The influence of quality parameters on plant N uptake from organic residues

Abstract

At a time when there is increasing concern about the decline in organic matter content in the tropical soils, the use of multipurpose tree prunings applied to the soil may playadualrole as a fertiliser and as a source Of organic ma.tter.Experiments were conducted in the FAO/IAEA-Agriculture and Biotechnology Laboratories,Seibersdorf, Austria, using. Krumbach sandy loam to test the influence of plant litter quality pariUtlefersOnplant N uptake from decomposing leaves of economically important trees and green manurespeceies. The direct approach of isotope dilution method was used in these experiments. In the first experiment the leaves were labelled with uN by tree injection,the labelled material was collected and incorporated a' a rate of 100 mgN kg-) soil and sown withryegrass (Lolium perenne L.). Ryegrass was harvested at 67days and nitrogen derived from.the residues (Ndfr) was determined by.the direct ISNmethod. In a second experiment )~' labelled residues of Casuarina, Leucaena, A/bizia. Medicago,G/iricidia and Euca/yptuswere add7dat a r~te of l?Omrs N kg-l soil, ryegrasswasgrownfor87daysandNdfrwas est1U1atedusmg,thedrrect·· ~.method. In 'ithethird el$perimepttwo months old leaves and twigs fr0ll1l.~yelU:oldtrees which had "be.encontinually, fertilisedwith ,)SNlabellednutrientsolution;(low~oncentrations) were used as dry and fresh materials. Ryegrass(Lolium perenne) was planted in .soils amended with tree litter already labelled with l~, isotope. The litter from Acacia auricu/iformis, Albizia lebbeck, Gliricidiasepium and Eucalyptus grandis were applied at a rate of 100 mg N kg-' soil as fresh and dry residues. Differences betWeen the two treatments in dry matter yield, total N, and N derived from residue (Ndfr) per pot in the ryegrass were insignificant. But there were significant differences when considering the different tree species. Relationships between residue quality characteristics (lignin, acid detergent fibre -ADF, cellulose and total extractable phenol (TEP) concentrations and the nitrogen derived from residue were determined. TEP concentrations were highly correlated with nitrogen in the ryegrass derived from residues. The correlations with other quality characteristics were not as strong. TEP concentration was the best predictor of plant N release in this experiment., It should also be noted that determination of TEP concentration is a straightforward procedure which could be developed as a simple method for field' use by extension workers. Results from this experiment suggests that drying procedure had little or no effect on the dynamics of N release and that the relatively small analytical differences in TEP concentration may not necessarily lead to differences in N release in the soil, or that drying at 70°C overnight does not affect TEP concentrations significantly.under well,..watered glasshouse conditions there is little difference in the mineralisation rates of residues whether added dry or fresh. The dry residues used in this experiment were dried at 70°C and not stored for long before use. Results from the EucalyptUs treatments suggested that there was a lag period, but whether this was due to the breakdown of polyphenolsor the establishment of more desirable C:N ratio is unclear. The Acacia treatments showed the lowest N release suggesting that other allelopathic compounds may have been released during breakdown as it did not have a particularly high TEP concentration or C:N ratio.