Study On Nutritional Physiology Of Nitrogen In Muskmelon Under Organic Farming System

Over the last decade, organic farming production and organic food consumption developed rapidly all over the world. The supply of nitrogen is of fundamental impact for the production in organic farming and the product quality. In this study, muskmelon nitrogen nutrition physiology was thoroughly analyzed based on the investigation of soil nitrogen transformation. We also carried out further work about the effect of organic and conventional treatments on muskmelon growth, yield and quality formation and N absorption. The findings of the present study may reveal the rule of soil organic nitrogen (ON) mineralization, and clarify the absorption and utilization capacity of ON and inorganic nitrogen by muskmelon, also enrich the theory of plant nitrogen nutrition further. The results can also provide technical support on a scientific basis for reasonable organic fertilizer application for muskmelon and for organic farming production.In this study, laboratory aerobic incubation technique was used to determine the dynamics of soils N mineralization in organic and conventional production systems. Potted and hydroponics cultures were used to investigate the absorption and utilization capacity of ON and inorganic nitrogen by muskmelon. The effect of different fertilization level on muskmelon growth, yield and quality formation along with N uptake in both organic and conventional production systems were also studied. The main results were indicated as follows:1 Analysis of soil nitrogen in organic and conventional production systemsSoil nitrogen characteristics in two organic farms in Shanghai were analyzed. The results showed that organic production management changed soil nitrogen characteristics, and in the situation of same or a little higher level of total N, plant available N in organic system reduced by 22.4%³6.8%, compared with conventional system.Dynamics of N mineralization in soils from organic, transitional and conventional production systems were studied. Mineralization of extra ¹⁴C-labled amino acids and peptides affected by different kinds of soils was also investigated. The results showed that nitrate and ammonium of soils in different production systems increased distinctly during the incubation period, and the increase extent of nitrate was much higher than ammonium. The content of free amino acid in soils from different production systems first increased and then decreased. Soluble organic N (SON) of different soils increased gradually during the incubation period, and the amount of SON showed organic soil (OS) > conventional soil (CS) > transitional soil (TS). The ratio of SON to soluble inorganic soil (SIN) decreased clearly, and it showed OS > TS≈CS. Mineralization potential, mineralization rate and ammonification rate in three different soils showed OS > CS > TS, while there was no difference in nitrification rate. The effect on mineralization of extra ¹⁴C-labled amino acids and peptides by different kinds of soils showed OS > TS > CS, and after 24 hours incubation, more than 50 percent of ¹⁴C-labled amino acids and peptides were absorbed by microorganism. Turnover rate of low molecular weight SON (LMW-SON) in soil was fast, and after 24 hours incubation the decompose rate of ¹⁴C-labled amino acids in soil took up 56.3%⁹5.4% compared with the initial amount added to the soil. The residue of ¹⁴C-labled amino acids in different kinds soils showed CS > TS > OS. When evaluate the characteristics of N mineralization in soil, both SIN and SON should be considered. During the incubation period, the bottleneck which affected the content of SON was not the absorption rate of LMW-SON by microorganism, but the rate of SON released from soil by organism.Under pot experiment conditions, soil N mineralization and its effect on muskmelon growth and N uptake were studied, with organic N (ON) and inorganic N (IN) treatments along with blank N (CK) as the contrast. The results showed that, SIN and SON content in the soil without plant indicated single peak curve. SIN content was significantly higher for IN treatment than for ON and CK treatments. The SON/SIN ratio of CK and ON treatments was 0.34⁰.59, which was higher than 0.25⁰.35 of IN treatment. The SON and SIN content in soil with muskmelon seedlings decreased steadily during incubation, and the ratio of SON/SIN showed rising tendency. Fertilization boosted muskmelon plant growth, N content and N uptake amount, and IN treatment was remarkable significant compared with the other two treatments. Though the total soluble N in the xylem sap per time showed IN > ON > CK, plants in CK and ON treatments took up higher ratio of SON from soil. SON amount mineralized from soil was high and it could be taken up by muskmelon plant directly. Although muskmelon mainly took up SIN it also took up SON as a supplement when SIN content in soil was low.2 Uptake of different forms of N by muskmelon——analysis on nutritional physiology of nitrogen in muskmelonThe effect of inorganic nitrogen (NO₃^--N, NH₄⁺-N) and amino acid nitrogen (Gly-N) on plant growth and nitrogen accumulation of muskmelon seedling were studied in hydroponic cultivation. Results showed that the effect of NO₃^--N, NH₄⁺-N and Gly-N were significant on plant growth, root morphology and nitrogen accumulation of muskmelon seedling. Compared with NO₃^--N, NH₄⁺-N and Gly-N significantly inhibited the growth of root and