No tillage and liming reduce greenhouse gas emissions from poorly drained agricultural soils in Mediterranean regions

Highlights

• The effect of tillage and liming on GHG was studied in poorly drained acidic soils.

• NT reduced N₂O emissions, global warming potential and greenhouse gases intensity.

• Liming reduced N₂O and CH₄ emissions under CT; no effect was observed under NT.

• NT and liming provide an opportunity for N₂O and CH₄ mitigation.

Abstract

No tillage (NT) has been associated to increased N₂O emission from poorly drained agricultural soils. This is the case for soils with a low permeable Bt horizon, which generates a perched water layer after water addition (via rainfall or irrigation) over a long period of time. Moreover, these soils often have problems of acidity and require liming application to sustain crop productivity; changes in soil pH have large implications for the production and consumption of soil greenhouse gas (GHG) emissions. Here, we assessed in a split-plot design the individual and interactive effects of tillage practices (conventional tillage (CT) vs. NT) and liming (Ca-amendment vs. not-amendment) on N₂O and CH₄ emissions from poorly drained acidic soils, over a field experiment with a rainfed triticale crop. Soil mineral N concentrations, pH, temperature, moisture, water soluble organic carbon, GHG fluxes and denitrification capacity were measured during the experiment. Tillage increased N₂O emissions by 68% compared to NT and generally led to higher CH₄ emissions; both effects were due to the higher soil moisture content under CT plots. Under CT, liming reduced N₂O emissions by 61% whereas no effect was observed under NT. Under both CT and NT, CH₄ oxidation was enhanced after liming application due to decreased Al^3 + toxicity. Based on our results, NT should be promoted as a means to improve soil physical properties and concurrently reduce N₂O and CH₄ emissions. Raising the soil pH via liming has positive effects on crop yield; here we show that it may also serve to mitigate CH₄ emissions and, under CT, abate N₂O emissions.

Introduction

Carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) are the most important greenhouse gases (GHGs) contributing to the total anthropogenic GHG emission in 2010, with about 76%, 16% and 6%, respectively (IPCC, 2014). Agriculture accounts for 12% of these global emissions and is considered the most important source of non-CO₂ GHGs (IPCC, 2014). The emission or consumption of these gases is the result of different and simultaneous microbial processes. Carbon dioxide is produced by autotrophic and heterotrophic respiration (Hanson et al., 2000), CH₄ is emitted by methanogenic and oxidized by methanotrophic microorganisms (Chan and Parkin, 2001) and N₂O production is mainly the result of nitrification and denitrification (Firestone and Davidson, 1989). All these processes are highly dependent on soil physicochemical properties which in turn regulate hydraulic conductivity and ultimately soil aeration and water filled pore space (WFPS) after water application by rainfall or irrigation. The soil profile is normally a heterogeneous mix of horizons with very different properties in which water infiltration is mainly conditioned by the horizon with lower hydraulic conductivity. This is the case of poorly drained soils with a low permeable Bt horizon (with a high clay content), that generates a perched water layer for a long period of time after rainfall and irrigation events. Planosolic soils, which cover large areas in subtropical and temperate regions, often present this problem maintaining a perched water table near the soil surface (5–10 cm) that affects crop yields and also GHG emissions. Soil management practices such as tillage can modify soil organic matter (SOM) content and distribution and specially soil structure thereby improving soils' hydraulic properties. In this line, Gómez-Paccard et al. (2015) demonstrated that no tillage (NT) increased SOM in the 0–5 cm layer and total porosity in the soil profile, improving water infiltration in these type of soils.

Adoption of NT is being promoted in the last decades because it can offer an array of environmental and economic advantages such as water conservation, reduction of erosion and runoff, enhanced soil C sequestration and reduction of production costs (Soane et al., 2012). In terms of GHG emissions, the effect of NT on N₂O emission has been variable and higher (Ball et al., 1999), similar (Omonode et al., 2011) and lower (Jantalia et al., 2008) fluxes than conventional tillage (CT) have been reported. This large uncertainty seems to be associated with interactions between physical soil properties (Gregorich et al., 2005, Rochette, 2008), climatic variability and duration of tillage practices (van Kessel et al., 2013). Several studies have shown that in NT, soil compaction as caused by the pass of sowing and harvest machines tends to increase bulk density and decrease soil porosity and aeration, especially in the rainy season, promoting N₂O losses by denitrification due to a higher soil WFPS compared with plowed soils (Linn and Doran, 1984). Conversely, other studies revealed that the greater amount of SOM and the increase of macroaggregate water-stability in the uppermost soil layer under NT would induce a reduction of anaerobic conditions and improved soil gas diffusivity which results in lower N₂O emission with respect to CT (Mutegi et al., 2010, Plaza-Bonilla et al., 2014). Rochette (2008) concluded that NT may increase N₂O emissions from many poorly drained agricultural soils located in regions with a humid climate. Regarding CH₄ emissions, previous research suggests a reduction in CH₄ oxidation with CT adoption (Ball et al., 1999, Ussiri et al., 2009), possibly due to the disturbance of the methanotrophic microbes and changes in soil gas diffusivity.

Liming is widely used to solve problems of soil acidity (e.g. Al^3 + toxicity and nutrient deficiency) and plays an important role in processes involved in N and C cycling. This agricultural practice offers potential to be an effective GHG mitigation strategy by decreasing N₂O emissions and increasing CH₄ uptake in semiarid arable soils (Barton et al., 2013a, Barton et al., 2013b). Liming may affect several mechanisms that regulate the production and consumption of soil GHGs, such as microbial activity and diversity (Barton et al., 2013b, Cuhel et al., 2010), enhancement of N₂O reductase which favors consumption of N₂O in soil (Simek and Cooper, 2002), and enhancement of nutrient availability and plant growth leading to improvements in nitrogen use efficiency (NUE) (Fageria, 2014). As some of these factors are also modified by tillage, a better understanding of how the interaction between tillage and liming affect denitrification, nitrification and CH₄ oxidation is crucial to propose effective GHG mitigation strategies. Yet, to date such effects remain to be investigated.

The objectives of this study were to: (1) quantify the effect of CT/NT on GHG emissions from soils with a low permeable Bt horizon in a rainfed crop; and (2) assess the influence of liming, and tillage × liming interaction on these emissions in acidic soils. Our main hypothesis was that the adoption of NT could decrease N₂O emissions and increase CH₄ oxidation as a result of the lower water saturation in the upper soil layer (0–5 cm), as reported by Gómez-Paccard et al. (2015), compared to CT. Because CT mixes thoroughly lime with the soil potentially maximizing the pH rise, we also hypothesized that liming would have a higher effect on GHG under CT conditions. To test these hypotheses, a field trial was performed in a degraded Ultisol with planosolic features during a rainfed triticale crop.