Monitoring of Potential N Losses from Dairy and Organic Farming Systems

Abstract

The project was carried out at Johnstown Castle and was concerned with monitoring potential nitrogen (N) leaching losses from organic and dairy farming systems. Some plots were cut for silage in year one; only grazed plots were used in year two. There were low input manure N plots in the organic system with low and high input fertiliser N plots in the dairy system. Soil samples, in 15 cm intervals from the surface to 90cm deep, were taken in triplicate from one plot of each treatment on three successive days per month from October to March in year 1 and from September to March in year 2. Extraction of nitrate-N (NO3-N) and ammonium-N (NH4- N) was carried out on un-bulked soil cores by taking a 20 g sub-sample and using 100 ml 2 molar KCl. A 20 g sub-sample of each day`s bulked replicates was dried at 105 degrees C overnight for moisture determination. The concentrations of NO3-N and NH4-N in the extracts were determined on an automatic analyser and the results were converted to kg per ha using the following bulk densities for the soil layers: 0-15, 15-30, 30-60, 60-90, 1.3, 1.4, 1.6, 1.75 g per cm3, respectively. In year 1 the results showed no significant difference between treatments in the level of NO3-N, NH4-N and total mineral N in the soil layers and total amounts to 90 cm. On four of the six dates, November, December, January, early-March, the level of total mineral N was lowest in the low N treatment. Among treatments, NH4-N was lowest in the low N treatment on four dates, November, December, January, early-March while NO3-N was also lowest on four dates, November, December, early-March and late-March. At the November, December, January and early-March sampling, the organic farm NH4-N data to 90 cm was highest which was reflected in the total mineral N results to 90 cm for November, December and early-March. In year 2 the results showed no significant difference between treatments in the level of NO3- N, NH4-N and total mineral N in the soil layers and total amounts to 90 cm. In September, October, November, December and February, total mineral N to 90 cm was highest in the high N treatment. This was a reflection of high NO3-N levels in that treatment for those sampling dates. Among treatments, total mineral N to 90 cm was lowest in the organic farm samples in September, October, November, December and February. This result reflected, among treatments, lowest NH4-N levels in September, October, November, December, February and lowest NO3- N levels in November and December. Estimates of the amounts of applied N leached, averaged over the two years of the experiment, were 22% for the Low N treatment and 12% for the High N treatment. An equation, developed from studies carried out in County Cork (Richards, 1999), gave predictions of N available for leaching in November from the N treatments in year 2 which were in very good agreement at low N and within 23% of those actually recorded at high N. Assessment of available N leaching models led to the conclusion that the relatively simple UK `N Cycle` model was most adaptable to Irish conditions since other European and US models require input parameters not readily available including those for soil texture, soil hydrology and soil organic matter.