Content for this page was originally written and provided by a collaboration between North Carolina State University, Department of Soil Science and Department of Crop Science; Understanding Systems, Inc; and the North Carolina Department of Environment and Natural Resources, Division or Water Quality. Original content published October 2008.
Agricultural rules both in the Neuse, Cape Fear and Lake Jordan River Basins have required a BMP accounting and tracking tool. The accounting and tracking tool that has been developed to meet the requirements of these river basin rules is the Nitrogen Loss Estimation Worksheet (NLEW). In addition, NLEW was adopted by the NC Division of Soil and Water Conservation in 1996 as the method to estimate BMP effects on relative nutrient dynamics for projects funded with Agriculture Cost-Share Program funds.
The purpose of the Aggregate Nitrogen Loss Estimation Worksheet (NLEW) is to:
Some inputs - soil types, the distribution of crops across soils, and applied N rates by crop - are aggregated. Aggregation of these parameters is at the county level.
Soil types are distributed into the soil management groups. The number of acres is determined by the overlay of digital soil maps with land coverage data. This overlay produces the soil management groups found within the local area (usually a county) and number of acres of those groups for a county. Land coverage data may not be exact. Therefore, the acreage of the soil management groups can be adjusted within the computer program.
The crops and their acreage are then entered. The crops are proportionally distributed across soil management groups. Again, the crop distribution by acres can be adjusted within the computer program. RYE N values are calculated for each soil management group and each crop. Realistic yield expectations are defined as the best three out of five years of yield. An RYE data table has recently been developed by an interagency group comprised of personnel from NCSU, NCDACS, NCDSWC, and USDA - NRCS for 16 agronomic crops and all the agricultural soils in North Carolina (Hodges, 2000). Once the RYE is obtained, it is multiplied by a N factor. This multiplied value is the N fertilization rate necessary to produce an optimum yield of that particular crop. (See table below for the N factors.) In NLEW, this N fertilization rate is referred to as the RYE N Rate. The applied average fertilizer rate for the period of 1991-1995 is entered for each crop.
| Crop | Use | Nitrogen, lb | Unit of yield or application area |
|---|---|---|---|
| Grain, Silage or Fiber | |||
| Barley | Grain | 1.4 to 1.6 | Bushel |
| Corn, grain | Grain | 1.00 to 1.25 | Bushel |
| Corn, silage | Silage | 10 to 12 | Ton |
| Cotton | 6 to 12 | 100 pounds of lint | |
| Oat | Grain | 1.0 to 1.3 | Bushel |
| Rye | Grain | 1.7 to 2.4 | Bushel |
| Sorghum | Grain | 2.0 to 2.5 | 100 pounds |
| Soybean | Grain | 3.5 to 4.0 | Bushel |
| Triticale | Grain | 1.4 to 1.6 | Bushel |
| Wheat | Grain | 1.7 to 2.4 | Bushel |
| Hay | |||
| Bermudagrass | Hay | 40 to 50 | Dry ton |
| Orchardgrass | Hay | 40 to 50 | Dry ton |
| Pearl Millet | Hay | 45 to 55 | Dry ton |
| Small grains | Hay | Dry ton | |
| Sorghum-sudangrass | Hay | 45 to 55 | Dry ton |
| Tall fescue | Hay | 40 to 50 | Dry ton |
| Timothy | Hay | 40 to 50 | Dry ton |
| Trees | |||
| Hardwood | 70 to 100 | Acre | |
| Pine | 40 to 60 | Acre | |
If the aggregate RYE N Rate is less than the aggregate Current N Rate (the current applied N rate), the extra N will be partitioned into Excess N - that is the N fertilizer that the crop cannot use. The Crop N Uptake is aggregate. This means that each crop is multiplied by 1- NUEcrop, which is the N that is not absorbed by the crop and can be leached through the soil and into the shallow groundwater.
