DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION 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 saliva L.), corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench], spring and winter wheat (Triticuin aestivum L.). Residues (20 g) in 10 by 10 cm, 1-inm 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" d ' for the five crop residues listed above, respectively. Potassium loss was first order with —r ranging from 29.3 to 4.4 g kg" initial K d", depending on crop and water. Averaged across water regimes, — r equaled 25,9,8,12, and 7 g kg" initial K d ' for the respective crops. The water effect indicated 150mm water removed 500 g kg' 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" during the first 34 d for the respective crop residues, after which nonlegume residues accumulated N (0.21 to 0.96 g kg" initial N d"), while alfalfa lost N (-0.37 g kg" initial N d"). Corn and winter wheat residue P increased from 0.7 to 1.2 and 0.5 to 1.0 g P kg", 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" initial P d", 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. R MANAGEMENT has become an important component of conservation tillage systems because surface residues help reduce water loss and soil erosion. Distribution of nutrients within the soil profile is altered in reduced tillage systems when compared with conventional tillage systems (Hargrove, 1985; Unger, 1991). Stratification of nutrients within the soil profile of notill systems may positively affect crop production and result in beneficial changes in soil physical properties (Bruce et al., 1995). Because surface residues decompose slower than incorporated residues (Douglas et al., 1980), there is a stratification of nutrients across time as well as space. Uneven distribution of residues by USDA-ARS, Southern Piedmont Conservation Research Center, Watkinsville, GA 30677-2373. All programs and services of the USD A are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap. Received 12 Aug. 1998. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 63:607-613 (1999). mechanical harvesters had a detrimental effect on nutrient availability to subsequent crops that was equivalent to poor distribution of fertilizer inputs (Douglas et al., 1992). Nutrient release from residues may play an equally important role in crop productivity through the effects of timing on nutrient availability. Several factors affect residue decomposition with the most important in agricultural systems being water, temperature, and residue properties such as N, cellulose, and lignin contents. The water regime of surface-placed residues is much different than that of incorporated residues (Douglas et al., 1980; Schomberg et al., 1994). Surface residues are subject to more frequent and extreme wetting and drying events than incorporated residues, which may alter the pattern and degree of residue decomposition (Franzluebbers et al., 1994). Wetting events can result in significant losses of soluble components from surface residues. Mass and nutrient losses £25% have been observed from surface residues during the first 30 d in the field (Douglas et al., 1980; Christensen, 1986; Collins et al., 1990). Decomposition rates usually decrease after initial losses of soluble C and other components (Christensen, 1985; Collins et al., 1990) because soluble carbohydrates serve as energy sources for the subsequent breakdown of more complex celluloses and lignins (Reinertsen et al., 1984). Most studies of residue dynamics have focused on changes in N and C, while only a few have evaluated changes in P (McLaughlin et al., 1988a, b, and c; Berg and McClaugherty, 1989; Sharpley and Smith, 1989; Buchanan and King, 1993) or K (Christensen, 1986) of decomposing residues. None of these studies evaluated multiple crop species decomposing under irrigated and nonirrigated field conditions or in an environment similar to that of the southern High Plains. Understanding water regime effects on surface-residue nutrient losses and residue decomposition is important when evaluating productivity of conservation tillage systems. This is especially true for the southern High Plains where under dryland conditions, crop and residue production is water-limited, but with irrigation nutrient availability becomes more important. Our objective was to evaluate water regime and crop effects on nutrient dynamics of