We adopted single-year application timing information for the nine major crops in each state (i.e., corn, soybean, winter wheat, spring wheat, cotton, sorghum, rice, barley, and durum wheat). According to USDA-ERS (2016), we grouped state-level crop-specific N fertilizer use into four application timings: fall (previous year), spring (before planting), at planting, and after planting. USDA-ERS (2016) has conducted the survey since 1996 and collected the data periodically for each crop. For example, corn producers were surveyed in 1996–2001, 2006, and 2010 (Supplement Table S5). Because the collected dates varied among different crops in each state, we adopted the latest survey for all nine crops. The raw data include crop-specific fertilizer use rates at four timings and percentages of the fertilized cropland in each state. As there is no available spatial information to identify the fertilized area, we assumed all cropland was fertilized at a lower application rate by multiplying the reported rate by the fertilized cropland percentage for all four application timings (Goebes et al., 2003). We used the fraction of the application rate of each timing to split the annual state-level crop-specific N use rate generated in Sect. 2.1 into four application timings.

We assumed farmers have the same preference regarding application timing for all crops in each state. We calculated the average application timing fraction based on the fraction of eight crops (excluding winter wheat) generated above for cropland pasture and other crops.

For spatial analysis, we downscaled the imputed state-level crop-specific N management data to gridded maps based on 1 km × 1 km historical land cover data (including crop density and crop type distribution maps) of the contiguous US from 1850 to 2015 developed by Yu and Lu (2017) and Yu et al. (2018). The cropland density maps were reconstructed to represent the area of cropped land each year while excluding summer idle/fallow areas; this was undertaken by incorporating various sources of inventory data and high spatial resolution satellite images. The crop type maps were reconstructed using satellite images and the USDA National Agricultural Statistics Service (NASS) survey data. The state-level land area of each crop type in each year is consistent with the USDA survey. More details about cropland maps can be found in Yu and Lu (2017) and Yu et al. (2018).

In this paper, for display purpose, we timed the cropland percentage with the N use rate in each grid to convert the unit of N use rate from g N m-2 cropland area per year to g N m-2 land area per year. We then resampled the N management maps at a 5 km × 5 km resolution with the average fertilizer use rate depicted in each pixel. To describe the regional difference of N management in the study area, we partitioned the entire study area into seven regions (Fig. 4): the Northwest (NW), the Southwest (SW), the Northern Great Plains (NGP), the Southern Great Plains (SGP), the Midwest (MW), the Southeast (SE), and the Northeast (NE) according to the US Fourth National Climate Assessment (2017).

N fertilizer consumption in the US was very low (< 0.3 Tg N yr-1) from 1850 to 1940, and then sharply increased to 9.8 Tg N yr-1 by 1980, followed by a slower rise to 11.4 Tg N yr-1 by 2015 (Fig. 2a). The N fertilizer use rate followed these total consumption trends and increased from less than 0.01 g N m-2 yr-1 in 1850 to 9.04 g N m-2 yr-1 in 2015 (Fig. 2a). In 1960, most agricultural areas received low N fertilizer input (less than 6 g N m-2 yr-1); however, due to expansion of cropland and the rapid increase in N fertilizer use from 1960 to 1980, the area receiving high N input (2–8 g N m-2 yr-1) greatly increased in 1980, and the usage frequency peaked at a few high N input values (insert figure in Fig. 2a); this may have been caused by intensive fertilizer use in certain crop types (e.g., maize and rice). In 2015, we found that fewer areas were characterized by low to medium-level fertilizer input (< 8 g N m-2 yr-1), and high-level N fertilizer application (8–18 g N m-2 yr-1) was more widespread compared to both 1960 and 1980 (inset figure in Fig. 2a). From 1940 to 1960, a moderate rise in the application rate was found in almost all crop types with increments ranging from 0.09 g N m-2 yr-1 in soybean to 5.5 g N m-2 yr-1 in rice (Fig. 2b). The 2 decades following 1960, was the period when the most dramatic increase of N fertilizer use occurred. The average increase during this time was 4.1 g N m-2 yr-1 with the largest increase found in corn (11.2 g N m-2 yr-1), while cotton received relatively stable amounts of N fertilizer (Fig. 2b). From 1980 to 2015, minor changes (< 2 g N m-2 yr-1) in the application rate of fertilizer were found in corn, yet large increases were seen in rice (7 g N m-2 yr-1), spring wheat (6.5 g N m-2 yr-1), and durum wheat (5.2 g N m-2 yr-1).

