The global challenge of meeting increased food demand and protecting environmental quality will be won or lost in cropping systems that produce maize, rice, and wheat. Achieving synchrony between N supply and crop demand without excess or deficiency is the key to optimizing trade-offs amongst yield, profit, and environmental protection in both large-scale systems in developed countries and small-scale systems in developing countries. Setting the research agenda and developing effective policies to meet this challenge requires quantitative understanding of current levels of N-use efficiency and losses in these systems, the biophysical controls on these factors, and the economic returns from adoption of improved management practices. Although advances in basic biology, ecology, and biogeochemistry can provide answers, the magnitude of the scientific challenge should not be underestimated because it becomes increasingly difficult to control the fate of N in cropping systems that must sustain yield i...

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Agroecosystems, Nitrogen-use Efficiency, and Nitrogen Management

Abstract The global challenge of meeting increased food demand and protecting environmental quality will be won or lost in cropping systems that produce maize, rice, and wheat. Achieving synchrony between N supply and crop demand without excess or deficiency is the key to optimizing trade-offs amongst yield, profit, and environmental protection in both large-scale systems in developed countries and small-scale systems in developing countries. Setting the research agenda and developing effective policies to meet this challenge requires quantitative understanding of current levels of N-use efficiency and losses in these systems, the biophysical controls on these factors, and the economic returns from adoption of improved management practices. Although advances in basic biology, ecology, and biogeochemistry can provide answers, the magnitude of the scientific challenge should not be underestimated because it becomes increasingly difficult to control the fate of N in cropping systems that must sustain yield increases on the world's limited supply of productive farm land.

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Agroecosystems, Nitrogen-use Effic iency, and Nitrogen Management

Human activities have roughly doubled the amount of reactive N that enters the element's biospheric cycle. Crop production is by far the single largest cause of this anthropogenic alteration. Inorganic fertilizers now provide 80 Tg N yr−1 (Tg = 1012 g), managed (symbiotic) biofixation adds about 20 Tg N yr−1, and between 28 and 36 Tg N yr−1 are recycled in organic wastes. Anthropogenic inputs (including N in seeds and irrigation water) now supply about 85% of 170 (151–186) Tg N reaching the world's cropland every year. About half of this input, 85 Tg N yr−1, is taken up by harvested crops and their residues. Quantification of N losses from crop fields is beset by major uncertainties. Losses to the atmosphere (denitrification and volatilization) amount to 26–60 Tg N yr−1, while waters receive (from leaching and erosion) 32–45 Tg N yr−1. These N losses are the major reason behind the growing concerns about the enrichment of the biosphere with reactive N. The best evidence suggests that in spite of some significant local and regional losses, the world's agricultural land accumulates N. The addition of 3–4 billion people before the year 2050 will require further substantial increases of N input in cropping, but a large share of this demand can come from improved efficiency of N fertilizer use.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GB900015

Nitrogen in crop production: An account of global flows

Balancing the amount of N needed for optimum plant growth while minimizing the NO 3 that is transported to ground and surface waters remains a major challenge for everyone attempting to understand and improve agricultural nutrient use efficiency. Our objectives for this review are to examine how changes in agricultural management practices during the past century have affected N in midwestern soils and to identify the types of research and management practices needed to reduce the potential for nonpoint NO 3 leakage into water resources. Inherent soil characteristics and management practices contributing to nonpoint NO 3 loss from midwestern soils, the impact of NO 3 loading on surface water quality, improved N management strategies, and research needs are discussed. Artificial drainage systems can have a significant impact on water quality because they behave like shallow, direct conduits to surface waters. Nonpoint loss of NO 3 from fields to water resources, however, is not caused by any single factor. Rather, it is caused by a combination of factors, including tillage, drainage, crop selection, soil organic matter levels, hydrology, and temperature and precipitation patterns. Strategies for reducing NO 3 loss through drainage include improved timing of N application at appropriate rates, using soil tests and plant monitoring, diversifying crop rotations, using cover crops, reducing tillage, optimizing N application techniques, and using nitrification inhibitors. Nitrate can also be removed from water by establishing wetlands or biofilters. Research that is focused on understanding methods to minimize NO 3 contamination of water resources should also be used to educate the public about the complexity of the problem and the need for multiple management strategies to solve the problem across agricultural landscapes.

