Fulvic acid (FA), the essence and most active component in humus, is widely used as a fertilizer synergistic agent and for soil improvement. As a synergist, FA can not only highly chelate microelements, but also play a key role as a growth promoter. Due to the small molecular weight and high solubility, FA is usually used by foliar spraying in vegetable production, yet the effect on fruit quality and nutrient absorption is still unclear. Here, ‘Jinpengdashuai’ tomatoes were used to investigate the effects of spraying FA on tomato fruit quality and soil Cu and Cd availability under stress of Cu and Cd by pot experiments. The results showed that the 1000 mg·L⁻¹ FA could significantly improve the biomass of tomato plants to some extents under different stresses of Cu and Cd. After spraying FA, the Cu and Cd content in different organs and the whole accumulation decreased; meanwhile, the transport efficiency of Cu and Cd was also reduced to some degree. The drops of FA significantly changed the chemical form of Cu and Cd in 0~10 cm soil, from the easily absorbed water soluble (or ion) form into the insoluble form, hard to absorb. The Cu content in the residual state increased by 93.8% and 172.5%, respectively, under single and compound stress, and the Cd content in the residual state increased by 16.7% and 58.6%. Foliar spraying FA could promote the absorption and transport of essential nutrients such as nitrogen, phosphorus, potassium, calcium, magnesium and zinc, and maintain the nutrient balance, which alleviates the inhibition of normal metabolism by Cu and Cd stress, to a certain extent. More distinctly, Vc, total sugar and lycopene increased by 11.4~45.9%, 19.2~48.5% and 30.9~84.5%, respectively, indicating that foliar spraying FA could improve the appearance and flavor quality of tomato fruits under stress of Cu and Cd.

In a wheat/maize rotation system, nitrogen (N) accounts for a large proportion of basal fertilizer, but soil N loss and the resulting environmental risk simultaneously exist worldwide. This study applied different urease/nitrification inhibitors together with basal fertilizers and investigated their effects on soil N level and grain yield. Six N stabilizing combinations consisted of two urease inhibitors (HQ and NBPT) and three nitrification inhibitors (DCD, DMPP, and Nitrapyrin). The treatments supplied with urease/nitrification inhibitors reduced, to some degree, the conversion rate of ${\mathsf{N}\mathsf{H}}_4^{+}$ into ${\mathsf{N}\mathsf{O}}_3^{-}$, and kept ${\mathsf{N}\mathsf{H}}_4^{+}$ content higher in surface soils for a longer time. Compared to CK, A1 treatment supplied with 1.5% HQ + 4% DCD well-maintained the levels of soil alkali-hydrolyzable N and ${\mathsf{N}\mathsf{H}}_4^{+}$. For example, alkali-hydrolyzable N and ${\mathsf{N}\mathsf{H}}_4^{+}$ contents at 0–20 cm soil layer under A1 were increased by 8.59–41.6% and 8.15–14.5% more than CK, respectively. Based on the entire growth period of wheat and maize rotation, urease/nitrification inhibitors improved soil available N in surface soils but did not prevent ${\mathsf{N}\mathsf{H}}_4^{+}$ and ${\mathsf{N}\mathsf{O}}_3^{-}$ leaching, especially in the intensive rainfall season. The combinations of HQ and DCD or Nitrapyrin significantly enhanced crop yield. Specifically, crop yields under A1 and A3 (1.5% HQ + 0.25% Nitrapyrin) were 16.3% and 14.3% higher than CK, respectively. The N stabilizing combinations also promoted N intake and transport at every growth stage. The maximum N accumulation was increased by 27% under A1, when compared to CK. The treatments supplied with urease/nitrification inhibitors also achieved higher apparent N recovery efficiency, N agronomic efficiency, and N partial factor productivity. Consequently, the combinations of urease/nitrification inhibitors could improve N availability at 0–40 cm soil layer, which in turn improved N use efficiency of wheat and maize. The results suggested that the two urease/nitrification inhibitor combinations, 1.5% HQ + 4% DCD (A1) and 1.5% HQ + 0.25% Nitrapyrin (A3), were optimal N stabilizing agents and worthy of further study.

