Nitrous oxide evolution from structurally intact soil as influenced by tillage and soil water content

Abstract

Among farmers there is a growing interest for adoption of reduced tillage practices, which has accentuated the need to understand the consequences for soil nutrient dynamics and losses. A laboratory study was conducted with structurally intact soil cores collected from two depths, 0–4 and 14–18 cm, within tillage experiments on contrasting soil types, both experiments with soil under mouldboard ploughing (MP) or shallow tillage (ST). The soil cores were adjusted to one of seven matric potentials ranging from −1500 to −15 hPa. The extent and regulation of nitrous oxide (N₂O) evolution as a function of tillage, depth and soil characteristics was studied by measurement of N₂O and CO₂ evolution rates, as well as nitrifying and denitrifying potentials, and subsequent data analysis by multiple linear regression models. At both sites, compaction of ST soil below the depth of tillage was significant. The vertical distribution of N₂O evolution was different in MP and ST soil, but no main effect of tillage on N₂O evolution was observed. Effects of soil variables on N₂O evolution were analysed using volumetric water content, water-filled pore space, or relative gas diffusivity (RD) to represent the effect of soil water. Using RD weakened interactions with tillage and C availability and strengthened main effects, suggesting that RD may provide a more general representation of the water effect. At 0–4 cm depth, N₂O evolution was related to NO₃⁻ availability in the soil with 5.1% C, but to C availability in the soil with 1.5% C. The contrasting patterns of dependencies in the different environments support the interpretation of reduced tillage and soil water content as indirect controls, via diffusional constraints, of N₂O evolution.

Introduction

Within arable agriculture, tillage is employed to improve soil tilth, to control weeds, and to incorporate crop residues, manure and fertilizers. In some regions, like the North American Great Plains, shallow tillage (ST) and no-till practices have been widely adopted to conserve soil water and to control soil erosion (Carter, 1994). Arable soils in Europe are typically mouldboard ploughed (Riley et al., 1994; Rasmussen, 1999), but the lower costs for fuel, machinery and labour associated with reduced tillage have stimulated the interest in such practices among farmers.

Reduced tillage further has a potential for mitigating anthropogenic greenhouse gas emissions to the atmosphere via carbon sequestration (Smith et al., 1998b). However, the absence of tillage may lead to soil compaction, reduced air-filled porosity and reduced availability of oxygen (Douglas et al., 1980; Schjønning, 1989; Ball et al., 1997; Gregorich et al., 2006; Sasal et al., 2006). Moreover, decomposition of crop residues is concentrated near the soil surface where the potential for leakage of gaseous products to the atmosphere is high. It has lead to concerns that benefits from C sequestration may be partly offset by an increase in emissions of the potent greenhouse gas nitrous oxide (N₂O). Nitrous oxide is a free intermediate of denitrification, as well as a by-product of ammonia oxidation and a product of nitrifier–denitrification (Oenema et al., 2005). The production of N₂O is stimulated under low oxygen conditions; Bollmann and Conrad (1998) found that the release of N₂O from both nitrification and denitrification peaked at partial pressures <0.5% O₂. Unless nitrite accumulates, denitrification is probably the only significant source of N₂O (Rudaz et al., 1991; Kester et al., 1997).

Many field studies have tried to link management effects on N₂O emissions to individual soil properties, but relationships are usually weak and system-specific, and soil water content often interacts with the effect of other soil variables (Robertson, 1994; Velthof et al., 1996; Burton et al., 1997; Bollmann and Conrad, 1998; Schmidt et al., 2000; Simek et al., 2006). Therefore, causal relationships have also been studied in manipulation experiments where some of the variability can be eliminated or controlled. In this way, for example, the effects of nitrite and nitrate availability (Firestone et al., 1979), air-filled porosity and nitrate (Letey et al., 1980), soil pH (Stevens et al., 1998) and C source (Gregorich et al., 2006) on N₂O and N₂ evolution have been investigated.

A significant proportion of annual N₂O emissions occurs after rainfall events (Li et al., 1992). The sudden increase in soil water content typically causes a flush of C and (net) N mineralization increasing substrate availability for nitrification and denitrification. This may be due to microbial stress (Kieft et al., 1987; Fierer and Schimel, 2003), soil organic matter exposure by physical disruption of aggregates (Goebel et al., 2005), or alleviation of diffusional constraints (Schjønning et al., 2003). However, wetting also causes a change in gas diffusivity that will affect the exchange of oxygen and denitrification products between soil and the atmosphere. Irrespective of the mechanism, it is imperative that in studies on the regulation of N₂O emissions the structural integrity of the soil–biota system is not disturbed prior to investigation.

We examined effects of tillage and soil water content on CO₂ and N₂O evolution in four distinct environments, i.e., two depth intervals of shallow-tilled and mouldboard-ploughed soil, by multiple linear regression analyses. Structurally intact soil cores were incubated at a range of matric potentials, the basic assumption being that effects of tillage system and soil water on microbial activities would reflect effects under field conditions. We hypothesized that effects and interactions of tillage and soil characteristics would lead to different patterns of N₂O regulation among the four environments and two soil types used in our study.