, Volume 195, Issue 1, pp 121-128

Failure of an iterative curve-fitting procedure to successfully estimate two organic N pools

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Abstract

Fitting a double negative exponential function to N mineralization data can be used to characterize two organic nitrogen pools; an ‘easily’ decomposable (Ndpm) and a ‘resistant’ one (Nrpm). The relevance of those two calculated N mineralization pools was investigated by adding ‘easily’ decomposable organic material to soils. Soil amended with crop residues of sugar-beet or bean was mixed with an equal amount of coarse sand, incubated at 35 °C and leached at specific time-intervals. Upon leaching, NH4 + and NO3 - were measured in the extracts. A double negative exponential function was fitted to the data and two organic N pools were defined. Fitting a double negative exponential function to N mineralization data to characterize an active and resistant organic N pool was sometimes impossible; the N mineralization data did not always resemble a negative exponential function. Additionally, the size of the two pools calculated were not constant with time and were often meaningless; the Nrpm pool was greater than the soil organic N content, the size of the Nrpm pool was smaller than the Ndpm pool or one of the N pools was negative. Relevant values for both Nrpm and Ndpm which were consistent with incubation time were only obtained when excessive amounts of organic material, normally not dealt with in the field, were applied.