Warm season leguminous cover crops

Sustainable intensification will help to meet food, fiber, and fuel needs of a global population projected to reach 12.3 billion by the end of this century (Gerland et al., 2014; Garnett et al., 2013). Nitrogen is a critical plant nutrient, but the environmental externalities of synthetic N production and use are a concern (Godfray et al., 2010). According to Menegat et al. (2022), synthetic N fertilizer production accounts for 10.6% of agricultural greenhouse gas (GHG) emissions and 2.1% of total global GHG emissions. In addition to carbon dioxide emissions in their production, the use of N fertilizer can also increase nitrous oxide emissions, a potent greenhouse gas (Viney et al., 2009). Further, due to low N-use efficiency, losses of N are relatively large and can negatively impact ground and surface water quality (Gaudio et al., 2015; Goolsby et al., 2001; Ran et al., 2019). Warm season leguminous cover crops (CCs) can fix large quantities of N. They differ from cool season legume CCs in their climatic requirements. Fall temperatures slow down metabolic processes and photosynthesis in cool season legumes, affecting overall vegetative growth and nitrogen fixation (Koehler et al., 2023; Kaye et al., 2019; Price and Norsworthy, 2013). Growth and N fixation tend to be more aggressive in warm season legume CCs compared to winter legume CCs (Rao and Northup, 2009a). By providing N to the following crop, N fertilizer use and associated carbon dioxide emissions from N-fertilizer production can be reduced. Nitrous oxide emissions often spike after fertilizer application and may also be reduced if N is supplied through legumes (Mahama et al., 2020). Although legumes are typically not very effective in reducing N-leaching, growing them in mixtures with non-legume CCs can increase their benefits by further reducing N leaching and adding organic carbon (Vogeler et al., 2019; Kakraliya et al., 2018). The combination of legume and non-legume CC has the potential to reduce nitrate (NO2−) leaching by up to 56% (Thapa et al., 2018). Warm season legumes, characterized by their vigorous growth and low input requirements, present a promising option for intercropping with major cash crops. For example, Manasa et al. (2018) found that maize intercropped with various summer legumes consistently resulted in a combined Land Equivalent Ratio (LER) greater than 1.5, indicating a significant advantage in land-use efficiency, due to maize’s wide spacing, slow early growth, and paired row planting that supported better legume development. Also Bybee-Finley et al. (2016) found that a mixture of pearl millet, sorghum sudangrass, and SH exhibited the highest evenness in biomass contribution compared to other intercropping methods. Warm season legume CCs also have the potential to improve overall weed control through competition for resources, allelopathic activity, and acting as physical barrier after termination in conservation tillage systems (Zannopoulos et al., 2024; Adler and Chase, 2007). These legumes can also add large quantities of biomass that may improve soil organic carbon content and soil health, and their taproots may alleviate soil compaction (Blanco et al., 2012; Lal, 2015). Summer legumes also diversify cropping systems, temporally if grown in rotation with main crops and spatially when intercropped in main crops (Bybee-Finley et al., 2016). Finally, they have the potential to generate economic returns if used as forage, if they replace N-fertilizer use for main crops, or if they boost following crop yields through a rotational effect. However, intercropping also presents challenges, such as competition with the cash crop for water, nutrients, and light, as well as complications associated with normal field operations (planting, weed control, harvesting). In this chapter, we will review research showing how warm season legume CCs can play an important role in creating sustainable solutions that decrease our dependence on synthetic fertilizers and alleviate the environmental impacts of crop production.