At this point the aggregate version of NLEW proceeds exactly like the field-scale version. The two subsurface N sources - from excess fertilizer applications and from fertilizer not utilized by the crop - are summed. If a cover crop is grown, then the Subsurface N is multiplied by 1 - % N Reduction (cover crop), leaving subsurface N that can be further reduced by additional BMPs. Then Subsurface N is multiplied by 1 - % N Reduction (BMP). The remaining N Subsurface Loss is added to the N Surface Loss to leave the Estimated N Leaving Targeted Area. This designation of a targeted area is not meant to imply an edge of field or stream loading N loss but rather the end of the accounting process.
Optimal crop production requires the application of nitrogen beyond the amount that can be retrieved by the crop. Agronomists have developed fertilizer nitrogen use efficiency (NUE) values to indicate the percent of applied N that is actually utilized by the crop and can be recovered in crop tissues. Fertilizer nitrogen use efficiency values were derived primarily from experiments conducted in North Carolina unless there was no data available. In general the NUE values are calculated for both the harvested portion of the crop and the stover. Since most NUE values were determined using the difference method and results are based on the stover from the prior year's crop being left in the field, we assumed that the system was at semi-steadystate for net N mineralization.
The Crop N Uptake is multiplied by 1 - NUE, which is the N that is not absorbed by the crop and can be leached through the soil and into the shallow groundwater. Since NLEW is a not a model and we are not trying to account for all N sources or N cycling such as net mineralization and denitrification). The assumption is made that all the fertilizer N not used by the crop moves below the root zone.
The two subsurface N sources - from excess fertilizer applications and from fertilizer not utilized by the crop - are summed. If a cover crop is planted, it is assumed that some of this excess N is absorbed by the unfertilized cereal cover crop. Much of this nitrogen absorbed by the cover crop will be released to the subsequent crop. Thus the N reducing value assigned to the cover crop is N that is stored in the soil organic matter pool and released in an unpredictable manner over a long time period. For a cropping season we have assumed this nitrogen is removed from the system completely (Table 4). In order to receive N-reducing credit for cereal cover crops, the crop must be planted by November 30 and killed no earlier than March 31 in the Coastal Plain and April 10 in the Piedmont. The range for N-reducing values for cover crops is between 5 to 15%, depending on the crop type. If a cover crop is grown, then the Subsurface N is multiplied by 1 - % N Reduction (cover crop), leaving subsurface N that can be further reduced by additional BMPs.
Research at NCSU has demonstrated that both riparian buffers and controlled drainage structures can reduce subsurface N flow to ditches and streams from 40 to 95% (Gilliam et al., 1997). In NLEW, the subsurface N can be affected by either of these BMPs. However, the area affected by these BMPs may not be the same area as the field area. Therefore, if the amount of area affected by the BMP is less than the field size, the area must be determined. Again the Subsurface N is multiplied by 1 - % N Reduction (BMP). The remaining N Subsurface Loss is added to the N Surface Loss to leave the Estimated N Leaving Targeted Area. This designation of a targeted area is not meant to imply an edge of field or stream loading N loss but rather the end of the accounting process.
NLEW currently partitions nitrogen into three pools: nitrogen in excess of crop needs if the crop is overfertilized, nitrogen absorbed by the crop, and nitrogen intercepted or transformed by BMPs. To determine the AVERAGE efficiency of crops that use nitrogen, a literature review on the nitrogen fertilizer use efficiency of the major crops was conducted from research results in North Carolina or other states. Once these values were collected and synthesized, they were reviewed and finalized by the following researchers: Drs. Steve Hodges, Gene Kamprath, Deanna Osmond, Noah Ranells, and Michael Wagger. In addition, the values were presented, discussed, and agreed upon by the entire NLEW committee (Table 2).
