Interactive effects of moisture content and bulk density of soils and addition of dried swine feeds on denitrification in upland soils

The effects of the moisture content and bulk density of soils and the addition of dried swine feces on denitrification in upland soils were investigated in a laboratory experiment using an acetylene inhibition technique. Low-humic andsols (LHA) or gray lowland soils (GLS) were used in the experiment with or without adding dried swine feces at four levels of moisture content. In addition, soil columns were prepared by packing each soil so that the bulk density of them was adjusted to four different levels. Denitrification rates of the soil columns prepared were determined at 25 deg C during 4 days. We also determined the in situ denitrification rate of two upland soils (LHA, GLS) receiving dried swine feces during the growing season of maize by the combination of an intact core method and an acetylene inhibition technique. The intact cores were taken separately from three layers (0-10 cm, l0-20 cm, 20-30 cm) on 8 July and 28 July, and incubated for 24 h under in situ conditions. On 8 July, denitrifying enzyme activity was determined for the sieved soils collected from the three different layers. The results obtained were as follows. l) In the laboratory experiment, denitrification rates increased with the increase in moisture content and/or bulk density of the soils. There were distinct boundary levels where denitrification rates were rapidly accelerated within the small increase of moisture content or bulk density of the soils. The levels were lowered by the addition of dried swine feces. 2) There was a negative correlation between the ratio of the gas phase and the logarithm of the denitrification rate if the soils were divided into 4 groups by the soil type (LHA or GLS) and organic materials (application or non-application). 3) The in situ denitrification rate of the soils on 8 July increased remarkably with the application of dried swine feces irrespective of soil type. The rates in the second layer (10-20 cm of the soil column) were the highest of all the layers, indicating that the effect of the bulk density of soils was much more significant than that of the organic content of soils in the experiment. 4) There was a positive correlation between denitrifying enzyme activities and CO2 emission rates from the soils. 5) The procedure to estimate the denitrification rate in upland soils where organic materials were applied was discussed. It should be emphasized that the relationship between the ratio of the gas phase and the denitrification rate obtained in this study would be a promising tool for the assessment of denitrification in soils.