NITROUS-OXIDE FLUXES FROM MANURE-AMENDED SOIL UNDER MAIZE

Nitrous oxide (N2O) emissions by agricultural soils are influenced by farming practices. The application of manure to cultivated land modifi es soil microbial activity by supplying additional quantities of C and N and changing soil physical and chemical properties. Nitrous oxide f luxes at the surface of a soil under maize (Zea mays L.) amended with dairy cattle manure were measured from April to October 1993 using clo sed chambers. The manure application rates were 0, 56, and 112 Mg ha(- 1) corresponding to 0, 170, and 339 kg ha(-1) of total N, respectively . Nitrate and NH4+ were measured in soil samples obtained at the same time that gas flux measurements were made. Nitrous oxide concentration s in the soil profile were quantified by sampling soil air at depths o f 5 and 15 cm using stationary air probes. On the manured plots 67% of the total N2O emitted during the growing season occurred during the f irst 7 wk following manure application. Fluxes of N2O occurred in epis odes with maxims that ranged from 0.070 mg m(-2) h(-1) on the soil wit hout manure amendment to 0.171 and 0.494 mg m(-2) h(-1) on sons that h ad received the low and high rates of manure, respectively. These high fluxes coincided with periods when NO3-N levels and soil water conten ts were relatively high. Fluxes were highest the first day after manur e application and returned to near preapplication levels 7 d later. Th is episode was followed by short-lived peaks of N2O flux that usually followed periods of rain. The absence of a strong correlation between N2O concentration in the soil profile and N2O flux at the soil surface suggested that high soil water content restricted gaseous diffusion i n the soil and/or that a variable part of the N2O produced via denitri fication was reduced to N-2. The accumulated N2O-N emissions at the so il surface of the manured plots over the snow-free season amounted to 1% of the manure N that was potentially mineralizable during the exper iment.