Abstract
To help reduce future N loads entering the Gulf of Mexico from the Mississippi River 45%, Iowa set the goal of reducing non-point source N loads 41%. Studies show that implementing winter rye cover crops into agricultural systems reduces N loads from subsurface drainage, but its effectiveness in the Mississippi River Basin under expected climate change is uncertain. We used the field-tested Root Zone Water Quality Model (RZWQM) to estimate drainage N loads, crop yield, and rye growth in central Iowa corn-soybean rotations. RZWQM scenarios included baseline (BL) observed weather (1991–2011) and ambient CO2 with cover crop and no cover crop treatments (BL_CC and BL_NCC). Scenarios also included projected future temperature and precipitation change (2065–2085) from six general circulation models (GCMs) and elevated CO2 with cover crop and no cover crop treatments (CC and NCC). Average annual drainage N loads under NCC, BL_NCC, CC and BL_CC were 63.6, 47.5, 17.0, and 18.9 kg N ha−1. Winter rye cover crop was more effective at reducing drainage N losses under climate change than under baseline conditions (73 and 60% for future and baseline climate), mostly because the projected temperatures and atmospheric CO2 resulted in greater rye growth and crop N uptake. Annual CC drainage N loads were reduced compared with BL_NCC more than the targeted 41% for 18 to 20 years of the 21-year simulation, depending on the GCM. Under projected climate change, average annual simulated crop yield differences between scenarios with and without winter rye were approximately 0.1 Mg ha−1. These results suggest that implementing winter rye cover crop in a corn-soybean rotation effectively addresses the goal of drainage N load reduction under climate change in a northern Mississippi River Basin agricultural system without affecting cash crop production.
Highlights
Hypoxic or dead zones in coastal oceans have been expanding since the 1960s and are forecast to increase with climate change if reduction strategies are not implemented [1,2,3]
We hypothesize that despite possible increased drainage N loads under climate change, including winter rye cover crop in corn-soybean rotations will reduce N loads in central Iowa below the reduction goal of 41% set to address the hypoxic zone in the Gulf of Mexico without reducing cash crop production, due to anticipated increases in rye uptake of crop available N and water under climate change
The analysis begins with crop production because of its role in accurate simulation of drainage N loads
Summary
Hypoxic or dead zones in coastal oceans have been expanding since the 1960s and are forecast to increase with climate change if reduction strategies are not implemented [1,2,3]. To address the growing dead zone in the Gulf of Mexico, the USEPA created the Mississippi River/Gulf of Mexico Nutrient Task Force in 1997 [5] This Gulf Hypoxia Task Force set a goal to reduce total nitrogen (N) loads from the Mississippi River Basin 45% by 2035, as compared with the baseline loads between 1980 and 1996 [6,7]. Central Iowa is one of the more important areas to reduce N loads to meet Iowa’s non-point source goal and to reduce N loading to the Mississippi River, because it has a large fraction of land in artificially drained row crops [11]. These conditions of relatively large subsurface drainage N loads from row crop production entering streams makes central Iowa a key region for nitrate reduction efforts
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