Hotspots of N fertilizer use have shifted from the southeastern and eastern US to the Midwest, the Great Plains, and the Northwest since the 1940s (Fig. 3). Application rates of commercial N fertilizer were very low (less than 0.2 g N m-2 yr-1) across the contiguous US before 1940. Mild increases (1–2 g N m-2 yr-1) in N fertilizer use were detected along the west coast and in the southeastern and eastern US in the 1940s. The application rates in these regions showed dramatic increases to above 7 g N m-2 yr-1 by 1980. In comparison, the Midwest received a N fertilizer input of over 10 g N m-2 yr-1, and the Southern Great Plains, the southern region of the Northern Great Plains, and the Northwest received N fertilizer at a rate of 4–7 g N m-2 yr-1 on average from 1960 to 1980. Not surprisingly, the most intensive N input was found in the central “Corn Belt” (Iowa, Illinois, Nebraska, and Minnesota) pinpointing the Midwest as the N fertilizer use hotspot in the US after 2000.

N fertilizer application rate in the four timings varied among regions and time periods. Here we present the seasonal variation of agricultural N input for the year 2015 as an example (Fig. 4). The contiguous US generally received low N input (< 0.5 g N m-2 yr-1) in fall, except California, North Dakota, the Midwest, and the Southern Great Plains (2–4 g N m-2 yr-1). Some areas in Washington, eastern Arkansas and southern Louisiana even received a fertilizer input of more than 4 g N m-2 yr-1. In comparison, the majority of N (> 4 g N m-2 yr-1) was applied in spring before planting, and the most intensive application took place in the Midwest, the Northern Great Plains, the Northwest, northern Kentucky, and parts of California, Florida, Arkansas and Louisiana (Fig. 4b). A small amount of N (< 1 g N m-2 yr-1) was applied at planting in most areas (Fig. 4c), although exceptions were found in the Northern Great Plains, northern California, and Washington (> 3 g N m-2 yr-1). In comparison, intensive N use (> 4 g N m-2 yr-1) after planting was identified in the eastern Midwest, the southern region of the Northern Great Plains, Idaho, California, and the western and southeastern parts of the Southeast (Fig. 4d).

High N input in fall prevailed in the Southern Great Plains, and Washington (Supplement Fig. S5a), and relatively high portions of N were also applied in Iowa (31 %) and southern Minnesota (32 %) during this period. In comparison, N applied in spring dominated across the contiguous US, especially in the Midwest, the Northwest, the eastern Northern Great Plains, Arkansas, and Virginia (> 50 %, Supplement Fig. S5b). Nitrogen fertilizer applied at planting, however, was generally found to be low (< 20 %) except in Montana (43 %), Wyoming (40 %), and New York (49 %) (Supplement Fig. S5c). Finally, N applied after planting was very high (> 50 %) in Ohio, Missouri, West Virginia, Vermont, New Hampshire, and coastal areas of the Southeast (Supplement Fig. S5d).

The fractions of NH4+-N and NO3--N varied greatly among seven regions of the US before the 1940s (Fig. 5). Ammonium sulfate was the major N fertilizer in the Midwest, the Northwest, and the Southwest, rendering high fractions of NH4+-N in these regions, while sodium nitrate was popular in the Northeast, the Southeast, and the Southern Great Plains, lowering the NH4+-N fraction in these regions. A wide variety of N fertilizers were adopted from 1940 through to 1980 in all regions, among which the high NH4+-N concentration fertilizers such as anhydrous ammonia, ammonium nitrate, and N solution were favored. Since then, the fractions of NH4+-N in the seven regions have remained at high and relatively stable level; however, farmers in these regions have also shifted from multiple nutrient fertilizers toward single nutrient fertilizers with high N concentrations, such as anhydrous ammonia, N solution, and urea.

The fraction of NH4+-N to total N use ranged from 0 to 0.65 in the 1900s to 0.8 to 0.9 after 2010 across the US (Fig. 6). NH4+-N fertilizer accounted for about 70 % of the total N fertilizer used in the Northern Great Plains region in 1900, while the Northeast and the Southeast were dominated by NO3--N fertilizer which accounted for about 95 % of the total N fertilizer use. These fractions showed a general downward trend during 1920–1940 except for a prominent peak value in 1932. In the following 3 decades from 1940 to 1970, sharp increases were detected in the Northeast (from 0.21 to 0.75), the Southeast (from 0.09 to 0.75), and the Southern Great Plains (from 0.6 to 0.9). During the period from 1970 to 2015, the fractions of NH4+-N in all seven regions were above 0.8 with four regions exceeding 0.9 (Fig. 6).

