Soil Nitrous Oxide Emission As Affected By Application Of Phosphorus

Phosphorus（P） is one of the most essential elements for living organisms on the planet, and is considered as one of the macronutrients for plant production. Most of the agricultural soils all over the world are deficient of phosphorus with a concentration of 0.05 mg g-1 in soil solution. Phosphorus deficiency in soil causes the reduction in crop growth and productivity. Phosphorus application also triggers the biogeochemical cycle by decomposition of organic matter, mineralization of nitrogen（N）, and influencing the growth and activities of microorganisms in the soil. Application of P to soils can substantially influence N₂ O emission through turnover of C and N. However, the effects of P application on N₂ O emissions are not well understood so far. Therefore, using phosphorous deficient soils, both pot as well as incubation experiments were conducted to investigate phosphorous effects on N₂ O emissions.Different levels of phosphorus（0, 20, 40 and 50 mg kg-1） were applied to air dried soil incubated under the control temperature at 25 °C. Nitrogen（N） as urea fertilizer was also applied in different doses（0 and 200 mg kg-1） to P treated soil under different moisture levels as water filled pore space（WFPS） of 60% and 90%. Moreover, biochar（0 and 300g/pot） alone as well as in combination of P was applied to different cultivars of rice（Oryza sativa L.） i.e. IR-64 and Huanghuazhan（HHZ）. Gas samples were taken at different time intervals and analyzed using a gas chromatograph for determination of N₂ O emissions. Soil samples were also obtained to analyze soil variables such as available P, dissolved organic carbon（DOC） and mineral N（NH4+-N and NO₃^--N）.The DOC contents were measured using a C/N elemental analyzer（Vario Max CAN; Elementar Analysensystme Gmb H, Hanau, Germany）. Soil ammonium（NH4+-N） and nitrate（NO₃^--N） concentrations were determined using an ultraviolet spectrophotometer. Soil p H was measured using a p H meter（p H electrode PB-10; Sartorius AG, Goettingen Germany）. Available P concentration was assessed using an ultraviolet spectrophotometer. Nitrous oxide concentrations were measured using a gas chromatograph（7890-A: Agilent Technologies Santa Clara, CA, USA）. The main results and conclusions of our several studies are following.Addition of P into soil significantly increased available phosphorus concentrations. Highest available P concentrations were measured at the initial stage of incubation, after day 20 concentrations gradually decreased until end of the study. The available phosphorus concentrations in different levels of P treatments ranged from 10-140 mg kg-1. Addition of different phosphatic fertilizes TSP, KH2PO4, MAP and Na2H2 PO were increased available phosphorus concentration in the soil. Furthermore, biochar treated soils have higher available P concentration as compared with CK. Soil moisture also increased available phosphorus contents and highest contents were observed at 90% WFPS, as compared with 60% WFPS.Highest ammonium（NH4+-N） concentrations were noted at day 10, and afterwards decreased gradually until the end of incubation at 50 days. Application of triple super phosphate（TSP） fertilizer increased NH4+-N concentrations. Application of biochar and N fertilizer also increased NH4+-N concentrations with increasing soil moisture.Nitrate（NO₃^--N） concentrations were continuously increased after commencement of the study and highest NO₃^-N concentrations were shown at the end of experiment due to NH4+-N oxidation converted to NO₃^--N. Incorporation of TSP fertilizer increased NO₃^--N concentrations during whole incubation period. The application of biochar and N fertilizers increased NO₃^-N concentrations with increasing soil moisture contents and higher concentrations were at 90% WFPS as compared to 60% WFPS. Application of different levels of P increased DOC contents and highest DOC in the soil was found at the high P levels as compared to low levels. Highest DOC contents were at the early stage of study, while gradually decreased until end of the study. The addition of biochar and N fertilizers increased DOC contents and higher contents were found at 90% WFPS as compared to 60% WFPS.Highest soil N₂ O emissions were at the beginning of the study and slowly decreased to ground level at end of the incubation. Addition of different levels of P increased N₂ O emissions and highest emission（0.12 ug kg-1h-1） at day 15 at higher phosphorus levels as compared to low level. Soil N₂ O emissions were increased by addition of different phosphatic fertilizers and higher emissions（0.10 ug kg-1 h-1） in the TSP treated soil. Application of N alone or combined with P treatments increased N₂ O emissions and high emissions were quantified at 90% WFPS as compared to 60% WFPS, because high moisture provides most conducive condition for N₂ O emission. Soil N₂ O emission was increased in HHZ rice cultivar because of higher N uptake than IR64.The results of incubation studies and pot experiment under control condition showed that the application of P at higher doses restore more P in soil. The high amount of P increases nitrifier and denitrifiers activities, which increased soil N₂ O emissions. The application of biochar increased readily available carbon in the soil, which acted as a substrate for microbial growth and metabolism, and hence stimulated higher N₂ O emission. Biochar application to P deficient soil has synergistic effect with P and improves P content in soil. Our findings suggest that application of P in the soil increases N₂O emission.