Agricultural nutrient management practices are an important component of the effort to improve water quality in the Mississippi River Basin. In particular, there is concern about nutrient export to the Gulf of Mexico via loss from subsurface drainage systems. Optimizing the use of fertilizers and animal manures in combination with other management practices has the potential to minimize negative impacts on water quality. Farmers are increasingly challenged to maximize crop production while utilizing fertilizers and animal manures in an efficient and environmentally friendly manner. An increasing awareness of the potential to address these issues by improving soil health has also led to significant interest in quantifying the impact of cropping management practices on soil health indicators.
The objectives of the first study in this thesis were to investigate the effects of crop rotation, tillage, corn residue removal, swine manure, and cereal rye (Secale cereale) cover crops on nitrate-N (NO3-N) and dissolved P (PO4-P) losses via subsurface drainage. The study was evaluated from 2008 through 2015 using thirty-six 0.4 ha plots outfitted with a subsurface drainage water quality monitoring system. Results showed that swine manure applied prior to both corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) had significantly higher flow-weighted NO3-N concentrations compared to swine manure applied before corn only. Total NO3-N losses from the treatments ranged from 15.2 to 29.5 kg N ha-1 yr-1. The cereal rye cover crops reduced NO3-N loss, whereas tillage and residue management had little impact. The treatments had minimal impact on PO4-P leaching. Total PO4-P losses averaged \u3c 32 g P ha-1 yr-1 from all treatments, which was very low in comparison to some drainage studies in the upper Midwest.
The objectives of the second study were to evaluate the effects of liquid swine manure application timing, crop rotation, cereal rye cover crops, and a nitrification inhibitor on drainage water quality and grain yields. The study was evaluated in 2016 and 2017 on the same plots as the first study. Results showed that early fall applied swine manure (EFM) with a rye cover crop resulted in significantly lower 2-yr average NO3-N concentrations and losses compared to EFM without a cover crop in a corn–soybean (CS) rotation. Spring application of urea ammonium nitrate (UAN) had significantly lower 2-yr average N losses compared to treatments receiving EFM or LFM in CS rotations. Average rye N uptake was significantly greater in plots receiving EFM application prior to corn (88 kg N ha-1) compared with plots receiving no manure prior to soybeans (51 kg N ha-1). There were no significant differences in 2-yr average NO3-N concentrations or losses in continuous corn (CC) treatments receiving either LFM or spring manure (SM). Yields in CC were significantly higher with SM compared to LFM. In CS rotations, 2-yr average corn yields were greatest with spring-applied UAN \u3e LFM without cover crop \u3e EFM without cover crop = EFM with cover crop. Soybean yields were lowest in the EFM with cover crop treatment. Delaying manure application until late fall rather than early fall, or spring rather than late fall, resulted in an economic advantage due to higher corn yields. The results indicate that the rye cover crop was effective for improving water quality and capturing N from manure.
The third study investigated the long term effects of tillage, corn residue removal, liquid swine manure applications, and cereal rye cover crops on soil health indicators. Total soil carbon (TC) levels were monitored annually to a depth of 120 cm over a 10-yr period from 2007 to 2016. In the spring of 2017, soil cores were taken to a depth of 15 cm and analyzed for TC, aggregate size distribution, bulk density, potentially mineralizable nitrogen, pH, P, and K levels. This data was evaluated with the Soil Management Assessment Framework to quantify an overall soil quality index (SQI) score in five different treatments. Results show that from 2007 to 2016, TC levels remained unchanged to a depth of 30 cm but increased in all treatments at a depth of 60 to 90 cm. There were also increases at the 30 to 60 cm and 90 to 120 cm depths in some treatments. The rate of change in TC at a given depth did not differ between treatments. In the 2017 soil cores, aggregate size distribution differed between treatments but there was no significant difference in the overall fraction of aggregates \u3e 212 ïÿým. Bulk density levels were significantly higher in no-till compared to treatments with tillage. Potentially mineralizable nitrogen levels did not differ significantly between treatments. Overall SQI scores in the five treatments differed significantly, with CC treatments having the highest SQI score and a no-till + cover crop treatment having the lowest SQI due to higher bulk density and lower TC than other treatments
