Saturday, 15 July 2006

Mineralization of Crop Residues and the Fate of Residue-Nitrogen as Affected by Tillage Practices and Litter Quality.

Martin Potthoff1, Horst H. Steinmann2, Friedrich Beese3, and Rainer G. Joergensen1. (1) Dept of Soil Biology and Plant Nutrition, Univ of Kassel, Nordbahnhofstrasse 1a, Witzenhausen, Germany, (2) The Research Centre for Agriculture and the Environment, Am Vogelsang 6, Goettingen, Germany, (3) Institute of Soil Science and Forest Nutrition, Buesgenweg 2, Goettingen, Germany

Crop residue derived nitrogen amounts up to 30 and 50 kg ha-1 a-1 for winter wheat and winter rape, respectively. The fate of this nitrogen and its contribution to the nutrition of subsequent crops was investigated in two different cropping systems in the field using a litterbag experiment and a mesocosm experiment to track 15N in decomposition of labelled plant litter. “Inversive ploughing” represented a treatment of conventional tillage and the litter was burrowed to a depth of 20 cm. “Chisel ploughing” represented a treatment of no inversation of the soil and the litter was slightly mixed into the upper 10 cm leaving a certain portion on the surface. As litter of different quality we used oil seed rape (C/N=40) and winter weed (C/N=100) in the experiments. “Inversive ploughing” was a three crop rotation starting with winter rape before summer barley that was followed by winter wheat. “Chisel ploughing” was a 4-year low input system starting with winter rape before oats that was followed by winter wheat. After the fourth year fields were left set aside before the rotation started again with rape. Therefore we investigated wheat straw decomposition in barley growings vs. set aside vegetation and rape straw decomposition in wheat growings vs. oats. Litterbag results showed a reduced breakdown of litter in the “Chisel ploughing” treatment over the winter period. In spring a strong mesh size effect was detected indicating a very important contribution of soil fauna to decomposition in the “Chisel ploughing” but not in the “Inversive ploughing” treatment. 65 to 80 % of residue derived N were recovered in the mesocosms. The rest was lost due to leaching and/or denitrification. Up to two thirds of recovered residue-N was obtained as organic soil N. 3 to 10 % of residue derived N was recovered in successive vegetation. However, the specific enrichment of residue-N in the plant biomass turned out to be a function of treatment rather than that it was a function of primary production. The enrichment of set-aside-vegetation in the “chisel ploughing” treatment after winter wheat clearly exceeded that of all other treatments. This indicates that chisel ploughing and wide C/N ratios of residue straw both are factors supporting the re-utilisation of residue-N due to a shift of larger N-releases from residues into spring. It is likely that the sowing of catch crops would make the set-aside-year in the low input system much more effective for saving residue nitrogen for the nutrition of following crops. We conclude that we watch a phenomena of increasing self-regulation in the reduced tillage treatment. Annual decomposition rates corresponding to “Inversive ploughing” were reached in “Chisel ploughing” only because of fauna activity that was supported by the reduced tillage intensity. Shifting the main mineralization into spring indicates an optimized synchronization of nutrient release in decomposition and nutrient uptake in primary production.

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