The proportion of ammonium N and nitrate N fertilizer use varied among regions. We used 2015 as an example to address the spatial variation of dominant fertilizer forms and their application rates across the US (Fig. 7). The ammonium N fertilizer applied in fall was mainly concentrated in the central area of contiguous US, Washington, and California displayed an application rate of 1–3 g N m-2 yr-1; however, a small amount of nitrate N fertilizer (< 0.1 g N m-2 yr-1) was applied in fall across the country except some areas in the Southern Great Plains and the Southeast (0.1–0.5 g N m-2 yr-1). In comparison, ammonium N fertilizer was intensively used in spring (> 5 g N m-2 yr-1) in the Midwest, the Northern Great Plains, the western Southeast, and the Northwest, where 0.5–1 g N m-2 yr-1 of nitrate N fertilizer was applied in the same time period. High-level ammonium N fertilizer applied at planting was mainly distributed in the Northern Great Plains, Iowa, Wisconsin, Washington, and California with application rates of 1–3 g N m-2 yr-1. High ammonium N fertilizer use after planting (above 3 g N m-2 yr-1) was found in California, Nebraska, the Midwest, the Northern Great Plains, the northern Southern Great Plains, and the western and southeastern regions of the Southeast; nitrate N fertilizer use rates were also high in these areas, especially along the southeast coast (Fig. 7).

In order to examine the validity of our data, we compared our data with other studies regarding temporal variations and spatial patterns of N fertilizer use; one regional dataset from IPNI (2018), and two global datasets from Lu and Tian (2017) and Nishina et al. (2017) were utilized (Figs. 8 and 9). IPNI (2018) developed the N fertilizer use dataset of the US spanning from 1987 to 2012 by adopting the method in Ruddy et al. (2006), which is based on county-level N consumption data derived from state-level N fertilizer sale and agricultural land acreage. Lu and Tian (2017) and Nishina et al. (2017) derived N fertilizer use data from the International Fertilizer Association (IFA) and the Food and Agriculture Organization of the United Nations (FAO), respectively. The four datasets showed the same interannual variation of national N fertilizer consumption from 1961 to 2014 but small difference in magnitude (Fig. 8). The N fertilizer consumption in our study is slightly smaller than the value from IPNI (2018) except for the year 2001 because our study excluded permanent pasture. The historical N fertilizer use amount in our study is also lower than Lu and Tian (2017). This is because the N fertilizer data from Lu and Tian (2017) contains permanent pasture and non-farm N fertilizer use and also covers territories of the US rather than just the continental US. While our data is marginally higher than Nishina et al. (2017) in most years, the overall spatial pattern of the N fertilizer use rate in the contiguous US in 2010 from our study is similar to the other three maps (Fig. 9). This indicates that hotspots of N fertilizer use occurred in the Midwest, on the west and east coast, and in some parts of the Northwest and the Southern Great Plains. Nevertheless, discrepancies were also found in the magnitude of N use rates between our data and the other studies: there were only a few states in Lu and Tian (2017) and no states in Nishina et al. (2017) which were reported as having applied over 7 g N m-2 yr-1 in the year 2010 (Fig. 9c and d). In comparison, two global datasets showed that vast regions received fertilizer at 2–4 g N m-2 yr-1, in which very little N fertilizer application was indicated in this study or IPNI (2018). This is because the global cropland maps were based on HYDE 3.2 (Klein Gildewijk, 2016) in Lu and Tian (2017) and HYDE 3.1 (Hurrt et al., 2011) in Nishina et al. (2017). The HYDE data overestimated cropland percentage in low crop-coverage states while underestimating crop percentage in the US “Corn Belt” (Yu and Lu, 2017). This explains the different spatial patterns revealed by these four N fertilizer datasets, even though their total consumption values are very close nationally. By comparison, our data considers cross-crop divergence in N fertilizer use, covers longer period, and shows more details with finer resolution.

