Nitrification in Dutch heathland soils

This thesis is the result of a study on the production of nitrate in Dutch heathland soils. Most of the heathlands are located on acid, sandy soils. Therefore , it has dealt mainly with the occurrence, nature and mechanisms of nitrification in acid soils. In the Netherlands, the production of nitrate in acid soils is a matter of great concern because it has become obvious that this process contributes to a continuous acidification of such soils that are exposed to high levels of deposition of ammonia and ammonium. Continuous acidification of sandy soils has a negative impact on the vitality of trees. Moreover, the groundwater may become polluted with nitrate and aluminum.As almost all Dutch heathlands are semi-natural in origin, management practices, such as sod- cutting, are needed to maintain the original dwarf-shrub vegetation. At present, the replacement of dwarf-shrubs by grasses is a big problem for management. This change in dominancy coincides with the increase in the availability of mineral nitrogen for plants which, in turn, is attributed to high levels of deposition of NO x and NH x originating from industry, traffic and intensive animal husbandry. Grasses may become dominant under conditions of relatively high nitrogen availability.In this study, it was indicated that the high availability of mineral nitrogen has also resulted in the general occurrence of nitrification in Dutch heathland soils (Chapter 2 and 4). Nitrification was not restricted to heathlands that were dominated by certain plant species. Nitrification in Dutch heathland soils appeared to be regulated by the availability of ammonium and oxygen whereas the availability of phosphate may also be important (Chapter 2 and 4). The presence of nitrification was not dependent on the pH of the heathland soils. Most of the production of nitrate in Dutch heathland soils was shown to take place in the organic horizon (Chapter 2 and 3).Using inhibitors it could be concluded that chemolithotrophic bacteria are the main nitrifying micro- organisms in Dutch heathland soils, despite their low pH (Chapter 3 and 4). The effect of pH on the activity of chemolithotrophic nitrifying bacteria was studied using suspensions that were maintained at pH 4 or 6. During these experiments nitrite never accumulated in detectable amounts. Therefore, it is concluded that the rate of production of nitrate is dependent on the activity of ammonium- oxidizing bacteria. Evidence is presented for the existence of chemolithotrophic ammonium-oxidizing bacteria that are active at pH 4 (Chapter 3). These acid-tolerant bacteria are present in many heathland soils (Chapter 4). In suspensions of pH 4 production of nitrate was exponential. This implies that the acid-tolerant ammonium-oxidizing bacteria are able to grow at low pH values (Chapter 3). The in situ oxidation of ammonium in many Dutch heathland soils is most likely caused by acid-tolerant, ammonium-oxidizing bacteria (Chapter 3 and 4). Unfortunately, little is known about the characteristics of these bacteria because attempts to isolate them have thusfar been unsuccesful. The isolation of acid-tolerant ammonium-oxidizing bacteria is hindered by their inability to be active at low pH without the presence of humus particles (Chapter 3 and 8).Acid-sensitive ammonium-oxidizing bacteria are also present in Dutch heathland soils. These bacteria are not able to nitrify in suspensions at pH 4. It is believed that the nitrifying activity of these bacteria in acid soils is mainly restricted to micro-sites with a relatively high pH. In this study two additional mechanisms were proposed to explain the activity of acid-sensitive ammonium- oxidizing bacteria in an acid soil without the need for assuming the presence of such micro-sites.- The first mechanism deals with the activation of acid-sensitive ammonium-oxidizing bacteria by ammonia produced by ammonifying micro-organisms (Chapter 5 and 8). This possibility was proposed to explain that, at low pH, nitrification seemed to be linked to net nitrogen mineralization in suspensions of heathland soils that contained only acid-sensitive ammonium-oxidizing bacteria (Chapter 5).- The second mechanism deals with the activation of acid-sensitive ammonium oxidizing bacteria by urea. Urea stimulated the production of nitrate at pH 5 in many suspensions of heathland soils containing acid-sensitive ammonium-oxidizing bacteria (Chapter 3, 4 and 6). An acid-sensitive ammonium-oxidizing strain isolated from a heathland soil showed urease activity (Chapter 6). This strain was able to nitrify at pH 4.5 in a medium containing urea, whereas it was not capable of doing so at pH values lower than 5.5, in a medium containing ammonium (Chapter 7).Acid-sensitive ammonium-oxidizing bacteria are however not believed to be important for in situ production of nitrate in Dutch heathland soils (Chapter 3 and 4).Information concerning nitrite-oxidizing bacteria in heathland soils is very scarce at the moment. Still, it is obvious that acid-tolerant nitrite-oxidizing bacteria are present in heathland soils that showed nitrate production in suspensions of pH 4. It appeared that, even at pH 4, acid-tolerant nitrite-oxidizing bacteria were able to efficiently oxidize the nitrite produced by ammonium-oxidizing bacteria (Chapter 8).