NH 4 + triggers the synthesis and release of biological nitrification inhibition compounds in Brachiaria humidicola roots

Abstract

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, NH 4 + or NO 3 -for 60 days and the release of BNI-compounds monitored. Only plants grown with NH 4 + released BNI-compounds from roots. The presence of NH 4 + 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 NH 4 + and NO 3 -grown plants responded to the stimulus from NH 4 + in the root environment. BNI-compounds found in root tissue and their release were nearly three times greater in NH 4 + grown than from NO 3 -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 NH 4 + is of Type-I. In the absence of NH 4 + , 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 BNIcompounds from B. humidicola roots is a regulated function and that presence of NH 4 + in the root environment is necessary for the sustained synthesis and release of BNI.

Soil nitrogen dynamics after Brachiaria desiccation

“…The presence of NH 4 + in the roots and the physiological consequences associated with N absorption, such as the acidification of the rhizosphere, appear to play important roles in the synthesis and release of BNIs by the roots of B. humidicola (Subbarao et al, 2007c(Subbarao et al, , 2009). The regulatory role of NH 4 + in the synthesis and release of BNIs appears to be adapted to protect NH 4 + from nitrifying agents and is thus a key factor in the evolutionary success of BNI production as a mechanism of plant adaptation (Subbarao et al, 2007c).…”

“…The regulatory role of NH 4 + in the synthesis and release of BNIs appears to be adapted to protect NH 4 + from nitrifying agents and is thus a key factor in the evolutionary success of BNI production as a mechanism of plant adaptation (Subbarao et al, 2007c). Thus, the lower pH observed in the rhizosphere of B. humidicola could also be attributed to its nitrification inhibition mechanism, at least in the rhizospheric region.…”

Evidence for biological nitrification inhibition inBrachiariapastures

“…Plants were grown with NH 4 ϩ or NO 3 Ϫ as sole N sources (38). Root exudates were collected using aerated treatment solutions and BNI activity was extracted (23); brachialactone levels were determined by using HPLC.…”

Repression of potential nitrification activities by matgrass sward species

“…As was shown by Subbarao et al (2006Subbarao et al ( , 2007cSubbarao et al ( , 2009a exudates from the roots of the grass species Brachiaria humidicola and Leymus racemosus were able to influence the activity of nitrifying bacteria. For B. humidicola, it was shown by Subbarao et al (2007b) that ammonium stimulates the synthesis and release of inhibition compounds.…”