ARS | Publication request: Dryland Soil Carbon and Nitrogen Influenced by Sheep Grazing in the Wheat-Fallow System

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 9, 2010
Publication Date: September 1, 2010
Repository URL: http://hdl.handle.net/10113/50497
Citation: Sainju, U.M., Lenssen, A.W., Goosey, H.B., Snyder, E.E., Hatfield, P.G. 2010. Dryland Soil Carbon and Nitrogen Influenced by Sheep Grazing in the Wheat-Fallow System. Agronomy Journal. 102:1553-1561.

Interpretive Summary: In the northern Great Plains, sheep grazing during fallow periods in wheat-fallow systems or after grain harvest in continuous wheat systems is often used to control weeds and pests, reduce feed cost, and increase nutrient cycling. In these systems, land is typically fallowed from 14 to 20 mo. Fallowing is used to conserve soil water, release plant nutrients, control weeds, increase succeeding crop yields, and reduce the risk of crop failure. Using tillage and herbicides to control weeds during fallow have been effective but are expensive, resulting in some of the highest variable costs for small grain production in Montana. Other disadvantages of using these practices are the exposure of soil to erosion due to tillage and increased risk of contamination of herbicides in soil, water, and human and animal health. Extensive application of commercial fertilizers in the 20th century increased crop yields but reduced environmental quality by increasing (1) N leaching from the soil profile to the groundwater, (2) surface runoff of N and P from agricultural lands to streams and lakes, causing eutrophication, and (3) greenhouse gas, such as N2O, emission. Increased soil acidity following application of commercial fertilizers, especially N fertilizers, also led to the development of infertile soils that did not respond well to increased fertilizer application for sustaining crop yields. In such conditions, integrated crop-livestock system can be used as an option to improve soil quality and sustain crop yields. We evaluated the effects of fallow management [sheep grazing (grazing), herbicide application (chemical), and tillage (mechanical)] for weed control and soil water conservation and cropping sequence [continuous spring wheat (CSW), spring wheat-fallow (SW-F), and winter wheat-fallow (WW-F)] on soil organic C (SOC), inorganic C (SIC), total N (STN), NH4-N, and NO3-N levels at the 0- to 120-cm depth and wheat yield. The experiment was conducted in a Blackmore silt loam (fine-silty, mixed, superactive, frigid, Typic Argiustolls) from 2004 to 2008 in southwestern Montana. Annualized wheat grain and biomass yields were greater in CSW than in SW-F and WW-F and greater in 2004 than in other years. From 2004 to 2007, SOC concentration at 0- to 15-cm declined by 2.99 g C kg-1 yr-1. In 2008, SOC content at 10- to 120-cm was greater in the mechanical or chemical than in the grazing treatment in CSW and SW-F. The STN content at 0- to 5-cm was greater in the chemical and mechanical than in the grazing treatment but at 30-to 60-cm was greater in the grazing than in the chemical treatment in CSW. From 2004 to 2006, NO3-N content at 0- to 60-cm was greater in SW-F or WW-F than in CSW. In 2008, NO3-N content at 30- to 120-cm was greater in CSW and SW-F than in WW-F and at 60- to 90-cm was greater in the mechanical than in the chemical treatment. The SIC and NH4-N contents were largely not influenced by treatments. Continuous tillage, followed by reduced amount of wheat residue returned to the soil from 2004 to 2007 probably reduced soil organic C and total N. In contrast, greater amount of N removed by wheat grain due to continuous cropping probably reduced soil NO3-N in CSW. For sustaining wheat yields and maintain soil C and N levels, reduced tillage with continuous cropping and less intensive sheep grazing that increase the amount of wheat residue returned to the soil could be adopted.

Technical Abstract: Sheep (Ovis aries L.) grazing during fallow for weed control in wheat (Triticum aestivum L.)-fallow systems may influence soil C and N levels and grain yields by returning part of consumed crop residue to the soil through feces and urine. We evaluated the effects of fallow management [sheep grazing (grazing), herbicide application (chemical), and tillage (mechanical)] for weed control and soil water conservation and cropping sequence [continuous spring wheat (CSW), spring wheat-fallow (SW-F), and winter wheat-fallow (WW-F)] on soil organic C (SOC), inorganic C (SIC), total N (STN), NH4-N, and NO3-N levels at the 0- to 120-cm depth and wheat yield. The experiment was conducted in a Blackmore silt loam (fine-silty, mixed, superactive, frigid, Typic Argiustolls) from 2004 to 2008 in southwestern Montana. Annualized wheat grain and biomass yields were greater in CSW than in SW-F and WW-F and greater in 2004 than in other years. From 2004 to 2007, SOC concentration at 0- to 15-cm declined by 2.99 g C kg-1 yr-1. In 2008, SOC content at 10- to 120-cm was greater in the mechanical or chemical than in the grazing treatment in CSW and SW-F. The STN content at 0- to 5-cm was greater in the chemical and mechanical than in the grazing treatment but at 30-to 60-cm was greater in the grazing than in the chemical treatment in CSW. From 2004 to 2006, NO3-N content at 0- to 60-cm was greater in SW-F or WW-F than in CSW. In 2008, NO3-N content at 30- to 120-cm was greater in CSW and SW-F than in WW-F and at 60- to 90-cm was greater in the mechanical than in the chemical treatment. The SIC and NH4-N contents were largely not influenced by treatments. Continuous tillage, followed by reduced amount of wheat residue returned to the soil from 2004 to 2007 probably reduced soil organic C and total N. In contrast, greater amount of N removed by wheat grain due to continuous cropping probably reduced soil NO3-N in CSW. For sustaining wheat yields and maintain soil C and N levels, reduced tillage with continuous cropping and less intensive sheep grazing that increase the amount of wheat residue returned to the soil could be adopted.