Disproportionately high N-mineralisation rates from green manures at low temperatures - implications for modeling and management in cool temperate agro-ecosystems

We examined the decomposition of Medicago lupulina, Melilotus alba and Poa pratensis at 3, 9, and 25 degreesC during 4 weeks. There was a strong tempe rature effect on the rate of CO2 evolution, and thus the extent of energy e xhaustion from the added substrates. However, there was no concomitant reta rdation of N mineralisation at low temperatures. In the analysis of varianc e of mineralized N the residue type gave a 10 times larger contribution to the regression than the temperature (T), whereas for CO2 evolution residue type and temperature were equally important contributors. This indicates th at although the temperature has a statistically significant effect on N-min eralisation it is substantially less than compared with the effect on carbo n mineralisation in the materials examined. The retardation of carbon miner alisation was least strong in Melilotus alba that had a relatively low cell ulose content, and a higher content of low molecular compounds. Though more research will be necessary to consolidate and explain this phenomena, it i s likely that an important factor is a decrease in the bioavailability of C -rich polymers at low temperatures, and thus a preferential utilization of N-rich low molecular substances. Nitrification was not effectively deterred at 3 degreesC. Thus, in terms of management, it is pertinent to reconsider the timing of green manure and catch crop incorporation in cool temperate climate regions, since the rapid release of nitrogen, coupled with the rela tively undeterred nitrification may result in a high N leaching risk by ear ly incorporation, but a low risk for N immobilization at late incorporation , if N rich residues are used.