Residue management is increasingly important as higher yielding corn and soybean varieties have led to a steady increase in the amounts of residue left on fields after harvest, in particular for corn residue in front of soybean. Managing the residues of corn as well as soybean to benefit soybean productivity offers options to increase nutrient use efficiency of fertilizer inputs and thus overall profitability. This is because fertilizer nutrients contained in residues such as nitrogen (N), sulfur (S) and potassium (K) are released into soil throughout the subsequent growing season, and thus are available to the following crop. For example, corn has a harvest index of approximately 60% for N, 20% for K, and 55% for S – meaning that 40% of N uptake, 80% of K uptake and 45% of S uptake by the corn crop will not be removed with grain harvest1 and thus are potentially available for the following soybean crop (=110 lb N, 160 lb K2O, 12 lb S per ac) depending on corn residue breakdown. Since soybeans acquire approximately half of their N from soil (the other half provided by biological fixation in soybean root nodules2, 3) this means that fertilizer N from the preceding corn phase may still be used by the soybean crop via decomposition of corn residues as well as residual fertilizer remaining in the soil, and similarly for K and S.

This project will use state-of-the-art nutrient isotope techniques to simultaneously track N, K and S from fertilization through soil and into the crop of the same-season and next-season following application via residue, under six major soybean management practices. This will provide, for the first time, a clear understanding of fertilizer usage by soybean and its management via residues. The overarching objective of this project is to determine the role of residues as a nutrient source in soybean production by directly tracking nutrients (N, K and S) from fertilizer to residue and/or soil and then to crop uptake, using stable isotopes. Though stable isotope labeling is possible and routine for N, it is only now possible for K and S with novel technological advances in the last five years (note: no stable isotopes exist for P) . We will therefore employ three-way stable isotope labeling to simultaneously track N, K and S throughout the corn-soybean rotation under different residue management practices.

Our specific objectives are to:
(1) Quantify nutrients stored and released in residues of corn for soybean production, as well as wheat and cereal rye cover crop residues.
(2) Determine the multi-season contributions of residue versus fertilizer versus soil for soybean nutrient uptake (N, K, S), with residues being corn (i.e., corn-soybean), corn + wheat (i.e., corn-wheat/soybean double crop), and cereal rye (i.e., cover crop).
(3) Evaluate how tillage and residue breakdown biologicals influence the nutrient release and value of residues for soybean production.
(4) Evaluate how fertilizer timing “in front” of corn vs “in front of” soybean influences soybean utilization of fertilizer N, K and S, including distinguishing soil vs residue contributions.
(5) Evaluate how cover cropping influences nutrient release for soybean from corn residues as well as from cover crop residues.
(6) Determine the contribution of soybean residues to the following corn crop nutrient uptake.

The proposed work will deliver the following three major deliverables:
(1) Residue nutrient tool. Guidelines on how much total and seasonally available N, K and S for soybean are stored in residues of corn, wheat, cereal rye as a cover crop. The nutrient value of residue will be expressed as a table (one per nutrient) in which the amount of residue can be estimated per bu of grain yield. Then, knowing how much residue is present based on grain yield, the lb/ac of N, K and S contained in the residues and credited to soybean uptake can be calculated. Similar guidelines will also be provided for corn based on soybean residue nutrient value and release.
(2). Residue management recommendations. Concrete recommendations on how tillage and residue breakdown biologicals can be used to favor nutrient release of residues for soybean uptake will be provided. Additional layers in these recommendations will include how corn vs wheat vs cover crop (cereal rye) residues are different or similar when it comes to maximizing N, K and S release for soybean uptake, by tillage, biologicals, and rotation (e.g., double cropping) practices.
(3) Fertilization recommendations. Our results will provide direct insight to soybean utilization of N and S from starter fertilizer (AMS) and of K from potash. This includes N, K and S added “in front of” soybean as well as “in front of” corn with soybean as a residual feeder, in full season and in double-cropped wheat/soybean rotations with corn. Additional recommendations on fertilizer timing will be provided in the context of tillage and cover cropping to maximize N, K and S fertilizer usage efficiency by soybean.

Residue management is a key component of any crop production operation. How residues are managed as part of a cropping system is important for soybean nutrition, because nutrient-containing residues have cross-over from the current growing season to the next season via decomposition. How to manage residues to maximize nutrient release in the following soybean phase carries multiple considerations, and requires clearly tracking nutrients from fertilizer to residue and soil and to the following soybean crop. Understanding fertilizer fate in soils and residues and residue nutrient release stands to provide returns to cropping systems by improving nutrient supply and rotation-wide efficiency of nutrient inputs. Increasing fertilizer prices and price volatility are further highlighting the need to increase nutrient use efficiency for profitability.