Document Type

Thesis - University Access Only

Award Date

1999

Degree Name

Master of Science (MS)

Department / School

Plant Science

Abstract

Leguminous cover crops may reduce erosion and nitrate leaching and improve soil qualities essential for long-term productivity. Some of these soil qualities include water infiltration (reflecting erosion potential) and N mineralization (reflecting N availability). One objective of this study was to determine the influence of medic interseeded into corn on water infiltration rate, aggregate stability, N mineralization, N uptake by corn, and corn yield. The experimental design was a randomized complete block with four replicates. The experiment was conducted at one site in 1997 (Aurora, SD) and two sites. in 1998 (Aurora, SD and Beresford, SD). Factorial treatments were two medic seeding rates (0 and 33 kg ha- 1) and two fertilizer rates (0 or 132 kg urea ha- 1) . Water infiltration was measured in June, July, and August with a double ring infiltrometer. Water infiltration rates were increased by medic in 1997 (33 .3 vs. 8.7 cm hr" 1) and in 1998 (48.4 vs. 12.6 cm hr"1). Medic also increased aggregate stability from May to June in 1997. Nitrogen mineralization was measured in field incubated soil cores. At Aurora, fertilized medic plots had a higher N mineralization rate 15 July to 16 August in 1997 in Aurora when compared to fertilized plots without medic (0.773 vs. 0.588 kg N ha·1 day" 1) . At Beresford in 1998, fertilized medic plots also had higher N mineralization rates compared with fertilized plots without medic between 4 August and 10 September in 1998 in Beresford (0.886 vs. 0.230 kg N ha·1 day" 1) . In fertilized plots, medic did not influence corn yield, although in unfertilized plots, medic reduced corn yield. The subsequent crop, oats, had a higher N percent (1.92 vs. 1.64 ¾N) in fertilized medic plots compared to fertilized plots without medic and plots without fertilizer. These findings indicate that sowing annual medic as a cover crop improved water infiltration and aggregate stability and changed temporal patterns of N mineralization. Measuring N mineralization is difficult in field soils. The problem becomes more complex when interactions between landscape position, treatments, and soil types occur. The second objective of this study was to develop a method for separating the impact of soil and landscape position on N mineralization in a field mineralization study. This experiment was conducted in a 10 ha field located in east central South Dakota. The rotation was continuous corn and the field was cultivated in the fall. Hog and/or cattle manure was disked or injected annually into the soil. In 1997, two transects (facing south and east) with grid points located 16 m apart were used for the study. Each transect contained summit, shoulder, backslope, and footslope positions. At each grid point, mineralization was measured in undisturbed soil and in a control soil from that landscape position. Nitrogen mineralization was measured by leaching field incubated soil columns (5 cm i.d. by 18 cm 1.) with 0.01 M CaCb monthly. Additional measurements included soil organic and total soil N, microbial biomass, corn silage N, and net N mineralization with rainout shelters. N mineralization was lower in shoulder, back- and footslope positions (43 to 96 kg N ha- 1) than in summit and toeslope positions (80 to 114 kg N ha-1) in the south-facing slopes. In addition to N mineralization, soil had low microbial biomass in the shoulder, back and footslope positions (9.6 to 27.8 mmol C mg- 1) and high biomass in the summit and toeslope positions (26.3 to 41.5 mmol C mg-1) at one of the two sites. The control samples, however, had opposite results, with higher mineralization rates in the shoulder, back- and footslope positions (86 to 154 kg N ha- 1) and lower rates in summit and toeslope positions (80 to 138 kg N ha- 1) on the south-facing slope. Mineralization rates are dictated in the field by the microclimate of each landscape position and by the characteristics of the soil at that particular site. By comparing the mineralization rate of intact soil cores and composited soil cores, we were able to separate differences due to landscape position from differences due to soil characteristics. For example, on the south-facing slope, intact core mineralization rates were low in the shoulder and backslope position. We know that mineralization at these positions was not due to climatic conditions because N mineralization in the composite cores was high at these positions. The south-facing aspect in this experiment had a higher overall mineralization rate, and higher potential for N mineralization. This is probably due to the higher temperatures in sites that face the sun. The highly sloping areas with the highest exposure to the sun, however, may have higher mineralization potential but lower actual mineralization because the sustained high N mineralization can lead to a depletion of N in those sites relative to C. The regression equation between mineralization estimated by mass balance and intact cores was y = 1.099 * x - 5.936. The equation indicates that the intact cores provided an unbiased estimate of actual field N mineralization. This approach can be used to: (i) improve estimates of N transformation kinetics in agricultural systems; (ii) improve fertilizer recommendations; and (iii) improve agricultural profitability. The intact and composite core approach for estimating field N mineralization is an improvement over the technique described by Burke et al. (1995) because it can be used to separate the impact of soil and climate on N mineralization. This method is very labor intensive and will work well with soils that drain adequately. This method will not work well with soils with high clay content and soils with many stones and rocks in the topsoil.

Library of Congress Subject Headings

Ground cover plants Medicago Soils -- Quality Soils -- Nitrogen content

Format

application/pdf

Number of Pages

86

Publisher

South Dakota State University

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