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Land use changes have significantly reduced nitrogen loadings in Walnut Creek watershed. With the conversion of row crop areas to native prairie and mandatory reduced nitrogen applications on refuge-owned croplands, reduced nitrogen loadings in Walnut Creek watershed have invariably occurred. However, accurately quantifying nitrogen reductions is problematic. A major confounding factor is the changing land use picture in the control watershed (Squaw Creek), where row crop land use has increased nearly 9.2 percent since 1990. Interpreting nitrogen loading reductions in the treatment watershed is difficult with a moving baseline condition indicated by the control watershed. Earlier estimations of nitrogen load reductions in Walnut Creek watershed were based on land use conditions in 1992 as the baseline condition (Schilling and Thompson, 1999). However, baseline land use conditions in 1992 do not represent conditions in 2005 since row crop land use increased substantially post-1996 with passage of the Freedom to Farm Act.

Nevertheless, nitrogen load reductions in Walnut Creek watershed were estimated using some of the same assumptions used previously (i.e., Schilling and Thompson, 1999; Schilling et al., 2002) and some hypothetical scenarios of land use conditions. First, a control condition in

Year 1985 1990 1995 2000 2005 2010 Ro w Cr o p P e rc e n ta g e 50 60 70 80 90 100 2.4% 26.1% Squaw Creek Walnut Creek

Figure 5. Change in row crop land cover in the Walnut Creek and Squaw Creek watersheds from 1990 to 2005.

Squaw Creek watershed was established. In 1990, 71.4% of the land in Squaw Creek watershed consisted of row crop. Schilling and Thompson (1999) showed that corn is the predominant row crop approximately 57% of the time, corresponding to a frequency of nearly two out of every three years in corn rotation. This percentage was confirmed by land use tracking in 2005 by CLU; corn again consisted of 57 percent of the total row crop in the Squaw Creek watershed. Typical nitrogen application in farmland around Prairie City was estimated to be 150 lbs/acre (Schilling and Thompson, 1999). Thus the amount of nitrogen applied to Squaw Creek watershed in 1990 was estimated by the following equation:

(11,622 acres) x (71.41% RC) x (57% corn)

x (150 lbsN/ac) = 709,885 lbs N (1)

Using the same equation for 2005 land use conditions and substituting 80.62% row crop for the 71.41% value, suggests that nitrogen applications in 2005 were 801,105 lbs, or a 12.8% increase over 1990 N application. In terms of unit loads, nitrogen application increased from 61.1 lbs/acre in 1990 to 68.9 lbs/acre in 2005.

Using the same approach for the Walnut Creek watershed, the Walnut Creek baseline condition in 1990 was estimated to be:

(12,981 acres) x (69.40% RC) x (57% corn)

x (150 lbsN/ac) = 764,913 lbs N (2)

In 2005, the percentage of row crop was reduced to 54.54 percent, thus reducing the nitrogen applications in the Walnut Creek watershed to 601,129 lbs N, or a 21.4% decrease over 1990 N application levels. Nitrogen application rates decreased from 66.4 lbs/acre in 1990 to 46.6 lbs/acre in 2005.

The amount of nitrogen application reduction provided by prairie restoration in Walnut Creek watershed was also estimated by considering the amount of row crop acres converted to prairie. From above, 2,213 acres out of total prairie plantings of 3,023 acres were established on 1990 row crop land. Using the same assumptions as above, prairie restoration from 1992 to 2005 reduced N applications as follows: (2,213 acres) x (57% corn) x (150 lbs N) =

189,220 lbs N (3)

The amount of nitrogen reduction provided by prairie restoration by the Neal Smith refuge is higher than the difference calculated from comparing the 1990 and 2005 land covers (163,784 lbs N). In addition, 479 acres of row crop land were owned by the refuge in 2005 but rented to area farmers on a cash-rent basis.

1 993 1 994 1 995 1 996 1 997 1 998 1 999 2 000 2 001 2 002 2 003 2 004 2 005 A n nual P rair ie P lantings (ac) 0 100 200 300 400 500 600 Avg. = 222 ac/yr Cumulative P rairie P lan tings (ac) 0 500 1000 1500 2000 2500 3000

3500 _{Figure 6. Annual and}

cumulative prairie plantings in Walnut Creek watershed.

In these areas, N applications were reduced from 150 lbs/acre to 100 lbs/acre. Assuming a 50% corn rotation in these areas mandated by the refuge, rental farmlands in 2005 reduced N applications by an additional 11,975 lbs N (479 ac x 50% corn x 50 lbs N). However, nitrogen application reductions provided by prairie conversion and reduced N application rates at the Neal Smith refuge are offset by the otherwise increasing trend in row crop land use in non-refuge portions of the watershed. For example, in the WNT1 subbasin, the 7.9 percent increase in row crop land cover from 1990 to 2005 would have contributed an additional 32,800 lbs of nitrogen per year in the watershed. Subtracting the additional N from WNT1 from the reductions provided by prairie restoration at the Neal Smith refuge (189,220 lbs) brings the estimated nitrogen application reductions resulting from the refuge in general agreement with the estimated total application reduction for the watershed (156,408 lbs N).

It is evident that a confounding factor in the analysis of nitrogen reductions is the otherwise increasing trend of row crop land cover in Squaw Creek and on non-refuge lands in Walnut Creek. Hence it is difficult to develop a true paired comparison of nitrogen application loads between Walnut and Squaw creek watersheds since N rates increased 12.8 percent in Squaw Creek watershed and decreased 21.4 percent in Walnut Creek. If the nitrogen application rates between Walnut and Squaw were compared in 1990, the rate of N application in Squaw Creek watershed was 92 percent of Walnut Creek watershed (61.1 compared to 66.4 lbs N/acre). In 2005, the ratio of nitrogen applications in Squaw Creek compared to Walnut Creek was 1.48 (68.9 compared to 46.6 lbs N/acre, respectively). Thus, one measure of the comparison in nitrogen loading reductions in Walnut Creek watershed compared to Squaw Creek would suggest that N applications have been reduced by 56 percent in Walnut Creek relative to Squaw Creek watershed (negative eight percent difference in 1990 plus 48 percent difference in 2005).

A second measure of quantifying reduced nitrogen applications in Walnut Creek watershed relative to Squaw Creek used the paired Squaw Creek land cover from 1990 and 2005 to adjust a hypothetical Walnut Creek baseline land cover. Provided that row crop land use in the Walnut Creek watershed increased by the same percentage as Squaw Creek from 1990 to 2005 (9.2%), nitrogen application loads in Walnut Creek watershed in 2005 would have been approximately 866,314 lbs N (assuming the row crop percentage is equal to 78.6% in equation 2). Comparing this hypothetical N load to the current condition of 601,129 lbs N (54.5% row crop in 2005) suggests that prairie restoration in Walnut Creek watershed may have reduced nitrogen applications by 30.6% from what applications might have been if the refuge had not been established. While speculative, the analysis nonetheless highlights the substantial effect of the Neal Smith refuge on nitrogen fertilizer loads in the Walnut Creek watershed