Can legume species, crop residue management or no-till mitigate nitrous oxide emissions from a legume-wheat crop rotation in a semi-arid environment?

Highlights

• Rotation of wheat with legume crops can reduce N₂O emissions in a semi-arid environment.

• Legume species (two pulses and two forage crops) gave similar N₂O emissions.

• Brown-manured legumes promoted more N₂O emissions than product-removed treatments.

• Compared to no-till, tillage increased N₂O emissions when legume crops harvested for grain or cut for hay.

Abstract

Soil emissions of nitrous oxide (N₂O) are generally low in Australian semi-arid cropping systems, but can be reduced further by incorporating legumes into cereal-based rotations. We used automated and manual chambers to compare N₂O emissions throughout a two-year legume-wheat field experiment. Two pulse crops [lupin (Lupinus angustifolius L.) and field pea (Pisum sativum L.)] and two forage legumes [vetch (Vicia sativa L.) and clover (a mixture of four Trifolium spp.)] were grown in the first year, followed by a wheat (Triticum aestivum L.) crop in the second year without additional nitrogen (N) fertilizer under either no-till or tillage systems. All legume crops were either (a) chemically terminated at anthesis (brown-manured, BM), or (b) cut for hay (forage legumes) or harvested for grain (pulses) (product-removed, PR). The fate of legume N in the BM treatment was traced using ¹⁵N labelled urea applied to soil in micro-plots. Results showed that N₂O emissions during the legume and wheat phases were mostly unaffected by legume species, the exception being lower emissions from the BM clover pasture under the no-till treatment. Under the PR treatment, tillage tended to increase N₂O emissions compared with the no-till treatment during both legume (121 vs. 104 g N₂O-N ha⁻¹ year⁻¹) and wheat phases (91 vs. 79 g N₂O-N ha⁻¹ year⁻¹). In contrast, a mixed result was found under the BM treatment with a significant tillage × crop management interaction during the legume phase, but no effect of tillage during the wheat phase. In general, the BM treatment promoted more N₂O emissions due to more N input into soil compared to the PR treatment, with the impact extending into the subsequent wheat crop. The N₂O emissions from the BM treatment during the legume (195 g N₂O-N ha⁻¹ year⁻¹) and wheat phase (181 g N₂O-N ha⁻¹ year⁻¹) were greater than those from the PR treatment for the corresponding phases (113 vs. 85 g N₂O-N ha⁻¹ year⁻¹). The ¹⁵N study showed that the majority of legume-derived N was retained in the soil at the end of the legume phase and likely to be readily available for the subsequent cereal crop. The mitigation of N₂O emissions derived from legumes in a semi-arid cereal-based cropping system can be optimized in a no-till system where product removal is practiced (cut for hay or harvested for grain) with legume species choice having little or no additional impact.

Introduction

Legume crops, grown either in rotations with cereals or oilseeds, or grown in livestock systems as forages for animal feed, provide a range of agroecosystem services (Jensen et al., 2012). Perhaps the most important role of legumes in agriculture is to obtain nitrogen (N) from the atmosphere through biological N₂ fixation, which reduces the amount of fertilizer N required to support crop and pasture production (Crews and Peoples, 2004). Although the emission of nitrous oxide (N₂O) is generally low (< 250 g N₂O-N ha⁻¹ year⁻¹) from rainfed cropping systems in semi-arid environments (Barton et al., 2011, 2008; Li et al., 2016), the large scale of land under this management in Australia and elsewhere equates to substantial N₂O emissions. Introducing legume crops into cereal-based rotations may further reduce N₂O emissions when considered across the whole rotation sequence. Compared to non-legume crops such as cereals or oilseeds, legume crops tend to emit less N₂O to the atmosphere. Although N₂O is also emitted from soils cropped with legumes, particularly in the post-crop period of residue mineralization (Barton et al., 2011), full-year emissions are generally less than for N-fertilised crops grown under the same conditions (Schwenke et al., 2015). A comprehensive review by Jensen et al. (2012) revealed that soil N₂O losses with legumes were generally lower than those from cereal crops with N fertilizer, especially when commercial rates of N fertilizer were applied. They found that the direct contribution of N₂O emissions from the biological N₂ fixation process to total N₂O emissions was insubstantial. Jeuffroy et al. (2013) demonstrated that including one pea crop in a three-year rotation could decrease total N₂O emissions by 20–25 % in a rain-fed farming system in the Paris Basin in France.

