agronomy Communication Catch Crops: A Nutrient Reservoir in Post-Harvest Residues under Water Deficit Magdalena Jastrz ˛ ebska * , Marta K. Kostrzewska , Maria Wanic, Marek Marks and Kinga Treder   Citation: Jastrz˛ ebska, M.; Kostrzewska, M.K.; Wanic, M.; Marks, M.; Treder, K. Catch Crops: A Nutrient Reservoir in Post-Harvest Residues under Water Deficit. Agronomy 2021, 11, 1501. https:// doi.org/10.3390/agronomy11081501 Academic Editor: Spyridon A. Petropoulos Received: 25 June 2021 Accepted: 27 July 2021 Published: 28 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Agroecosystems and Horticulture, Faculty of Agriculture and Forestry, University of Warmia and Mazury in Olsztyn, Plac Lódzki 3, 10-718 Olsztyn, Poland; marta.kostrzewska@uwm.edu.pl (M.K.K.); maria.wanic@uwm.edu.pl (M.W.); marek.marks@uwm.edu.pl (M.M.); kinga.treder@uwm.edu.pl (K.T.) * Correspondence: jama@uwm.edu.pl Abstract: Undersowing catch crops (CCs) in cereals provides many environmental benefits and potentially contributes to building agricultural resilience to climate change. The increasing soil water deficit due to global warming is becoming a challenge for the sustainability of Central European agriculture. Some of the multiple functions of CCs may be altered under water shortage. Two pot experiments were conducted in Poland to assess the effect of water deficit on N, P, K, and Mg accumulated in post-harvest residues left by spring barley undersown with Italian ryegrass or red clover, and in the soil under these crops. In both experiments, barley grown alone provided a reference, and two levels of water supply were adopted: higher (sufficient for barley) and lower (reduced by 50%). Under water deficit, CCs undersown in spring barley maintained their function of capturing and storing nutrients. Post-harvest residues of barley undersown with CC and stressed with water shortage accumulated the same or higher amounts of N, P, K, and Mg than residues of barley grown alone under sufficient water supply. Soil nutrient contents were negatively correlated with crop biomass. Further research with other CC species and studies based on field experiments under rainout shelters are recommended. Keywords: intercropping; Hordeum vulgare L.; Lolium multiflorum Lam.; Trifolium pratense L.; water stress; nitrogen; phosphorus; potassium; magnesium 1. Introduction Many pathways towards agricultural sustainability, including organic farming, inte- grated farming, ecological intensification, climate-smart agriculture, and conservation agriculture, promote the cultivation of crops that serve production and environmen- tal/ecological functions [1–5]. Catch crops (CCs) undersown in cereals are one of the best examples of multifunctional crops [6]. The main aim of growing CCs is to prevent leaching of nutrients, mainly nitrogen (N), in the fall after harvest and during the following winter by capturing (catching) and fixing them into living plant tissues [7,8]. An undersown catch crop is a plant sown simultaneously with the main crop (usually cereal species) or during its initial vegetation, which stays after the main crop harvest until fall of the same year or until the following season (over the fall and winter periods) [9]. This form of catch cropping is particularly effective in regions where climatic conditions after harvest of the main crops may reduce the development of late-sown CCs [7]. Grasses and legumes are the most common undersown CCs, and, among them, Italian ryegrass (Lolium multiflorum Lam.) and red clover (Trifolium pratense L.), respectively, introduced into spring barley (Hordeum vulgare L.) field, are particularly prevalent [10–13]. Apart from preventing nutrient leaching, undersown CCs perform other important functions for cropping systems’ resilience and long-term stability. Left in the field after harvesting the main crop, they serve as ground covers (cover crops, mulch) and reduce Agronomy 2021, 11, 1501. https://doi.org/10.3390/agronomy11081501 https://www.mdpi.com/journal/agronomy