|Metrics for Nitrate Contamination of Ground Water at CAFO Land Application Sites - Iowa
Swine Study (EPA/600/R-09/045) June 2009
Nitrate (NO3-) is the most common chemical contaminant found in ground water and there are increasing indications that agriculture contributes to this contamination. In the United States, concentrated animal feeding operations (CAFOs) are a common agricultural practice. CAFOs lead to concentrated production of animal waste and manure. In most instances, this manure is then utilized as an input for crop production because the manure is relatively rich in plant nutrients, including nitrogen (N) and phosphorus (P). Manure disposal on agricultural land by CAFOs is usually dictated by a Comprehensive Nutrient Management Plan (CNMP or NMP). The stated intention of the CNMP is to utilize the manure as beneficially as possible without a high risk of contaminating surface and ground water. The objectives of this research were to monitor changes in soil nutrient composition at various depths in response to various scenarios of swine manure applications according to an approved CNMP and determine if site characteristics or management protocols that pose a risk to ground water can be identified.
A study was conducted for one year (2006) on a swine-row crop farm in central Iowa. The row crop operation consisted primarily of corn (Zea mays L.) -soybean (Glycine max (L.) Merr.) rotation. Swine production consisted of growing-finishing operation of 4,200 head. Swine waste was stored in pits for up to a year before being applied. Land application consisted of injecting the effluent into a slit approximately 20 cm below the soil’s surface. Eight plots (10 x 10 m) were established. Two plots were in a field in which swine manure effluent was applied in the fall to supply a corn crop’s N requirement (approximately 150 kg N ha-1). Four plots were in a field in which swine manure effluent was applied in the spring to supply approximately 100 kg N ha-1 with the additional crop N (50 kg ha-1) being supplied post-planting as sidedressed fertilizer. The last two plots were in a soybean field; one received swine effluent application in 2005 and the other did not. Soil core samples (0-15, 15-30, 30-45, 45-60, and 60-120 cm depths) were taken in May (planting time) and October (after harvesting). Soil samples from the top 22.5 cm were also collected biweekly throughout the growing season. These samples were analyzed for soluble components (nitrate, ammonium, SRP, pH, and EC), as well as exchangeable ammonium and Mehlich III extractable P. Plant samples were also collected and analyzed for biomass and N content.
Detailed soil sampling revealed that soil N and P concentrations were greatest in the upper 20 and 10 cm, respectively. In addition, the variations in soil P and N were greater at the soil surface than at lower depths. Concentrations of soil P and N at all depths decreased during the growing season. The largest decrease in soil P and N concentrations was found in the upper 10 cm, and at 20-40 cm, respectively. Nutrient removal from the soil was also calculated from changes in soil concentrations. Changes in soil N concentration indicated that the soybean and corn crop removed approximately 140 and 200 N ha-1. Analyses of the plant biomass indicated that approximately 140 and 200 kg N ha-1 had been accumulated in the soybean and corn crop, respectively. Similarly, P removals based on soil removal versus grain and biomass removal were not significantly different and averaged 62 kg ha-1. Thus, crop removal by the two methods was in excellent agreement for both P and N. There were no differences in the P or N removal rates between the two management practices, one in which all of the N requirement was supplied by manure and another where sidedressed N supplemented the amount of N added by manure application. These results suggest that P additions to the soil from manure application can be reduced without affecting crop production if sufficient N is added from sidedressed fertilizer.
The results from this study indicate that application of swine manure effluent at this farm according to the existing CNMP should supply N and P in sufficient amounts for crop production without leading to a further accumulation of N or P in the soil. There were no significant differences in the corn yields between the two manure management practices. Sparse rainfall during the early part of the 2006 growing season resulted in weather that was not typical of central Iowa. Therefore, far reaching conclusions from this research may not be possible. The use of soil characteristics in the topsoil as indicators of the potential of downward movement of soil N and P will be made more difficult by the large variations in soil N and P concentrations in this zone.
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