Cereal crops following legumes often emit less N₂O compared to a continuous cereal crop rotation, principally due to the reduced requirement for synthetic N fertilizer after the legume (Schwenke and Haigh, 2019). Where legumes in rotation lead to reduced N fertilizer use, further environmental benefits arise from the reduction in greenhouse gas emissions associated with the production of synthetic N fertilizers. Huth et al. (2010) reported a 34 % reduction in the amount of fertilizer N required by cereal crops when grown in rotation with legumes in subtropical agricultural systems. Therefore, legume crops provide an effective way to mitigate N₂O emissions across a broad range of farming systems and environments.

Legumes grown in a cropping rotation may be either pulse crops, typically harvested for grain, or annual/perennial pastures (forages), typically cut for hay or grazed directly by animals. Legumes may also be included in a cropping rotation as a manure crop, whereby all biomass grown is retained in the paddock. The so-called “brown-manure” crops are chemically terminated at peak biomass to increase soil N fertility, and in many cases, to control herbicide-resistant weeds while reducing the incidence of root diseases (Swan et al., 2015). When legumes are brown-manured, legume-N is returned to the soil through decomposition of plant residues, in contrast to product removal systems where a significant proportion of legume-N is exported from the system as grain, hay or animal product (Roper et al., 2012).

Legume residues, typically with a low in C:N ratio, mineralize rapidly and produce ammonium-N which can lead to N₂O loss during the subsequent aerobic nitrification or anaerobic denitrification (Butterbach-Bahl et al., 2013; De Antoni Migliorati et al., 2015). Schwenke et al. (2015) estimated that 75 % of the annual N₂O losses associated with pulses occurred post-harvest in subtropical agricultural systems, but found no difference in cumulative emissions between chickpea (Cicer arietinum L.), faba bean (Vicia faba L.) and field pea (Pisum sativum L.). Although post-harvest decomposition of legume residues may represent a significant source of N for N₂O losses, the slow release of legume-fixed N may better match the N demand by the subsequent non-legume crop than does fertilizer N (Crews and Peoples, 2005). Hence, there could be a reduction of the overall quantity of N₂O loss. Therefore, the management of legume residues may help to further reduce N₂O emissions.

No-till practice has been widely adopted for dryland grain production in Australia (Llewellyn and d’Emden, 2010) with benefits of reduced erosion, increased water infiltration, reduced compaction, reduced energy consumption, improved soil structure, and possibly increased soil organic carbon sequestration (Hobbs, 2007; Thomas et al., 2007). However, with the increased instances of soil and stubble-borne diseases and the development of more herbicide-resistant weeds under no-till farming (Chauhan et al., 2006), occasional strategically timed tillage can be warranted (Crawford et al., 2015). To ameliorate soils with constraints such as acidity, sodicity and compaction, tillage is generally required to incorporate amendments into the soil (Page et al., 2018). However, tillage could potentially increase N₂O emissions. The incorporation of crop residues into soil through tillage increases aeration and microbial activity by exposing more residue surfaces to microorganisms, thereby enhancing the rate of mineralization and N₂O emissions (Muhammad et al., 2019). Previous research at the field site featured in this paper found that two years of pre-sowing tillage had no measurable effect on N₂O emissions in a canola crop studied during the drier-than-average year in 2013 (Li et al., 2016). On the other hand, no-till practice could also promote N₂O emissions by improved soil moisture status via increased soil water infiltration, favoring the activity of denitrifying bacteria (Skiba et al., 2002). Helgason et al. (2005) analyzed N₂O emissions in Canada using over 400 datasets from a range of farm management regimes, including tillage practice, and found that no-till practice increased N₂O emissions in humid climates, but decreased it in semi-arid climates. Therefore, there is inconsistent information on our understanding of the effect of tillage on N₂O emissions in semi-arid climates.

The objectives of the present study were to investigate the impacts on N₂O emissions of (a) legume crop species, (b) legume residue management (brown-manured vs. product-removed), and (c) tillage practice (tillage vs. no-till). Soil N₂O emissions were monitored for two years (legumes in year 1, non N-fertilised wheat in year 2) in this semi-arid environment in south-eastern Australia.