Date Published: April 12, 2019
Publisher: Public Library of Science
Author(s): Jason Kaye, Denise Finney, Charles White, Brosi Bradley, Meagan Schipanski, Maria Alonso-Ayuso, Mitch Hunter, Mac Burgess, Catalina Mejia, Jose Luis Gonzalez-Andujar.
Cover crops have the potential to be agricultural nitrogen (N) regulators that reduce leaching through soils and then deliver N to subsequent cash crops. Yet, regulating N in this way has proven difficult because the few cover crop species that are well-studied excel at either reducing N leaching or increasing N supply to cash crops, but they fail to excel at both simultaneously. We hypothesized that mixed species cover crop stands might balance the N fixing and N scavenging capabilities of individual species. We tested six cover crop monocultures and four mixtures for their effects on N cycling in an organically managed maize-soybean-wheat feed grain rotation in Pennsylvania, USA. For three years, we used a suite of integrated approaches to quantify N dynamics, including extractable soil inorganic N, buried anion exchange resins, bucket lysimeters, and plant N uptake. All cover crop species, including legume monocultures, reduced N leaching compared to fallow plots. Cereal rye monocultures reduced N leaching to buried resins by 90% relative to fallow; notably, mixtures with just a low seeding rate of rye did almost as well. Austrian winter pea monocultures increased N uptake in maize silage by 40 kg N ha-1 relative to fallow, and conversely rye monocultures decreased N uptake into maize silage by 40 kg N ha-1 relative to fallow. Importantly, cover crop mixtures had larger impacts on leaching reduction than on maize N uptake, when compared to fallow plots. For example, a three-species mixture of pea, red clover, and rye had similar maize N uptake to fallow plots, but leaching rates were 80% lower in this mixture than fallow plots. Our results show clearly that cover crop species selection and mixture design can substantially mitigate tradeoffs between N retention and N supply to cash crops, providing a powerful tool for managing N in temperate cropping systems.
One of the grand challenges for agriculture is to minimize nitrogen (N) losses to the environment while maintaining adequate N supply for high cash crop yields . Cover crops (CCs) are a key strategy for meeting this challenge. Cover crops are typically planted outside of the cash crop growing season to scavenge N from soil and prevent erosion and leaching . Then, when CCs are killed, the N in their tissues can be microbially mineralized to supply inorganic N to subsequent cash crops [3, 4]. While this sounds like an ideal N regulator for agricultural systems, the N dynamics are quite challenging to manage. Cover crops that are good at scavenging N from soil (e.g. grasses) often have high C:N ratios when they are killed. During microbial decomposition of such CC residues, N is immobilized [5, 6], reducing availability to cash crops to an extent that can limit yields in some cases [7–9]. Conversely, legume CC tissues have low C:N ratios and thus, microbes decomposing their tissues mineralize N and increase N availability to cash crops. Unfortunately, these legume CCs can be poor scavengers for soil N and N leaching can be high under them [10–12]. Thus, a major research gap is discovering strategies that enable CCs to prevent N leaching while increasing N supply to and yields of subsequent cash crops. In this paper, we explore CC species selection and CC mixture design as two strategies to improve the balance between N supply and N retention from CCs.
We observed substantial variation among CC monocultures in N dynamics including biomass N accumulation, N leaching, and N supply to a maize cash crop. Thus, CC selection can be a valuable tool for N management. A key result is that all CCs reduced leaching relative to fallow plots, including legumes and winter-killed non-legumes. We found that pea monocultures increase maize yields in comparison with fallow or non-legume monocultures and that they increase maize N uptake even more than our large manure application. However, non-legume CCs, especially winter-hardy grasses, need to be managed carefully so that large stands of high C:N CCs do not lead to N limitation in subsequent maize crops.