The impact of organic farming on the soluble organic nitrogen pool in horticultural soil under open field and greenhouse conditions: a case study

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The aim of this study was to investigate the abundance and composition of soluble organic nitrogen (SON) and its labile components (protein and total free amino acids) in horticultural soils managed by organic (ORG) or conventional (CNV) practices under greenhouse or open field conditions. Protein concentration accounted for a high proportion of the SON fraction in all sampled sites, while free amino acids (FAAs) only constituted a small proportion. The contents of soil SON, protein, total FAAs and nitrate-nitrogen (NO3 −-N) increased in organic soils more than those under conventional practice. Soils cultivated in greenhouse conditions contained greater pools of total soluble nitrogen (TSN) and SON as well as NO3 −-N and ammonium-nitrogen (NH4 +-N) compared to open fields. Seasonal changes also affected the total abundance of SON, protein and FAAs in all the treatments, and the seasonal variation depends on the N form being measured. Electrophoretic patterns of soil proteins in greenhouse soils were clearly different from those in open field soil, but these significant differences were not observed between ORG and CNV soils. The soil amino acid pools were dominated by the same five amino acids, glycine, serine, asparagic acid, threonine and tyrosine, among four treatments. Greenhouse cultivation significantly affected the FAA profiles, while ORG management did not have the same effect. We propose that the management practice (ORG vs. CNV) has an effect on the quality of the SON labile pools, while it plays a lesser role in the SON quality compared to cultivation method (greenhouse vs. open field).