Impacts of long-term application of poultry manure on subsurface drain water quality
With a steady growth of egg industry in Iowa as the leading state in the US, there is a huge amount of poultry manure released into the environment and need to be treated properly. Land application of poultry manure on crop production and pasture has been considered as proper management practice to utilize the valuable nutrient content of poultry manure (N, P, K and other minerals) for crop development instead of chemical fertilizer. However, there is an environmental risk of over-applying poultry manure on soil and water quality especially for field plots having tile-drained systems. A few long-term studies have been done to evaluate the impacts of poultry manure application rates on soil and subsurface water quality, and crop production to expose the experiment over a wide range of weather conditions for better understanding and being able to provide informed recommendations. A long-term study (1998-2009) was conducted to evaluate the environmental impacts of surface applied poultry manure on crop production, nitrate and phosphorus leaching in tile drain water as well as nitrate and phosphorus built-up in top soil. Application rates include two poultry manure application rates (168kg-N/ha - PM and 336 kg-N/ha - PM2) with three replications for each rate, urea ammonium nitrate - UAN (168kg-N/ha) with four replications and a control - None (0kg-N/ha). These treatments are assigned on eleven field plots equipped by a state-of-the-art single subsurface tile drain. Tile drain is intercepted by a sump to collect the tile flow and sample water. Corn is planted on one half and soybean on the other half of the plot. Only corn areas receive the treatments. Corn and soybean are rotated yearly. Soil NO3-N and PO4-P concentration are collected in Spring (before applying manure) and Fall (after harvesting) at the depth of 120 cm from the surface, then divided into 5 depths (0-15, 15-30, 30-60, 60-90, 90-120 cm). The long-term trends showed the increase of crop yield when applying poultry manure in compared with other treatments (UAN and None). However, PM2 treatment yields much higher NO3-N and PO4-P concentration residual in soil profile, especially on the top soil (0-30cm) than those of PM treatment. Seasonal effects of soil nutrient were significantly at the top soil (0-30cm) for PO4-P in all treatments in which PM and PM2 had a tendency to increase PO4-P concentration at post harvesting while UAN and None showed somewhat declining trends over 12 years. In addition, PM treatment gives the lowest tile flow, NO3-N, PO4-P concentration and losses in tile drain water lower than those of PM2 and UAN treatments on both yearly and monthly average. Seasonal effects of wet-dry-normal weather cycle also showed that the carried over of NO3-N concentrations were lower with PM treatments than those of PM2 and UAN treatment. These findings are significant in term of reducing nutrient losses in subsurface drainage at the early stage of corn development. Thus, poultry manure at low rate (168kg-N/ha) appears to be suitable for corn-soybean rotation production with tile-drainage in Iowa.