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Managing Nitrogen Contaminated Soils

Benefits of N₂–Fixing Alfalfa

^a USDA-ARS-US Dairy Forage Research Center (Minnesota Cluster), Saint Paul, MN 55108
^b USDA-ARS, 1991 Upper Buford Cir., Saint Paul, MN 55108
^c Center for Watershed Science and Education, College of Natural Resources, Univ. of Wisconsin–Stevens Point, Stevens Point, WI 54481
^d Ground Water Quality Bureau, New Mexico Environ. Dep., P.O. Box 26110, Santa Fe, MN 87502

View larger version (34K): [in this window] [in a new window]   Fig. 1. Schematic diagram of plot layout with well nests. Down-gradient wells were actually located a few meters within the plots, but were drawn on the border to show their relationship to the water table.

View larger version (45K): [in this window] [in a new window]   Fig. 2. Kriged contours of anaerobically mineralizable N in topsoil (0–0.15 m) over the plot area in July 1998. Paired plots for plant sampling are indicated by the 12 clear rectangles.

View larger version (39K): [in this window] [in a new window]   Fig. 3. Mean monthly air temperature in the Central District of Wisconsin and monthly precipitation at Waupaca, WI, during the study period, and the 1971–2000 long-term means of these data.

View larger version (48K): [in this window] [in a new window]   Fig. 4. Spatial distribution of soil nitrate N at the feedlot site in November 1998. Soil depth increments are represented by different planes in the figure, with kriged surface contours shown as in the two-dimensional maps in other figures.

View larger version (15K): [in this window] [in a new window]   Fig. 5. Mean and SE of alfalfa herbage dry matter (DM) yield and N yield for N2–fixing (black bars) and non-N2–fixing (gray bars) alfalfa by harvest at an abandoned feedlot (averaged across 2 forage production years). Bars with the same letter did not differ (P > 0.05).

View larger version (22K): [in this window] [in a new window]   Fig. 6. Mean and SE of average fine root length and root mass by depth for alfalfa. Fine roots had diam. < 1 mm. No differences were detected among alfalfa entries or for the interaction of depth and N2 fixation status. Differences between adjacent depth increments are indicated by the asterisks (* P < 0.05; ** P < 0.01).

View larger version (18K): [in this window] [in a new window]   Fig. 7. Plot of the fraction of N derived from the atmosphere (fNdfa) against total harvested N in N2–fixing alfalfa shoots. Data are from paired 1-m2 plots across two growing seasons; fNdfa (fraction of N derived from the atmosphere) was estimated by the natural abundance 15N method.

View larger version (19K): [in this window] [in a new window]   Fig. 8. Response of total N (closed symbols) and fixed N (open symbols) in N2–fixing alfalfa herbage to soil and manure N uptake in an abandoned feedlot. Data are from paired 1-m2 plots during two growing seasons based on the natural abundance 15N method.

View larger version (42K): [in this window] [in a new window]   Fig. 9. Kriged contour diagrams of total N, NH4–N, and NO3–N in topsoil (0–0.15 m) across time at the abandoned feedlot site. In 1998, NO3–N concentrations ranged up to 120 mg N kg–1 soil. Figures are shown in consistent scale, but the sampling frame for inorganic N was expanded by 7.6 m in all directions in 2000 and 2001.

View larger version (21K): [in this window] [in a new window]   Fig. 10. Mean nitrate N concentrations and mean change in water table depth in all monitoring well ports (except two wells affected by focused recharge, discussed in the text), 1999–2002. Each sampling depth was screened over adjacent 0.6-m intervals. The shallowest of the four installed wells was dry during the study period, and the next shallowest (Upper) was dry on many dates.