A multi-spatial scale analysis of land use and climate change impacts on water quality and crop productivity for major US cropping systems

Abstract

Agricultural production of food and fuel within the Corn Belt region of the Midwest, US plays a crucial role in the US economy. Nutrient loss under agricultural production within this region has a significant impact on water quality at both local and national scales and is expected to worsen under future climate conditions. Future climate projections include increases in carbon dioxide concentrations ([CO2]), temperature, vapor pressure deficit (VPD), and variability of extreme precipitation events, which have the potential to alter crop productivity, nutrient transformations, and energy and water budgets. Here we present an approach that quantifies the potential impact of future climate on crop productivity and nutrient loss and how this could affect water quality. Our approach also quantifies the impact of mitigation strategies that have the potential to improve water quality and total crop production within this region. One mitigation strategy that has the potential to reduce nitrogen (N) losses at multiple spatial scales is the integration of perennial grasses, like miscanthus (Miscanthus × giganteus Greef et Deu.; Chae et al., 2014) and switchgrass (Panicum virgatum L.). However, N reduction will depend on the land on which perennials are planted and the crops they displace. A multi-scale economic and biophysical modeling approach was used to quantify the impact of land use and climate change on crop productivity and water quality under contemporary and future climates. Using an integrated economic-agroecosystem modeling approach, we analyzed the impact of land use change driven by a national scale economic policy (i.e., Renewable Fuel Standard (RFS2)) on water quality at the scale of the Mississippi Atchafalaya River Basin (MARB) under contemporary climate conditions. While the RFS2 mandate could be met with less cropland area, our results indicated that reaching the full RFS2 mandate with both corn stover and perennial grasses would require additional corn area and N inputs to meet the demand of cellulosic biofuel production. By including both corn stover and perennial grasses as viable feedstocks, perennial grasses were not placed on regions of active, high N leaching cropland and therefore did not improve water quality relative to the baseline. However, a reduction in N loss within the MARB could be obtained if perennial grasses are used to reach the full cellulosic portion of the RFS2 mandate. To quantify the sensitivity of elevated [CO2], temperature, and VPD on crop productivity under future climate conditions, we improved an agroecosystem model to account for reproductive heat stress on corn and soybean yield. We conducted a high-resolution, field-scale modeling analysis for the Raccoon River Basin (RRB) in west-central Iowa. Our results suggest that corn yields could decrease and soybean yields could increase by the end of the 21st century relative to the baseline, even with the inclusion of reproductive heat stress on soybeans. However, if the hybrid growing season length is increased for corn cultivars, one could obtain corn yields comparable to today by the end of the century. Furthermore, sensitivity to high VPD and temperature on yield was reduced at higher [CO2] for soybean, but not corn, due to the CO2 fertilization effect. Building off the integrated modeling approach and improved version of the agroecosystem model, we conducted simulations to analyze the impact of climate-driven crop productivity changes and the strategic integration of miscanthus based on profit and N loss on water quality for the RRB under contemporary and future climate conditions. Three scenarios were created to isolate the effect of future climate and miscanthus on N loss when compared to a baseline scenario. Our results indicate that N loss could be greater under future climates in the late 21st century relative to contemporary climate due to a reduction in crop productivity and the associated reduction of N uptake, as well as increased mineralization rates due to higher temperatures. The inclusion of miscanthus provided a reduction in N loss under both contemporary and future conditions. However, more miscanthus replaced annual row crops under future climate relative to contemporary climate due to lower profitability and higher N loss under corn and soybean production. While the inclusion of miscanthus under future climate did not reduce N loss below levels obtained under the baseline scenario, it did result in a reduction of total N loss below levels obtained in the historical evaluation simulation. Together, these analyses suggest that the integration of strategically placed miscanthus has the potential to reduce N loss under contemporary climate conditions at both the small and large basin scales. Under future climate conditions, corn yields are projected to decrease while soybean and miscanthus yields are projected to increase relative to contemporary conditions. Mineralization rates were also projected to increase under warmer conditions in future climate projections, resulting in an increase of N loss under current land use, especially with a reduction in crop productivity. However, the strategic implementation of miscanthus on land with low profitability and high N leaching resulted in a greater N loss reduction under future climate than contemporary climates.