Cereal rye is the most common cover crop utilized in corn-soybean systems, due to its ease of establishment and termination, ability to uptake residual N, winter hardiness, and relatively low seed cost (SARECTIC, 2017). Furthermore, cereal rye has been demonstrated to decrease N losses in tile-drained landscapes (Ruffatti et al., 2019), increase C sequestration (Poeplau and Don, 2015), improve N cycling (Lacey and Armstrong, 2014; 2015), and reduce erosion (Basche et al., 2016). However, while the environmental and soil health benefits of cereal rye are well established, soybean agronomic benefits remain uncertain. Studies in the Midwest region of the U.S. have reported that cereal rye increased (Moore et al., 2014), decreased (Eckert, 1998), and did not affect soybean yield (De Bruin et al, 2005). This uncertainty in the literature for soybean following cereal rye termination demonstrates a level of economic risk for adopting farmers and that more detailed studies are needed to better understand the system and to develop adaptive management plans for more consistent optimization of yield production.
Dominantly in the research literature, soybean yield reduction following cereal rye termination has been linked to planter interference and failed termination that results in stand reduction (Schipanski et al. 2014). However, little focus has been given to the potential of cereal rye growth resulting in deficient soil N and S levels that affect soybean development and nodulation. Dr. Armstrong’s lab has demonstrated that cereal rye has the potential to scavenge >1-118 lbs/A of N from the soil but only 10% of its residue N released during decomposition is recovered by the following soybean plant. Additionally, cereal rye has the potential to scavenge 0.2-6 lb/A of sulfur (S) from the soil (Figure 1A) and because S must be released from the residue similar to N, its return to soil is likely as slow as N release. A preliminary on-farm trial conducted by Dr. Casteel’s lab found that AMS increased soybean yield following cereal rye relative to no AMS application (Figure 1B).
Soybean takes up ~5 lb of N and 0.35 lb of S per bushel, so a 70-bu soybeans would need 350 lb N and 24.5 lb S per acre. Nearly 75% of the N and over 50% of the S are removed in the grain (largely in amino acids of the protein), and thus ~260 lb N and ~13 lb S per acre are removed from the field in the grain. Soybean takes up N in the “easiest” forms possible (e.g., soil N and fertilizer N) followed by biological N fixation (BNF). The direct N uptake from the soil or supplemental N requires less energy than BNF for the soybean plant. Biological N fixation can provide anywhere from 48-93% of a soybean’s N needs, with the United States averaging about 60% (Tamagno et al., 2018). Sulfur is a macro-nutrient that is needed as a co-factor for proper nodulation and fixation by Bradyrhizobium japonicum for soybean. If soil N supply is limited, greater demand is placed on BNF to supply N. Furthermore, if S supply is limited (e.g., limited mineralization, scavenged or leached S, low S deposition), then BNF is compromised and could have cascading effects on soybean growth and development, yield, and quality.
Therefore, the overarching goal of the study is to Maximize soybean production by advancing cover crop and intense/nutrient management synergies (i.e., N and S). Our specific research goals are to (1) Understand the paths that advance soybean yield with cereal rye inclusion (i.e., water holding capacity later in the season, N and S release, etc.); (2) investigate the impact of cereal rye N and S scavenging on soybean nutrient utilization (N and S), seed yield, and seed quality; and (3) determine the mechanisms that impede soybean yield under cereal rye inclusion (i.e., immobilization of N and S, limited microbial activity, delayed soybean development, and nodulation/fixation).