Influence of long-term application of stockpiled feedlot manure with straw or wood-chip bedding on net nitrogen mineralization and nitrification in a clay loam soil

Little research has been conducted on the influence of land application of stockpiled feedlot manure (SM) containing either wood-chip (WD) or straw (ST) bedding on soil net N mineralization (Nm) and nitrification (Nn) rates during the barley (Hordeum vulgare L.) silage growing season. The stockpiled manure containing ST or WD bedding at 77 Mg (dry weight) ha−1 yr−1 was annually applied for 13–16 yr to a clay loam soil in a field experiment in southern Alberta. The net Nm and Nn rates were measured using the “in situ-soil core” method over 30–33 d (Nm1, Nn1) and 46–50 d (Nm2, Nn2) in each of 4 yr (2011–2014). Net Nm1 rates were generally significantly (P ≤ 0.05) greater for ST (1.0–1.9 mg N kg−1 d−1) than WD (0.1–0.9 mg N kg−1 d−1). Net Nn1 rates were also generally significantly greater for ST (0.9–2.0 mg N kg−1 d−1) than WD (0.03–0.9 mg N kg−1 d−1). Similar trends were found for Nm2 and Nn2. The Nn rates, however, were limited by NH4 supply during the incubations as Nm:Nn ratios were typically <1 with relatively low initial ammonium levels. Hence, the nitrification rates reflected the Nm rates and they would have been considerably lower than potential nitrification rates. A shift from ST to WD bedding by feedlot producers may decrease crop N supply 2- to 10-fold by lowering Nm (2- to 10-fold) and Nn (2- to 30-fold), and supplemental inorganic N fertilizer may be required when WD bedding is used.