Abstract of PhD Thesis

Assessment and regulation of N-mineralisation in a grassland soil

Patrick Dundon (2006) - University of Limerick

Nitrogen (N) is a crucial determinant of grassland productivity and the majority of contemporary grassland enterprises are dependent on anthropogenic additions of organic and/or inorganic N to maintain sward yields. In recent years, concerns have grown over the impact of N-fertilisation of grassland pastures on the environment, in particular, water quality. In order to address these concerns, the European Union published Directive 91/676/EEC in December 1991, commonly referred to as the ‘Nitrates Directive’. The objective of this directive was to reduce water pollution caused by N loss from agricultural sources.

One of the key stipulations of the Nitrate Directive was the grassland managers should take into account the natural supply of N through mineralisation of organic N in soil. The absence of a standardised methodology for measuring values of N-mineralisation in grassland soil constitutes a major obstacle to grassland managers who wish to take into account the contribution of mineralised N to the pool of inorganic N in soil. Furthermore, mineralisation of biologically fixed N in organic grassland soils is poorly understood, and hence, the release of mineralised N from soil organic matter is poorly controlled. Thus, the twin aims of this thesis were to examine the in vitro arginine ammonification method as an applicable measure of N-mineralisation in soil and to investigate regulation of N-mineralisation by readily available carbon (C) substrates and inorganic N nutrients in soil.

This study was conducted on a grassland farm at Patrickswell, Co. Limerick. Values of in vitro arginine ammonification and soil factors known to influence N-mineralisation were measured at both the micro-plot scale and the plot scale. Values of in vitro arginine ammonification reflected the influence of depth of soil, soil moisture, organic matter, and pH on values of N-mineralisation in the field. Values of in vitro arginine ammonification were also compared with values of N-mineralisation measured in the field, via the in situ incubation method. No relationship was found between in vitro and in situ values of N-mineralisation with the exception of values of in vitro arginine ammonification measured at the conclusion of the seven-day incubation period. Seven-day field incubation of in situ cores affected the ability of soil microorganisms to mineralise native soil organic matter, and hence, the applicability of the in situ method as a legitimate measure of N-mineralisation in soil is questionable. Values of in vitro and in situ N-mineralisation were correlated with values of grassland production namely, sward height and herbage dry-matter production. Values of in vitro N-mineralisation were positively related to the quantity of readily available C substrates in soil. Readily available C substrates constitute a pool of readily mineralisable N in soil and it appears that readily available C substrates regulate N-mineralisation at the level of sufficient quantity of enzymes present to use the available substrate. In contrast, under conditions of readily available C- and inorganic N-limitation in soil, it appears that microbial mineralisation of organic N is regulated by ammonium-N (NH4 +-N) catabolite repression.