Moot exudates of Brachiaria humidicola inhibit nitrification characterization and quantification of this unique biological phenomenon

摘要

A major portion of the fertilizer-N (which is usually or converted to NH_4~+ form) is rapidly converted into NO_3 by soil nitrifiers, Nitrosomonas and Nitrobacter. Nitrification associated nitrogen losses result in poor N recovery (often less than 30% of the fertilizer N added) in most agricultural systems. Tropical pasture grasses such as Brachiaria humidicola have been reported by us earlier to have the ability to suppress nitrification in soils. The objective of this investigation is to characterize the inhibitory effect on nitrification from root exudates of B. humidicola. A bioassay that uses a recombinant construct of Nitrosomonas to detect and quantify this inhibitory effect from root exudates (NI activity) has been developed. Root exudates from hydroponically grown plants of B. humidicola and soybean are collected by keeping the intact plant roots in aerated distilled water, and the root exudates are condensed at 45℃ using rotary-evaporators. The nitrification inhibitory (NI) activity of root exudates was determined using bioassay and soil incubation studies. Substantial NI activity was detected in the root exudates of B. humidicola, but not with soybean, where root exudates have stimulated nitrification. The inhibitory effect from B. humidicola root exudates on soil nitrification was stable (up to 75 days) and effective (ranging from 50% to 70% from 10 AT units of NI activity added to g~(-1) soil) than the most commonly used synthetic nitrification inhibitor, Nitrapyrin of 4.5 ppm (about 20% inhibition). Root exudates have specifically blocked the AMO (aminomonooxygenase) enzymatic pathway in Nitrosomonas, thus making the nitrifying bacteria dysfunctional. Bioassay that is developed to quantify NI activity from roots can serve as a powerful tool in evaluating field crops and pastures for their ability to inhibit nitrification in soil. From a genetic improvement perspective, this powerful bioassay can open the way for evaluation of germplasm for selecting high NI activity genetic stocks in. B. humidicola and other tropical pastures.