Upendra Sainju
USDA-ARS, Northern Plains Agricultural Research Laboratory, USA
Title: Particulate and active soil nitrogen fractions in response to sheep grazing in dryland cropping systems
Biography
Biography: Upendra Sainju
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, 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 3 weed management practices (sheep grazing [grazing], herbicide application [chemical], and tillage [mechanical]) and 2 cropping sequences, continuous spring wheat CSW (Triticum aestivum L.), spring wheat-pea (Pisum sativum L.) or barley (Hordeum vulgaris L.) mixture and 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 and 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.