Evaluation of laboratory procedures for prediction of available soil nitrogen in Nebraska

Abstract

Soil organic matter decomposition and the resulting mineralization of N is a source of plant-available N seldom considered in the US when making fertilizer recommendation, and may contribute to a significant portion of the soil NO\sb3-pool. Large mineral N pools created by excessive fertilization and soil organic N mineralization may lead to less efficient use of N and potential for N pollution of the nation\u27s surface and ground water. Although several soil N availability indexes have been set forth, none of those tests have gained cohesive acceptance in terms of commercial application. Electro-ultrafiltration (EUF) is one such procedure that has been recently proposed to effectively quantify soil potentially mineralizable N. The technique is based on vacuum extraction of a soil-water suspension at different voltages and temperatures across anode and cathode. This procedure provides for extraction of NO\sb3\sp- and NH\sb4\sp+ and of readly soluble N compounds from soils using the principles of ultrafiltration and electrodialysis. The investigation encompassed both laboratory and field study. Nitrogen availability indexes differed in the amount of N extracted for a group of Nebraska surface soils. The data showed a correlation coefficient of 0.71 between the results obtained with the waterlogged method and those achieved using the EUF technique. However, the autoclave, KCl, pH 11.2 phosphate-borate buffer, and NaHCO\sb3-UV methods were more highly correlated (r $\geq$ 0.87) with the EUF technique than was the waterlogged method. The results obtained with the alkaline KMnO\sb4 method yielded the lowest correlation coefficient (r = 0.25) with the EUF technique. The slopes of the regression equations between EUF and the chemical indexes tested indicated that the EUF procedure was a relatively stronger extractant than the KCl method. Soil organic N mineralized determined by plant N uptake varied between geographical locations and ranged from 16 to 174 kg ha\sp{-1}. $\Sigma$EUF-N\sb{\rm organic} correlated poorly (r = 0.03) with total N uptake for four different locations. Multiple regression including $\Sigma$EUF-N\sb{\rm NO3}, $\Sigma$EUF-N\sb{\rm organic} and growing degree days as independent variables resulted in a high correlation (r = 0.95) with total N uptake. Within each field location, $\Sigma$EUF-N\sb{\rm extractable} correlated well with total N uptake (r $\geq$ 0.75). Autoclave Labile-N + residual NH\sb4- + NO\sb3-N exponentially correlated (r = 0.90) with total N uptake for all sites. Potentially mineralizable N differed widely in the amounts of N extracted from soils under various land treatments. Total NO\sb3-N loading to a depth of 9.50 m in the soil profiles associated with various kinds of land use was feedlot $\u3e$ irrigated corn fields $\u3e$ lawn $\u3e$ grassland. The data showed that the amount of NO\sb3-N moving through the vadose zone was very small under native grassland and urban lawns. In contrast, the potential for ground water pollution appeared relatively high under the feedlot and irrigated corn fields receiving manure and fertilizer N