| Crop | N Use Efficiency. (%) | Reference |
|---|---|---|
| Bermuda Grass | 75 | Woodhouse (1969) |
| Flue-cured Tobacco | 50 | Sisson (1991) |
| Burley Tobacco | 40 | MacKown (1996) |
| Corn - Coastal Plain | 55 |
Chancy (1982)
Kamprath (1986)
Wagger (1992)
|
| Corn - Tidewater | 40 |
Chancy (1982)
Kamprath (1986)
Wagger (1992)
|
| Corn - arenic | 40 |
Chancy (1982)
Kamprath (1986)
Wagger (1992)
|
| Corn - Coastal Plain & irrigated | 65 |
Chancy (1982)
Kamprath (1986)
Wagger (1992)
|
| Piedmont - Conventional | 40 | Wagger (1996) |
| Piedmont - No-till | 55 | Wagger (1996) |
| Sweet Potato | 40 | Ortega (1996) |
| Cotton | 50 | Torbert (1994) |
| Cucumber | 30 | Osmond (1999) |
| Wheat | 45 | Scharf (1993) |
The entire NLEW committee, along with additional faculty from the Soil Science Department at NCSU went through the North Carolina State agricultural cost-share list to determine which of the cost-shared BMPs were capable of reducing the nitrogen NOT absorbed by the crop. This excess nitrogen that is left in the soil profile or moved into the shallow ground water can either be absorbed by a subsequent crop (for example, a rye cover crop) or intercepted and transformed (for example, riparian buffers).
As a result of that meeting, the following BMPs were determined to reduce excess nitrogen. Most of these values have years of research to support the reduction value associated with each practice. These values were sent to Dr. Robert Evans for his review and comment. He concurred with the values that were presented. There were several people in the committee that felt that 90% effectiveness for riparian buffers was too high and that a more realistic value of 80% should be used. The riparian buffer value was reduced to 85%.
Sediment/Nutrient Delivery Reduction from Fields
| Practice | N Reductions (%) |
|---|---|
| Grade stabilization | 0 |
| Nutrient management | ?* |
| Riparian buffer (20 feet) | 30 |
| Riparian buffer (30 feet) | 40 |
| Riparian buffer (50 feet) | 50 |
| Riparian buffer (70 feet) | 55 |
| Riparian buffer (>100 feet) | 60 |
| Rock-lined outlet | 0 |
| Sediment control basin | 0 |
| Water control structure | 40 |
| Streambank stabilization | 0 |
* NLEW considers nutrient management through reduction in fertilizer application. The efficiency of nutrient management will vary.
Erosion Reduction/Nutrient Loss Reduction in Fields
An underlying assumption of NLEW is that the majority of nitrogen lost is subsurface soluble nitrogen. Erosion practices, although effective in reducing erosion and surface losses of nitrogen, do not have a proven effect on the majority of N in agricultural systems. Since most nitrogen is lost through subsurface flow, these practices have little effect on the overall nitrogen loss budget. In addition, some of these practices will be accounted for in the current year that it is implemented. For example, if cropland is converted to pasture, the reduced nitrogen loading will be accounted for in the pasture crop and its higher nitrogen use efficiency.
Agricultural Chemical Pollution Prevention
Proper Animal Waste Management Practices
Animal waste application is treated as any nutrient application. No specific numbers will be assigned to these practices.
Stream Protection from Animals
While some of the above BMP systems have demonstrated a significant effect on water quality of receiving waters, there is no standard effect with regard to nitrogen reduction. The sitespecific nature of BMPs is relative to pre-BMP management practices as well as topographic and hydrologic aspects of a particular site.
The effectiveness of cover crops to reduce nitrogen in the soil profile is a function of how much the crop can absorb (which depends on planting date, kill or plow-down date, and growth patterns) and the subsequent mineralization and release of the nitrogen in the cover crop residue the following year. Researchers at NC State University conducted 15N research on cover crops. At the realistic corn fertilizer rate of 150 kg N/ha, rye accumulated approximately 35 kg of nitrogen/ha by the middle of April whereas wheat had accumulated only 20 kg/ha of nitrogen. This difference in nitrogen content is due primarily to difference in total biomass accumulation: 3,000 kg/ha (rye) vs 2,000 kg/ha (wheat). Although wheat accumulates about 2/3 the nitrogen and biomass as rye, soil nitrate concentrations and subsequent release of nitrogen are very different. Soil nitrogen concentrations are much higher under wheat than rye, in March and April. In addition, wheat releases its nitrogen faster than rye although both crops release most of the nitrogen accumulated during their growth by the 16th week after the crop is killed.
