Optimization of Nitrogen Demand in Vegetables by Different Impacts on Autotrophic and Heterotrophic Nitrification

Aims The understanding of the interactions between N transformations and N uptake by plants in greenhouse soils with large N accumulation is still not clear. The aim is to understand the plant- soil interactions (vegetables) on N transformations with respect to N supply. Methods 15N tracing studies were conducted in two greenhouse soils to simultaneously quantify soil gross N transformation and plant N uptake rates using the Ntraceplant tool. Results There were significant feedbacks between vegetable N uptake and soil gross N transformation rates, whether soil N accumulation occurred or not. Plant NO3– uptake rates (UNO3) were higher than the NH4+ uptake rates (UNH4), which is consistent with the NO3–-preference of the vegetable plants studied. While UNH4 was still responsible for 6-49% of total N uptake rates, significantly negative relationships between UNH4 and NH4+ immobilization rate and autotrophic nitrification rate (ONH4) were observed. ONH4 was significantly inhibited in the presence of plants and decreased with time. ONH4 (1.11 mg N kg-1 d-1) was much lower than UNO3 (8.29 mg N kg-1 d-1) in the presence of plants. However, heterotrophic nitrification rate (ONrec), which ranged from 0.10 to 8.11 mg N kg-1 d-1 was significantly stimulated and was responsible for 5-97% of NO3– production in all plant treatments, providing additional NO3– to meet N requirements of plants and microorganisms.Conclusions The management of organic N fertilizers should be improved to stimulate inorganic N production via heterotrophic nitrification in greenhouse cultivation.

Effects of Changing Temperature on Gross N Transformation Rates in Acidic Subtropical Forest Soils

Soil temperature change caused by global warming could affect microbial-mediated soil nitrogen (N) transformations. Gross N transformation rates can provide process-based information about abiotic–biotic relationships, but most previous studies have focused on net rates. This study aimed to investigate the responses of gross rates of soil N transformation to temperature change in a subtropical acidic coniferous forest soil. A 15N tracing experiment with a temperature gradient was carried out. The results showed that gross mineralization rate of the labile organic N pool significantly increased with increasing temperature from 5 °C to 45 °C, yet the mineralization rate of the recalcitrant organic N pool showed a smaller response. An exponential response function described well the relationship between the gross rates of total N mineralization and temperature. Compared with N mineralization, the functional relationship between gross NH4+ immobilization and temperature was not so distinct, resulting in an overall significant increase in net N mineralization at higher temperatures. Heterotrophic nitrification rates increased from 5 °C to 25 °C but declined at higher temperatures. By contrast, the rate of autotrophic nitrification was very low, responding only slightly to the range of temperature change in the most temperature treatments, except for that at 35 °C to 45 °C, when autotrophic nitrification rates were found to be significantly increased. Higher rates of NO3− immobilization than gross nitrification rates resulted in negative net nitrification rates that decreased with increasing temperature. Our results suggested that, with higher temperature, the availability of soil N produced from N mineralization would significantly increase, potentially promoting plant growth and stimulating microbial activity, and that the increased NO3− retention capacity may reduce the risk of leaching and denitrification losses in this studied subtropical acidic forest.

Effects of pH and mineralisation on nitrification in a subtropical acid forest soil

Effects of pH on nitrification have been explored widely; however, few researchers have investigated the compound effects of pH and mineralisation on nitrification and responsible nitrifiers in subtropical forest soils. An acid subtropical forest soil was subjected to pH treatments by liming for 40 days at 25°C and 60% water holding capacity. After 40 days, gross N transformation rates in the samples were evaluated using the 15N tracing technique and model. Ammonia-oxidising bacteria (AOB) and ammonia-oxidising archaea (AOA) were quantified by quantitative polymerase chain reaction. The results showed that gross rates of mineralisation (P < 0.01), NH4+ oxidation to NO3− (P < 0.01), and dissimilatory NO3− reduction to ammonium (DNRA, P < 0.05) significantly increased with elevated soil pH. The NH4+ oxidation to NO3− rates and N mineralisation rates were significantly correlated (P < 0.05). The abundance of AOB (P < 0.05) but not AOA amoA genes dramatically increased in the elevated pH treatments after 40 days. We suggest that the N mineralisation was sensitive to elevated pH in the subtropical acid forest soil, which increased the supply rate of nitrification substrate and the favourable pH microsites for nitrifiers. Interactions between elevated pH and mineralisation have a synergistic and stimulatory effect on development of AOB and their associated nitrifying activity.