Soil and Cover Crop Responses to Liquid Swine Manure Application
By Cynthia A. Cambardella, Jeremy W. Singer, and Thomas B. Moorman, USDA-ARS National Soil Tilth Laboratory, Ames, Iowa
Large-scale pork production is a major agricultural enterprise in the Midwest. Large numbers of confined hogs produce about 50 million tons per year of swine manure in Iowa alone. Rapid expansion of concentrated animal feeding operations (CAFOs) has resulted in increased concentrations of manure nutrients in surface waters, which contribute about 15 percent of the total nitrate load in the Mississippi River Basin. Producers are being encouraged to develop manure management practices that fulfill crop production requirements while minimizing the potential for environmental pollution.
The most commonly used manure management practice in the Midwest involves fall application to land where corn will be grown in the subsequent growing season. Fall planted annual cover crops can capture manure nutrients and immobilize them in plant biomass, subsequently reducing the potential for nutrient loss through run-off or leaching. Decomposition of cover crop residue the following spring may help synchronize manure nitrogen (N) availability and corn N uptake, improving nutrient-use efficiency within the crop rotation.
Description of Experiments
We conducted experiments to evaluate the effects of integrating a rye/oat cover crop with liquid swine manure application on retention of manure N in a corn-soybean cropping system (Figure 1a and 1b). Our objectives were to compare soil N changes after manure application with and without a cover crop and to evaluate cover crop and soil N response for three manure-N rates (Table 1). Target N rates for manure application were 0, 100, 200, or 300 lb N/ac. Liquid swine manure was injected about six to eight weeks after a 70 percent rye/30 percent oat cover crop mixture was drop-seeded in soybean. Manure was injected to a depth of 5 inches using a narrow-profile knife designed to minimize soil disturbance.
We measured cover crop shoot biomass and N and phosphorus (P) uptake in mid-November and mid-April following manure injection. Surface soil (0-8 in) inorganic N in the manure injection band was quantified every week for up to 6 weeks after manure application and in the following spring before and up to 6 weeks after killing the cover crop prior to corn planting. Soil profile (to 48 inches in 8 inch increments) inorganic N was also quantified before manure application in the fall and before the cover crop was killed the following spring.
Soil Inorganic Nitrogen
Surface soil nitrate-N concentrations were more than 30 times higher in the fall of 2005 than in 2006. Nitrate-N was significantly lower under the rye/oat cover crop at 22 days after manure application in the fall of 2006 (0.80 ppm with cover crop; 3.22 ppm without cover crop), but in 2005, the difference wasn’t apparent until 42 days after manure application (56 ppm with cover crop; 89 ppm without cover crop). September and October were significantly warmer and drier in 2005 than in 2006 (Table 2). Soil nitrate-N production increases with increasing temperature and nitrate-N leaching potential increases with increasing rainfall. Therefore, significant amounts of soil nitrate-N were probably lost from the top 8 inches of soil in 2006 compared to 2005.
The rye/oat cover crop reduced nitrate-N in the surface soil and nitrate-N leaching beneath the manure band measured in April of 2006 (Figure 2). Total soil profile inorganic N content was positively related to manure N application rate and was significantly lower under the rye/oat cover crop (Table 3).
Cover Crop Production and Nutrient Uptake
Aboveground cover crop biomass production was greater in the spring (1201 lb/ac) than in the fall (268 lb/ac) for both years. Increasing the manure N rate from 0 to 300 lb N/ac had no effect on cover crop shoot biomass in the fall (368 lb/ac in 2005; 167 lb/ac in 2006) or spring (1588 lb/ac in 2006; 815 lb/ac in 2007) of either year.
Similarly, aboveground cover crop biomass N and P uptake didn’t differ for the three manure N rates and the non-manured control in the fall (11.1 lb N/ac and 1.1 lb P/ac in 2005; 5.7 lb N/ac and 0.3 lb P/ac in 2006), but N and P uptake after application of at least 200 lb N/ac of manure N was significantly greater than the control in the spring (34.7 vs 76.4 lb N/ac and 6.9 vs 12.6 lb P/ac in 2006; 29.0 vs 43.4 lb N/ac in 2007).
We have demonstrated that a rye/oat cover crop reduces soil inorganic N after liquid swine manure injection. Cover crop impacts on soil N are observed within a month after application and persist into the following spring. Cover crop nutrient uptake was higher than the control in the spring when at least 200 lb manure N/ac was applied. These results quantify the potential for cover crops to enhance plant nutrient uptake and reduce N leaching potential in farming systems using manure. Our future research will investigate when cover crop nutrients are released and become available to subsequent crops.