Rye cover crop effects on maize: A system-level analysis
Introduction
Inclusion of winter cover crops in high-input rain-fed maize (Zea mays L.)-based cropping systems is a conservation practice for enhancing the environmental performance of these systems (Kaspar and Singer, 2011, Thorup-Kristensen et al., 2003). Cover crop shoots protect soil from erosion (Kaspar et al., 2001), and roots take up residual NO3-N from the soil during the fall-to-spring fallow period, reducing the movement of nutrients into surface and ground water (Dinnes et al., 2002, Kaspar et al., 2012, Kaspar et al., 2007, Salmerón et al., 2010). The use of cover crops also has the potential to provide long-term soil quality benefits such as improving carbon sequestration and soil physical properties (Basche et al., 2016a, Blanco-Canqui et al., 2015, Kaspar and Singer, 2011, Moore et al., 2014), and other ecosystem services such as weed and pest suppression and beneficial insect conservation (Schipanski et al., 2014). Water quality degradation, especially NO3 pollution of surface waters, is the most pressing environmental impact of these systems in the US Midwest. Cover crops have been promoted as one of the most viable options for reaching water quality goals set in the Midwest (e.g. Iowa Nutrient Reduction Strategy; Iowa Deptartment of Agriculture and Land Stewarship, 2013) because of their lower cost of adoption compared to built infrastructure such as denitrifying bioreactors and wetlands (Christianson et al., 2013, Dinnes et al., 2002).
Despite the evidence of the benefits of cover crops and the existence of incentives such as cost-share programs, adoption of the practice lags behind targets. Current records indicate that cover crops are used in only 1.55% of Iowa row-crop farmland (National Agricultural Statistics Service, 2016). In the Midwest, winter rye (Cereale secale L.) is a commonly used cover crop species (Singer, 2008) because it can withstand harsh winter conditions and has superior growth and N uptake compared to other species (Johnson et al., 1998, Kaspar and Bakker, 2015). Some studies have reported reductions in maize yield following a rye cover crop (Iqbal et al., 2015, Johnson et al., 1998, Kaspar and Bakker, 2015, Krueger et al., 2012, Krueger et al., 2011, Pantoja et al., 2015, Singer and Kohler, 2005, Singer et al., 2008), although rye and other grass winter cover crops do not consistently reduce maize yields in the Midwest (Basche et al., 2016a, Miguez and Bollero, 2005). Nonetheless, concerns regarding possible negative yield impacts of rye on maize have been found to be an impediment to the adoption of cover crops by producers (Arbuckle and Roesch-McNally, 2015). To promote the adoption of the practice, quantification of the actual risks and the trade-offs associated with cover crop use, along with the development of risk abatement strategies, are necessary (Arbuckle and Roesch-McNally, 2015; Carlson and Stockwell, 2013).
Miguez and Bollero (2005) identified that the effect of grass cover crops on maize yields throughout US studies was neutral, although significant variation existed across these studies. Similarly, rye cover crops generally reduce NO3-N loss but the magnitude of the leaching-reduction effect also varies widely across years, locations and management (Dabney et al., 2010, Dinnes et al., 2002, Kaspar and Singer, 2011, Thorup-Kristensen et al., 2003). This indicates that rye effects on the maize system depend on specific combinations of management choices and environmental conditions. Most studies have focused on quantifying rye effects on final maize yields and/or annual NO3-N losses, and many knowledge gaps still exist regarding the mechanisms by which rye affects these systems. Broadly speaking, rye effects on maize can be grouped into biotic and abiotic factors. Biotic factors include pests and disease pressure (Acharya et al., 2016, Bakker et al., 2016) and allelopathy (Dhima et al., 2006, Duiker and Curran, 2005, Raimbult et al., 1991, Tollenaar et al., 1993), and at present are not well understood (Blanco-Canqui et al., 2015). A larger body of evidence exists for abiotic factors, which allowed us to develop a generalized framework of the abiotic effects of rye on the maize system (Fig. 1). Literature findings have shown maize yield reductions following rye cover crop to be related to depletion of soil moisture (Krueger et al., 2011, Mirsky et al., 2015, Raimbult et al., 1991, Unger and Vigil, 1998) and/or plant available N (Crandall et al., 2005, Kessavalou and Walters, 1999, Krueger et al., 2011, Tollenaar et al., 1993), or to a mulching effect that reduces soil temperature and seedling growth (Munawar et al., 1990, Teasdale and Mohler, 1993). More specifically, rye abiotic effects on the maize system can arise from changes in the soil via: 1) the addition of organic C and N (shoot and root); 2) changes in soil surface cover that alter soil temperature and water dynamics; and 3) changes in the state variables such as inorganic N and soil water at the time of cover crop termination (Fig. 1). The magnitude of these changes affects the system differently, which may explain the wide variation in yield and NO3-N leaching responses to rye cover crops across different studies.
