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 (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.

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 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.

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.

Introduction

Legumes are often grown for incorporation into soil as a green manure providing benefits such as off season soil cover, stimulated soil biological activity and improved plant nutrition (Breland, 1994). Most interest has been attached to the legume's ability to furnish subsequent crops with readily available nitrogen (N) (Ladd and Amato, 1986, Breland, 1994). However, this depends on the inherent qualities of the plant material (Amato et al., 1984), environmental factors such as soil temperature and moisture (Kladivko and Keeney, 1987) and soil type (Christensen, 1985). Despite a considerable understanding of these factors, little research has examined the effects of soil (0–100 mm) temperatures normally experienced during winter months (0–15 °C) in temperate regions on the release of N from plant residues (Breland, 1994, De Neve et al., 1996, Andersen and Jensen, 2001).

It has generally been observed that the conversion of soil organic N to ammonium (NH₄⁺-N) occurs more readily than subsequent nitrification at soil temperatures below 10 °C (Campbell and Biederbeck, 1972, Emmer and Tietema, 1990). Nitrification has also been reported to be more sensitive than mineralization to temperature fluctuations (Campbell and Biederbeck, 1972, Biederbeck and Campbell, 1973), as nitrite oxidizers appear to be more sensitive than NH₄⁺-N oxidizers to low temperatures (Tyler and Broadbent, 1960). Furthermore, it appears that immobilization of mineral-N increases with decreasing temperatures (Ledgard et al., 1989, Nicolardot et al., 1994).

Net N transformation rates represent only the sum of competing processes, and do not indicate the rates of individual processes. Quantifying gross mineralization, nitrification and immobilization as separate microbial processes provides for a more fundamental understanding of the N cycle and patterns of residue decomposition. Adding substrate to substrate-limited processes also confounds tracer ¹⁵N techniques, which are most commonly used to measure net N transformations. In contrast, gross N transformation rates can be estimated by ¹⁵N dilution techniques without adding a substrate (Davidson et al., 1991). Briefly, gross mineralization is measured by adding ¹⁵NH₄⁺-N (e.g. the product) and examining the rate at which the atom% ¹⁵N enrichment of the NH₄⁺-N pool decreases as microorganisms mineralize soil organic ¹⁴N to ¹⁴NH₄⁺-N. Similarly, gross nitrification is measured by adding ¹⁵NO₃⁻-N and examining the rate at which the atom % ¹⁵N enrichment of the nitrate (NO₃⁻-N) pool decreases as microorganisms nitrify ¹⁴NH₄⁺-N to ¹⁴NO₃⁻-N. Consumptive processes (e.g. immobilization, gaseous losses, nitrification) change the size of these pools but not their ¹⁵N enrichment, permitting calculation of gross mineralization and gross nitrification from the rate of dilution of pool enrichment (Davidson et al., 1991). Gross immobilization can then be determined as the difference between gross mineralization and net mineralization rates.

The objectives of this study were to determine net and gross mineralization, nitrification and immobilization rates in clover residue amended and unamended soils as affected by constant or changing cold temperatures (2–15 °C) in a laboratory incubation study and under winter field conditions. The intention was to assess the potential risk of over winter leaching of N after the application of fresh green manure with low C/N ratio.