9 research outputs found
Ammonium sorption and ammonia inhibition of nitrite-oxidizing bacteria explain contrasting soil NâO production
Better understanding of process controls over nitrous oxide (NâO) production in urine-impacted 'hot spots' and fertilizer bands is needed to improve mitigation strategies and emission models. Following amendment with bovine (Bos taurus) urine (Bu) or urea (Ur), we measured inorganic N, pH, NâO, and genes associated with nitrification in two soils ('L' and 'W') having similar texture, pH, C, and C/N ratio. Solution-phase ammonia (slNHâ) was also calculated accounting for non-linear ammonium (NHââș) sorption capacities (ASC). Soil W displayed greater nitrification rates and nitrate (NOââ») levels than soil L, but was more resistant to nitrite (NOââ») accumulation and produced two to ten times less NâO than soil L. Genes associated with NOââ»oxidation (nxrA) increased substantially in soil W but remained static in soil L. Soil NOââ»was strongly correlated with NâO production, and cumulative (c-) slNHâ explained 87% of the variance in c-NOââ». Differences between soils were explained by greater slNHâ in soil L which inhibited NOââ»oxidization leading to greater NOââ» levels and NâO production. This is the first study to correlate the dynamics of soil slNHâ, NOââ», NâO and nitrifier genes, and the first to show how ASC can regulate NOââ» levels and NâO production. © 2015 Macmillan Publishers Limited
Nitrification gene ratio and free ammonia explain nitrite and nitrous oxide production in urea-amended soils
The atmospheric concentration of nitrous oxide (NâO), a potent greenhouse gas and ozone-depleting chemical, continues to increase, due largely to the application of nitrogen (N) fertilizers. While nitrite (NOââ») is a central regulator of NâO production in soil, NOââ» and NâO responses to fertilizer addition rates cannot be readily predicted. Our objective was to determine if quantification of multiple chemical variables and structural genes associated with ammonia (NHâ)- (AOB, encoded by amoA) and NOââ» -oxidizing bacteria (NOB, encoded by nxrA and nxrB) could explain the contrasting responses of eight agricultural soils to five rates of urea addition in aerobic microcosms. Significant differences in NOââ» accumulation and NâO production by soil type could not be explained by initial soil properties. Biologically-coherent statistical models, however, accounted for 70â89% of the total variance in NOââ» and NâO. Free NHâ concentration accounted for 50â85% of the variance in NOââ» which, in turn, explained 62â82% of the variance in NâO. By itself, the time-integrated nxrA:amoA gene ratio explained 78 and 79% of the variance in cumulative NOââ» and NâO, respectively. In all soils, nxrA abundances declined above critical urea addition rates, indicating a consistent pattern of suppression of Nitrobacter-associated NOB due to NHâ toxicity. In contrast, Nitrospira-associated nxrB abundances exhibited a broader range of responses, and showed that long-term management practices (e.g., tillage) can induce a shift in dominant NOB populations which subsequently impacts NOââ» accumulation and NâO production. These results highlight the challenges of predicting NOââ» and NâO responses based solely on static soil properties, and suggest that models that account for dynamic processes following N addition are ultimately needed. The relationships found here provide a basis for incorporating the relevant biological and chemical processes into N cycling and NâO emissions models
Temperature alters dicyandiamide (DCD) efficacy for multiple reactive nitrogen species in urea-amended soils: Experiments and modeling
Dicyandiamide (DCD) is a nitrification inhibitor (NI) used to reduce reactive nitrogen (N) losses from soils. While commonly used, its effectiveness varies widely. Few studies have measured DCD and temperature effects on a complete set of soil N variables, including nitrite (NOâÂŻ) measured separately from nitrate (NOââŸ). Here the DCD reduction efficiencies (RE) for nine N availability metrics were quantified in two soils (a loam and silt loam) using aerobic laboratory microcosms at 5â30 °C. Both regression analysis and process modeling were used to characterize the responses. Four metrics accounted for NOâ⟠production and included total mobilized N, net nitrification, maximum nitrification rate, and cumulative NOâ⟠(cNOââŸ). The REs for these NOâ⟠-associated production variables decreased linearly with temperature, and in all cases were below 60% at temperatures â„22 °C, except for cNOâ⟠in one soil. In contrast, REs for NOâ⟠and nitric oxide (NO) gas production were less sensitive to temperature, ranging from 80 to 99% at 22 °C and 50â95% at 30 °C. Addition of DCD suppressed nitrous oxide (NâO) production in both soils by 20â80%, but increased ammonia volatilization by 36â210%. The time at which the maximum reduction efficiency occurred decreased exponentially with increasing temperature for most variables. The two-step nitrification process model (2SN) was modified to include competitive inhibition coupled to first-order DCD decomposition. Model versus data comparisons suggested that DCD had indirect effects on NOâ⟠kinetics that contributed to the greater suppression of NOâ⟠and NO relative to NOââŸ. This study also points to the need for NIs that are more stable under increased temperature. The methods used here could help to assess the efficacy and temperature sensitivity of other NIs as well as new microbial inhibitors that may be develope