The amount of biomass produced by crops is strongly related to their water and N use (Gastal and Lemaire, 2002, Sinclair and de Wit, 1975, Sinclair and Rufty, 2012). For rye cover crops, this could mean that the greater the biomass, the higher the potential to alter water, N and temperature dynamics, resulting in increases in the potential for both yield penalty and reductions in NO3-N losses. Krueger et al. (2011) and Pantoja et al. (2015) found rye biomass production to have a direct relationship to maize yield penalty, while Malone et al. (2014) found in a modeling study that rye N uptake had a strong relationship with NO3-N losses. In this study, we hypothesized that the magnitude of rye cover crop abiotic effects on maize yields and environmental performance variables such as drainage water and NO3-N losses would be proportionally related to its biomass production. We tested this hypothesis and examined the underlying crop-soil dynamics that would support such a scenario by analyzing six years of data from a no-till continuous maize (with and without rye cover crop) experiment carried out in central Iowa, US. This dataset was collected over years that crops experienced drought, flood and historically average weather, and included measurements of many system variables shown in Fig. 1. We supplemented our analysis by using a calibrated cropping systems model for this site (Dietzel et al., 2016) to better understand growth-limiting factors and soil-crop dynamics with variability in both weather (30 years) and management (four simulated rye termination dates within a year). To our knowledge, current literature lacks a system-level analysis of the effect of rye on maize in which the most important system variables are analyzed simultaneously. Such analysis is necessary to further our understanding of the abiotic mechanisms by which rye impacts maize and the environmental performance of the system, and to provide baselines for quantifying trade-offs and risks associated with this practice.
Section snippets
Soil
The dataset used in this study was derived from observations collected from 2009 to 2014 in the Comparison of Biofuel Systems (COBS) experiment. This experiment was conducted in a 9-ha field that is part of the Iowa State University Agronomy and Agricultural Engineering Research Farm, in Boone County, Iowa (41.92°N, 93.75°W). The soil is a Webster silty clay loam (fine-loamy, mixed, superactive, mesic Typic Endoaquoll) and Nicollet loam (fine-loamy, mixed, superactive, mesic Aquic Hapludoll),
Rye shoot and root biomass and C:N
Over the 6-yr period, rye shoot biomass at termination day varied from 120 to 2499 kg ha−1 (Table 4). The low rye biomass production in 2009 and 2013 coincided with relatively cool spring weather conditions in those years (Fig. 2). In 2014, poor growth was related to winterkill, caused by extremely harsh conditions during that winter. On the other hand, the unusually warm temperatures in February and March of 2012 allowed rye to produce the highest biomass over the 6-year period even when it was
Discussion
In this study, we approached rye cover crop abiotic effects on maize yields and environmental performance from a systems perspective (Fig. 1). Initially, we hypothesized that the magnitude of rye effects on maize yields and environmental performance variables such as cumulative drainage water and NO3-N losses would be proportionally related to rye biomass production, an easily measurable trait. Experimental, literature and modeling results did not support the hypothesized relationship for yield
Conclusions
Coupling experimental and literature findings with modeling, we provided a system-level analysis of rye cover crop effects on maize and extrapolated results beyond the study period to obtain a more complete picture of the abiotic effects of the inclusion of a rye cover crop in rain-fed maize-based systems. Modeling scenario analysis showed that in the long term, rye improves environmental performance (26% reduction in NO3-N losses) without consistently reducing maize yields. However,
Acknowledgements
This project was supported by the Iowa Nutrient Research Center (project: ‘Quantifying temporal and spatial variability in NO3-N leaching across Iowa’), the United States Department of Agriculture’s National Institute of Food and Agriculture (competitive grant No. 2012-67011-1966) and the Agriculture and Food Research Initiative (Hatch project No. 1004346). We thank Robert Ewing, Dave Sundberg, Carl Pedersen and other personnel from the COBS project for contributing to the data collection. We
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