Comment on "Legacy nitrogen may prevent achievement of water quality goals in the Gulf of Mexico".

Van Meter et al (Reports, 27 April 2018, p. 427) warn that achieving nitrogen reduction goals in the Gulf of Mexico will take decades as a result of legacy nitrogen effects. We discuss limitations of the modeling approach and demonstrate that legacy effects ranging from a few years to decades are equally consistent with observations. The presented time scales for system recovery are therefore highly uncertain.

Illinois River Nitrate-Nitrogen Concentrations and Loads: Long-term Variation and Association with Watershed Nitrogen Inputs.

The Illinois River is a major contributor of nitrate-N to the Mississippi River and the Gulf of Mexico, where nitrate is a leading cause of summertime benthic hypoxia. Corn-soybean production on tile-drained land is a leading source of nitrate-N in this river system, in addition to municipal wastewater discharge. We calculated annual nitrate-N loads in the Illinois River at Valley City from 1976 to 2014 by linear interpolation. Although there was not a significant trend in annual loads during the entire study period, there was a significant downward trend in flow-weighted nitrate-N concentration after 1990 despite high concentrations in 2013 after the 2012 drought. Multivariate regression analysis revealed a statistically significant association between annual flow-weighted nitrate-N concentration and cumulative residual agricultural N inputs to the watershed during a 6-yr window. This suggests that declines in flow-weighted nitrate-N concentration may reflect increasing N use efficiency in agriculture and a depletion of legacy N stored in the watershed. The watershed appears to have transitioned from a state of stationarity in nitrate concentration to nonstationarity. The average annual nitrate-N load at Valley City from 2010 to 2014 was 10% less than the 1980-1996 average load, indicating recent progress toward Illinois' nutrient loss reduction milestone of 15% reduction by 2025 and ultimate target of 45% reduction.

Managing Multiple Mandates: A System of Systems Model to Analyze Strategies for Producing Cellulosic Ethanol and Reducing Riverine Nitrate Loads in the Upper Mississippi River Basin.

Implementing public policies often involves navigating an array of choices that have economic and environmental consequences that are difficult to quantify due to the complexity of multiple system interactions. Implementing the mandate for cellulosic biofuel production in the Renewable Fuel Standard (RFS) and reducing hypoxia in the northern Gulf of Mexico by reducing riverine nitrate-N loads represent two such cases that overlap in the Mississippi River Basin. To quantify the consequences of these interactions, a system of systems (SoS) model was developed that incorporates interdependencies among the various subsystems, including biofuel refineries, transportation, agriculture, water resources and crop/ethanol markets. The model allows examination of the impact of imposing riverine nitrate-N load limits on the biofuel production system as a whole, including land use change and infrastructure needs. The synergies of crop choice (first versus second generation biofuel crops), infrastructure development, and environmental impacts (streamflow and nitrate-N load) were analyzed to determine the complementarities and trade-offs between environmental protection and biofuel development objectives. For example, the results show that meeting the cellulosic biofuel target in the RFS using Miscanthus x giganteus reduces system profits by 8% and reduces nitrate-N loads by 12% compared to the scenario without a mandate. However, greater water consumption by Miscanthus is likely to reduce streamflow with potentially adverse environmental consequences that need to be considered in future decision making.

Variation in riverine nitrate flux and fall nitrogen fertilizer application in East-central illinois.

In east-central Illinois, fertilizer sales during the past 20 yr suggest that approximately half of the fertilizer nitrogen (N) applied to corn ( L.) occurs in the fall; however, fall fertilizer N sales were greatly reduced in 2009 as wet soil conditions restricted fall fieldwork, including fertilizer N applications. In 2010, we observed unusually low flow-weighted nitrate concentrations (approximately 40% below the long-term average) in two east-central Illinois rivers (5.7 mg N L in the Embarras River and 5.6 mg N L in the Lake Fork of the Kaskaskia River). Using long-term river nitrate data sets (1993-2012 for the Embarras and 1997-2012 for the Kaskaskia), we examined nitrate concentrations and developed regression models to estimate the association between fall fertilizer N application on riverine nitrate yields in these tile-drained watersheds. During these periods of record, annual riverine nitrate yields ranged from 8 to 57 kg N ha yr (30 kg N ha yr average) for the Embarras River and 2.6 to 59 kg N ha yr (32 kg N ha yr average) for the Kaskaskia. Multivariate linear regression relationships with the current and previous year's annual water yields, previous year's corn yield, and nine-county fall fertilizer sales accounted for 96% of the annual variation in nitrate yield in both watersheds. Running the regression models with fall fertilizer sales set to the 2009 amount suggests that the average reduction in nitrate yield (for the period of record) would be 17 and 20% for the Embarras and Kaskaskia Rivers, respectively. These data suggest that shifting fertilizer N application to the spring can be detected in watersheds as large as 481 km.

Sources of nitrate yields in the Mississippi River Basin.

