of biofuels As a side note, I will be happy to show the simulation model Cycles to those interested in the C and N 2 O angles 14

15 Questions? Contact information: 15