Nitrogen Management Strategies to Reduce Nitrate Leaching in Tile-Drained Midwestern Soils

ricultural fields, and increased use of manufactured N fertilizers Balancing the amount of N needed for optimum plant growth while Less than 50 yr ago, corn (Zea mays L) was generally minimizing the NO3 that is transported to ground and surface waters grown in rotation with cereal crops and forage legumes remains a major challenge for everyone attempting to understand such as alfalfa (Medicago sativa L), red clover (Trifoand improve agricultural nutrient use efficiency Our objectives for lium pratense L), and sweetclover (Melilotus spp) this review are to examine how changes in agricultural management practices during the past century have affected N in midwestern soils Through biological N fixation, the legumes generally and to identify the types of research and management practices needed increased the amount of residual N in the soil profile to reduce the potential for nonpoint NO3 leakage into water resources Cycling of this residual, biologically fixed N along with Inherent soil characteristics and management practices contributing N mineralized from SOM added with animal manure to nonpoint NO3 loss from midwestern soils, the impact of NO3 loading or deposited through rainfall was the primary process on surface water quality, improved N management strategies, and through which corn and other grain crops obtained N research needs are discussed Artificial drainage systems can have a Following World War II, increased availability of significant impact on water quality because they behave like shallow, commercial N fertilizer and decreased demand for fordirect conduits to surface waters Nonpoint loss of NO3 from fields age crops led to a significant reduction in crop rotations to water resources, however, is not caused by any single factor Rather, and a general substitution of purchased N for biological it is caused by a combination of factors, including tillage, drainage, N In Iowa, forage pasture represented more than 336% crop selection, soil organic matter levels, hydrology, and temperature and precipitation patterns Strategies for reducing NO3 loss through (3 389 160 ha) of the state’s total cropped area at the drainage include improved timing of N application at appropriate end of World War II (US Dep of Commerce, Bureau rates, using soil tests and plant monitoring, diversifying crop rotations, of the Census, 1945) By 1997, forage pasture area in using cover crops, reducing tillage, optimizing N application techIowa comprised only 128% (1 393 451 ha) of the state’s niques, and using nitrification inhibitors Nitrate can also be removed total cropped area (USDA Natl Agric Stat Serv, from water by establishing wetlands or biofilters Research that is 1997) Incorporation of legumes into a crop rotation focused on understanding methods to minimize NO3 contamination was no longer needed as commercial N inputs gradually of water resources should also be used to educate the public about replaced biological N fixation the complexity of the problem and the need for multiple management The increased availability of commercial N also facilistrategies to solve the problem across agricultural landscapes tated specialization and a national trend for separating crop and animal production enterprises Animal manure no longer served as an important crop nutrient resource, N is essential for growth and reproduction and meadow legumes were not required on farms that of all life forms, and except for legume crops and began to specialize in corn and soybean [Glycine max virgin soils with relatively high soil organic matter (L) Merr] production Although there is considerable (SOM), soil N must usually be supplemented to sustain variation among years and regions in N fertilizer usage, food, feed, and fiber production During the past 20 yr, the net result of this farming-system change was a napublic concern regarding N movement from agricultural tional average increase in commercial N fertilizer use nonpoint sources into broader water resources has inof 24 kg ha 1 yr 1 (Fig 1) between the mid-1960s and creased as problems such as hypoxia (Rabalais et al, the late 1990s The rise in commercial N fertilizer usage 1996) became more evident To understand current within Midwest states (Illinois, Iowa, Indiana, Michigan, questions about N management in the US northern Minnesota, Missouri, Nebraska, Ohio, and Wisconsin) Corn and Soybean Belt, it is necessary to examine the has slowed, with average use of commercial N fertilizer changes that have occurred in agriculture during the past increasing by 4 kg ha 1 (05 kg ha 1 yr 1 ) from 1991 to century These changes include the use of less diversified 1999 (Fig 2) However, the trend of steadily increased crop rotations, separation of crop production and animal usage of commercial N fertilizer may change if fossil fuel enterprises, changes in tillage intensity, drainage of agprices increase substantially and the fertilizer industry is Abbreviations: LCD, localized compaction and doming; LSNT, lateUSDA-ARS Natl Soil Tilth Lab, 2150 Pammel Dr, Ames, IA 50011 spring nitrate test; NIR, near infrared; PSNT, presidedress soil nitrate Received 27 Apr 2001 *Corresponding author (dinnes@nstlgov) test; SI–CD, subirrigation with controlled drainage; SOM, soil organic matter; WTM, water table management Published in Agron J 94:153–171 (2002)

