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Nutrient Cycling in Microdose Fertilized West African Soils Under Indigenous Vegetable Production

Date

2020-07-29

Journal Title

Journal ISSN

Volume Title

Publisher

ORCID

0000-0002-1181-307X

Type

Thesis

Degree Level

Doctoral

Abstract

Fertilizer microdosing is the application of a reduced fertilizer rate next to the seed within ten days of sowing. It is being promoted as a part of an Integrated Soil Fertility management (ISFM) technique in sub-Saharan Africa (SSA) that seek to grow indigenous vegetables economically and sustainably. The objective of this research is to understand how this ISFM technique affects yield, atmospheric nutrient losses, and soil properties in soils of two ecozones of SSA: i) the dry savanna (DS) (lna, Republic of Benin) and ii) rainforest (RF) (Ilesha, Nigeria) under cultivation with two indigenous vegetable species: local amaranth (Amaranthus cruentus (AC)) and African eggplant (Solanum macrocarpon (SM)). A combination of chemical and spectroscopic techniques was employed in field and phytotron studies. In the field studies, farm fields from both sites were cultivated with AC and SM, the soils were treated with 5 t ha-1 poultry manure and variable rates of urea. Soil samples were collected before planting and after harvest for analysis. RF soils had higher vegetable yields than the DS for both vegetables, and yield in soils of both ecoregions increased with increasing nitrogen (N) input up to 60 kg N ha-1 but decreased at 80 kg N ha-1. Using manure+urea increased vegetable yields in both ecoregions, and cultivating AC caused a reduction in soil pH and CEC in soils of both ecoregions. We found that the combination of manure and 40 kg N ha-1 is the optimum rate to increase the organic carbon content in soils of both ecoregions. X-ray absorption near-edge structure (XANES) spectroscopy was used to determine C, N, and P speciation in these soils. XANES analysis identified plant-derived C and N functional groups, and showed that soil texture and pH influenced the turnover rate of SOM in both ecoregions, with decomposition happening more rapidly in the DS than in the RF. Adsorbed and organic P were the two dominant P species in the manure amended DS soils before planting and after harvest in soils cultivated with both AC and SM, with the addition of urea (40 kg N ha-1) increasing the organic P form in DS soils cultivated with AC. RF soils cultivated with AC initially had large amounts of apatite P in the manure amended soils, which was transformed to adsorbed and organic P after harvest. Urea addition to the RF soils shifted the dominant P species from organic P to adsorbed and apatite P, which may limit P availability. In the phytotron study, soils from both ecoregions were cultivated with AC and treated with 5 t ha-1 poultry manure and and/or urea (80 kg N ha-1). Vegetable yield were higher in the RF soils than in the DS. Cumulative CO2 emissions over 21 days ranged from 497.06 to 579.47 g CO2-C kg-1 soil-1 day-1 in the RF, and 322.96 to 624.97 g CO2-C kg-1 soil-1 day-1 in the DS, while cumulative N2O emissions ranged from 60.53 to 220.86 mg N2O-N kg-1 soil-1 day-1 in the RF, and 24.78 to 99.08 mg N2O-N kg-1 soil-1 day-1 in the DS. In the RF soils, when compared to the use of urea alone, the use of manure and manure+urea reduced N2O emissions but led to an increase in emissions in the DS samples. ATR-FTIR analysis showed that the combined use of manure and manure+urea increased the rate of microbial decomposition in the DS soils, but no such effect was observed in the RF soils. We conclude that combining manure and urea fertilization has different effects on soils of the two ecoregions, and that RF farmers can reduce agricultural N2O emissions without compromising soil productivity and yield potential.

Description

Keywords

Sub-Saharan Africa, fertilizer microdosing, African leafy vegetables, synchrotron, sustainability, nutrient stewardship

Citation

Degree

Doctor of Philosophy (Ph.D.)

Department

Soil Science

Program

Soil Science

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DOI

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