Corn, wheat, and potato crop residue decomposition and nitrogen mineralization in sandy soils under an irrigated potato rotation

Transformation of organic nitrogen (N) from both soil organic matter and from crop residues into inorganic forms is an important process which determines the pool of available N for the subsequent crop. This study was conducted in the Pacific Northwest (PNW) on an irrigated Quincy fine sand where potato (Solanum tuberosum) was grown in three or four year rotations with either corn (Zea mays L.) or wheat (Triticum sativa L.). The decomposition of crop residue and the mineralization of nitrogen (N) provides a source of plant available N to the next crop. An in-situ column incubation technique was used to determine the N mineralized during January through September 2000 from corn, wheat, and potato crop residues. The soil was sieved to determine the amount of crop residues present in the soil which contributes to N mineralization. The dry weight of the crop residue in January soil samples, taken at the top 30 cm depth, ranged from 8.4 for potato to 26.5 Mg ha(-1) for corn, but decreased to 4.6 for potato and 12.7 Mg ha(-1) for corn in March. Total N content in the crop residue was used to estimate potentially mineralizable N (PMN) at the time of sampling. The PMN in the top 30 cm soil depth, on the basis of residues sampled in January, were 398, 378, and 121 kg ha(-1) for the corn, wheat, and potato crop residues, respectively, but decreased to 189, 114, and 68 kg ha(-1), respectively, in March samples. Cumulative N mineralized in the top 30 cm depth of soil during January through September was 172, 128, and 72 kg ha-1 for corn, wheat, and potato residues, respectively. Cumulative N mineralized during January through May accounted for 53 percent of the total N mineralized from January through September. The fate of the mineralized N early in the cropping season is uncertain, since this period represents minimal N uptake by crops.