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

Abstract

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 (NH 4 +-N) and nitrate (NO 3 −-N) concentrations and gross rates of mineralization, nitrification and immobilization. The fate of 15N labelled clover residue was also measured in the field experiment. In the 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. Gross nitrification was initially (7–56 days) inhibited in amended soil incubated at 2 or 5 °C, causing an accumulation of NH 4 +-N. However, after 77 days at 2 or 5 °C, gross nitrification rates increased, such that NO 3 −-N increased to concentrations which were greater than those of NH 4 +-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 NO 3 −-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 NO 3 −-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. This research 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.