Sulfur fertiliser use in the Midwestern US increases as atmospheric sulfur deposition declines with improved air quality

The management of nitrogen (N) and phosphorus (P) fertilizers has been a significant concern in agricultural systems globally. Extensive research focuses on optimizing N and P application to maximize yields while minimizing environmental impact, including greenhouse gas emissions, soil degradation, and water eutrophication. Sulfur (S), another essential plant nutrient, also carries environmental consequences, highlighted by acid rain research in the 1970s and 80s. Historically, crop S demand was met by substantial atmospheric S deposition. However, improvements in air quality through regulation and shifts in energy production have led to a decline in atmospheric S deposition in the US, Europe, and elsewhere. This necessitates a shift from diffuse atmospheric inputs to targeted fertilizer applications, requiring a critical analysis of agricultural S loads, soil S dynamics, and their environmental effects. The Midwestern US, a major producer of maize and soybean, experienced high atmospheric S deposition before the Clean Air Act and Amendments. While this initially met crop S demand, increased crop acreage and yields have driven increased S demand. Despite considerable research on optimizing N and P application, comparable attention hasn't been given to understanding and optimizing agricultural S inputs. A challenge in assessing the environmental consequences of large-scale S applications has been a lack of publicly available data. This study utilizes a compiled dataset of S-containing fertilizers from the Association of American Plant Food Control Officials (AAPFCO) from 1985-2015 as a proxy for S inputs to Midwestern croplands to address this gap.

The paper references numerous studies on nitrogen and phosphorus fertilizer management, highlighting the existing knowledge and concerns regarding their environmental impact. It also cites research on the effects of acid rain, demonstrating the environmental consequences of high sulfur deposition. Prior research has shown a shift in sulfur cycle manipulation, from atmospheric emissions to agricultural additions, indicating the increasing importance of understanding this change in the sulfur cycle. The authors note a lack of comparable research on sulfur fertilizer application compared to nitrogen and phosphorus, justifying the need for this study.

This research compiled fertilizer sales data from the Association of American Plant Food Control Officials (AAPFCO) for 1985–2015. Data were reported annually by farmers and aggregated at the county level. Fertilizers were categorized based on their nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) content, irrespective of whether each nutrient was the primary addition or a carrier. The total load of each nutrient was calculated for the study region. Where nutrient content wasn't reported, a range of possible values was calculated using publicly available product information and literature. County-level data were aggregated to evaluate trends across a 12-state Midwestern region, acknowledging potential discrepancies between fertilizer purchase and application locations. To account for unused fertilizer, the unused mass from one year was subtracted from the value of the following year, leading to some negative values year-to-year. The researchers normalized N, P, K, and S loads by dividing the total weight of each fertilizer by the total crop acreage data from the USDA. This total acreage includes maize, soybean, and other crops, with maize and soybean dominating. The S export in maize tissues was estimated using tissue data and maize acreage planted from the USDA. Atmospheric S deposition was estimated using annual volume-weighted sulfate concentrations from the National Atmospheric Deposition Program (NADP), dry S deposition estimates from the Clean Air Status and Trends Network (CASTNET), and precipitation data from PRISM. Spatial interpolation was used for unmonitored regions. The uncertainty in flux estimates includes those associated with precipitation volume and sulfate concentrations (wet deposition), gaseous sulfur dioxide and particulate sulfate concentrations and deposition velocity (dry deposition), and spatial extrapolation. Statistical analysis, using R's `lm` function, determined best-fit linear regression lines for atmospheric S deposition, maize and soybean yields, and fertilizer sales to evaluate trends and statistical significance.

Between 1987 and 2019, atmospheric S deposition declined at a rate of −0.12 kg S ha⁻¹ yr⁻¹, from 4.7 to 1.1 kg ha⁻¹ yr⁻¹, across the Midwestern US. Comparatively, all S-containing fertilizer products increased at a rate of 0.10 kg S ha⁻¹ yr⁻¹, from 1.3 to 4.9 kg ha⁻¹ yr⁻¹ between 1985 and 2015. When considering only products with >85% S content, the increase was still significant. The rate of change in average S fertilizer load was substantially higher than that of N, P, and K. While N and P increased, their rates of change weren't significantly different from zero. The increase in S fertilizer use was more pronounced after 2009. The increase in S fertilizer is attributed to several factors: a decline in atmospheric S deposition; increased maize and soybean acreage and yields, leading to higher nutrient export; spatial heterogeneity in soil S pools and cycling rates; and farmer management decisions regarding S deficiencies and other agricultural practices. The study found that atmospheric S deposition is now close to pre-industrial background levels. The mass balance estimate indicates that while inputs may recently meet crop S demand, the estimate is conservative because it doesn't account for S export in soybean (often double-cropped with maize) or riverine export of sulfate-S. This highlights a need to investigate soil S cycling processes that regulate mobility and residence time.

The study's findings highlight a significant shift in sulfur management in agriculture. The decline in atmospheric S deposition, coupled with increased crop yields and acreage, has necessitated a substantial increase in sulfur fertilizer application. The significant difference in the rate of change of sulfur fertilizers compared to other major nutrients emphasizes the need for focused attention on sulfur management. The authors caution against interpreting the findings as a positive effect of dirtier air, citing research demonstrating the substantial benefits of air quality improvements on crop yields. The study emphasizes that the increased pressure to add S fertilizers is likely to continue with air quality regulation and high agricultural productivity as priorities. The need to understand the dynamics of S released from the soil pool and the long-term consequences of S applications is stressed, drawing parallels to research on excess N and P fertilizers.

This study demonstrates a substantial increase in sulfur fertilizer use in the US Midwest, driven by declining atmospheric sulfur deposition and increased crop production. The authors highlight the need for improved sulfur management strategies to optimize fertilizer application, minimize environmental impacts, and ensure sustainable agricultural practices. Further research is needed to quantify the dynamics of soil sulfur cycling and the long-term ecological consequences of increased sulfur fertilization.

The study acknowledges limitations in the data, including the potential mismatch between fertilizer sales and application locations and the absence of annual or comprehensive data across all crops. The mass balance estimate is considered conservative, as it does not fully account for all potential sulfur losses. The study also acknowledges that the explanation for the broad trend is likely a combination of factors and that further research is needed to fully quantify each component.