Riverine nitrate N in the Mississippi River leads to hypoxia in the Gulf of Mexico. Several recent modeling studies estimated major N inputs and suggested source areas that could be targeted for conservation programs. We conducted a similar analysis with more recent and extensive data that demonstrates the importance of hydrology in controlling the percentage of net N inputs (NNI) exported by rivers. The average fraction of annual riverine nitrate N export/NNI ranged from 0.05 for the lower Mississippi subbasin to 0.3 for the upper Mississippi River basin and as high as 1.4 (4.2 in a wet year) for the Embarras River watershed, a mostly tile-drained basin. Intensive corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] watersheds on Mollisols had low NNI values and when combined with riverine N losses suggest a net depletion of soil organic N. We used county-level data to develop a nonlinear model ofN inputs and landscape factors that were related to winter-spring riverine nitrate yields for 153 watersheds within the basin. We found that river runoff times fertilizer N input was the major predictive term, explaining 76% of the variation in the model. Fertilizer inputs were highly correlated with fraction of land area in row crops. Tile drainage explained 17% of the spatial variation in winter-spring nitrate yield, whereas human consumption of N (i.e., sewage effluent) accounted for 7%. Net N inputs were not a good predictor of riverine nitrate N yields, nor were other N balances. We used this model to predict the expected nitrate N yield from each county in the Mississippi River basin; the greatest nitrate N yields corresponded to the highly productive, tile-drained cornbelt from southwest Minnesota across Iowa, Illinois, Indiana, and Ohio. This analysis can be used to guide decisions about where efforts to reduce nitrate N losses can be most effectively targeted to improve local water quality and reduce export to the Gulf of Mexico.

Miscanthus and switchgrass production in central Illinois: impacts on hydrology and inorganic nitrogen leaching.

Biomass crops are being promoted as environmentally favorable alternatives to fossil fuels or ethanol production from maize (Zea mays L.), particularly across the Corn Belt of the United States. However, there are few if any empirical studies on inorganic N leaching losses from perennial grasses that are harvested on an annual basis, nor has there been empirical evaluation of the hydrologic consequences of perennial cropping systems. Here we report on the results of 4 yr of field measurements of soil moisture and inorganic N leaching from a conventional maize-soybean [Glycine max (L.) Merr.] system and two unfertilized perennial grasses harvested in winter for biomass: Miscanthus x giganteus and switchgrass (Panicum virgatum cv. Cave-in-Rock). All crops were grown on fertile Mollisols in east-central Illinois. Inorganic N leaching was measured with ion exchange resin lysimeters placed 50 cm below the soil surface. Maize--soybean nitrate leaching averaged 40.4 kg N ha(-1) yr(-1), whereas switchgrass and Miscanthus had values of 1.4 and 3.0 kg N ha(-1) yr(-1), respectively. Soil moisture monitoring (to a depth of 90 cm) indicated that both perennial grasses dried the soil out earlier in the growing season compared with maize-soybean. Later in the growing season, soil moisture under switchgrass tended to be greater than maize-soybean or Miscanthus, whereas the soil under Miscanthus was consistently drier than under maize--soybean. Water budget calculations indicated that evapotranspiration from Miscanthus was about 104 mm yr(-1) greater than under maize-soybean, which could reduce annual drainage water flows by 32% in central Illinois. Drainage water is a primary source of surface water flows in the region, and the impact ofextensive Miscanthus production on surface water supplies and aquatic ecosystems deserves further investigation.

Nitrogen mass balance of a tile-drained agricultural watershed in East-Central Illinois.

Simple nitrogen (N) input/output balance calculations in agricultural systems are used to evaluate performance of nutrient management; however, they generally rely on extensive assumptions that do not consider leaching, denitrification, or annual depletion of soil N. We constructed a relatively complete N mass balance for the Big Ditch watershed, an extensively tile-drained agricultural watershed in east-central Illinois. We conducted direct measurements of a wide range of N pools and fluxes for a 2-yr period, including soil N mineralization, soybean N(2) fixation, tile and river N loads, and ground water and in-stream denitrification. Fertilizer N inputs were from a survey of the watershed and yield data from county estimates that were combined with estimated protein contents to obtain grain N. By using maize fertilizer recovery and soybean N(2) fixation to estimate total grain N derived from soil, we calculated the explicit change in soil N storage each year. Overall, fertilizer N and soybean N(2) fixation dominated inputs, and total grain export dominated outputs. Precipitation during 2001 was below average (78 cm), whereas precipitation in 2002 exceeded the 30-yr average of 97 cm; monthly rainfall was above average in April, May, and June of 2002, which flooded fields and produced large tile and riverine N loads. In 2001, watershed inputs were greater than outputs, suggesting that carryover of N to the subsequent year may occur. In 2002, total inputs were less than outputs due to large leaching losses and likely substantial field denitrification. The explicit change in soil storage (67 kg N ha(-1)) offsets this balance shortfall. Although 2002 was climatically unusual, with current production trends of greater maize grain yields with less fertilizer N, soil N depletion is likely to occur in maize/soybean rotations, especially in years with above-average precipitation or extremely wet spring periods.

Long-term changes in mollisol organic carbon and nitrogen.

