Particulate and active soil nitrogen fractions are reduced by sheep grazing in dryland cropping systems

Sheep (Ovis aries L.) grazing, a cost-effective method of weed control compared to herbicide application and tillage, may influence N cycling by consuming crop residue and weeds and returning N through feces and urine to the soil. The objective of this experiment was to evaluate the effect of sheep grazing compared to tillage and herbicide application for weed control on soil particulate and active soil N fractions in dryland cropping systems. Our hypothesis was that sheep grazing used for weed control would increase particulate and active soil N fractions compared to tillage and herbicide application. Soil samples collected at the 0–30 cm depth from a Blackmore silt loam were analyzed for particulate organic N (PON), microbial biomass N (MBN), and potential N mineralization (PNM) under dryland cropping systems from 2009 to 2011 in southwestern Montana, USA. Treatments were three weed management practices [sheep grazing (grazing), herbicide application (chemical), and tillage (mechanical)] as the main plot and two cropping sequences [continuous spring wheat (Triticum aestivum L.; CSW) and spring wheat–pea (Pisum sativum L.)/barley (Hordeum vulgare L.) mixture hay–fallow; W–P/B–F] as the split-plot factor arranged in randomized complete block with three replications. The PON and MBN at 0–30 cm were greater in the chemical or mechanical than the grazing treatment with CSW. The PNM at 15–30 cm was greater in the chemical or mechanical than the grazing treatment in 2009 and 2011 and at 5–15 cm was greater with W–P/B–F than CSW in 2010. From 2009 to 2011, PON at 0–30 cm and PNM at 15–30 cm reduced from 2 to 580 kg N ha−1 year−1 in the grazing and chemical treatments, but the rate varied from −400 to 2 kg N ha−1 year−1 in the mechanical treatment. Lower amount of labile than nonlabile organic matter returned to the soil through feces and urine probably reduced soil active and coarse organic matter N fractions with sheep grazing compared to herbicide application and tillage for weed control. Reduction in the rate of decline in N fractions from 2009 to 2011 compared to the herbicide application treatment, however, suggests that sheep grazing may stabilize N fractions in the long-term if the intensity of grazing is reduced. Animal grazing may reduce soil N fractions in annual cropping systems in contrast to known increased fractions in perennial cropping systems.