Objectives: To slow down the chemical fixation of phosphate fertilizer, reduce the risk of active phosphorus leaching, stimulate the inherent phosphorus resource activity of soil, and improve phosphorus supply capacity. Methods: This study utilized a combination of field experiments and indoor chemical analysis. Six types of exogenous organic matter (fulvic acid, biochar, compound microbial fertilizer, high-energy microbial inoculum, pig manure-vermicompost, cow manure-vermicompost) were added based on conventional fertilization. The experiment was conducted under the wheat-maize rotation system in the Huang-Huai-Hai region. Results: Compared with control (CK) without exogenous organic matter (EOM), all the other treatments with EOM had an enhancing effect on the available phosphorus of the cultivated soil. During the maize harvest, the combined application of biochar, pig manure-vermicompost and cow manure-vermicompost treatment significantly increased the content of available phosphorus in 0–20 cm soil by 45.87–56.59% compared with CK. The combined application of fulvic acid, biochar, pig manure-vermicompost and cow manure-vermicompost treatment significantly increased the content of Ca₂-P in 0–20 cm soil by 34.04–65.14%. The content of Ca₁₀-P in each treatment with EOM exhibited a lower level compared to CK. EOM could slow down the fixation of phosphorus to some degree. Correlation analysis revealed significant associations between Ca₂-P, Ca₈-P, Al-P, Fe-P, neutral phosphatase activity, acid phosphatase activity, and the available phosphorus content in the soil. The combined application of fulvic acid, biochar, and cow manure-vermicompost could enhance the activity of neutral and acid phosphatase in topsoil to a certain extent, thereby facilitating the conversion of phosphorus into highly available Ca₂-P. EOM could enhance the soil phosphorus availability and decelerate the conversion of soil phosphorus into O-P and Ca₁₀-P forms with low availability. Among all treatments, biochar exhibited the most pronounced efficiency in mitigating phosphorus leaching downward. Conclusions: All the EOMs had the potential to enhance the conversion of phosphorus into soluble phosphorus (Ca₂-P), thereby mitigating the chemical fixation of soil phosphorus and ameliorating non-point source pollution caused by phosphorus. EOM enhanced the activity of neutral and acid phosphatase, which was beneficial to the conversion of organic phosphorus to inorganic phosphorus and increasing the content of available phosphorus. All EOMs had good effects on the retention of soil effective phosphorus, among which biochar had the best effect on retaining effective phosphorus in the tillage layer and blocking phosphorus leaching downward.

The aim was to improve the fertilizer utilization efficiency and alleviate environmental pollution risk under a wheat-maize rotation system. Here, the combinations of different nitrogen stabilizers and phosphorus activators were used to reduce nitrogen loss and phosphorus fixation in the field experiment. Compared to the control, the combination of 1.5%HQ + 0.5%DMPP + biochar showed the most significant effect on the retention of alkali-hydrolysable nitrogen (Nah), the highest with an increase of 22.6% at the 0~20 cm layer soil; and the combination of 1.5%HQ + 3.5%DCD + CMFs (compound microbial fertilizers) showed the most significant effect on the maintenance of available phosphorus (Pa), with the highest increase of 41.3%. N, P synergists combined with a basal fertilizer could effectively slow down the transformation from ${\mathrm{N}\mathrm{H}}_4^{+}$ to ${\mathrm{N}\mathrm{O}}_3^{-}$, and keep ${\mathrm{N}\mathrm{H}}_4^{+}$ at an increase of 7.38%~19.6%. Moreover, the N, P synergists could efficiently lock the available nutrients around the roots, preventing the migration of ${\mathrm{N}\mathrm{O}}_3^{-}$, ${\mathrm{N}\mathrm{H}}_4^{+}$, Nah, and Pa to the deeper layers. Especially for ${\mathrm{N}\mathrm{O}}_3^{-}$, the total accumulation at 0~60 cm decreased by 32.1%, and the activation of Pa was mainly concentrated at 0~40 cm. Under the same nutrient inputs, the combination of 0.3%NBPT + 0.5%DMPP + CMFs obtained the highest wheat yield. The combination of 1.5%HQ + 0.5%DMPP+ biochar gained the highest maize yield. Overall, the application of N, P synergists could increase the effective duration of Nah, Pa, and ${\mathrm{N}\mathrm{H}}_4^{+}$ in the surface soil, and reduce the accumulation of ${\mathrm{N}\mathrm{O}}_3^{-}$ in the 0~60 cm soil layer. The capacity of holding and keeping nutrients from leaching rose obviously; simultaneously, the assimilative capacity of crops for nitrogen and phosphorus increased distinctly, which could lower the eutrophia risks from nitrogen and phosphorus.