Winter soil temperature (2–15 °C) effects on nitrogen transformations in clover green manure amended or unamended soils; a laboratory and field study

Few studies have examined the effects of winter soil temperatures typical of temperate regions (0–15 °C) on the release of nitrogen (N) from plant residues. Similarly, few have studied gross N transformation rates (mineralization, nitrification and immobilization) as an interactive unit. N cycling was examined in clover amended or unamended soil incubated under constant laboratory temperatures of 2, 5, 10 or 15 °C for 161 days. Under laboratory conditions we also examined the impact of a sudden change in soil temperature whereby amended soil previously incubated for 98 days at 2, 5 or 10 °C was subsequently incubated at 15 °C, while amended soil previously incubated at 15 °C was incubated at 2 °C for a further 63 days. The effect of fluctuating winter temperatures was studied using intact soil cores under winter field conditions for 35 days. The kinetics of N transformations were determined in the laboratory incubation and field experiment by measuring soil ammonium (NH4+-N) and nitrate (NO3−-N) concentrations and gross rates of mineralization, nitrification and immobilization. The fate of 15N labelled clover residue was also measured in the field experiment. laboratory incubation and field experiments, soil mineral-N concentration was significantly (P<0.001) higher in amended, compared with unamended soil. Under laboratory conditions mineral-N concentration significantly (P<0.05) increased with increasing incubation temperature in amended soil. In unamended soil, mineral-N concentration was significantly (P<0.05) greater when incubated at 15 °C than at 2, 5 or 10 °C alone. Under winter field conditions all mineral-N released from clover residues was at risk of leaching during winter rainfall. nitrification was initially (7–56 days) inhibited in amended soil incubated at 2 or 5 °C, causing an accumulation of NH4+-N. However, after 77 days at 2 or 5 °C, gross nitrification rates increased, such that NO3−-N increased to concentrations which were greater than those of NH4+-N. This suggests that nitrifying bacteria took longer to acclimatize to the cold conditions than ammonifying microorganisms. Nitrate-N was the dominant form of mineral-N throughout the incubation experiment in amended soil incubated at 10 or 15 °C. In unamended soil, gross immobilization rates generally followed the same pattern as gross mineralization rates throughout the incubation. Unamended soil incubated at 10 °C and below produced negligible NO3−-N, indicating that N or carbon limited nitrification at these temperatures. Increasing incubation temperature from 2, 5 or 10 °C to 15 °C caused a rapid increase in soil NO3−-N concentration and gross mineralization and nitrification rates, but significantly (P<0.05) less mineral-N was released than if incubated at a constant 15 °C. This suggests that intermediate substrates may have been depleted during the initial incubation period at 2, 5 or 10 °C, hence limiting mineralizable-N. Decreasing soil temperature from 15 to 2 °C caused an initial increase in mineral-N, which was quickly followed by rapid immobilization of mineral-N; gross immobilization rates were up to 2.8 fold greater than gross mineralization rates. Similarly, under field conditions, microbial biomass N and gross immobilization increased with decreasing soil temperature suggesting there was population growth of adapting micro-flora. The release of mineral-N from clover residues in the incubation experiment also seemed to occur in two-phases, interpreted as first the mineralization of the labile and then the more recalcitrant fractions of the residues. esearch has shown that significant mineral-N is released from soil amended with clover residues at temperatures as low as 2 °C. Therefore, the incorporation of N-rich plant material should be delayed until spring to avoid winter N leaching.