The influence of nitrification in determining the supply, distribution and fate of nitrogen in grassland soils

Abstract

The microbiology of nitrification has been extensively studied, but the ecology and environmental impact of the process has received less attention. The reason for this has more to do with the difficulties of conducting field experiments to examine the links with other processes which cause losses of nitrogen, than a failure to appreciate its importance in agricultural systems. This project was designed to overcome some of the limitations of existing field techniques to enable simultaneous measurements of nitrification and the major processes of N supply (mineralization) and N loss (denitrification and leaching) to be examined. The study proceeded in three distinct phases: firstly, soils with contrasting N management histories were examined, using laboratory assays for potential activities. Clear differences which resulted from higher N inputs were established, with correspondingly higher nitrifying activities. For example, in a fertilized soil, ammonia-oxidizers produced 48.4 compared with 1.3 nM N02 gˉ¹ soil hˉ¹ in an unfertilized soil. Potential nitrite-oxidizing rates were 93.4 and 62.5 nM gˉ¹ hˉ¹ respectively. Assays of enzyme kinetics, therefore confirmed the higher nitrifying activity in the fertilized soil, but demonstrated a lower affinity of the enzyme for N02- substrate, with K, values of 436 and 310 µM N02 -N, respectively. Nitrifying rates in soils from grass-clover swards were intermediate between the fertilized and unfertilized soils. Secondly, a new field incubation technique was developed and used to obtain actual rates by concurrent measurements of the major N cycling processes. A strong correlation was established between nitrification and denitrification (r² = 0. 98). The measurements were verified by comparison with other independent methods. Net rates of nitrification in the same soil type ranged from 0.55 - 1.17 kg N haˉ¹ dˉ¹ , with the highest rates in the fertilized soil. Over 70% of the mineralized N was nitrified, of which 80% was subsequently lost (i.e. either denitrified or leached). Thirdly, the practical implications of these findings were examined in greater detail using 15N labelling techniques which enabled process rates (net and gross) to be established in a model of the N cycle. When nitrification was inhibited, there were no significant differences between gross or net mineralization rates in the soils from the three swards, which indicated that N-immobilization could be directly influenced by the level of nitrifying activity in these soils. The influence of nitrification in determining the pathways of N loss from grassland soils was quantified in this study. From a detailed investigation of the processes involved in N cycling, it was deduced that nitrification was also one of the major factors in determining the outcome of competition for inorganic N between plant and microbial biomasses.