Influence of Legacy Applications of Feedlot Manure Amendments on Soil Carbon and Nitrogen Fractions

Utilizing a field site that received 17 annual applications of cattle feedlot manure and compost in southern Alberta, followed by 3 years with no application, we investigated the manure/compost legacy effect on selected soil carbon (C), nitrogen (N), and phosphorus (P) fractions in surface- and sub-soil layers. Amendment treatments were two feedlot manure management systems [stockpiled (SM) or composted (CM)] and two bedding materials [straw (ST) or wood-chip (WD)], with a field application rate of 77 Mg ha-1 (dry basis). An unamended control (CK) was also included for total of 5 treatments (CK, SM-ST, SM-WD, CM-ST and CM-WD). Soil samples (n= 60) were collected in spring 2018 at 0-15, 15-30 and 30-60 cm depths and analyzed for soil organic C (SOC), total N (TN), δ15N, permanganate oxidizable carbon (POXC), water-extractable organic carbon (WEOC), available N (KCl extractable NO3-N and NH4-N), and available P (Olsen-P). Additionally, the 0-15 cm layer soil was physically fractionated into light, clay, silt and sand using a densimetric-based method and analyzed for SOC, TN and δ15N levels.

Our preliminary results indicate that, regardless of whether feedlot pens are bedded with straw or wood chips, or whether the manure is applied to soil as SM or CM, there is a strong manure/compost legacy effect on soil nutrients and organic matter fractions; SOC, TN, and P concentrations were still elevated in manure-amended soil three years after discontinuing SM or CM inputs. The proportion of light fraction in soil was also affected by the type of amendment with SM-WD = CM-WD > SM-ST = CM-ST > CK. On-going research progress and future work on physically fractionated soil samples will be presented and discussed at the 57th annual ASSW.