Nutrient Management Research Database

When appraising the impact of food and fiber production systems on the composition of the Earth’satmosphere and the ‘greenhouse’ effect, the entire suite of biogenic greenhouse gases – carbon dioxide(CO2), methane (CH4), and nitrous oxide (N2O) – needs to be considered. Storage of atmospheric CO2intostable organic carbon pools in the soil can sequester CO2while common crop production practices canproduce CO2, generate N2O, and decrease the soil sink for atmospheric CH4. The overall balance betweenthe net exchange of these gases constitutes the net global warming potential (GWP) of a crop productionsystem. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site inMichigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeasternColorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillageand crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissionscomprised 40–44% of the GWP from both rain-fed sites and contributed 16–33% of GWP in the irrigatedsystem. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether asystem is a sink or source of CO2, i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOCaccumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately1100 kg CO2equivalents ha
1y
1. Carbon accrual rates were about three times higher in the irrigatedsystem. The rainfed systems had been in NT for >10 years while the irrigated system had been converted toNT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in theirrigated system or if N2O emission rates decline or increase with time after conversion to NT.

This study examined how three agricultural ecosystems (agroecosystems) contribute to climate change, including greenhouse gas production and carbon storage. Soil carbon storage removes CO2 from the air, reducing the overall effect the cropping system has on climate change.

The three overall systems studied were:

1. Michigan
   1. Corn-soybean-wheat with conventional tillage
   2. Corn-soybean-wheat with no-till
   3. Continuous alfalfa
2. ColoradoDryland
   1. Wheat-corn-fallow (WCF)
   2. Opportunity continuous cropping (OC)
   3. Prairie grass mixture
3. Colorado irrigated
   1. Continuous corn no-till (NT)
      1. Fertilized at 0, 120, and 180 lbs N/ac
   2. Corn-soybean no-till
   3. Continuous corn conventional tillage (CT)
      1. Fertilized at 0, 120, and 180 lbs N/ac

Greenhouse gas emissions were measured (carbon dioxide - CO2, nitrous oxide - N2O, and methane - CH4) along with changes in soil carbon storage.

Michigan

The total global warming potential (GWP - refers to the sum of the strength of greenhouse gases emitted minus any greenhouse gas removed from the air, such as through soil carbon storage) was 1140 kg CO2-equivalents per hectare. Around half of that was attributed to N2O. The remainder was N-fertilizer production, lime, and farm operations.

N2O and CH4 emissions were similar between tillage types, though farm operation emissions were lower in NT due to less tractor traffic. The NT system was able to store significantly more carbon than CT, resulting a substantial greenhouse gas mitigation. As a result of this mitigation, NT overall global warming potential was only 140 kg CO2-equivalents per hectare. Continuous alfalfa was a net sink, meaning emissions were lower than the total greenhouse gases removed from the air. This was primarily the result of not applying N fertilizer.

Colorado Dryland

As a result of little rainfall at the start of the study, corn yield and N2O emissions were minimal. Methane consumption was higher in cropped soils (i.e. compared to perennial grasses). Given the minimal N2O emissions, overall emissions were largely controlled by CO2 from fertilizer production. GWP was lower in the rotations that did not include a fallow, as they were able to accumulate more soil carbon.

Colorado Irrigated

The irrigated corn plots emitted more CH4 than they consumed, which the authors attribute to the large amounts of irrigation applied at planting to moisten the rooting zone. N2O emissions increased along with fertilizer rates. The NT plots showed slightly reduced N2O emissions compared to the CT plots. Soils that included soybean in their rotations showed higher N2O emissions than those that did not. Tillage prevented the accumulation of soil carbon, subsequently increasing the overall GWP of CT plots. On the other hand, NT soils had a negative GWP, which means they consumed more greenhouse gases than they emitted.