Degree Type

Dissertation

Date of Award

2020

Degree Name

Master of Science

Department

Agronomy

Major

Soil Science ( Soil Fertility)

First Advisor

Marshall D McDaniel

Abstract

We lack an understanding of how long-term management practices, especially interactions of contrasting practices, affect plant nutrient uptake and this limits our ability to make recommendations and improve agroecosystem nutrient-use efficiency. Management practices like phosphorus (P) fertilization and no-tillage alter plant nutrient uptake individually, but how they interact to alter nutrient dynamics at the rhizosphere scale and in the initial root morphology is entirely unknown. In this study, I examined the main and interactive effects of 25-years of severe P limitation (and excess P fertilization) with no-tillage on the dynamics of phosphate (PO4-P), sulfate (SO4-S), and nitrate (NO3-N) and cations and extracellular phosphatase activity in rhizosphere soil; additionally, we examine the changes in root morphology and P uptake.Maize and soybeans were grown for 14 days in rhizoboxes with undisturbed soil samples that were collected from a 25-year, 2×2 factorial experiment crossing P fertilization (0 and 63 kg P ha-1) with tillage management [chisel plow (CP) and no-tillage (NT)]. Soil solution was collected from the rhizosphere (< 3mm from a root) and bulk soil (> 8mm from a root) through micro-suction cup lysimeters and analyzed on an ionic chromatograph for PO4-P and cations. Rhizosphere and bulk soil samples were also collected directly from the rhizosphere to analyze phosphatase activity. Root morphology was measured using a scanner and an image analyzer software, along with crop biomass and nutrient content. 25-years of no P fertilizer induced a significant depletion in plant-available P. Under excessive P fertilization, NT soils increased efficiency of P uptake in maize rhizosphere solution compared to CP by 20%. Other cations were not affected by P fertilization or tillage treatments. Unexpectedly, rhizosphere and bulk soil phosphatase activity did not differ; however, there were complex treatment effects that depended on the crop. NT reduced phosphatase activity by 360%, but only under maize. While under soybean long-term P fertilization reduced the enzyme activity by 75% compared to no P fertilizer for 25 years. Total root biomass, for both maize and soybean, were increased by 80% with fertilization. In maize, NT soils increased root diameter by 10%, root length by 67%, root surface area by 87%, and root forks by 114%. Fertilization did not change maize root morphology. In soybean, fertilization reduced length of fine roots. NT soils increased RL by 68 %, RS by 73%, RF by 77%. Crop nutrient uptake was not affected by tillage, rather only P uptake was increased by adding P fertilizer. Long-term effects of P fertilization and no-tillage had complex, interactive effects on maize and soybean rhizosphere nutrient dynamics. This highlights our lack of understanding of how even well-established management practices can have complex interacts to affect rhizosphere nutrient cycling, and provide insight into mechanisms for the individual and interactive effects on nutrient cycling. In these Midwest USA Mollisol soils, long-term P fertilization and no-tillage can affect early root growth and morphology, but ultimately not total plant nutrient uptake. Converting to NT seems to increase root biomass, increase metrics of root size and particularly with smaller root classes. These early effects of NT on root biomass and morphology may in part explain why NT consistently decreases yield, according to resource allocation theory whereby plants that are allocating resources to roots will not be to shoots.

DOI

https://doi.org/10.31274/etd-20210114-138

Copyright Owner

Tomas Javier Sitzmann

Language

en

File Format

application/pdf

File Size

136 pages

Share

COinS