Soluble organic nitrogen (SON) represents an important component of biogeochemical cycles in terrestrial ecosystems.However, in comparison to soluble inorganic nitrogen (SIN), SON has received little attention, particularly with regard to the pools of N present due to the uncertainty of the availability of SON for direct uptake by plants, its ecological significance, and analytical limitations. Moreover, many studies suggest that SON can be a major pathway for nitrogen (N) loss in soils and a principal factor affecting water quality. It is apparent that SON can be an essential nitrogen form for plant uptake, microbial transformation, and leaching. Ge et al. also confirmed that SON can be a direct N source for tomato plants grown in soil. The amount of SON in soils varies greatly with soil and land use type, vegetation cover, soil management strategy, environmental conditions and analytical methods used. Studies on SON pools in agricultural soils and, for example, Christou et al. found that SON constituted 57 ± 8 % of the total dissolved N pool. Unfortunately, few studies have investigated the impact of different soil management strategies,such as the use of greenhouse conditions or the effect of open field cultivation and organic or conventional farming, on the amount of SON in the soil, although some studiessuggest that differences can be expected to occur. Therefore,the aims of this study were to quantitatively analyze the amounts of SON and SIN in the soils under different methods of horticultural management, and to identify the relationships among these parameters, i.e. SON, dissolved organic carbon (DOC) and SIN (NH4+ and NO3-).

Soils used in greenhouses and open field cultivation were sampled in Shanghai, China, where organic farming has been conducted for 3 years or conventional faming has been continued in the same area.The results indicated that the amounts of SON, nitrate (NO3-) and ammonium (NH4+) were greater in the greenhouse soils than those under open field cultivation, which indicated a higher degree of soil management was imposed under greenhouse conditions. Greenhouse cultivation is also known to accelerate the turnover of SON in the soils, which may explain the significantly higher amounts of SON present in these soils. Organic farming, which does not use artificial fertilizers and pesticides, also resulted in significantly higher amounts of SON(average 42.10 mg kg-1) compared with soils under conventional faming (24.59 mg kg-1). The reasons for the observed differences in pool sizes of soluble inorganic nitrogen (SIN) and NO3- in the greenhouse soils and the open fields include (a) the heavy application of both complex fertilizer and organic fertilizer that exceeded crop requirements and (b) warmer temperatures and moist soils in the greenhouses, which are likely to lead to greater rates of N cycling compared with the open field soils. These results suggest that SON may be an important source of N in all horticultural systems, representing a pool of labile N readily available for plant growth. However, its concentration is less sensitive to different management practices than SIN. In contrast to SON, the total soluble nitrogen and inorganic N (SIN) pools varied widely with the different management practices although they were dominated by NO3- in all treatments. Soil organic N was positively related to dissolved organic carbon and NO3- contents. This relationship indicates that NO3-and dissolved organic matter play a key role in the retention of SON in soil.

Collectively, the amounts of SON extracted by KCl varied greatly among the horticultural soils collected in this study, which is likely to be regulated in part by land management practices. The amount of SON present was greater in soils from greenhouse conditions than in soils from open field cultivation. Organic management, which is known primarily for the abstinence of artificial fertilizers and pesticides, also resulted in significantly higher amounts of SON. However, there are still many gaps in our understanding of SON dynamics such as quantifying the size and composition of SON pools in different systems, identifying the fractions of SON that are involved in N mineralization, microbial assimilation and plant uptake, and determining the ultimate fate of SON. The recent development of simple methods to measure SON and improved techniques to characterize SON will stimulate more research and enable workers to begin to answer these challenging questions.