I have a number of interesting (at least to me) things that could be said if I had time to say them. For now I'll launch this drive-by post about this paper.

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
-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 (NH4+) in the root environment. Brachialactone release is restricted to those roots that are directly exposed to NH4+. 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 [biological nitrification inhibition] function could become a powerful strategy toward the development of low-nitrifying agronomic systems, benefiting both agriculture and the environment.

The earlier post Science Babble asserted that:

nitrous oxide emissions are the natural nitrogen cycle. Atmospheric nitrogen is fixed and then subsequently digested by bacteria in multi-step processes that end in N2, but with intermediate steps - coming and going - that include gases such as ammonia and nitrous oxide. Sooner or later, one way or another, all of the nitrogen in every plant will be recycled back to the atmosphere where it came from in the first place. It will not accumulate in the biosphere since it's like candy to living things and they eat it until it is all gone.

There's fierce competition for nitrogen and so it's not really surprising that there are plants that have found ways to do battle with soil bacteria that would otherwise contribute to loss of food for the plants. What is interesting is the indirection. The plants don't inhibit the denitrifying bacteria directly, they inhibit nitrifying bacteria that provide food for the denitrifiers, since plants can use ammonium as a nitrogen source.

The paper further asserts that plants that take up nitrate emit nitrous oxides from their leaves wheras those that take up ammonium for their nitrogen source do not. Since ammonium is a cation it is less mobile in soil and doesn't leach away like nitrate. Inhibiting nitrifying bacteria would then benefit the plant even more.

Nitrate fixing bacteria that either associate with legumes or are free living bypass inhibition since they directly produce nitrate, but the majority of commerical nitrogen fertilizers and all organic nitrogen goes though an ammonium step on the way to nitrate. It's a good place to intervene in the nitrogen cycle.

Some grasses such as wild wheat can do this trick, which suggests that it might be possible to teach other food grasses such as wheat and maize to do it too.