Effects Of Ferrihydrite On Nitrification In Acid Soils

Nitrification is the key N transformation process in soil which may lead to N losing and environmental problems. Soil nitrification is a typical biological process and is highly sensitive to soil pH. Increasing evidence indicates the existent of nitrification in acid soil. Great variations of nitrification in acid soils has been reported, but the mechanisms are not entirely clear. Differences in the occurrence of nitrification cannot be solely explained by abiotic factors such as pH and N availability and biotic factors including the composition of the soil bacterial community. But that still uncaptured factors contributed to suppression of nitrification. Fe is rich in the subtropical acid soil and transformed easily. Recently, we found that nitrification in acid soil was significantly affected by soil Fe oxides. The co-existence of biological-chemical processes influenced by Fe oxides may explain the variations of nitrification in subtropical acid soils.Researches on different amount of iron oxide and different water conditions were carried out to investigate the nitrification of acid soils. The ferrihydrite/hydrous ferric oxide (HFO) which was a weak crystalline iron oxide and widely dispersed in the environment was selected as experimental material. The purple and yellow soils were collected from Yongchuan and Beibei, Chongqing. The red soil was collected from Yingtan, Jiangxi. The soil basic physical and chemical properties were analyzed and nitrification potential was assayed. According to the result, the red soil contained high Fe oxide levels and acidic fluvo-aquic soil were selected as the treatments with neutral fluvo-aquic soil as control. The fresh soils were collected and removed impurities, passed through 2 mm sieve and kept in a 4℃ refrigerator until use. The HFO was prepared by artificial synthesis and adjusted to the selected soil pH before adding to the soil. The experiments were conducted as follows:(1) different amount of HFO (0%, 0.5%,1%,3%,5%,10% HFO) was added to soil with 6 mmol N kg-1 dry soil under 60% WHC (water holding capacity) and 28℃ condition to investigate the nitrification process; (2) Different amount of HFO (0%,0.5%,1%,3%,5%,10% HFO) was added to soil with 6 mmol N kg-1 dry soil KNO3 under 60%WHC and 28℃ condition to investigate the nitrification process; (3) The effect of different water content of soil (40%,80%,120% and 200% WHC) on acidic fluvo-aquic soil with 3% HFO during the 14-day 28℃ incubation. The soil NO3--N, NH4+-N, mineral N and pH were measured. The main results were as follows:(1) Effects of HFO on nitrification in red soilIn the incubation with (NH4)2SO4 as N source, the soil NO3--N concentrations decreased with incubation time. The net nitrification rates were-0.27±0.02 to-0.43± 0.01 mg N kg-1 d-1. In the incubation with KNO3 application, the dynamic changes of NO3--N and NH4+-N concentrations did not cooperated with ferrihydrite gradient. Nitrification has not occurred in the red soil and the addition of HFO have no effect on the nitrification in red soil.(2) Effects of ferrihydrite on nitrification in fluvo-aquic soilIn the incubation with (NH4)2SO4 as N source, the net nitrification rates had no significant difference between 0% and 0.5% HFO (2.78±0.05,2.74±0.09 mg N kg-1 d-1 for 0% and 0.5% HFO, respectively; p>0.05) in acidic fluvo-aquic soil. The net nitrification rate was significantly reduced when 1% HFO was added into the soil (2.46 ±0.11 mg N kg-1 d-1). And, the soil net nitrification rate was reduced by 37% when the HFO level increased from 1% to 3%. In neutral fluvo-aquic soil, the net nitrification rate was 5.62±0.05 mg N kg-1 d-1 for 0%HFO, it was significantly higher than other treatments. The net nitrification rates were inhibited when HFO was added in both acidic and neutral fluvo-aquic soil. The higher the HFO level was added, the larger the net nitrification was inhibited. The dynamic changes of NH4+-N concentrations were different for acidic and neutral fluvo-aquic soil when HFO was added. In acidic fluvo-aquic soil, NH4+-N concentrations decreased after HFO addition. However, the NH4+-N concentrations increased in the neutral fluvo-aquic soil on the contrary.For the neutral fluvo-aquic soil, the nitrifers was poisoned by HFO so the oxidation of ammonia was inhibited and the N was kept in the form of NH4+-N. As a result, a reduction of net nitrification was observed.In acidic fluvo-aquic soil, the poison to nitrifers cannot explain the reduction of net nitrification solely. Possible mechanisms for these reactions could be that the NH4+-N was oxidized into gases through Feammox process or Fe oxide stimulated the NO3--N immobilization by which promoted the abiotic formation of nitrogenous polymers,In the incubation with KNO3 as N source, the NO3--N concentration decreased 7.22-10.80 mg N kg-1 in acidic fluvo-aquic soil. The difference was significant only when the addition of HFO was higher than 3%.(3) In acidic fluvo-aquic soil, the nitrification pattern of different water content were changed after HFO addition. Without HFO, the net nitrification rate increased first, then decreased along with the increase of soil water content. The net nitrification rate was minimal under 200%WHC. After 3% HFO was added, soil net nitrification rate increased along with the soil water content, and the maximum net nitrification rate was observed in 200% WHC. The soil O2 was restricted along with the increase of soil water content to promote the formation of anaerobic conditions. HFO promoted the ammonia oxidation to nitrates in anaerobic conditions.This research investigated the effect of HFO to nitrification of acidic fluvo-aquic, neutral fluvo-aquic and red soil and the effect of HFO to different water content of acidic fluvo-aquic soil. The main conclusions were as follows:(1) no nitrification was observed in red soil, and the addition of HFO has no effect on red soil. (2) HFO inhibited the net nitrification of both acidic and neutral fluvo-aquic soil. The higher the HFO was added, the larger the net nitrification was inhibited. While, the inhibitory mechanisms may be different. (3) In acidic fluvo-aquic soil, the nitrification patterns of different water content were changed after HFO addition. HFO promoted the ammonia oxidation to nitrates in anaerobic conditions. Our results showed that the co-existence of biological-chemical processes influenced by Fe oxides were important in N transformation not only in the subtropical flooded soils but also upland soils.