Using these results, we determined that an average 15% net reduction (uptake minus mineralization) in nitrogen was an appropriate value to assign early planted (before November 30), late killed (April 1st in the Coastal Plain, April 10th in the Piedmont), and unfertilized rye cover crop. Oats are intermediate to rye and wheat in its ability to absorb nitrogen. A value of 10% for early-planted, late killed, nonfertilized oats would be reasonable. Based on the lower total crop nitrogen content of wheat, its faster mineralization rate, and the fact that soil nitrate levels were much higher for wheat, we assigned a 5% nitrogen reduction value. Again this is for early planted, late killed, unfertilized wheat.
During a national conference on no-till, it was concluded that no-till had little effect on increasing or decreasing N movement into shallow ground water (Logan, 1987). Under some unique conditions, with certain crops, no-till may reduce subsurface losses of nitrate-N. The available data from the Piedmont of North Carolina demonstrate that at the same N fertilization rates, yields of no-tilled corn are much greater. The very large and sustained corn yield increases in the Piedmont region of North Carolina suggest that N is used more efficiently under no-till systems and subsurface N losses are probably reduced. This is why we have assumed greater fertilizer N use efficiency with no-till corn than conventional corn in the Piedmont.
Most of the data reviewed from both the U.S., the Southern region, and North Carolina supports the conclusion that tillage type has no proven effect on N movement into the shallow ground water. After summarizing the available data and submitted the product to outside reviewers, it is our determination that no-till systems without a cereal winter cover crop do not represent an Nreducing BMP in the Coastal Plain of North Carolina for any crop. This does not preclude the use of no-till or strip-till planting techniques to reduce production costs and reduce sediment losses. Agricultural systems of best management practices should include practices that reduce all pollutants. For a more comprehensive analysis of nitrogen losses from no-till and strip-till, please see Osmond et al., 2000.
Based on available research and professional judgement, the follow nitrogen reducing credits for the different best management practices have been determine (Table 3).
| Best Management Practice | Nitrogen Reduction (%) |
|---|---|
| Filter strip (Minimum of 20 feet) | 40 |
| Waster control structures (flashboard risers) | 40 |
| Riparian buffers | |
| Minimum 50 foot width = 30 feet trees and 20 feet grass | 85 |
| NRCS standard | 85 |
| Minimum 20 feet trees only | 75 |
| Minimum 30 feet grass only | 65 |
| Cover crop | |
| Rye & Triticale | 15 |
| Oats & Barley | 10 |
| Wheat | 5 |
We request that users of the NLEW tool provide feedback to the authors or other NC agricultural agency personnel so that we can provide future versions that incorporate user comments and suggestions. The NLEW working group will continue to consider modifications to the current version as results from applied research in NC, the Southeast, and elsewhere becomes available.
Chancy, H.F. and E.J. Kamprath. 1982. Effects of deep tillage on N response by corn on a sandy Coastal Plain soil. Agron. J. 74:657-662.
Gilliam, J.W., D.L. Osmond, and R.O. Evans. 1997. Nitrogen reducing best management practices to control nitrogen in the Neuse River Basin. Tech. Bull. #311. North Carolina State University. Raleigh, NC.
Hodges, S.C. 2000. Realistic yield expectations for soils of North Carolina: Alphabetical listings by series. http://ces.soil.ncsu.edu/nmp/RYE_Alpha.PDF.
Hodges, S.C. 2000a. Soil management groups for North Carolina. http://ces.soil.ncsu.edu/nmp/SMG_Final_2000.pdf.
Jacobs, T.J. and J.W. Gilliam. 1985. Riparian losses of nitrate from agricultural drainage waters. J. Environ. Qual. 14:472-478.
Kamprath, E.J. 1986. Nitrogen studies with corn on Coastal Plain soils. North Carolina Agricultural Research Service, North Carolina State University, Raleigh, NC.
Logan T.J. (ed.). 1987. Effects of Conservation Tillage on Groundwater Quality. Lewis Publishers, Chelsea, MI.
