Nutrient Dynamics of Crop Residues Decomposing on a Fallow No-Till Soil Surface

Conservation practices retain crop residues on the soil surface that affect nutrient distribution in time and space. We hypothesized that nutrient mineralization from surface residues may not be correlated to mass loss but may depend on crop type and water availability. Frequent, moderate, and no irrigation were used to evaluate water effects on N, P, K, and mass dynamics of alfalfa (Medicago sativa L.), corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench], spring and winter wheat (Triticum aestivum L.). Residues (20 g) in 10 by 10 cm, 1-mm mesh bags were placed on a Pullman clay loam (fine, mixed, thermic Torrertic Paleustolls) at Bushland, TX, in August 1991 and collected monthly until August 1992. Water regime did not influence mass, N, or P dynamics but did affect K. Mass declined exponentially with decomposition coefficients (−r) of 4.4, 1.5, 2.0, 1.7, and 1.1 g kg⁻¹ d⁻¹ for the five crop residues listed above, respectively. Potassium loss was first order with -r ranging from 29.3 to 4.4 g kg⁻¹ initial K d⁻¹, depending on crop and water. Averaged across water regimes, -r equaled 25, 9, 8, 12, and 7 g kg⁻¹ initial K d⁻¹ for the respective crops. The water effect indicated 150-mm water removed 500 g kg⁻¹ initial K from residues. Residue N declined from 38.7 to 16.0, 10.9 to 5.1, 12.2 to 6.4, 9.5 to 4.5, and from 7.6 to 3.4 g kg⁻¹ during the first 34 d for the respective crop residues, after which nonlegume residues accumulated N (0.21 to 0.96 g kg⁻¹ initial N d⁻¹), while alfalfa lost N (−0.37 g kg⁻¹ initial N d⁻¹). Corn and winter wheat residue P increased from 0.7 to 1.2 and 0.5 to 1.0 g P kg⁻¹, respectively, during the first 34 d, after which all residues lost P (−1.4, −2.1, −1.3, -2.0, and -2.8 g kg⁻¹ initial P d⁻¹, respectively). Nutrient dynamics were not directly related to mass loss. Water regime effects were small, so nutrient availability from residues should be similar in irrigated and dryland systems in the southern High Plains. All programs and services of the USDA are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap.