Elsevier

Agricultural Systems

Volume 122, November 2013, Pages 73-78
Agricultural Systems

Life cycle assessment of the potential carbon credit from no- and reduced-tillage winter wheat-based cropping systems in Eastern Washington State

https://doi.org/10.1016/j.agsy.2013.08.004Get rights and content

Highlights

  • No-till increases soil carbon sequestration in winter wheat dry lands.

  • N2O emissions constitute two thirds of the winter wheat carbon footprint.

  • Tillage reduction is more profitable in the moderate and high rainfall zones.

  • Carbon credit does not breakeven with tillage reduction cost in low rainfall zones.

Abstract

Integrated environmental and economic assessment studies are required to support tillage reduction decisions. In this paper, carbon sequestration and nitrous oxide emissions from winter wheat-based cropping systems were evaluated in eastern Washington, USA, using computer simulation. System boundaries were expanded to consider fertilizer production and use of machinery in a standard life cycle assessment (LCA) study. Variations in rainfall, tillage intensity and crop rotation were considered. Potential earnings from carbon credits obtained by converting to reduced and no-till management were evaluated and compared with the corresponding changes in farm budgets. No-till increased the average soil carbon sequestration by 0.5, 0.3, 0.2 Mg-C ha−1 yr−1 (30-year average) in high, middle and low rainfall zones, respectively. On-farm N2O emissions contributed 60–70% of the total CO2-equivalent emissions (CO2e ha−1 yr−1) in high and middle rainfall scenarios and 30–40% in low rainfall scenarios. Production of fertilizers contributed 13 ± 3% of total emissions. Emissions from fuel consumption varied across sites due to differences in machinery use with different topography, tillage, soils and number of fallow years. Reduced tillage systems are more profitable in the moderate and high rainfall zones with 2011 crop price and input cost assumptions but they are less profitable in the drier rainfall zone. Even a more inclusive accounting with LCA that includes credits for reductions in N2O emissions, fuel usage and fertilizer production was insufficient to compensate for the lower returns. With the net market price assumption of 2.48 $ CO2e1 ha−1 yr−1, the CO2e credits for reducing tillage ranged from 0.27 to 1.63 $ CO2e1 ha−1 yr−1 across the region.

Introduction

Climate change concerns have motivated interest in incentive-based agricultural practices that help reduce greenhouse gas (GHG) emissions. The United Nations Framework Convention on Climate Change and the Kyoto Protocol (UNFCCC, 2008) have framed agreements and protocols to reduce GHG emissions. The protocols utilize market-based mechanisms such as emission trading aimed at creating a carbon market where emissions/reductions can be exchanged. The US-EPA is applying cap-and-trade programs (Paltsev et al., 2007) on industrial emissions. So far, the capped emissions are focused on the power sector, which creates potential opportunities to lower emissions in other sectors such as agriculture, providing incentives for reductions that can be traded as carbon credits. Agriculture is the second largest global source of GHG emissions after the power sector (Minamikawa et al., 2009). It is also the largest land use category worldwide (Dutaur and Verchot, 2007). Because agriculture is the second leading emitter and leading land-use, and because agricultural emissions are not capped, it can be argued that agriculture should be a target for incentive-based emission reductions.

Reduced-till (RT) or no-till (NT) agricultural practices reduce disturbance to topsoil layers and reduce oxidation of soil organic carbon (SOC). Global and temperate climate regional studies indicated RT and NT may therefore increase SOC storage compared to conventional-till (CT) (West and Post, 2002, Chatskikh and Olesen, 2007, Sainju et al., 2008). On the contrary, studies under tropical and semi-arid conditions indicated minimal effect of tillage reduction on increasing SOC storage (Sanderman et al., 2010). Carbon sequestration and emission offset depends on climate feedback loops in the C and N cycles (Thornton et al., 2007). Therefore, assessment of carbon footprint reduction due to tillage reduction should consider regional climatic conditions which favor the use of modeling based approaches to evaluate long term effects of climatic conditions. Some researchers found that NT and RT decreased nitrification and denitrification rates, which reduced N2O emissions (Chatskikh and Olesen, 2007, Kroeze et al., 1999), while others found higher N2O emissions during the first NT years followed by a decrease due to soil aggregation (Six et al., 2004). Alluvione et al. (2009) found that methane (CH4) emissions tended to increase due to less CH4 oxidation capacity of the soil under NT and RT compared to CT. However, other authors state that there is no significant difference in CH4 fluxes of CT compared to NT and RT (Jacinthe and Lal, 2005, Mosier et al., 2006, Omonode et al., 2007). In fact, GHG emissions are not dependent on tillage only. For instance, Omonode et al. (2007) investigated different tillage scenarios and reported that CO2 emissions were mainly dependent on crop rotation. It is difficult to determine the outcome of converting CT to RT and NT based on field measurements only and simulation models can be useful in this regard. Cropping system models analyze interacting mechanisms giving predictions of emissions from different land operations (Hammer et al., 2002).

Agriculture uses external resources with associated GHG emissions such as fuel, fertilizers, and pesticides. To fully evaluate agricultural GHG emissions, a more comprehensive view of environmental impacts should also consider emissions from on-farm fuel consumption by equipment used for transportation and land operations (Koga et al., 2003, Mileusnic et al., 2010) and from production of fertilizers (Das and Kandpal, 1998). Furthermore, factors such as fuel used for machinery operations and use of fertilizers affect the standard costs within an enterprise budget and thus influence decisions to change tillage operations to RT or NT. A complete view of emission analyses should consider the economics of all factors.

Life Cycle Assessment (LCA) is an integrated approach to expand the field evaluation of emissions to broader boundaries that include external resources used in agricultural production. Finnveden et al. (2009) define LCA as a tool to assess the environmental impacts and resources used throughout a product’s life cycle. LCA can also be extended beyond the evaluation of GHG emissions and Global Warming Potential (GWP). LCA is also used to evaluate the impacts of pesticides (Margni et al., 2002) on human health and ecosystems, and to compare fertilizer choices for cropping major commodities such as wheat.

In this paper, an LCA-based methodology was developed to evaluate potential carbon credits resulting from conversion of CT to NT or RT for winter wheat (WW) in selected locations in the dryland region of eastern Washington, USA and for selected WW-based rotations. Carbon sequestration and GHG emissions were evaluated using results from a simulation study reported by Stöckle et al. (2012). The systems’ boundaries were expanded to consider emissions from fuel consumption and fertilizer production. The market value of RT and NT was evaluated by considering budgets for the selected cropping systems (Painter, 2009) and carbon credits per unit area expanded to include carbon sequestration and GHG emissions from farming practices including tillage, fertilizer usage and manufacture, and fuel usage.

Section snippets

Methodology

To evaluate tillage reduction implications at the local farm level, the conceptual approach of integrating techno- and eco-spheres LCA impacts with value-sphere parameters (Hofstetter et al., 2000) was simplified as depicted in Fig. 1. Analysis in the value-sphere is focused on carbon credits and typical farm budgets comparing NT, RT and CT scenarios as applied to dryland WW production in Eastern Washington, USA. In the techno-sphere, the cropping systems model, CropSyst (Stöckle et al., 1994,

Techno-sphere emissions

Simulated sequestered carbon and cumulative N2O emissions are presented in Fig. 2. Carbon sequestration under CT shows little change over the 30-year simulation time. In general, NT carbon sequestration was higher than RT. The amount of sequestered soil carbon started to level off for the highest rainfall zone by the end of the simulation period at year 30. Starting the simulation from the stable content for CT at 104.7 Mg C ha−1, NT began to level off at 108.8 Mg C ha−1 storing 4.1 Mg C ha−1 in 30 

Discussion

Converting from CT to NT increases SOC sequestration and reduces GWP from wheat-based dry land cropping systems of eastern WA. NT increased soil carbon sequestration by 0.5, 0.3, 0.2 Mg-CO2e ha−1 yr−1 in high, middle and low rainfall zones, respectively. An increase in rainfall intensity increases crop yield and carbon sequestration due to the increased residue biomass recycled to the soil. Thomson et al. (2002) showed that winter wheat yield in this region increases proportionally to the rainfall.

Conclusion

A complete, LCA-based view of the CO2e savings from tillage reduction expands the evaluation of emissions on the larger ecological scale, and focuses the economic analyses on the scale of representative farm budgets by tillage and across the region. Crop system models estimate the emissions from agricultural lands providing insight to changes that accompany tillage reduction under climate specific conditions. No and reduced-till management of dryland wheat-based cropping systems are more

Acknowledgement

This research was supported in part by a Grant from the Paul G. Allen Family Foundation.

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