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Title:   Author  item
                                            Sainju, Upendraitem
                                            Barsotti, Joyitem
                                            LENSSEN, ANDREW - Iowa State Universityitem
                                            HATFIELD, PATRICK - Montana State University
  Submitted to: Nutrient Cycling in Agroecosystems
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/20/2014
                                    Publication  Date: 6/19/2014
                                    Citation: Sainju, U.M., Barsotti, J.L., Lenssen, A.W., Hatfield, P.G. 2014. Particulate and active soil nitrogen fractions are reduced by sheep grazing in dryland cropping systems. Nutrient Cycling in Agroecosystems. 99(1-3):79-93. DOI: 10.1007/s10705-014-9619-8.Interpretive Summary: Sheep grazing during fallow periods (e.g. before crop planting, after grain harvest, and during summer fallow) is a cost effective method of weed control compared to herbicide application and tillage. Sheep grazing is often used to control weeds and pests, reduce feed cost, and increase nutrient cycling in dryland cropping systems. Tillage and fallow can expose soil to erosion and herbicide application can contaminate soil, water, and air, all of which can increase risks to human and animal health. Little is known about the effect of sheep grazing on soil nitrogen cycling. 
We evaluated the effect of sheep grazing compared to tillage and herbicide application for weed control on soil particulate organic nitrogen (PON), microbial biomass nitrogen (MBN), and potential nitrogen 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 [CSW] and spring wheat-pea/barley 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 nitrogen inputs through feces and urine probably reduced soil active and coarse organic matter nitrogen 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. Reducing the grazing intensity either by decreasing the number of sheep per plot, duration of grazing, or both may enhance soil nitrogen storage and mineralization and therefore soil productivity.
                                Technical Abstract: 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.


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