Objectives The leaching of soil dissolved carbon (C) and nitrogen (N) is the primary form of C and N loss in greenhouse soils. Research on the effects of different organic materials replacing part of the chemical N fertilizer to reduce the leaching of soil dissolved C and N, as well as the key influencing factors, provides a theoretical basis for the scientific application of N fertilizers in greenhouse soils.

Methods An in-situ leaching experiment was carried out in typical greenhouse soils at the Luhe Base of Jiangsu Academy of Agricultural Sciences, with five treatments included: no fertilization (CK), chemical fertilizer only (CF), composted straw replacing 30% of chemical N fertilizer (STR), chicken manure replacing 30% of chemical N fertilizer (CHI), and spent mushroom replacing 30% of chemical N fertilizer (MUS). The leaching dynamics of dissolved organic C (DOC), dissolved organic N (DON), nitrate (NO₃⁻), and ammonium (NH₄⁺) were analyzed. The fluorescence spectra of dissolved organic matter (DOM) in the leachates were analyzed by excitation-emission matrix (EEM) fluorescence spectroscopy and fluorescence index was determined. The components of DOM was clarified in combination with modeling of parallel factor (PARAFAC) analysis. The correlations between the leaching amount of dissolved C and N and characteristics of DOM components were further analyzed.

Results The total leaching of TDN varied from 12.63 to 69.15 kg/hm² in all treatments during the whole growing period of vegetables. Nitrate and DON accounted for 90.0%−92.7% and 7.21%−9.72%, respectively. In comparison to CF, organic fertilizer substitution treatments significantly decreased the leaching amount of NO₃⁻ and DON by 30.2%−53.4% and 20.7%−49.1%, respectively, with the largest decrease both observed in MUS treatment. In opposite to DON leaching, DOC leaching increased in organic fertilizer substitution treatments, and was significantly higher in CHI treatment (C 6.97 kg/hm²) compared with CF. Both DOC and DON leaching had significantly positive relationships with C1 component (microbial-derived substances with high aromaticity and high degree of humification) and C3 component (humic-like substances existing in high nutrient environment), and had significantly negative relationship with C4 component (fulvic-like substances with low molecular weight). But DOC and DON leaching had no significant relationship with C2 component (plant residues derived humic-like substances). The leaching amount of DOC, DON, TDN and NO₃⁻ had significantly positive relationships with fluorescence index (FI), biological index (BIX) and freshness index (β/α) of DOM in the leachates.

Conclusions Under the condition of 30% substitution of chemical N fertilizer with organic fertilizers, the CHI treatment significantly increased the proportion of the refractory C1 component in the leachate, thereby promoting the leaching of DOC. MUS and CHI treatments significantly reduced the proportion of the C3 component in the leachate, thus reducing more leaching amount of DON compared to STR treatment. Partial substitution of chemical N fertilizer with composted straw, chicken manure and spent mushroom could mitigate dissolved N leaching from greenhouse soil, mainly because of the decrease of chemical N fertilizer supply and humic-like C3 component of DOM in the leachates. Therefore, leaching of dissolved inorganic and organic N were both decreased in the organic fertilizer substitution treatments. With the consideration of avoiding the increase of dissolved C leaching and achieving the highest effectiveness in mitigating dissolved N leaching, it was recommended to adopt 30% substitution of chemical N fertilizer with spent mushroom manure.