Several tropical grasses, particularly Urochloa humidicola (Rendle) Schweick.) (Syn. Brachiaria humidicola), have been associated with low soil nitrate content and reduced nitrogen (N) losses from pasture systems. Previous studies have detected that root exudates of certain Urochloa genotypes can inhibit ammonium oxidation in in vitro bioassay with Nitrosomonas and reduce net nitrification in soils, a phenomenon termed biological nitrification (BNI). However, net nitrification rates reflect the result of several N transformation processes that co-occur and together determine changes in NO₃⁻ concentrations in soils. Thus, to better understand the mechanisms underlying BNI, gross rates of nitrification, ammonification and inorganic N immobilization need to be assessed. In this study we used, for the first time, stable isotopes techniques to disentangle the different gross N transformation processes that determine net nitrification rates in soil of Urochloa genotypes differing in BNI. Intact soil cores were collected from two experimental sites in Colombia under different Urochloa genotypes, classified as low-BNI and high-BNI based on net nitrification rates determined in previous studies. Soil cores were labeled with (¹⁵NH₄)₂SO₄ and K¹⁵NO₃ to quantify gross N ammonification and nitrification, respectively and calculate the immobilization rates. Our results do not confirm lower gross nitrification rates under the high-BNI genotypes, despite reduced potential net nitrification rates and lower abundance of ammonia oxidising organisms, hence not supporting the widely accepted mechanism of BNI. Instead, the low potential nitrification rates could be explained by increased inorganic N immobilization under high-BNI genotypes, both in NH₄⁺-N and NO₃⁻-N form. The lack of differences might be explained by a strong N-limitation in our experiment, unlike previous BNI studies when large amounts of fertilizers were applied. Our results suggest that a combination of different mechanisms, particularly stimulation of N immobilization may be responsible for the BNI capacity observed as low NO₃⁻ soil content and reduced N losses.

几种热带草，特别是Urochloa Humticola（Rendle）Schweick。）（Syn。Brachiaria cubaticola），与土壤硝酸盐含量低和牧场系统中的氮（N）损失减少有关。先前的研究已经发现，某些Urochloa基因型的根系分泌物可以在亚硝化单胞菌体外生物测定中抑制铵氧化，并减少土壤中的净硝化，这种现象称为生物硝化（BNI）。然而，净硝化速率反映几个氮转化流程，同时出现并一起确定NO变化的结果₃ ^-在土壤中的浓度。因此，为了更好地理解BNI的机理，需要评估硝化，氨化和无机氮固定的总速率。在这项研究中，我们首次使用稳定的同位素技术来分解不同的总氮转化过程，这些过程决定了BNI不同的Urochloa基因型土壤的净硝化率。从哥伦比亚的两个不同Urochloa基因型的实验地点收集了完整的土壤核心，根据先前研究确定的净硝化率将其分类为低BNI和高BNI。土壤核心用（¹⁵ NH ₄）₂ SO ₄和K ¹⁵ NO标记₃分别量化总氨氮和硝化作用并计算固定率。我们的结果并未证实在高BNI基因型下的总硝化率降低，尽管潜在的净硝化率降低且氨氧化生物的丰度降低，因此不支持BNI的广泛接受机制。相反，在低电位硝化速率可以通过增加无机氮固定在高-BNI基因型解释的，无论是在NH ₄ ⁺ -N和NO ₃ ^--N形式。与以前的BNI研究不同，在大量施用肥料的情况下，差异的缺乏可能是由我们的实验中强烈的N限制所致。我们的研究结果表明，不同的机制，N固定化的特别刺激的组合可能是造成所观察到的低NO的BNI容量₃ ^-土壤含量和减少的氮损失。