1. Spectral Reflectance-based Nitrogen Management for Subsurface Drip Irrigated Cotton Raj Yabaji, Kevin Bronson, Cary Green, Eduardo Segarra, and Adi Malapati Texas A & M Univ. – Texas Agric. Exp. Stn, and Texas Tech Univ, Lubbock, TX

2. Introduction The Southern High Plains of Texas average 3000-4000 ft in elevation, 18 in. rainfall/yr and 80 in. open pan evaporation/yr. The densest concentration of cotton production in the world is here. 3 to 4 million ac of cotton are planted each yr. Cotton is mono-cropped. Half of the cotton acreage is dryland, half is irrigated from the declining Ogallala aquifer. % 70 of the irrigation is center pivot, 25 % is furrow-irrigated, & 5 % is subsurface drip.

3. In-season sensing of N status for Cotton

4. Nitrogen management for subsurface drip irrigated cotton

5. Objectives Test spectral reflectance as an N management guide for SDI cotton Refine the window of N fertilizer injection in SDI cotton Compare 32-0-0 with 28-0-05S as N fertilizer sources in SDI cotton Measure denitrification in SDI cotton as affected by N management

6. Materials & Methods

7. Plot plan 600 ft 8, 40-in rows

9. Treatments 32-0-0 or 28-0-0 5S injected early bloom to mid bloom (5 weeks) 32-0-0 or 28-0-0 5S injected early bloom to peak bloom (8 weeks) Zero-N (1 plot/station/rep) Reflectance-based mgt: 0.5 X N early to peak bloom, N injection rate adjusted upwards by 100 % when GVI (Green vegetative index)reflectance/GVI soil test < 0.95 or statistical significance. Amount of N fertilizer = 150 lb N/ac (for 2.5 bale/ac) – 0-24 in soil test NO 3 -N – irrigation water NO 3

10. Nitrogen requirements for high-yielding cotton 1 Nitrogen fertilizer plus 0-24 inch NO 3 -N

11. Cropscan MSR 16 Passive sensor (natural light) 16 wavebands (up & down) 48 inches above canopy Within 2 hrs solar noon Percent reflectance = Refltd irrad @ λ / Incoming irrad @ λ

12. Vegetative indices (Bausch and Duke, 1996) Green vegetative index = R 820 /R 550 Red vegetative index = R 820 /R 650 R = percent reflectance at λ (nm)

15. Results

16. Green vegetative index for SDI cotton Lubbock, 2005 * *

17. Green vegetative index for SDI cotton Lubbock, 2006 * * * *

18. Nitrogen fertilizer injections Lubbock, 2005

19. Nitrogen fertilizer injections Lubbock, 2006

20. Spring soil nitrate, N fertilizer amounts injected, well water nitrate, and total N supply, Lubbock, TX, 2005

21. Spring soil nitrate, N fertilizer amounts injected, well water nitrate, and total N supply, Lubbock, TX, 2006

22. Correlations of Green vegetative index (GVI), chlorophyll meter (SPAD) readings, leaf N, biomass and lint yield, SDI cotton Lubbock 2005-6

23. Correlations with chlorophyll meter (SPAD) and spectral reflectance at early bloom Lamesa, 2003 Means: Biomass – 704 kg/ha, Leaf N – 3.9%, NDVI_cropscn – 0.43, NDVI_grnskr – 0.52

24. Leaf N SPAD NDVI_cropscan NDVI_greenskr 550nm Biomass 0.30 0.34 -0.29 Leaf N 0.53 0.52 0.43 SPAD -0.30 NDVI_crops 0.47 -0.31 NDVI_grnsk Cropscn_550 Correlations with chlorophyll meter (SPAD) and spectral reflectance at early bloom Lamesa, 2004 Means: Biomass – 1954 kg/ha, Leaf N – 3.7%, NDVI_cropscn – 0.73, NDVI_grnskr – 0.86 0.21 -0.32

25. Mid-bloom biomass, green vegetative index, chlorophyll meter readings as affected by N management, Lubbock, TX, 2005

26. Mid-bloom biomass, green vegetative index, chlorophyll meter readings as affected by N management, Lubbock, TX, 2006

27. First open boll biomass, N uptake, seed and lint yields as affected by N management, Lubbock, TX, 2005

28. First open boll biomass, N uptake, seed and lint yields as affected by N management, Lubbock, TX, 2006

29. Recovery efficiency of 90 lb fertilizer-N/ac in cotton plants

30. Summary Green vegetative index related to leaf N and biomass, and responded to N fertilizer treatments. GVI reflected net N mineralization in zero-N plot. We tested spectral reflectance-based N management, i.e. initial N injections of ½ X of soil test recommendations, and adjusting upwards as GVI falls significantly below soil test management for 2 ½ bale/ac yield goal. Spectral-reflectance N injection saved 28 and 15 % N compared to soil test-based management for 2005 and 2006, respectively.

31. Summary cont. Recovery efficiency of N fertilizer injected daily in subsurface drip irrigated cotton is high, i.e. 62-75 % Denitrification from N fertilizer injections in SDI cotton was not detectable.