Assessing crop residue cover using shortwave infrared reflectance

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Abstract

Management of crop residues is an important consideration for reducing soil erosion and increasing soil organic carbon. Current methods of measuring residue cover are inadequate for characterizing the spatial variability of residue cover over large fields. The objectives of this research were to determine the spectral reflectance of crop residues and soils and to assess the limits of discrimination that can be expected in mixed scenes. Spectral reflectances of dry and wet crop residues plus three diverse soils were measured over the 400–2400 nm wavelength region. Reflectance values for scenes with varying proportions of crop residues and soils were simulated. Additional spectra of scenes with mixtures of crop residues, green vegetation, and soil were also acquired in corn, soybean, and wheat fields with different tillage treatments. The spectra of dry crop residues displayed a broad absorption feature near 2100 nm, associated with cellulose-lignin, that was absent in spectra of soils. Crop residue cover was linearly related (r2=0.89) to the Cellulose Absorption Index (CAI), which was defined as the relative depth of this absorption feature. Green vegetation cover in the scene attenuated CAI, but was linearly related to the Normalized Difference Vegetation Index (NDVI, r2=0.93). A novel method is proposed to assess soil tillage intensity classes using CAI and NDVI. Regional surveys of soil conservation practices that affect soil carbon dynamics may be feasible using advanced multispectral or hyperspectral imaging systems.

Introduction

Crop residues are the portions of a crop that are left in the field after harvest. Crop residues on the soil surface decrease soil erosion, increase soil organic matter, and improve soil quality (Lal et al., 1999). By reducing the movement of eroded soil into streams and rivers, the movement of nutrients and pesticides is reduced. The overall result is less soil erosion and improved water quality. Thus, management of crop residues is an integral part of many conservation tillage systems.

Rapid, accurate, and objective methods to quantify residue cover in individual fields are needed for management decisions. The standard technique for measuring crop residue cover used by the USDA Natural Resources Conservation Service (NRCS) is visual estimation along a line-transect (Morrison et al., 1993). Regional assessments of conservation tillage practices in the US are compiled from annual surveys of crop residue levels after planting by the Conservation Technology Information Center (CTIC, 2000). Reviews of crop residue measurement techniques document recent modifications and illustrate the unresolved problems of current techniques Corak et al., 1993, Morrison et al., 1995.

The reflectance spectra of both soils and crop residue lack the unique spectral signature of green vegetation in the 400–1000 nm wavelength region Aase & Tanaka, 1991, McMurtrey et al., 1993, Daughtry et al., 1995. Crop residues and soils are often spectrally similar and differ only in amplitude for visible and near infrared wavelengths Baird & Baret, 1997, Streck et al., 2002, which makes discrimination between crop residues and soil difficult or nearly impossible using reflectance techniques. One promising approach for discriminating crop residues from soil is based on detecting a broad absorption feature near 2100 nm that appears in all compounds possessing alcoholic –OH groups, such as cellulose (Murray & Williams, 1988). Two other broad absorption features near 1730 and 2300 nm are associated primarily with nitrogen and lignin and may also serve as a basis to distinguish crop residues from soils Curran, 1989, Elvidge, 1990, Kokaly & Clark, 1999. The absorption feature near 2100 nm is clearly evident in the reflectance spectra of the dry crop residues and is absent in the spectra of soils Daughtry, 2001, Nagler et al., 2000, Streck et al., 2002. A new spectral variable, cellulose absorption index (CAI) that quantified the relative depth of this absorption feature was defined (Daughtry et al., 1996) using reflectance in three bands—two on the shoulders at 2021 and 2213 nm and one at 2100 nm (absorption maximum). However, this wavelength region is also strongly affected by water content (Palmer & Williams, 1974). Moisture content, age of the residue, and degree of decomposition affected the spectral reflectance and CAI of crop residues (Nagler et al., 2000). Water significantly altered the reflectance spectra of wet crop residues, but did not prevent the discrimination of crop residues from soils using CAI (Daughtry, 2001). Additional work is needed to determine the effects of mixed pixels (soil+residue) on discrimination of crop residues from soils and to test the CAI algorithm with hyperspectral images of fields and watersheds.

The objectives of this research were to (1) determine the spectral reflectance of crop residues and soils as a function of water content, and (2) assess the limits of discrimination that can be expected in mixed scenes. This research provides the scientific foundation required for assessing crop residue cover and tillage practices over large areas.

Section snippets

Laboratory reflectance spectra

Reflectance spectra were acquired with a spectroradiometer (FieldSpec Pro, Analytical Spectral Devices, Boulder, CO) over the 400–2500 nm wavelength region at 1 nm intervals. The samples were illuminated by two 300-W quartz-halogen lamps mounted on the arms of a camera copy stand at 50 cm over the sample at a 45° illumination zenith angle. The 18° fore optic of the spectroradiometer was positioned 90 cm from the sample surface at a 0° view zenith angle which resulted in a 28 cm diameter field

Lab spectra

Mean reflectance spectra of dry and wet corn and soybean residues are presented in Fig. 1. For each crop residue, the upper-most spectrum is the oven-dried residue and the lowest spectrum is for the water-saturated residue. The presence of water in the crop residue reduced reflectance across all wavelengths. Two major water absorption bands near 1450 and 1960 nm dominate the reflectance spectra at wavelengths >1300 nm. A broad absorption feature near 2100 nm is also evident in the reflectance

Acknowledgments

We are indebted to Andrew Russ and Adam Booher who acquired and processed much of the data used in this paper.

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