Dryland Soil Carbon and Nitrogen Influenced by Sheep Grazing in the Wheat–Fallow System

Sheep (Ovis aries) 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 carbon (SOC), inorganic carbon (SIC), total nitrogen (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 C and N storage. 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 maintaining 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.