Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 14, 1999
Publication Date: June 11, 2000
Interpretive Summary: Management of crops in the U.S. has changed substantially from practices involving intensive tillage to practices such as no-tillage, ridge tillage, and other conservation tillage systems, that leave residues undisturbed following harvest. Crop residue cover affects erosion, water quality, integrated pest management, nutrient management, and other issues, so we need to understand how long residues last in conservation tillage systems. Relationships used in many agricultural models to estimate residue cover assume that residues are randomly distributed and flat on the ground, but a large portion of crop residue remains standing after harvest. Our objective was to determine how residue mass and soil cover relationships change in no-tillage small grain fields as residues decompose and shift from standing to flat. Winter and spring wheat, winter barley, and spring oat were grown at Bushland, TX, using different seeding rate, fertilization, and irrigation to produce a range of crop and residue yields. During decomposition, differential irrigation increased environmental variability. Residue mass was measured 7 times in 14 months, after taking photographs to determine soil cover. For crop-date combinations, cover coefficients were determined using total or flat mass. Using total mass, a given amount of residue provided more cover as it aged, because more of the residues were flat on the surface. Using flat biomass, residue cover could be estimated following harvest through extended decomposition periods using a single coefficient. Our findings can improve estimation of residue cover for no- tillage fields in erosion, water quality, and other agricultural and natural resource management models so that they can be applied to ensure good environmental management.
Technical Abstract: Maintaining residue cover provides diverse conservation benefits. Exponential relationships have been developed to estimate cover from biomass of randomly distributed, flat residues, but a large portion of crop biomass remains standing after harvest. Our objective was to determine how relationships between biomass and soil cover change in no-tillage small grain fields as residues decompose and shift from standing to flat. Winte and spring wheat (Triticum aestivum L.), winter barley (Hordeum vulgare L.) and spring oat (Avena sativa L.) were grown at Bushland, TX, on Pullman clay loam (fine, mixed thermic Torrertic Paleustoll) in 12 field plots - in three randomized complete blocks. For each crop differential seeding rate, fertilization, and irrigation produced a range of biomass. During decomposition, differential irrigation increased environmental variability (13, 5, and 0 applications to sub-sub-plots). Ash-free residue biomass was smeasured 7 times in 14 months, after taking photographs to determine soil cover of 1 m2 sites. For crop-date combinations, cover coefficients were determined using total (kt, m2/g) or flat (kf, m2/g) biomass. Regression indicated kt increased with time (p<0.0001 for all crops, except spring wheat with p<0.0041). Across crops, the relationship kt = 0.0037 + 0.000047 (DAH) (r2 = 0.54, p<0.0001) indicated that decomposition affects cover provided by total biomass. Across crops, the weak relationship kf = 0.0136 + 0.000023 (DAH) (r2 = 0.17, p<0.016) indicated that cover could be estimated from flat biomass using kf=0.0175 for extended periods. Our findings can improve estimation of residue cover for no-tillage fields but residue orientation should be considered.