Konrad Egenolf

University of Hohenheim, Institute for Plant Production and Agroecology of the Tropics and Subtropics

Challenges

Nitrate leaching and denitrification are the mayor pathways of N-losses from agroecosystems
and both are environmentally detrimental (Canfield et al., 2010). Nitrate
leaching negatively affects the quality of belowground water bodies with consequent
harmful effects on humans. In more detail, nitrate leaching contributes to the Novercharge
of groundwater bodies (and often encountered critical levels in our drinking
water) as well as terrestrial and maritime aquatic ecosystems, leading to the well-known
consequences of eutrophication and formation of so called coastal dead zones (Diaz et
Rosenberg, 2008). Hence, sustainable solutions for reducing nitrate losses from agroecosystems
are urgently required.
Biological Nitrification Inhibition (BNI) is one potential strategy to reduce nitrate levels
within the soil and hence the above mentioned consequences of leaching and
denitrification. BNI refers to the capacity of plants to retard the soil microbial
nitrification processes through the production and release of nitrification inhibitors into
the rhizosphere (Subbarao et al., 2006). Up to date, this effect has been described for
several, mainly graminaceous species and especially for perennial grasses (Subbarao et
al., 2007a). The tropical forage grass Brachiaria humidicola (B.h.) has gained major
attention due to its high BNI-activity

Objectives

Objective of the proposed PhD-work is to elucidate the influence of soil characteristics
(i.e. soil-pH and soil-type) on the BNI-performance of Brachiaria humidicola. The
relation will be assessed through growth chamber / rhizobox experiments (UHOH) and
fundamental results will be validated on contrasting field cites in Colombia (CIAT).

The proposed PhD work consists of three objectives:

• Objective I: Development of new biochemical and molecular methodologies allowing theaccurate detection of NI-compounds and their effects on microbial nitrification withspecial emphasis on bacteria AND archaea.
  

• Objective II: To use the developed methods to advance the currently limited understanding about the influence of soil-pH and soil-type on BNI-expression.

• Objective III: To validate the fundamental processes first observed under controlled conditions (rhizobox experiment) in field experiments allowing the environment specific quantification of BNI-performance of diverse B. humidicolas varieties.

 Methodology

• Objective I

Identification and quantification of mayor NI-compounds (e.g. brachialactone) by
HPLC-techniques will be supplemented by the determination of plant extracts/root
exudates NI-activity. The latter will be achieved by assessing the inhibitory effect on
bacterial and archaeal nitrifier communities (incubation studies).
Molecular techniques targeting the quantification of bacterial and archaeal amoA genes
(encoding the α-subunit of the enzyme ammonia monooxygenase) will be improved to
provide the basis for assessing microbial abundance (DNA) and nitrifying activity
(RNA) within investigated soils (field validation experiments).

• Objective II

The soil parameter dependent expression of BNI will be assessed within three-factorial
experiments with soil-type, soil-pH (adjustment at different levels) and plant BNIactivity
(contrasting B.h.-hybrids from current CIAT-breeding program) as investigated
factors.
The experiments will be conducted as greenhouse / rhizobox-trials, enabling sampling
of root material (
quantification of plant NI-secondary compounds), collection of root
exudates (
quantification of NI-compound exudation into the rhizosphere) anddetermination of overall decrease in soil nitrification potential in the soil (
incubation studies). NI-compounds will be quantified by techniques developed under Objective I.

• Objective III

Field validation of encountered relationships will be approached by assessing soil
microbial communities (molecular techniques developed under Objective I) and overall
N-recovery-efficiency (well-established 15N stable isotope techniques at Institute 380a)
on selected B.h. sites within the current CIAT project.

 Expected Results

The identification of soil-environmental conditions favouring BNI-expression and the
quantification of environment specific agronomical and environmental benefits are
essential to give sound cropping recommendations and make use of potential benefits of BNI.
The proposed PhD-work will contribute to a deeper understanding of this plant-microbe
interaction by especially clarifying the influence of the soil parameters pH and type on
the plant BNI-expression and the resulting inhibition of the microbial counterpart
(Bacteria versus Archaea) in different soil environments.