MacKown, C.T. and T.G. Sutton. 1996. Recovery of Fertilizer N Applied to Burley Tobacco. Agron. J. 89: 183-189.
Ortega, M.R.V. 1996. Analysis of sweet potato growth under differing rates of nitrogen fertilization. Doctor of Philosophy Thesis. North Carolina State University. Raleigh, NC.
Osmond, D.L. and J. Schultheis. 1999. Personal Communication.
Osmond, D.L., N.N. Ranells, G.C. Naderman, M.G. Wagger, G.D. Hoyt, J.L. Havlin, and S.C. Hodges. 2000. Considering no-till as a N-reducing best management practice. North Carolina State University, Raleigh, NC.
Scharf, P.C., M.M. Alley, and Y.Z. Lei. 1996. Spring nitrogen on winter wheat: I. Farmer-field validation of tissue test-based rate recommendations. Agron. J. 85:1181-1186.
Sisson, V.A., T.W. Rufty, and R.E. Williamson. 1991. Nitrogen-use efficiency among fluecured tobacco genotypes. Crop Sci. 31:1615-1620.
Torbert, H.A. and D.W. Reeves. 1994. Fertilizer Nitrogen Requirements for Cotton Production as Affected by Tillage and Traffic. Soils Sci. Soc. of Am. J.58:1416-1423.
Wagger, M.G., M.J. Vepraskas, and H.P. Denton. 1992. Corn grain yield and nitrogen utilization in relation to subsoiling and nitrogen rate on Paleudults. Agron. J. 84:888-892.
Wagger, M.G. 1996. Reduction of nitrate leaching in agricultural soils via cover crops. Water Resources Research Institute of the University of North Carolina. Report no. 303. Raleigh, NC.
Woodhouse, W.W. 1969. Long-term fertility requirements of Coastal Bermuda. Nitrogen, phosphorus, and lime. Agron. J. 61:251-156.
The Nitrogen Loss Estimation Software (NLEW) Aggregated version software has been developed under the auspices of the Neuse Basin Oversight Committee - Agricultural Rule 0.0238. The Neuse Basin Oversight Committee asked a joint interagency committee to develop NLEW. The development of this tool has been cooperative effort between the N.C. Department of Agriculture, N.C. Department of Environment and Natural Resources, N.C. State University, and USDA - Natural Resource Conservation Services. This software was designed to meet the nitrogen loss assessment and best management practice tracking requirement under Agricultural Rule 0.0238 for those entities participating in the Local Option.
The conceptual development of NLEW should be attributed to the following individuals: D.L. Osmond, N.N. Ranells, S.C. Hodges, and E.J. Kamprath (NC State University); L. Xu (NCDENR - Division of Water Quality); T.E.. Jones (formerly NCDENR - Division of Soil and Water Conservation); J.R. Hansard (USDA-NRCS), and; J.R. Cummings (NCDA).
The overall software design was by S.H. Pratt and K. May, while the programming was provided by K. May and S.H. Pratt (Understanding Systems, Inc). Algorithms for the calculations within the program were provided by D.L. Osmond and L. Xu (NCSU and NCDENR). Data base management is provided by D. Crouse (NC State University)
The NLEW aggregated software should be cited as follows: Osmond, D.L., D. Crouse, K. May and S.H. Pratt. 2007. Aggregate Nitrogen Loss Assessment Worksheet (NLEW) for the Neuse River Basin. NCDA, NCDENR, NCSU, NRCS. Raleigh, NC.
We would like to express our appreciate to Natalie Jones and Kelly Fulford from the North Carolina Division of Soil and Water Conservation who tested the software and made useful and important suggestions. We would also like to thank Noah Ranells (Crop Science Department, NCSU) for reviewing the algorithms. Finally, funding of this tool was provided through DENR from USEPA Section 319 funds.
The contents and views expressed in this document are those of the authors and do not necessarily reflect the policies or positions of the North Carolina State University, or other organizations named, nor does the mention of trade names for products or software constitute their endorsement. North Carolina State University is not responsible for any undesirable outcomes attributed to decisions made by users based on NLEW recommendations.