Conversions of Mollisols from prairie to cropland and subsequent changes in crop production practices in the Midwestern USA have resulted in changes in soil organic matter. Few studies have used archived samples, long-term resampling of soils to a depth of 1 m, and space for time studies to document these changes. We resampled soils by depth (0-100 cm) in fields at 19 locations in central Illinois on poorly drained Mollisols that were in corn (Zea mays L.) and soybean (Glycine max L. Merr.) rotations, were tile drained, and had no known history of manure application in recent decades. Three fields were paired with virgin prairie remnants, two had grass borders that were sampled, and 16 had been previously sampled in 1901 to 1904 or 1957 under various land uses (virgin prairie, cultivation, grass cover). The soils had large amounts of C and N in the profile, with mean values of 175 [corrected] Mg C ha(-1) and 16.1 Mg N ha(-1) for the 18 cultivated fields sampled in 2001 and 2002. We confirmed a large reduction in organic C and total N pools from conversion of prairies to annual cultivation and artificial drainage and documented no change in these organic matter pools of cultivated soils during the period of synthetic fertilizer use (1957--2002). Cultivated fields had soil C and N concentrations typically 30 to 50% less than virgin prairie soils. Smaller but significant declines in C and N concentrations were found when comparing 1900s cultivated fields to concentrations in 2002, after another 100 yr of cultivation, and in comparing 1957 grass covered fields that had been converted to annual cultivation before 2002. The reduction in organic matter after cultivation of prairies occurred mostly in the top 50 cm of soil, with evidence of translocation of C and N from these upper layers to the 50- to 100-cm depth, possibly enhanced by tile drainage. For these Mollisols, declines in organic matter were likely completed by the 1950s, with organic matter pools in a steady state under the production practices in place from the late 1950s through 2002.

Evaluation of the ADAPT model for simulating nitrogen dynamics in a tile-drained agricultural watershed in central Illinois.

Assessing the accuracy of agronomic and water quality simulation models in different soils, land-use systems, and environments provides a basis for using and improving these models. We evaluated the performance of the ADAPT model for simulating riverine nitrate-nitrogen (NO3-N) export from a 1500-km2 watershed in central Illinois, where approximately 85% of the land is used for maize-soybean production and tile drainage is common. Soil chemical properties, crop nitrogen (N) uptake coefficient, dry matter ratio, and a denitrification reduction coefficient were used as calibration parameters to optimize the fit between measured and simulated NO3-N load from the watershed for the 1989 to 1993 period. The applicability of the calibrated parameter values was tested by using these values for simulating the 1994 to 1997 period on the same watershed. Willmott's index of agreement ranged from 0.91 to 0.97 for daily, weekly, monthly, and annual comparisons of riverine nitrate N loads. Simulation accuracy generally decreased as the time interval decreased. Willmott's index for simulated crop yields ranged from 0.91 to 0.99; however, observed crop yields were used as input to the model. The partial N budget results suggested that 52 to 72 kg N ha(-1) yr(-1) accumulated in the soil, but simulated biological N fixation associated with soybeans was considerably greater than literature values for the region. Improvement of the N fixation algorithms and incorporation of mechanisms that describe soybean yield in response to environmental conditions appear to be needed to improve the performance of the model.

Relating net nitrogen input in the Mississippi River basin to nitrate flux in the lower Mississippi River: a comparison of approaches.

A quantitative understanding of the relationship between terrestrial N inputs and riverine N flux can help guide conservation, policy, and adaptive management efforts aimed at preserving or restoring water quality. The objective of this study was to compare recently published approaches for relating terrestrial N inputs to the Mississippi River basin (MRB) with measured nitrate flux in the lower Mississippi River. Nitrogen inputs to and outputs from the MRB (1951 to 1996) were estimated from state-level annual agricultural production statistics and NOy (inorganic oxides of N) deposition estimates for 20 states that comprise 90% of the MRB. A model with water yield and gross N inputs accounted for 85% of the variation in observed annual nitrate flux in the lower Mississippi River, from 1960 to 1998, but tended to underestimate high nitrate flux and overestimate low nitrate flux. A model that used water yield and net anthropogenic nitrogen inputs (NANI) accounted for 95% of the variation in riverine N flux. The NANI approach accounted for N harvested in crops and assumed that crop harvest in excess of the nutritional needs of the humans and livestock in the basin would be exported from the basin. The U.S. White House Committee on Natural Resources and Environment (CENR) developed a more comprehensive N budget that included estimates of ammonia volatilization, denitrification, and exchanges with soil organic matter. The residual N in the CENR budget was weakly and negatively correlated with observed riverine nitrate flux. The CENR estimates of soil N mineralization and immobilization suggested that there were large (2000 kg N ha-1) net losses of soil organic N between 1951 and 1996. When the CENR N budget was modified by assuming that soil organic N levels have been relatively constant after 1950, and ammonia volatilization losses are redeposited within the basin, the trend of residual N closely matched temporal variation in NANI and was positively correlated with riverine nitrate flux in the lower Mississippi River. Based on results from applying these three modeling approaches, we conclude that although the NANI approach does not address several processes that influence the N cycle, it appears to focus on the terms that can be estimated with reasonable certainty and that are correlated with riverine N flux.