Browsing by Subject "BNI"

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    Biochemical and ecophysiological characterization of BNI (Biological Nitrification Inhibition) by Brachiaria humidicola
    (2021) Egenolf, Konrad; Rasche, Frank
    In perennial grasslands, especially Brachiaria humidicola (syn. Urochloa humidicola) dominated swards, one hypothesized mechanism of high N efficiency is the plant exerted control of nitrification via the synthesis and release of nitrification inhibitors (NI) into the soil. This phenomenon has been conceptualized as Biological Nitrification Inhibition (BNI). This doctoral thesis was conducted with the aim of broadening our fundamental understanding on BNI ecophysiology, with a special emphasis on the edaphic parameters soil pH and soil texture as factors shaping the soil microbial community composition. The overarching objectives were to (1) screen root exudates of B. humidicola for major bioactive secondary metabolites with nitrification inhibiting activity, to (2) proof the significance of rhizosphere pH and nutritional N form for NI release and understand the underlying exudation mechanism, and to (3) elucidate the influence of soil pH and soil texture on the ammonia oxidizer community composition and BNI performance of B. humidicola. Root exudate screening via LC-MS and NMR techniques revealed several novel NI compounds with significantly higher NI activity compared to previously described brachialactone, i.e. the brachialactone isomers/derivatives 3-epi-brachialactone (ED50 ~ 20 µg ml-1, ED80 ~ 40 µg ml-1) and 3,18-epoxy-9-hydroxy-4,7-seco-brachialactone (ED50 ~ 40 µg ml-1) as well as the phenol aldehyde vanillin (ED50 ~ 12.5 µg ml-1, ED80 ~ 20 µg ml-1). In the case of the described brachialactone derivatives, internal tissue concentrations were extremely low (2-8 µg g-1 root DM), suggesting so far undiscovered cytosolic precursors. In the case of vanillin, its chemical proximity to other phenolic compounds previously described as NI, i.e. methyl-coumarate, methyl-ferulate and methyl 3-(4-hydroxyphenyl) propionate, drew the attention to phenylalanine and coumaric acid as common precursors and possible BNI breeding target. With regard to the NI exudation, the hypothesized positive effect of low rhizosphere pH and NH4+ nutrition was confirmed for both the brachialactone isomers/derivatives and vanillin. However, for 3-epi-brachialactone it was demonstrated that NH4+ did not constitute an essential prerequisite for NI synthesis and release. In contrast, NI release correlated with the transmembrane proton gradient, which in turn depends on soil pH and is favored by rhizosphere acidification occurring under cation-dominant nutrition (e.g. NH4+). These findings were considered as evidence for an active NI release via secondary transporters (possibly MATE transporters). The effects of soil pH and soil texture on the ammonia oxidizer community and BNI performance of B. humidicola were investigated through a three-factorial pot trial including liming and different soil types as experimental factors. No clear conclusion could be drawn with regard to the hypothesized effects of soil pH, soil texture and the ammonia oxidizer community composition on BNI performance of B. humidicola. In the presented pot trial, B. humidicola reduced net nitrification rates by 50-85% compared to the non-planted control, but this reduction was observable irrespective of soil pH, soil texture and the ammonia oxidizer community composition. Furthermore, the reduction of net nitrification was largely dependent on microbial N immobilization and efficient plant N (probably NO3-) uptake rather than BNI. This absence of a clear BNI effect was mainly attributed to high N inputs, which is in accordance with previous studies indicating that BNI was impaired in high nitrifying environments. The argument was underlined by theoretical enzyme-kinetic calculations, revealing a strong influence of substrate (NH4+) availability on soil nitrification dynamics, but as well BNI performance: Assuming soil NI concentrations at ED50 (~ effective dose 50% inhibition), it could be shown that – with the only exception of AOA populations suppressed by non-competitive inhibitors – the efficacy of NI is severely disrupted by increasing soil NH4+ availability. Besides contrasting AOA and AOB sensitivities towards NI, the inter-domain differences of ammonia-monooxygenase (AMO) kinetics probably delivers an additional explanation for the observation, that under field conditions BNI has mainly been confirmed for AOA, and to a lesser extent for AOB. Based on the findings of the presented doctoral thesis, it was concluded that BNI may play an important role in extensive B. humidicola pasture systems, especially on acid, coarse textured and AOA dominated soils. Intensification, especially increasing N amendments, will most likely disrupt the nitrification inhibiting effect and under these circumstances, N immobilization and efficient plant N uptake may display the dominant factors controlling net nitrification rates.