(Different crops will pull up depending on user choice. Users will assign an acreage value to each crop type)
Σ Crop acres = Total Crop Acres
Ex:
Crop_a (acres) Crop_b Crop_d Crop_f Σ Soil management group acreage = Total SMG Acres
Ex:
SMGx (acres) SMGy Total Crop Acres = Total SMG Acres
This should occur automatically through the overlay between cropped acres, soil type and the soil management group data base. However, the acreage for each SMG can be changed.
Cropa_% = Crop_a (acres)/Total Crop Acres (acres) Cropb_% = Crop_b/Total Crop Acres (acres) Cropd_% = Crop_d/Total Crop Acres (acres) Cropf_% = Crop_f/Total Crop Acres (acres)
Cropa_Nx (lb) = SMGx (acres) * Cropa_% * RYE_SMGx_cropa (bu/A) * Nfactor (lb/bu) Cropb_Nx = SMGx * Cropb_% * RYE_SMGx_cropb * Nfactor Cropd_Nx = SMGx * Cropd_% * RYE_SMGx_cropd * Nfactor Cropf_Nx = SMGx * Cropf_% * RYE_SMGx_cropf * Nfactor Crop_RYE_Nx (lb) = Σ Cropi_Nx (lb)
Cropa_Ny (lb) = SMGy (acres) * Cropa_% * RYE_SMGy_cropa (bu/A) * Nfactor (lb/bu) Cropb_Ny = SMGy * Cropb_% * RYE_SMGy_cropb * Nfactor Cropd_Ny = SMGy * Cropd_% * RYE_SMGy_cropd * Nfactor Cropf_Ny = SMGy * Cropf_% * RYE_SMGy_cropf * Nfactor Crop_RYE_Ny (lb) = Σ Cropi_Ny (lb) Total_RYE_N (lb) = Σ Crop_RYE_NI (lb)
N_App_Crop_a (lb) = Crop_a (acres) * N rate Crop_a (lb/A) N_App_Crop_b = Crop_b * N rate Crop_b N_App_Crop_d = Crop_d * N rate Crop_d N_App_Crop_f = Crop_f * N rate Crop_f Total_N_Applied (lb) = Σ N_App_Crop_I (lb)
IF Total_N_Applied (lb) > Total_RYE_N (lb)
Excess_N (lb) = Total_N_Applied (lb) - Total_RYE_N (lb)Surface_N (lb) = Excess N (lb) * 0.05 Subsurface_N (lb) = Excess N (lb) * 0.95
Part_Cropa (lb) = Σ Cropa_Ni (lb) * (1-NUECropa) Part_Cropb = Σ Cropb_Ni * (1-NUECropb) Part_Cropd = Σ Cropd_Ni * (1-NUECropd) Part_Cropf = Σ Cropf_Ni * (1-NUECropf) Subsurface_crop (lb) = S Part_CropI (lb)
ELSE
Surface_N (lb) = 0 Subsurface_N (lb) = 0
Part_Cropa (lb) = N_App_Crop_a (lb) * (1-NUECropa) Part_Cropb (lb) = N_App_Crop_b (lb) * (1-NUECropb) Part_Cropd (lb) = N_App_Crop_d (lb) * (1-NUECropd) Part_Cropf (lb) = N_App_Crop_f (lb) * (1-NUECropf) Subsurface_crop (lb) = Σ Part_CropI (lb)
Total subsurface (lb) = Subsurface_crop (lb) + Subsurface_N (lb)
(Determine % of total area that the cover crop or BMP affects)
%area_a = Affected_area_a/Total acreage %area_b = Affected_area_b/Total acreage %area_d = Affected_area_d/Total acreage Subsurface_BMP (lb) = (Total subsurface (lb) * NReda * %area_a) + (Total subsurface (lb) * NRedb * %area_b) + (Total subsurface (lb) * NRedd * %area_d) Subsurface_Loss (lb) = Total subsurface (lb) - Subsurface_BMP (lb)
Total_N_lost (lb) = Subsurface_Loss (lb) + Surface_N (lb)