Nitrogen fertilization is critical to optimize short-term crop yield, but its long-term effect on soil organic C (SOC) is uncertain. Here, we clarify the impact of N fertilization on SOC in typical maize-based (Zea mays L.) Midwest U.S. cropping systems by accounting for site-to-site variability in maize yield response to N fertilization. Within continuous maize and maize-soybean [Glycine max (L.) Merr.] systems at four Iowa locations, we evaluated changes in surface SOC over 14 to 16 years across a range of N fertilizer rates empirically determined to be insufficient, optimum, or excessive for maximum maize yield. Soil organic C balances were negative where no N was applied but neutral (maize-soybean) or positive (continuous maize) at the agronomic optimum N rate (AONR). For continuous maize, the rate of SOC storage increased with increasing N rate, reaching a maximum at the AONR and decreasing above the AONR. Greater SOC storage in the optimally fertilized continuous maize system than in the optimally fertilized maize-soybean system was attributed to greater crop residue production and greater SOC storage efficiency in the continuous maize system. Mean annual crop residue production at the AONR was 22% greater in the continuous maize system than in the maize-soybean system and the rate of SOC storage per unit residue C input was 58% greater in the monocrop system. Our results demonstrate that agronomic optimum N fertilization is critical to maintain or increase SOC of Midwest U.S. cropland.

Funding: This study was funded by the United States Department of Agriculture National Institute of Food and Agriculture grant number 2014-67019-21629 to MJC, DCO, JES and JS, https://nifa.usda.gov/ . The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.

In this study, we attempted to resolve the inconsistent effects of N fertilization on SOC by evaluating change in surface SOC across a range of N rates empirically determined to be insufficient, optimum, or excessive for maximum maize yield. We measured long-term SOC change in two dominant Midwest U.S. cropping systems (continuous maize and maize-soybean rotation) at four Iowa locations spanning a range of climates and soils that is representative of rainfed maize production in the Midwest U.S.

The relative importance of these processes in controlling SOC response to N fertilization likely depends on the response of crop growth to N fertilization. We hypothesize that when N inputs are below the rate that maximizes yield (agronomic optimum N rate; AONR), added N stimulates crop growth, which increases crop residue inputs to the soil and thereby increases SOC. However, when N inputs are above the AONR, added N imparts no change in crop residue production but increases residual inorganic N [ 17 ], which alleviates microbial N limitation and thereby enhances SOC mineralization [ 16 ]. Crop response to N fertilization is site-specific and regional N fertilization recommendations do not account for this variability [ 18 ]. Hence, the same recommended N rate applied at two different locations could favor SOC storage through enhanced crop growth at one location but favor SOC mineralization through increased residual inorganic N at the other location. This site-to-site variability in crop response to N fertilization may generate inconsistent conclusions about the effect of N fertilization on SOC.

Nevertheless, the overall effect of N fertilizer application on cropland SOC remains unresolved [ 9 , 11 – 14 ]. Nitrogen fertilization has been reported to increase, decrease, or have no effect on SOC [ 9 – 12 , 15 ]. Positive effects of N fertilization on SOC have been explained by increased crop growth leading to greater residue inputs to soil [ 15 ]. Negative effects of N fertilization on SOC have been explained by enhanced SOC mineralization when microbial decomposition is otherwise N-limited [ 16 ].

The quantity of C stored in a soil represents a balance between C inputs and outputs. Crop residue (biomass excluding harvested material) is the most important C input in most conventional cropping systems. And in maize-based cropping systems, N fertilization is perhaps the greatest management factor affecting crop residue inputs [ 7 ]. Nitrogen fertilization can increase maize residue production by 40 to 50% in a maize-soybean rotation [ 8 , 9 ] and by 50% to >100% in a continuous maize system [ 8 – 10 ].

Maize and soybean cropping occupies more than 70 million ha in the U.S., representing a large stock of intensively managed soil organic C (SOC). Maintenance of this SOC is essential for future food production because crop yields are positively associated with SOC in the long term [ 1 ]. Soil organic C can increase crop yield by enhancing soil water holding capacity and nutrient retention [ 2 – 4 ]. Furthermore, N bound to C in soil organic matter (SOM) is frequently the largest source of N for the crop and the largest sink of N fertilizer inputs in modern grain cropping systems [ 5 , 6 ]. Accordingly, SOC impacts both crop yield and N losses to the environment.

