What Soil Nitrogen Indices and Climatic Variables Are Needed to Predict Grass Growth Under a Temperate Maritime Climate?.

Current nitrogen (N) fertilizer recommendations for grassland fail to account for the supply of N provided through the mineralization of soil organic matter (SOM). The “one soil fits all” approach risks under- or over-supplying N fertilizer. Recent Irish studies have identified a number of soil N indices suitable for predicting mineralizable N (MN) and grassland N uptake in controlled environments. The objective of this study was to examine the temporal variability and effectiveness of these soil N indices along with measured climatic variables to predict daily N supply for herbage N uptake and grass dry matter (DM) production over a 15 month sampling period on two contrasting Irish grassland sites: (1) moderately drained gleyic luvisol and (2) well drained cambisol, in untreated control plots (T0) and lime treated plots (TL). Mineralizable N, as measured using the Illinois soil N test (ISNT-N) and 7d anaerobic incubation (AI-7) test were more stable than soil mineral N (SMN = total oxidized N (TON) & ammonium (NH₄-N)) under the precipitation, soil temperature and drainage conditions experienced over the experimental period at both sites. Climatic and soil conditions influenced soil N supply and plant growth. In this study, models that combined precipitation levels, soil temperature, soil NH₄-N concentration and their interactions best explained the variability in daily grass herbage DM production and N uptake. These early results indicate potential to provide improved site specific fertilizer N recommendations for grasslands using predictive models that incorporate soil N indices measuring available N pools and reliable local meteorological forecasts. The results from this study will be discussed in further detail in this paper.