Nitrates and Row Cropping

Transcription

1 Nitrates and Row Cropping Matthew Helmers Dean s Professor, College of Ag. & Life Sciences Professor, Dept. of Ag. and Biosystems Eng. Iowa State University

2 Corn Grain Harvest (~100 lbs N/acre/year) Corn Residue Return (~65 lbs N/acre/year) Corn Nitrogen Cycling & Budget Corn Nitrate Use (~165 lb N/acre) Gaseous Loss (~10 lbs N/acre/year) Fertilizer to Corn (~150 lbs N/acre/year) Native Soil Organic Matter Nitrogen ~ 10,000 lb N/acre Microbial production of nitrate from native soil ( lbs N /acre/year) Microbial re-uptake of nitrate ( lbs N/acre/year) NITRATE Nitrate leaching to water (~30 lbs N/acre/year) 3

3 Soybean Grain Harvest (~165 lbs N/acre/year) Soybean Residue Return (~65 lbs N/acre/year) Soybean Nitrogen Cycling & Budget Soybean Nitrogen Use (~230 lb N/acre) Soybean Nitrate Use (~130 lb N/acre/year) Atmospheric Fixation (~100 lbs N/acre/year) Gaseous Loss (~2 lbs N/acre/year) Native Soil Organic Matter Nitrogen ~ 10,000 lb N/acre Microbial production of nitrate from native soil ( lbs N /acre/year) Microbial re-uptake of nitrate ( lbs N/acre/year) NITRATE Nitrate leaching to water (~30 lbs N/acre/year) 4

4 Soil Nitrate Production vs. Crop Nitrate Uptake Rate of soil nitrate production from native soil organic matter Rate of corn or soybean nitrate uptake The majority of nitrate used by corn and soybean comes from soil nitrate production. Corn gets the difference from fertilizer while soybean gets the difference from legume fixation of atmospheric nitrogen. March February In the shaded areas, the soil produces nitrate, but there is no crop to use it. As a result, some nitrate is lost to waterways. 6

5 Summary: Why Is Nitrate Lost? An average Iowa soil contains 10,000 lbs of nitrogen/acre in organic matter. This organic nitrogen is not susceptible to rapid loss. When the soil is warm and moist (not saturated), microbes transform the organic nitrogen into nitrate. Nitrate is susceptible to rapid loss. If there is no crop to use soil nitrate, it can be lost to waterways (especially during heavy precipitation). Almost all nitrate loss to Iowa waterways occurs when soils are warm and moist and crops are not rapidly using soil nitrate. While both spring and fall periods lack crops, more nitrate loss usually occurs in the spring because soils are also warm and wet. 8

6 Nitrate Loss Pathways Nitrate is leached from the plant/crop root zone Nitrate is primarily transported in subsurface flow pathways In tile drained lands that nitrate moves to tile lines and is exported to the stream In non-tile drained lands that nitrate moves to shallow groundwater

7 Water Flow Pathways Naturally Well-Drained Soils Soils with Poor Natural Drainage Land management and land use impacts ET and infiltration which in turn impact surface runoff, subsurface drainage, deep percolation

8 Relationship to Landuse

9 Nitrate-N Comparison Nitrate concentrations estimated based on land use and N application rates overestimate the observed nitrate concentrations by about 17% on the basis of a least squares statistical model. This 17% difference could be largely explained by in stream loss of nitrate and by dilution due to surface runoff. Overall, empirically based modeling approach to predict impacts of practice implementation (83% subsurface flow and 17% surface runoff

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11 Existing Conditions N Rate

12 Estimated Nitrate Concentrations at NRTN N-rate

13 Summary Nitrate is leached through the soil profile below the crop root zone In tile drained areas this nitrate can be transported to the streams through this drains In non-tiled areas the nitrate may move through longer subsurface flow pathways to the stream but may still make it to the stream