Materials and methods

Nitrogen fertilization experiments Long-term N fertilization experiments were established at Iowa State University Research Farms to study the effect of N fertilization rate on crop yield and SOC storage in continuous maize and maize-soybean cropping systems. The experiments were established at the Central (42°01’ N; 93°47’ W), Southeast (41°11’ N; 91°29’ W), and South (40°58’ N; 93°25’ W) locations in 1999 and at the Northwest location (42°55’ N; 95°32’ W) in 2000 (Fig 1). The research sites span a climatic gradient from 790 to 1000 mm mean annual precipitation and 8.3 to 11.0°C mean annual temperature. Soils at all four locations are within the Mollisol order, and the suborders represent the three dominant suborders of the U.S. Maize Belt–Udolls, Aquolls and Ustolls [19]. The soil texture classes at the four locations are loam, silt loam, or silty clay loam and initial SOC concentrations ranged from 20.8 g kg-1 to 28.1 g kg-1 (Table 1). The Northwest, Central, and Southeast locations are underlain by artificial subsurface drainage, while the South location is not. All four research sites were previously managed as conventional maize and soybean production systems for at least 10 years before experimental establishment. PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 1. Locations of long-term N fertilization experiments. Maps show study locations within the most productive region of the U.S. rainfed Maize Belt (eastern Nebraska, southern Minnesota, Iowa, and central and northern Illinois) [20]. a) Mean annual precipitation, b) mean annual minimum temperature, c) mean annual maximum temperature, d) Major Land Resource Areas [21]. All climate data were averaged over 1981–2010 [22]. Cardinal directions for the Major Land Resource Areas are abbreviated in the legend (N, S, E, W, and C for north, south, east, west, and central, respectively). https://doi.org/10.1371/journal.pone.0172293.g001 PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Table 1. Soil properties and weather conditions at four experimental locations in Iowa. https://doi.org/10.1371/journal.pone.0172293.t001 The experimental design at each location is a split-plot randomized complete block design with four replicate blocks. Each block consists of three main plots differing in crop sequence: continuous maize, maize-soybean, and soybean-maize. The main plots are split into individual treatment plots, each receiving one of five or seven N rates to maize; soybean does not receive N fertilizer. The N rates applied at the Central location are 0 to 269 kg N ha-1 in 67 kg N ha-1 increments, while those at the Northwest, Southeast, and South locations are 0 to 269 kg N ha-1 in 45 kg N ha-1 increments. Each individual treatment plot measures 6.1 m x 15.2 m (Northwest), 4.6 m x 15.2 m (Central and South) or 6.1 m x 19.8 m (Southeast). Crops are planted lengthwise in the plots with a row spacing of 0.76 m. The trials are managed with fall chisel plowing and spring secondary tillage before planting. Nitrogen fertilizer is applied as either urea incorporated at planting or urea ammonium nitrate solution injected as an early side-dress. Other nutrients and soil pH are maintained based on soil testing for optimum production.

Yield measurement and residue carbon inputs Maize and soybean grain yields were measured annually from the center three to six rows of each individual treatment plot using a small-plot combine. Mean grain yields were calculated by averaging across years (2000–2014 for Central and South locations, 2001–2014 for Northwest, and 2000–2015 for Southeast) by individual treatment plot. Maize and soybean grain yields are reported at standard 155 g kg-1 and 130 g kg-1 moisture contents, respectively. Mean grain yields were adjusted to dry matter and used to estimate total aboveground dry matter production using harvest indices of 0.50 grain dry matter/total aboveground dry matter for maize and 0.42 grain dry matter/total aboveground dry matter for soybean [24,25]. Aboveground residue inputs were calculated as the difference between total aboveground dry matter production and grain dry matter. Belowground residue inputs were calculated as the product of total aboveground dry matter production and a root/shoot ratio (0.18 root dry matter/total aboveground dry matter for maize and 0.15 root dry matter/total aboveground dry matter for soybean) [25]. Residue dry matter inputs were converted to residue C inputs assuming a tissue C concentration of 400 g kg-1 [26]. We estimated mean annual residue C inputs for each individual treatment plot by taking an average of maize and soybean total residue C inputs (aboveground plus belowground) weighted by the number of years in each crop phase.

Soil sampling Soil samples were collected at the onset of each experiment and again in 2014 (Northwest, Central, and South locations) or 2015 (Southeast location). For the initial sampling event, soil samples were collected by main plot at the Northwest and Central locations and by individual treatment plot at the Southeast and South locations. For the final sampling event, soil samples were collected by individual treatment plot at all locations. Each soil sample was composited from 15 cores of 2.5 cm diameter x 15 cm depth that were collected in the fall after harvest and before chisel-plow tillage. Soils were air dried and finely ground for total C and N determination by dry combustion elemental analysis using a Leco CN analyzer (initial sampling event; Leco Corp., St. Joseph, MI) or a Vario Max CN analyzer (final sampling event; Elementar Americas, Mt. Laurel, NJ). A subset of archived soil samples from the initial sampling event was also analyzed on the Vario Max CN analyzer to confirm consistency of results between instruments. Carbonates were not present in surface soil at these locations. Because the plots at all locations are chisel-plowed annually to a depth of approximately 30 cm, the long-term N rate was not expected to influence bulk density in the surface soil. This assumption was corroborated at one location by previous research [10]. Accordingly, we used bulk density measurements taken from each cropping system within each block to scale SOC to a mass per area basis for the 0–15 cm depth. Rates of change in SOC and the C/N ratio were calculated for each individual treatment plot by first calculating the difference in surface SOC or C/N ratio between sampling times and then dividing this difference by the number of years between sampling times. Although historical sampling practices restricted our analysis of SOC to the surface 15 cm, changes observed in surface SOC may be representative of changes occurring at greater depths. An analysis of long-term SOC change data from regional agricultural studies showed that change in surface SOC was positively correlated with change in total soil profile SOC (S1 Fig).