Quantitative synthesis on the ecosystem services of cover crops

Abstract

The maintenance of soil health in agro-ecosystems is essential for sustaining agricultural productivity. Through its positive impacts on various soil physical and biological processes, cover cropping can be an important component of sustainable agricultural production systems. However, the practice of cover cropping can be complex, and possible trade-offs between the benefits and side effects of cover crops have not been examined. To evaluate these benefits and potential trade-offs, we quantitatively synthesized different ecosystem services provided by cover crops (e.g., erosion control, water quality regulation, soil moisture retention, accumulation of soil organic matter and microbial biomass, greenhouse gas (GHG) emission, weed and pest control, as well as yield of the subsequent cash crop) using data from previous publications. We used a simple indicator (δ), defined as the ratio of an observed variable (i.e., ecosystem service) under cover crop and under fallow condition, to evaluate the impacts of cover crops on a given ecosystem service. Our results showed that cover crops provided beneficial ecosystem services in most cases, except for an increase in GHG emission (δ_CO2 = 1.46 ± 0.47 and δ_{N2 O} = 1.49 ± 1.22; $\overline{x}$ ± SD) and in pest (nematode) incidence (δ_{nematode abundance} = 1.29 ± 1.61). It is also important to highlight that, in some cases, tillage could offset the extent of ecosystem service benefits provided by cover crops. Based on this synthesis, we argue that cover crops should be incorporated into modern agricultural practices because of the many environmental benefits they offer, particularly the maintenance of soil and ecosystem health. More importantly, there was generally an increase in cash crop yield with cover cropping (δ_yield = 1.15 ± 0.75), likely due to improvement in various soil processes. Despite its benefits, the complexity of cover crop management should not be overlooked, and site-specific factors such as climate, soil type, cover crop species and tillage practices must be considered in order to optimize the benefits of cover cropping. In addition to crop yield, detailed economic analyses are needed to calculate the direct (e.g., reduction in the amount of chemical fertilizer) and indirect monetary benefits (e.g., the improvement of soil quality) of cover crops. Such a comprehensive analysis could serve as incentive for producers to integrate cover crops into their management practices.

Introduction

The inappropriate uses of conventional agriculture technology such as heavy machinery (e.g., for tillage) and chemical inputs (e.g., fertilizers, pesticides, herbicides), as well as practices such as monoculture crop cultivation and, in some cases, groundwater exploitation for supplemental irrigation (Lal, 2015) have led to unprecedented environmental consequences, including serious declines in soil quality. Soil compaction, erosion, salinization, and water pollution are common characteristics of degraded landscapes, with soil loss being the most visible manifestation of that condition (Sumner and McLaughlin, 1996). Yet soils are the foundation of multiple ecosystem service provisions, defined as the services that the ecosystem provide for human well-being (e.g., biomass and raw material production, nutrient cycling, biodiversity conservation, physical and cultural environment, carbon sequestration and archive to geological and archaeological heritage) and are critical to achieving the United Nations Sustainable Development Goals (Keesstra et al., 2016). Therefore, a shift towards ‘nature-based solution’ (NBS) practice as an alternative to conventional agriculture has been recommended (Keesstra et al., 2018). Examples of such practices include minimizing mechanical soil disturbance, allowing permanent soil cover with crop residue, and increasing species diversification (FAO, 2002).

Cover cropping is among some of the most recognizable NBS practices that enhances the provisioning of various ecosystem services in agro-ecosystems (Keesstra et al., 2018). Residue cover has well documented effects on the intensity and seasonal variability of numerous soil processes relevant to nutrient transport and transformation in soils (e.g., soil temperature and soil moisture) (Kahimba et al., 2008; Siczek and Lipiec, 2011). Cover crop has long been recognized as a beneficial practice not only for its impact on nutrients retention, but also for soil organic matter (SOM) accretion (Sainju and Singh, 1997; Lal, 2015). With the escalating cost of N fertilizer, legume cover crops have received significant attention due to their N fixation potential (Shrestha et al., 1999; Ladha et al., 2005). Cover crops also provide additional agronomic services including increased arbuscular mycorrhizal fungi (AMF) inoculation (Galvez et al., 1995), reduced incidence of certain soil pathogens (Bagayoko et al., 2000; Fageria et al., 2005; Sainju et al., 2005), and early-season weeds, particularly those that require light for germination (Teasdale, 1996).