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Review and Interpretation: Nitrogen Management Strategies to Reduce Nitrate Leaching in Tile-Drained Midwestern Soils

Nitrogen utilization by corn (Zea mays L.) is influenced by the form of inorganic N present in the root zone. A field experiment was conducted on Enon sandy loam (fine, mixed, thermic Ultic Hapludalf) to determine N use efficiency and its partitioning in various plant parts of corn. Ammonium nitrate labeled either as NH4-N or NO3-N and applied at 50, 100, or 200 kg N ha" was evaluated. Microplots (1.06 m) were established in the main N plots for N fertilizer application, and main plots were used to determine grain yield. After the crop harvest, soil N alone, fertilizer N used by the crop, and fertilizer N remaining in inorganic and organic forms in the top 75 cm of the soil were measured. Grain yield response to N application was significant (P < 0.01). Recovery of applied fertilizer N in corn and weeds ranged from 43 to 57% and 3 to 5%, respectively. Nitrogen sources showed no significant differences with respect to N accumulation in corn. Only 17 to 20% of the fertilizer N was recovered in the grain. The amounts of soil N utilized by the corn was about threeto sixfold higher than the fertilizer N, suggesting extensive turnover of soil and fertilizer N through immobilization and mineralization. More N was recovered (21-63% of added N) in the soil from NH4 than from NO, (6-38%). Loss of N (unaccounted for) ranged from 11 to 18% at 100 kg N ha-' and from 34 to 48% at 200 kg N ha-. Nitrogen loss was higher in the plots receiving NO, than NH4. Most of the fertilizer N remaining in the soil at the end of the growing season was in the organic fraction, suggesting immobilization into microbial and root biomass. N USE EFFICIENCY by corn is influenced by source, rate and time of fertilizer application, and hydrologic conditions such as rainfall distribution and water-table fluctuations during the growing season. The use of fertilizer labeled with the stable isotope N provides a meaningful way to differentiate between the total amount of fertilizer N utilized by the crop and N losses during crop production for both environmental and agronomic reasons. Many researchers (e.g., Carter et al., 1967; Westerman et al., 1972; Bijeriego et al., 1979; Olson, 1980) have studied the recovery of labeled fertilizers in corn and sudangrass [Sorghum x drwnmondii (Steudel) Millsp. & Chase] at the time of harvest. Kitur et al. (1984) reported that 71 to 75% of the fertilizer N was accounted for in harvested grain and stover of corn and soil. Nitrogen rate had no large effect on the fraction of fertilizer N unaccounted for and presumed to be lost by denitrification and leaching. Chabrol et al. (1988) found that, when a mixture of (NH4)2SO4 and KNO3 (122 kg N ha") was applied to corn 12 d after planting, 61% was present in the crop and 19% remained in the soil; the 20% loss observed was assumed to be due to a combination of leaching and denitrification. Hera (1977) observed that, when labeled NH4NO3 was • applied at the rate of 160 kg N ha ~ in split applicaG.B. Reddy, Dep. of Plant Science and Technology, North Carolina AT and K.R. Reddy, Dep. of Soil and Water Science, Univ. of Florida-IFAS, Gainesville, FL 32611. Joint contribution from North Carolina AT (ii) determine the N use efficiency by corn; and (iii) estimate fertilizer N loss using mass balance. MATERIALS AND METHODS The field experiment was conducted in 1986 on an Enon sandy loam soil, at the North Carolina AT CEC, 7.0 cmolc kg-; soil total N, 0.31 g kg-'; total inorganic N, 21 mg kg-; and organic matter content, 8.1 g kg-. The field plots were tilled before planting. The plot size was 4.57 by 7.62 m and was planted with six rows of corn =0.91 m apart. Two weeks prior to planting, 3.51 L habutylate (S-ethyl diisobutylthiocarbamate) and 2.35 L haatrazine (2chloro-4-ethylamino-6-isopropylamino-s-triazine) were mixed in an appropriate volume of water and applied to the plots. Pioneer 3369AR corn was planted on 8 May 1986, at a rate to obtain 43 078 plants ha-. Ammonium nitrate was applied at the rate of 0, 50, 100, or 200 kg N ha-. Recommended rates of P (60 kg ha-) and K (60 kg ha-) were applied to all the plots. Planting and fertilizer application were done by hand. Fertilizers were applied in bands ~5 cm deep and 8 cm away from the center of each row. Treatments were replicated three times in a randomized complete-block experimental design. Average ambient minimum and maximum air temperatures during the growing season were 16 and 27 °C, respectively. The total precipitation during the growing season was 170 mm. For determination of fertilizer N recovery, N-enriched fertilizer was applied to microplots (1.16 by 0.92 m) within each large plot. Two microplots were established permanently in the center of each main plot by inserting a galvanized metal (1-mm-thick) barrier into the soil to a depth of 0.31 m. Two microplots in each main plot were 2.14 m apart. One microplot was treated with NH4NO3 (10.387 atom %) and another with NH4NO3 (10.396 atom %), at respective N levels of each treatment. Enriched N fertilizer was placed in bands as described for the main plot. Due to the dry season, plots were irrigated (50 mm ha-) once in May and twice in June. Plots were hand weeded and weed plants were washed, dried, and saved for chemical analysis. At harvest, plants in the center rows were harvested for Abbreviations: CEC, cation-exchange capacity; Ndff, plant N derived from labeled fertilizer N; Ndfs, soil N plus nonlabeled fertilizer N; Nds, native soil N.

