Long-term application of poultry manure on crop yield, n-uptake and residual soil nitrate-nitrogen under corn-soybean rotation system

A long-term study (1998–2009) evaluated the impacts of poultry manure application rates (168 kg N ha⁻¹ and 336 kg N ha⁻¹) on crop yields, soil NO₃-N residual concentrations, corn stalk N uptake, and potential correlations with crop yield response to soil NO₃-N and corn stalk N from a corn-soybean rotation system in Iowa. Experimental treatments included urea ammonium nitrate (UAN) with 4 replications, poultry manure at two different rates (168 kg N ha⁻¹ and 336 kg N ha⁻¹) with 3 replications for each rate, and a control (0 kg N ha⁻¹). The treatments were applied on eleven field plots (0.1 ha to 0.4 ha) in an unbalanced randomized complete block design. In each field plot, corn was planted on one half and soybean on the other half. Corn and soybeans were rotated within each plot yearly, with the fertilizer or manure treatment applied to the half of the plot planted to corn. Three soil cores were collected from each plot half planted to corn in the early spring (before planting and treatment application) and in late fall (after harvesting and before tillage) at a depth of 120 cm from the soil surface. Each core sample was divided into 5 different depths (0–15, 15–30, 30–60, 60–90, and 90–120 cm) for comparative analysis. The results of the twelve year study indicated that long-term trends for fall top soil (0–30 cm) NO₃-N concentration showed higher NO₃-N residual concentrations with the PM2 treatment than the other treatments (PM, UAN and None), with the average fall soil NO₃-N concentrations of 13.6 vs. 9.9, 9.6, and 6.8 ppm, respectively. The soil NO₃-N concentrations for all treatments varied between spring and fall soil samples, with higher average spring soil NO₃-N concentrations, demonstrating the effect of the previous year's soybean production on residual soil NO₃-N concentrations. While the spring soil analysis was an indication of influence of the previous year's soybean crop, the fall soil analysis was a more direct indication of the impact of the fertilizer treatment on the soil NO₃-N concentrations. The comparable soil NO₃-N concentrations with PM and UAN treatments suggest that applying poultry manure at the lower rate (168 kg N ha⁻¹) resulted in less NO₃-N residual concentration in the soil profile than applying poultry manure at the double rate (336 kg N ha⁻¹).