Masters Theses

Date of Award

8-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Michael D. Mullen

Committee Members

Don Tyler, Gary Lessman

Abstract

This study was conducted to compare the effects of three types of cover situations, wheat, hairy vetch, and previous corn residue, and two rates of nitrogen fertilizer, 0 and 168 kg N ha-1 on soil biological and biochemical properties in long term, no-till corn plots. The research plots were located in Milan,TN at the Milan Experiment Station on a Loring silt loam (fine-silty, mixed, thermic, Typic Fragiudalf), and were arranged in completely randomized split plot with four replicates. Plots were randomly sampled twelve times, four samples within the row and four times at 18.3 and 37.5 cm. from the row center. Samples were separated into three depths: 0-5,5-10, and 10-20 cm. Four 37.7 by 37.7 cm. squares of mulch were randomly collected from each plot. Soils were analyzed for total carbon, total nitrogen, soil pH, total exchangeable bases, bacteria, fungi, Aspergillus flavus, fluorescent pseudomonads, cellulose degrader, and Bacillus spp. enumerations, microbial biomass carbon and nitrogen, acid phosphatase, alkaline phosphatase, β- glucosidase, arylsulfatase, L-asparaginase, and wet aggregate stability. Mulch samples were analyzed for total carbon and nitrogen content. Soil carbon and nitrogen values were significantly greater under vetch cover, due to the higher carbon and nitrogen concentration in the vetch mulch. Soils of the 168 N rate plots had significantly greater carbon and nitrogen than those of the low rate, due to increased crop mulch and available nitrogen associated with the fertilizer. Soil pH was significantly lower in vetch plots compared to wheat or previous corn residue plots, due to higher nitrogen mineralization and nitrification from fixed and/or fertilizer nitrogen. Fertilizer significantly decreased soil pH, because nitrification generates acidity. Exchangeable potassium and phosphorus concentrations were higher in previous corn residue soils due to increased corn yields in cover plots. Soil potassium and phosphorus concentrations were higher in 0 N rate plots due to decreasing availability with decreasing pH. Vetch mulch contained greater carbon and nitrogen due to the ability of symbiotic nitrogen fixation. Mulch from plots receiving the 168 N rate held greater carbon and nitrogen due to increased nutrient availability. Vetch plots yielded the greatest quantity of corn due to the nitrogen-rich vetch mulch. Plots receiving the 168 N rate yielded more corn due to greater nitrogen availability. Vetch plots with the 0 N rate and wheat plots with the 168 N rate yielded the greatest cover crop dry matter. The wheat yields in the ON treatments were lowest due to lack of available nitrogen. The pH was significantly lower under vetch with 168 N treatments. Lower pH results in less vigorous legume growth. Vetch plots had greater numbers of bacteria, fungi, and Bacillus spp. due to the nitrogen-rich vetch mulch. Bacterial numbers were significantly greater in vetch plots with the 0 N rate, because fertilizer suppresses the high microbial activity of vetch plots. Aspergillus flavus, actinomycetes, and cellulose degrader counts were greater under wheat mulch based on their fungal nature to degrade cellulosic material. Fluorescent pseudomonad numbers were greater under vetch due to carbon and nitrogen concentrations in the mulch. Bacteria, actinomycetes, and Bacillus numbers were greater under plots receiving the 0 N rate due to soil acidification ensuing fertilizer amendment. Fungi, Aspergillus flavus, fluorescent pseudomonads, and cellulose degraders flourish in plots receiving the 168 N rate due to fungal and pseudomonad resistance to acidity. Microbial biomass nitrogen was greater in wheat plots with the 168 N rate and vetch plots with the 0 N rate, supporting enumeration data. All enzyme activities were significantly greater under vetch covers, due to the high carbon and nitrogen content of soils under vetch mulch. All enzyme activities except acid phosphatase and β-glucosidase were significantly greater in plots receiving the 0 N rate. Acid phosphatase and β-glucosidase are active at a lower pH range than the others. Soil aggregates were larger and more stable under vetch cover. A greater portion of total soil aggregates were retained on the 2 mm sieve in vetch plots. Overall, the greatest microbial numbers and activities came from soils of vetch plots receiving no nitrogen. Vetch efficiently fixes a sufficient amount of nitrogen. Any excessive nitrogen fertilizer on these plots causes reduced enzymatic activities and proliferation due to soil acidification.

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