https://soils.ifas.ufl.edu/wetlands/publications/PDF-articles/177.Fate%20of%2015N.pdf

Fate of nitrogen-15 enriched ammonium nitrate applied to corn

Limited water availability is a major constraint for cultivation of rice (Oryza sativa L.) in the traditional flooded systems, particularly in the semiarid regions of the world. Aerobic rice cultivation provides feasible alternative to traditional rice production in these regions, allowing significant water savings. Field experiments were conducted at the ANGR University Agricultural Research Station, India during 2009–2010 and 2010–2011 to compare crop growth, yield, and water savings under aerobic rice–maize (R–M) and flooded R–M rotation systems. The effect of aerobic rice on the succeeding maize crop was also studied. The total amount of water applied (including rainfall) in the aerobic plots was 967 and 645 mm compared to 1546 and 1181 mm in flooded rice system, during 2009 and 2010, respectively. This resulted in 37 to 45% water savings with the aerobic method. The soil moisture in aerobic treatment was maintained in the –30 to –40 kPa range throughout the crop growth. The aerobic rice system produced significantly lower grain yields in 2009 and 2010, where differences between flooded and aerobic rice were 39 and 15.4%, respectively. The yield differences were attributed to the differences in spikelet number per panicle and grain weight. Significant increase in yields was recorded in both systems with increased N rates up to 120 kg ha−1. Significantly higher yields were obtained in no-till maize grown subsequent to the aerobic rice than flooded rice, possibly due to residual soil N and improved soil physical conditions.

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Impact of Aerobic Rice Cultivation on Growth, Yield, and Water Productivity of Rice–Maize Rotation in Semiarid Tropics

Nitrogen transformations in ponds receiving polluted water from nonpoint sources

The sustainability of traditional rice (Oryza sativa L.) cultivation in many Asian countries is being questioned due to severe water shortage conditions, envisaging the need for development of water-saving rice production technologies. A 2-year-field study on a typic Haplustalf soil was conducted to compare traditional transplanted rice–maize system with water-saving aerobic rice–maize system, with an overall objective of investigating the fate of fertilizer nitrogen (N) using 15N-labeled urea. Results from the field experiments showed that the rice plants positively responded to N fertilizer application. The average fertilizer N recovery by rice crop over the 2 years in aerobic rice was 26 kg per 100 kg of applied fertilizer N in the main field and 21 kg per 100 kg of applied N in the microplot, while the recoveries were 41 and 32 kg ha−1 per 100 kg of applied N in traditionally cultivated rice under flooded conditions. The fraction of 15N that was found in soil after the harvest of rice crop ranged from 11.4 to 47.1 kg ha−1 in aerobic rice and 14.2–51.4 kg ha−1 in flooded rice. Average recovery of 15N fertilizer in maize after the first growing season was 3.3 %, and the corresponding recovery in soil was 19 %. An additional 1.3 % of the fertilizer was recovered by crops during the two subsequent seasons. This study indicates the need to develop management practices that improve N use efficiency in aerobic rice by reducing losses to improve yields and reduce N export to the environment.

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G. B. Reddy

Papers

Fate of nitrogen-15 enriched ammonium nitrate applied to corn

Impact of Aerobic Rice Cultivation on Growth, Yield, and Water Productivity of Rice–Maize Rotation in Semiarid Tropics

Nitrogen transformations in ponds receiving polluted water from nonpoint sources

Uptake efficiency of 15 N-urea in flooded and aerobic rice fields under semi-arid conditions