aerial parts of the seedling. The treatment of different types of nitrogen affected root length, volume and surface area as the following tendency: NO₃^--N > Gly-N > NH₄⁺-N (p < 0.05). Also the chlorophyll content, average nitrogen content and nitrogen accumulation amount showed the same tendency. Compared with the NO₃^--N treatment, NH₄⁺-N and Gly-N treatments increased nitrogen partitioning to muskmelon root. NH₄⁺-N treatment decreased the pH of nutrient solution significantly, and Gly-N treatment increased the pH of nutrient solution, while in the NO₃^--N treatment, it remained the same as the initial. The changes of pH value in nutrient solution for different types of nitrogen treatments affected plant growth and nitrogen accumulation of muskmelon seedling significantly as was proven by the experiment. Though muskmelon prefer to uptake NO₃^--N, amino acid nitrogen also could be a good nitrogen source.Uptake kinetics of different forms of nitrogen (NO₃^--N, NH₄⁺-N and Gly-N) by muskmelon, also the effect of NH₄⁺ compared to NO₃^- uptake, were studied under aseptic hydroponic conditions. The results indicated that uptake kinetics of different forms of nitrogen by muskmelon accorded with the Michaelis-Menten equation. Both the maximum uptake rate (Vmax) and the affinity (1/Km) followed the tendency NO₃^- > NH₄⁺ > Gly. NH₄⁺ boost the uptake ability of NO₃^- by muskmelon, by enhancing both the uptake rate and the affinity with the uptake site.3 Uptake and utilization of N by muskmelon in organic production and the formation of yield and fruit qualityThe transformation of available N in soil and its uptake by muskmelon were analyzed in both organic and conventional farming systems. The results indicated that the organic fertilizer mineralized the same level of available N to the soil during muskmelon plant development, compared with conventional treatment. Though N content in plant tissue reduced gradually during muskmelon plant development, total accumulated N amount increased gradually. The conventional treatments were significantly higher than the organic on N content and its accumulation amount in plant, though there was no significant difference caused by fertilization level within each production system. N uptake rate of conventional treatments was much higher than that of organic, also nitrate N accumulated in muskmelon plant was higher, but their N utilization efficiency was lower. In both organic and conventional farming systems, muskmelon mainly took up inorganic N, and the protein N was significantly higher than amino acid N, also muskmelon could took a higher ration of organic N in organic system.Effects of fertilizer level on plant development, yield and quality formation of muskmelon in spring and autumn were studied under organic and conventional production systems. The results indicated that there were no significant difference in plant biomass and yield in the two farming systems, and fertilization level within each farming system had no significant effect. Plant biomass and fruit yield in spring were larger than in autumn, due to higher growing degree days. Dry mass partitioning of muskmelon mainly went into leaf in stem growth period and to fruit in later period. Production system and fertilizer level had no effect on the rate of dry mass partitioning in different muskmelon organs.Both organic and conventional farming production systems and fertilizer amount did not change the tendency of fruit growth and sugar accumulation. The fruit mainly accumulated glucose and fructose before entering mature stage and later sucrose accumulated remarkably, thus total sugar content rose continuously. Both the production system and fertilization amount had no effects on fruit TSS and sugar content. Fruit vitamin C content was significantly increased in organic farming system than that of conventional, by 16% in spring and 21% in autumn on average, respectively. Fruit nitrate content was significantly decreased in organic farming system compared with that of conventional system, by 12% in spring and 16% in autumn on average, respectively, and this was related to lower nitrogen content in fruit pulp. All this indicated that fruit growth of muskmelon was not affected by organic and conventional farming systems, and organic fruit quality was increased in a certain extent, but it was not affected by fertilization amount.From all above, we can draw conclusions that the SON content was increased during soil aerobic incubation, and the bottleneck which affected the content of SON was the rate of SON released from soil organism. Uptake kinetics of Gly-N and inorganic N by muskmelon accorded with Michaelis-Menten equation. Compared with NH₄⁺-N, Gly-N also could be a good nitrogen source. Though muskmelon mainly took up SIN, it also could uptake SON as a supplement when SIN content in soil was low. N uptake rate of muskmelon in conventional system was much higher than that of organic, but their N utilization efficiency was lower. Muskmelon grown in organic system gained the same plant biomass and yield as in conventional system, and increased fruit quality to a certain extent. The results of the study claryfied the mineralization of organic N in soil and the nutritional physiology of N in muskmelon. It also clarified the potential ability of SON uptake by muskmelon. All these findings enriched the theory of plant N nutrition, and provided the scientific base of reasonable fertilization for organic farming.