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

Abstract

Fitting a double negative exponential function to N mineralization data can be used to characterize two organic nitrogen pools; an ‘easily’ decomposable (N_dpm) and a ‘resistant’ one (N_rpm). 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 N_rpm pool was greater than the soil organic N content, the size of the N_rpm pool was smaller than the N_dpm pool or one of the N pools was negative. Relevant values for both N_rpm and N_dpm which were consistent with incubation time were only obtained when excessive amounts of organic material, normally not dealt with in the field, were applied.