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Landwirtschaftliche Fakultät - Jahrgang 2012

 

Titel Use of natural abundance of δ15N as indicator of long-term N management on grassland farms
An estimation of long-term N efficiency
Autor Melanie Kriszan
Publikationsform Dissertation
Abstract Productive agroecosystems usually rely on the addition of Nitrogen (N) generally as chemical synthetic N fertilizers and animal excreta, but inefficient N management nitrogen losses contribute to environmental pollution, global warming, and a decline in biodiversity. This thesis was designed to test the hypothesis that the natural abundance of δ15N signatures of various N pools may be used to reconstruct the efficiency of long-term N management of grassland farms retrospectively. Two types of long-term fertilizer experiments in a low mountain range pasture ecosystem at the former Grassland Research Station of the University Bonn, Germany, were considered to examine the changes in natural N isotope composition of various N pools. In a first experiment, calcium ammonium nitrate (δ15N = -1.0 ‰) was applied at a rate of 0 to 240 kg N / ha-1a-1 combined with a two and four cut management throughout 22 years. In a second study, a field lysimeter experiment, calcium ammonium nitrate and cattle slurry (δ15N = 8.9 ‰) were applied. N was applied at rates of 0 to 480 kg N / ha a for a time period of 22 years. Sampling different plant species and soils or particulate organic matter (>250 µm, 63-250 µm) then allowed to relate excess N fertilization and resulting N losses to the cumulative changes of δ15N signatures. Based on these N isotope data the study was extended by a survey on nine common grassland farms, located in North Rhine-Westphalia and Rhineland-Palatinate in Germany and once in Styra, Austria. These farms comprised different N management, and could be grouped into low and high N input categories thus served as an independent set of test sites to verify the use of δ15N signature as an indicator of practically relevant N management on grassland farms. Samples from these farms consisted of soil, plant biomass, hay,silage, milk, hair, urine, faeces, slurry, and manure. The results showed that the mean δ15N values of plant biomass (-1.3 to 0.95 ‰) were approximately 2-5 delta units lower compared with those of the topsoils (3.1 to 3.7 ‰), suggesting that light N was lost during soil organic matter formation. The variations in plant δ15N values can be attributed to differences in plant growth, resource acquisition strategies, or δ15N values of the soil N pools for which they compete. If higher N fertilizer rates were applied (up to 480 kg N / ha), the mean δ15N increased from -1.2 ‰ to 4.8 ‰ for plants, and from 1.8 ‰ to 6.0 ‰ for the topsoils, respectively. The largest enrichment of 15N was found in samples that were manured with organic fertilizer. As total soil N contents did not change significantly during the fertilization experiment, and since N losses via leaching were negligibly small, it was concluded that beside plant uptake and removal of N with biomass, most N was lost from the field plots through the gaseous phase. Particularly, N volatilization processes may account for a significant 15N isotope enrichment. In soils, this enrichment of δ15N values was preserved, suggesting that the respective N was incorporated into soil organic matter pools with slow N turnover. The screening of different farm types confirmed a significant variation in the δ15N values of harvested plant materials (-2.18 ‰ to 6.79 ‰) and the topsoils (1.47 ‰ to 7.91 ‰). Relative to farms under low N input management, the 15N of samples taken from high N input managed grasslands were elevated by 2.8 delta units. Moreover, mean δ15N values of plant biomass, soil and hair were closely correlated with stocking rate and milk production. High N input farms with a stocking rate larger than 1.4 livestock units/ha were characterized by higher amounts of organic N return per area and hence, higher losses of N from the slurry or farmyard manure, being reflected in elevated δ15N values of the studied N pools. In turn, lower δ15N values of topsoils and plants were related to low emission fertilizer application techniques and larger distances between stable and field. The results consistently show that high N input management on grassland farms systematically influences the δ15N values of soils and plant biomass. Hence, the δ15N signatures of different farm N pools, notably plant biomass, soil and hair could be identified as useful indicators for the efficiency of N management, i.e. the δ15N tracing technique represents an attractive approach to reconstruct the retrospective intensity and efficiency of fertilizer management in agricultural practice.
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© Universitäts- und Landesbibliothek Bonn | Veröffentlicht: 12.10.2012