Soil Nitrification Characteristics And N 2 O And NO Emission Patterns Of Typical Tea Gardens In China

China is a traditional tea growing country. The total area of tea in China accounted for about 70.2% of the world’s tea plantation area in 2015. The tea plant is a cash crop which prefers acid and aluminum. It usually grows in the soil of pH 3.0～6.8, and the most suitable pH is 4.5～5.5. It prefers ammonium nitrogen rather than nitrate nitrogen. Soil acidification aggravates with an increase in stand age. As a result, nitrogen（N） cycling characteristics in tea ecosystem may be different from those in other ecosystems. However, the current research on soil N cycling mainly focuses on four major food crops, especially rice, wheat and maize, and few studies have investigated the mechanism of soil N turnover in tea plantation. Tea plant prefers soil ammonium nitrogen, so nitrification process is not conducive to the microbial use of N in tea soils, but can increase N losses to environment.Therefore, the study of soil nitrification characteristics of tea garden will help us to control the soil nitrification and acidification in tea garden. In this study, the typical tea garden soil in China was selected and the objective were to explore the effects of the conversion of forest to tea plantation on the net nitrification rate and N₂O emission, exploring the pathways of nitrification, clarifying the effects of N fertilizer on soil net nitrification rate and gas emission and to investigate the effects of biochar on soil net nitrification rate and the gas emission in the tea plantation. The results will provide theoretical basis and technical support for the rational use of N fertilization, the sustainable development and the control of soil nitrification and acidification in tea plantation.At present, there are many studies focus on the conversion of forest land into rice fields such as paddy fields and maize fields. However, there are few studies on the nitrogen cycling of tea crops.This study focuses on its effects on soil nitrification and N₂O emissions when forest land transforms into tea plantation. An incubation experiment at 25 ℃ and 60% water holding capacity（WHC） was carried out to investigate soil net nitrification rate and N₂O emission in three typical tea plantations（Anhui, Zhejiang, Fujian） and their respective adjacent woodlands in China. The results indicated that conversion of woodland to tea plantation significantly decreased soil net nitrification rates in Anhui Province, whereas soil net nitrification rates in tea plantations and their respective adjacent woodlands in Zhejiang and Fujian Province were very low （<0.2 mg N/（kg·d））, and were not significantly different between woodlands and tea plantations. In Anhui Province, soil cumulative N₂O emissions over the 28-day incubation in tea plantations with more than 10 years of plantation ages were significantly lower than their respective adjacent woodlands while there was no difference in soil cumulative N₂O emission between tea plantation with 10 years of plantation ages and its respective adjacent woodland. In Zhejiang and Fujian Provinces, soil cumulative N₂O emissions over the 28-d incubation in tea plantations were higher than those in their respective adjacent woodlands. Soil cumulative N₂O emissions was significantly positively correlated with pH value in Anhui Province.This result indicated that the conversion of woodland to tea plantation decreased soil pH, and thus decreased soil net nitrification rate and N₂O emission in Anhui Province.The nitrification process is divided into autotrophic nitrification and heterotrophic nitrification.Tea soil is often acidic, which is prone to the presence of heterotrophic nitrification, and simulataneously long-term fertilization application can stimulate autotrophic nitrification. However,it is not clear whether nitrification is mainly from heterotrophic nitrification or autotrophic nitrification and the relative contribution. In order to distinguish the autotrophic nitrification and heterotrophic nitrification of tea soils, we carried out a 4-d 15N tracing incubation experiment at 25℃and 60% water holding capacity with and without acetylene. The results showed that the relative contribution of autotrophic nitrification and heterotrophic nitrification was 100% and 0, respectively,indicating that nitrification in Anhui tea garden soil is all from autotrophic nitrification. The conversion of woodland to tea plantation and the increase of plantation ages did not change the pathways of nitrification. The gross nitrification rates of AH10, AH20, AH30 and AH40 were 0.56,0.58, 0.58 and 0.55 mg N/（kg·d）, respectively, which were not significantly different from that of AHCK （0.76 mg N/（kg·d））. The gross nitrification rate of AH60 was 1.67 mg N/（kg·d）, 1.2 times higher than that of AHCK. There was no significant correlation between soil gross nitrification rate and pH value.An incubation experiment at 25 ℃ and 60% water holding capacity was carried out to investigate soil net nitrification rate, NO and N₂O emission in response to different N fertilizers （ammonium bicarbonate,urea and ammonium sulfate） in tea soils. The results showed that the application of ammonium bicarbonate and urea significantly increased soil pH and net nitrification in tea soils, while the application of ammonium sulfate significantly reduced soil pH and net nitrification. The net nitrification rates of tea soil were positively correlated with soil pH values on day 2, which indicated that N fertilizer affected soil nitrification by changing the pH value of tea soil. Ammonium bicarbonate and ammonium sulfate addition had no significant effect on soil N₂O emission. And the application ofurea with theN application rate of200 mgN/kg significantiy stimulated the cumulative N₂O emissions in tea soil. During the whole incubation, the cumulative NO emissions of ammonium sulfate （HS） with the N application rate of 200 mg N/kg treatment were the lowest, at 102.8 jig N/kg,compared with CK treatment （116.1 μg N/kg）. There was no significant difference in cumulative NO emissions among all treatments （106.6 and 126.0 μg N/kg）. The correlation analysis showed that the cumulative NO emissions in tea soil were positively correlated with the pH values and net nitrification rates on day 2.At present, soil acidification has become increasingly serious in tea plantations, and the application of biochar into tea soil can may alleviate soil acidification. Few studies have investigated the effects of biochar on soil nitrification, N₂O and NO emissions in tea plantation. To investigate the effects of biochar on soil nitrification, N₂O and NO emissions in tea plantation, we carry out an incubation experiment at 25 ℃ and 60% water holding capacity with different biochar treatments（1%BC, 2%BC, 5%BC, 10%BC, 10%BC + 0.27%CP）. The results showed that the soil pH was significantly increased after application of biochar in Yixing tea soil. The application of 1% BC,2%BC, 5%BC and 10%BC increased the soil pH in the tea garden by 0.28, 0.35, 0.68 and 0.84 pH units,respectively. The net nitrification rate, N₂O and NO emissions were affected to varying degrees.There was no significant difference in net nitrification rates among 1%BC, 2%BC and 5% BC treatments, but 10%BC significantly inhibited the net nitrification rate. The addition of 0.27% CP on the basis of 10%BC further inhibited the net nitrification rate with the net nitrification rate only 0.29 mg N/（kg·d）. The application of 1%BC and 2%BC had no effect on the cumulative N₂O emissions in tea soil, while the application of 5%BC and 10%BC significantly stimulated cumulative N₂O emissions in tea garden soil. In the 10%BC treatment, adding 0.27%CP significantly inhibited the cumulative N₂O emissions. The application of biochar had no effect on cumulative NO emissions in tea soil. The ratios of cumulative NO emissions and cumulative N₂O emissions were greater than 1.