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Particulate and active soil nitrogen fractions are reduced by sheep grazing in dryland cropping systems
- Upendra M. Sainju, Joy L. Barsotti, Andrew W. Lenssen, Patrick G. Hatfield
- Nutrient cycling in agroecosystems 2014 v.99 no.1-3 pp. 79-93
- urine, organic matter, arid lands, Pisum sativum, weed control, Hordeum vulgare, feces, Triticum aestivum, mineralization, crop residues, soil, weeds, soil sampling, pesticide application, nitrogen, sheep, tillage, cropping systems, microbial biomass, spring, spring wheat, grazing intensity, cropping sequence, Montana
- 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 organic N (PON), microbial biomass N (MBN), and potential N mineralization (PNM) at the 0-30 cm depth in a Blackmore silt loam 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]) and two cropping sequences (continuous spring wheat [Triticum aestivum L.] [CSW] and spring wheat-pea [Pisum sativum L.]/barley [Hordeum vulgaris L.] mixture hay-fallow [W-P/B-F]). The PON and MBN at 0-30 cm were 126 to 620 kg N ha-1 greater in the chemical or mechanical than the grazing treatment with CSW. The PNM at 15-30 cm was 7 to 13 kg N ha-1 greater in the chemical or mechanical than the grazing treatment in 2009 and 2011 and at 5-15 cm was 7 kg N ha-1 greater with W-P/B-F than CSW in 2010. From 2009 to 2011, PON at 0-30 cm reduced from 0.43 Mg N ha-1 yr-1 in the grazing treatment to 0.59 Mg N ha-1 yr-1 in the chemical treatment. Similarly, PNM at 15-30 cm reduced from 0.7 kg N ha-1 yr-1 in the mechanical treatment to 4.1 kg N ha-1 yr-1 in the chemical treatment and at 5-15 cm from 1.8 kg N ha-1 yr-1 with CSW to 5.1 kg N ha-1 yr-1 with W-P/B-F. Removal of crop residue during grazing but negligible N inputs 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. Decline in the rate of change from 2009 to 2011 suggests that sheep grazing may stabilize N fractions in the long-term, especially in continuous cropping system.