We further compared our map with that from Ruddy et al. (2006). Ruddy et al. (2006) developed a map showing the spatial patterns of N fertilizer use rate in the contiguous US in 1997, using the same approach as IPNI (2018) to derived county-level N use rate, but downscale to gridded maps based on the enhanced 1992 National Land Cover Data (NLCDe 92) (Nakagaki and Wolock, 2005); this approach provides more details regarding spatial N fertilizer use. The overall pattern of N fertilizer use from our study is similar to Ruddy et al. (2006), indicating hotspots of N fertilizer use (> 8 g N m-2 yr-1) in the Midwest, on the west and east coast, and in some parts of the Northwest, the Southern Great Plains, Nebraska, California, Texas, and Florida (Supplement Fig. S6). Nevertheless, the higher N use rates in California, southern Florida, and eastern North Carolina (> 8 g N m-2 yr-1 in Ruddy et al. (2006) and 2–6 g N m-2 yr-1 in our study) displayed in Ruddy et al. (2006) may be caused by amplified fertilizer use in areas with low cropland coverage, whilst our maps removed such “apparent peaks” by accounting for cropland density in each grid cell. In comparison, the N use rate of Indiana and Ohio (4–8 g N m-2 yr-1 in Ruddy et al., 2006) is lower than that in our study (> 8 g N m-2 yr-1). This may be caused by using the fixed cropland map from the year 1992 instead of the 1997 map in their study and overlooking cropland area change information between years. In addition, our map provides more detail on spatial heterogeneity by adopting state-level crop-specific N fertilizer application rates in each state and generating spatial maps based on 1 km land use history data (Yu and Lu, 2017). Thus, our map is advantageous in that it can characterize cross-crop differences in using N fertilizers, especially in intensively cultivated regions, such as the Midwest, the Great Plains, and southern Southeast.

We further compared the ratios of N application timing across corn, cotton, spring wheat, and winter wheat nationwide with the report provided by Wade et al. (2015), in which nitrogen management practices were reported regionally and nationally across the US (Supplement Table S9). Overall, the differences between our study and Wade et al. (2015) range between 0 and 6 %, with the largest difference found in after-planting fertilizer use in cotton (6 %). In our study, the application timing showed that the majority of N applied in spring was found in corn (50 %) and spring wheat (43 %), in which fall application accounts for the second largest share of annual fertilizer consumption (18 % for corn and 22 % for spring wheat). In comparison, cotton and winter wheat producers applied 53 and 44 % of N after planting, respectively. According to Bierman et al. (2012), corn in Minnesota received 32.5 and 58.8 % of N fertilizer in fall of 2008 and spring of 2009, respectively, which is close to our estimates (31.9 % in fall and 59.2 % in spring). These comparisons imply that N fertilizer application timing across the nation and across crops was well depicted in our study.

Compared to natural organic N fertilizer, chemical N fertilizer gradually became the major agricultural N input in the US from 1850 (6.67 %) to 1930 (83.86 %); however, the total consumption and per unit area rate during this period were still low (Mehring et al., 1957). Although cropland area declined between 1940 and 1970, especially for corn and cotton (Gunjal et al., 1980; Nickerson et al., 2011), the total N consumption at a national scale had been increasing as the result of the rising N fertilizer use rate and widespread adoption of N fertilizer (Beddow, 2012) along the west coast, the Southeast, the Southern Great Plains, and the Midwest. With the expansion of cropland to 383 million acres in 1982 (Nickerson et al., 2011), in addition to the marked increase in the application rate of all crops except cotton, the major agricultural regions received a tremendous amount of N during 1970–1985, except for the conspicuous drop in the year 1983 due to large cropland abandonment (i.e., cropland converted to other land use types, Yu and Lu, 2017). Driven by a change in grain demand and fertilizer prices, N fertilizer use in the US gradually increased; however, there have been fluctuations since 1985, such as the drop in N fertilizer consumption in 2008–2009, which may have been due to the high price of N fertilizer caused by the 2008 financial crisis (USDA-ERS, 2013). The increase in N fertilizer use in the seven regions throughout the US can be attributed to different driving factors. For example, the increase of N use in regions dominated by spring wheat and rice, such as the northern regions of the Northern Great Plains, western regions of the Southeast, and southern Texas, were largely due to the increased fertilizer use rate. In contrast, the increases in the Midwest and southern regions of the Northern Great Plains were mainly due to corn expansion (Yu and Lu, 2017), while the N use rate of corn has been relatively high and stable since the 1980s. Along with the rise in N fertilizer use, the fraction of NH4+-N to total N use also increased from 1900 to the present day. Before the 1960s, the fraction varied greatly among the seven regions of the US due to the different species of N fertilizers used, which may have been determined by fertilizer prices, farmer preference, and cultivated crop types, among other factors. Since the 1970s, the NH4+-N fraction has remained at a high level (> 80 %) across the US, which may largely lower the risk of N leaching (Gentry et al., 1998). Conversely, it may also increase the potential for N loss through ammonia volatilization. In addition, the shift from multiple NH4+-N fertilizers to single NH4+-N fertilizers with high N concentrations since the 1960s may also enhance the risk of gaseous N emission due to large amounts of N being available in soil (Harrison and Webb, 2001).