Despite their potential benefits to ameliorate soil conditions, the introduction of cover crops can add to the complexity of farming operations. In the case of a legume cover crop like hairy vetch (Vicia villosa Roth.) that can provide a large portion of the N required by the subsequent crop (ex. corn), late cover crop termination is usually recommended because this practice allows higher N accumulation in the cover crop biomass (Clark et al., 1997), and for better synchronization of N release from the decomposing cover crop and corn N uptake (Ladan and Jacinthe, 2017). In contrast, early termination of cover crop might be appropriate in situations where rainfall amount is low and depletion of soil moisture reserve by cover crops is a concern (Mitchell et al., 2015). At times the potential side effects of cover cropping could offset potential benefits. For example, prolonged dry periods may diminish the benefits of cover crops, due to continued evapotranspiration by the growing cover crop (Dabney et al., 2001; Rusinamhodzi et al., 2011) or water competition with the main crops (Unger and Vigil, 1998), although a recent study has shown that cover crops with deeper rooting system (e.g., palisadegrass, Brachiaria brizanta or Urochloa brizanta (Hochst. ex A.Rich.) R Webster) allow the subsequent cash crop to develop a more extensive rooting system and consequently better drought tolerance (Balbinot Junior et al., 2017). Similarly, additional N supply is often required for producing high cover crop biomass (i.e., to build organic matter stock), such as those from the Poaceae family, but they produce residue with high C:N ratios, leading to temporary soil N immobilization (Zhu et al., 2012). In many agricultural regions, climate change is expected to result in pronounced summer droughts, and recent studies have suggested that repeated soil drying and moistening cycles can potentially exacerbate the export of nutrient loss in agricultural runoff (Smith and Jacinthe, 2014; Daryanto et al., 2017a, Daryanto et al., 2017b). At the present, it is unclear whether cover cropping can help mitigate the impact of climate variability on nutrient use efficiency and loss from agroecosystems.

Globally, there has been growing interest in considering cover crops as a component of NBS practice and, as an illustration of that interest, numerous studies involving cover crops have been conducted in both temperate and tropical regions (Fig. 1) (Hwang et al., 2015; Basche et al., 2016; García-González et al., 2016; Almeida et al., 2018). For example, in the United States, the area planted with cover crops has doubled during the last five years (SARE, 2017) and in Brazil, the use of tropical grasses from the genus of Urochloa or Brachiaria has been promoted in combination with no-till (NT) to improve phosphorus (P) availability in Oxisols and Ultisols (Almeida et al., 2018). Yet, the number of cover crop users is still limited, indicating numerous challenges associated with adoption of the practice. The lack of knowledge and skills, access to cover crop seeds, training and technical assistance are potential barriers to cover crop adoption, particularly for smallholders (Mwangi et al., 2015; Pratt and Wingenbach, 2016). Therefore understanding the determining factors of farming practices adoption is critical to effectively promote cover cropping among farmers and harness the conservation and ecosystem services benefits that this practice can provide (Mol and Keesstra, 2012).

Aside from production cost, crop yield is the ultimate factor that determines the willingness to adopt cover cropping. Most farmers believe that cover crops must be grown for a full year, halving the number of cash crop cycles (Tonitto et al., 2006). Yield variability with cover cropping has also been reported. For example, lower rice yield was observed following cover crops of the Poaceae family (Nascente et al., 2013), but increased yield was measured with leguminous cover crops (Dabney et al., 1989). Similarly, considerable difference in corn yield was recorded depending on the cover crop species that preceded the corn crop (Kaspar and Bakker, 2015). Although numerous factors may contribute to the variable results reported in the literature, a comprehensive quantitative analysis of these factors on crop yield variability and other ecosystem services associated with cover crops is still lacking. The need for such an analysis has previously been acknowledged (Blanco-Canqui et al., 2015). To better understand the extent of each benefit relative to the potential adverse impact associated with cover crops, we quantitatively synthesized and compared different ecosystem services associated with the management, including services unrecorded by prior assessments such as weed and pest control (Lal, 2015; Brennan, 2017; Kaye and Quemada, 2017). By using data from field experiments across the globe, this review is to complement the previously modeled ecosystem services provided by cover crops (Schipanski et al., 2014) and to thoroughly assess the NBS in the context of a changing climate.