NH4+ triggers the synthesis and release of biological nitrification inhibition compounds in Brachiaria humidicola roots
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- Subbarao, G.V., Wang, H.Y., Ito, O. et al. Plant Soil (2007) 290: 245. doi:10.1007/s11104-006-9156-6
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The release of chemical compounds from plant roots that suppress soil nitrification is termed biological nitrification inhibition (BNI). Determining the environmental factors that control the synthesis and release of BNI-compounds from Brachiaria humidicola (Rendle) Schweick, a tropical pasture grass that thrives on acid soils, is the focus of this investigation. Because the BNI trait is related to the N status of the plant, we investigated the possibility that the expression of this trait would be related to the forms of N found in the root environment. Plants were grown with two sources of N, NH4+ or NO3− for 60 days and the release of BNI-compounds monitored. Only plants grown with NH4+ released BNI-compounds from roots. The presence of NH4+ and possibly the secondary effect of its uptake (i.e., acidic pH) in the root environment significantly enhanced the release of BNI-compounds. Both the NH4+ and NO3− grown plants responded to the stimulus from NH4+ in the root environment. BNI-compounds found in root tissue and their release were nearly three times greater in NH4+ grown than from NO3− grown plants. The BNI-compounds released from roots composed of at least three active components—Type-I (stable to pH changes from 3.0 to 10), Type-II (temporarily loses its inhibitory effect at a pH higher than a threshold pH of 4.5 and the inhibitory effect is reestablished when the root exudate pH is adjusted to <4.5) and Type-III (inhibitory effect is irreversibly lost if the pH of the root exudate reaches 10.0 or above). A major portion of BNI-compounds released in the presence of NH4+ is of Type-I. In the absence of NH4+, mostly Type-II and Type-III BNI-compounds were released. The BNI-compounds inhibited the function of Nitrosomonas europaea through the blocking of both ammonia monooxygenase and hydroxylamino oxidoreductase pathways. These results indicate that the release of BNI-compounds from B. humidicola roots is a regulated function and that presence of NH4+ in the root environment is necessary for the sustained synthesis and release of BNI.