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Agriculture and livestock production systems are two major emitters of greenhouse gases. Methane with a GWP (global warming potential) of 21, and nitrous oxide (N2O) with a GWP of 300, are largely emitted from animal production agriculture, where livestock production is based on pasture and feed grains. The principal biological processes involved in N2O emissions are nitrification and denitrification. Biological nitrification inhibition (BNI) is the natural ability of certain plant species to release nitrification inhibitors from their roots that suppress nitrifier activity, thus reducing soil nitrification and N2O emission. Recent methodological developments (e.g. bioluminescence assay to detect BNIs in plant root systems) have led to significant advances in our ability to quantify and characterize the BNI function. Synthesis and release of BNIs from plants is a highly regulated process triggered by the presence of NH4 + in the rhizosphere, which results in the inhibitor being released precisely where the majority of the soil-nitrifier population resides. Among the tropical pasture grasses, the BNI function is strongest (i.e. BNI capacity) in Brachiaria sp. Some feed-grain crops such as sorghum also have significant BNI capacity present in their root systems. The chemical identity of some of these BNIs has now been established, and their mode of inhibitory action on Nitrosomonas has been characterized. The ability of the BNI function in Brachiaria pastures to suppress N2O emissions and soil nitrification potential has been demonstrated; however, its potential role in controlling N2O emissions in agro-pastoral systems is under investigation. Here we present the current status of our understanding on how the BNI functions in Brachiaria pastures and feed-grain crops such as sorghum can be exploited both genetically and, from a production system's perspective, to develop low-nitrifying and low N2O-emitting production systems that would be economically profitable and ecologically sustainable.

Nitrification, a key process in the global nitrogen cycle that generates nitrate through microbial activity, may enhance losses of fertilizer nitrogen by leaching and denitrification. Certain plants can suppress soil-nitrification by releasing inhibitors from roots, a phenomenon termed biological nitrification inhibition (BNI). Here, we report the discovery of an effective nitrification inhibitor in the root-exudates of the tropical forage grass Brachiaria humidicola (Rendle) Schweick. Named "brachialactone," this inhibitor is a recently discovered cyclic diterpene with a unique 5-8-5-membered ring system and a gamma-lactone ring. It contributed 60-90% of the inhibitory activity released from the roots of this tropical grass. Unlike nitrapyrin (a synthetic nitrification inhibitor), which affects only the ammonia monooxygenase (AMO) pathway, brachialactone appears to block both AMO and hydroxylamine oxidoreductase enzymatic pathways in Nitrosomonas. Release of this inhibitor is a regulated plant function, triggered and sustained by the availability of ammonium (NH(4)(+)) in the root environment. Brachialactone release is restricted to those roots that are directly exposed to NH(4)(+). Within 3 years of establishment, Brachiaria pastures have suppressed soil nitrifier populations (determined as amoA genes; ammonia-oxidizing bacteria and ammonia-oxidizing archaea), along with nitrification and nitrous oxide emissions. These findings provide direct evidence for the existence and active regulation of a nitrification inhibitor (or inhibitors) release from tropical pasture root systems. Exploiting the BNI function could become a powerful strategy toward the development of low-nitrifying agronomic systems, benefiting both agriculture and the environment.

The tropical pasture grass, Brachiaria humidicola (Rendle) Schweick, produces nitrification inhibitory compounds (termed biological nitrification inhibitors or BNIs) in its shoot and root tissues and releases BNIs from its roots. In the present study, two BNI compounds were isolated and identified from the shoot tissue of B. humidicola using activity-guided fractionation. The recombinant Nitrosomonas europaea containing luxAB genes derived from the bioluminescent marine gram-negative bacterium Vibrio harveyi, were used to determine BNI activity. The BNI compounds in the shoot tissue were identified as linoleic acid (LA) and linolenic acid (LN) using authentic-chemicals obtained from ©Sigma (ED80 16.0 μg ml−1 for both LA and LN) for verification. None of the other tested free fatty acids namely stearic acid, oleic acid, arachidonic acid, and cis-vaccenic acid showed any inhibitory effect on nitrification. Among the fatty acid methyl esters (FAME) evaluated [methyl oleate, methyl linoleate (LA-ME) and methyl linoleneate (LN-ME)], only LA-ME showed an inhibitory effect (ED80 8.0 μg ml−1). The inhibitory effect of LA, LN and LA-ME in the soil was stable for 120 days at 20°C. Soil treated with LA, LN and LA-ME showed a very low accumulation of NO3 − and the maintenance of soil inorganic N in the NH4 + form. The inhibitory effect of LA-ME on soil nitrification was greater than that of LA or LN. In addition to BNI activity, both LA and LA-ME showed a suppressive effect on urea hydrolysis in soil. Both LA and LN blocked the AMO (ammonia monooxygenase) and HAO (hydroxylamino oxidoreductase) enzymatic pathways in Nitrosomonas. Since LA and LN can be produced from vegetable oils such as soybean, flax or sunflower, they have the potential for use as nitrification inhibitors in production agriculture. [ABSTRACT FROM AUTHOR]

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 or NO for 60 days and the release of BNI-compounds monitored. Only plants grown with NH released BNI-compounds from roots. The presence of NH 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 and NO grown plants responded to the stimulus from NH in the root environment. BNI-compounds found in root tissue and their release were nearly three times greater in NH grown than from NO 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 is of Type-I. In the absence of NH, 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 NH in the root environment is necessary for the sustained synthesis and release of BNI. [ABSTRACT FROM AUTHOR]