The winter wheat in the Southern Great Plains and the northern region of the Northwest received most of its annual ammonium N and nitrate N fertilizer in fall, which contributed to the growing grain yield (Mahler et al., 1994). Corn in the Midwest, however, especially in Minnesota, Iowa, and Illinois, received over 30 % of its annual NH4+-N in fall, implying a high potential for N loss in this region (Dinnes et al., 2002; Parkin and Hatfield, 2010). Corn farmers that adopted fall application in the Midwest usually applied nitrification inhibitor with fertilizer; however, sudden NH4+-N input exceeding plant demands may cause tremendous ammonia volatilization (Sommer et al., 2004). Soybean and winter wheat in the Midwest, northern regions of the Southeast, and eastern sections of the Northern Great Plains received more than 65 % of N fertilizer in fall, which is after planting for winter wheat. However, due to the large planting area of corn with two-fold higher N application rates than other crops, these regions are still characterized by high N fertilizer input in spring. According to the USDA, spring application of N fertilizer occurs approximately 1 month before planting, which may cause large amounts of N to be lost through processes such as leaching of soluble nitrate; this leaching is triggered by intense rainfall in spring, contributing to hypoxia in the Gulf of Mexico (Goolsby et al., 2001). In comparison, fertilizer use for spring wheat and barley were only found at-planting in Montana and North Dakota in the form of ammonium N. Moreover, cotton farmers along the southeast coast and in western Texas preferred to apply both ammonium- and nitrate-form fertilizer after planting, which is also the case for corn and cotton farmers in Ohio and Nebraska and winter wheat in the Southern Great Plains.

The uncertainties of this database are mainly due to the following: (1) N fertilizer use in nine major crops and cropland pasture combined accounts for nearly 85 % of the national agricultural fertilizer consumption (excluding N fertilizer use in permanent pasture and non-farm areas, Table S2 in the Supplement), and the rest fertilizer goes to other crops. We grouped these crops (e.g., oilseeds, sugar crops, vegetables, fruits, other grains) into the category “others” and equally assigned the rest N fertilizer amount (rate, application timing, and the fraction of NH4+-N and NO3--N) to all the crops within this category. This may bias the estimated N fertilizer use of some crop types. (2) Due to the paucity of finer resolution information, state-level crop-specific N management data (rate, application timing, and the fraction of NH4+-N and NO3--N) were obtained to characterize the way in which farmers use fertilizer to maximize their profits. Although this is the best data we can obtain for national coverage, crop-specific practices, and centurial duration, sub-state (e.g., county) details are still lacking. (3) Timing information regarding the crop-specific N application derived from the latest survey years was assumed unchanged over time due to the lack of interannual survey data. This assumption may cause an underestimation of the fall application before the 2000s, as urea and N solution-forms of N, which are suitable for spring application, have been increasingly used to replace fall-applied anhydrous ammonia since the 1960s (Randall and Sawyer, 2008). (4) To separate the amount of NH4+-N and NO3--N in fertilizer use, we adopted the N form fraction of 11 major single N fertilizer types, which accounted for 85 % of the N consumption in the US after the 1980s, and assumed the NH4+-N: NO3--N ratio was 1:1 for the rest fertilizer types (Supplement Fig. S7). However, mixed N fertilizers were favored before single N fertilizers became popular (Sheridan, 1979). The temporal change of the NH4+-N fraction in mixed N fertilizer before the 1980s may be biased in estimations. For example, ammonium N only accounted for 2 % of mixed fertilizer N in 1900, but has been favored since 1925 and gradually rose to 72 % of mixed N fertilizer by 1944 (Mehring et al., 1946). (5) The NH4+-N fraction was assumed to be constant across crop types in each state in a year. This may cause biases because farmers may apply different type of N fertilizers to different crops. For example, NH4+-N was favored in rice paddies due to higher oxidizability and the tendency toward N loss via denitrification from NO3--N, implying a higher NH4+-N fraction in the N fertilizer of rice (Norman et al., 2003). (6) The historical crop type maps were reconstructed using USDA survey data at state-level. However, the spatial distribution of N fertilizer use was uncertain at a sub-state level due to the lack of finer scale data for crop type map reconstruction. Therefore, a finer-scale spatial survey of crop-specific N fertilizer use (e.g., county-level), annual application timing data, and the development of crop-specific fractions of NH4+-N and NO3--N data will be beneficial for further improving the characterization of geospatial and temporal patterns of N fertilizer management in the US.