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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Crop Residue Decomposition in No-Tillage Small-Grain Fields

^a USDA–ARS, 1420 Experiment Station Rd., Watkinsville, GA 30677 USA
^b USDA–ARS, P.O. Drawer 10, Bushland, TX 79012 USA

jsteiner{at}arches.uga.edu

Conservation tillage fields provide different environments for biological and chemical processes than tilled fields. Our understanding of decomposition does not adequately account for post-harvest residue distributions or field environment variability. We hypothesized that temperature and moisture could be used to normalize field environments to optimal conditions that produce maximum decomposition rates; and biomass density could be normalized based on the fraction of initial biomass remaining over time. Four small grains were grown at Bushland, TX, to produce high-, medium-, and low-biomass densities in 36 field subplots using different seeding rate, fertilizer, and irrigation on Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll). During decomposition, differential irrigation increased environmental variability (13, 5, and 0 applications to sub-subplots). Ash-free crop residue biomass was measured seven times during 14 mo. Climate indices related field to optimal conditions, based on the daily minimum of air temperature and precipitation coefficients. First-order decomposition coefficients, k, were determined by plot, using the cumulative climate index to represent time. Irrigation did not affect k (P < 0.45), indicating that the moisture index accounted for irrigation effects; but crops had different coefficients (P < 0.062). Initial biomass density was inversely related to k (P < 0.008), indicating that climate-based indices inadequately normalized environments across density treatments. The k was correlated to initial biomass , fraction-standing initial biomass , and initial N concentration in standing biomass . Climate indices may allow normalization of field environments important to decomposition and other agroecosystem processes if density effects on atmosphere–soil-residue interactions can be better quantified.

Abbreviations: A, constant in temperature coefficient equation • Biomass Density Treatments: H, high • M, medium • L, low. Crops: B, barley • O, oat • S, spring wheat • W, winter wheat. DAH, days after harvest • DD, decomposition day • k, decomposition coefficient • M_o, initial biomass • M_t, total biomass at time t • P, precipitation (or irrigation) • PC, precipitation coefficient • T, temperature • TC, temperature coefficient • T_opt, optimum temperature for decomposition

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