ACKNOWLEDGMENTS

The authors gratefully acknowledge additional financial support for this research from the National Science Foundation - RANN (Grant No. G134733X), the Kearney Foundation of Soil Science, and the University of California Agricultural Experiment Station. The laboratories and mass spectrometer facilities of the Department of Land, Air and Water Resources provided strong support for this research.

The authors gratefully acknowledge the able assistance, ingenuity, and dedication of the three technicians primarily responsible for the establishment, operation, and analytical procedures of this research, David L. Hoffman, David A. Goldhamer, and Dianne W. Toy. Several students including Paul Brooks, Nora H. Monette, and Brian D. Brown assisted in analyses, and their help is greatly appreciated. The assistance of Tyler Nakashima and James Burgess, in mass spectrometer maintenance is greatly appreciated. The use of the gas chromatograph under the control of Professor C. C. Delwiche was indispensable and is gratefully acknowledged.

SECTION 1

INTRODUCTION

The amount of NO

NO3

reaching the ground water of irrigated lands is dependent upon each of the components of the N cycle in soils. The amount of fertilizer N applied and crop uptake of N are easily measured parameters of the total balance. The other components of the N balance, however, are not as easily measured or determined. The leaching component of the N balance has been a subject of much study and is the primary concern in terms of in ground water. The natural spatial variability of the leaching component has been demonstrated to be large (Biggar and Nielsen, 1976). The other components such as residual soil N, denitrification, and NH3 volatilization losses are also not easily measured. The residual soil N is difficult to measure due to the large organic N pool of most soils and complicated by the natural spatial variability of that pool (Rolston, 1977). The transient processes of immobilization and mineralization within the large organic N pool also contribute to the complexity of the measurement. Volatilization loss of fertilizer as NH occurs to varying degrees in calcareous soils. This component can generally be minimized by incorporating NH fertilizer below the soil surface. Another loss of N from the soil system for which absolute amounts and rates are not well known is denitrification. Volatile denitrification products, primarily N20 and N2, are evolved whenever anoxic sites develop within the soil and when sufficient carbon as supplied by soil organic matter, plant materials, and manure is available.

Simulation models of the N balance in soil systems attempt to predict the amount and concentration of NO in irrigation return flow water as a function of irrigation and cropping practices (Mehran and Tanji, 1974; Donigian and Crawford, 1976; Shaffer et al., 1976; Tanji and Gupta, 1977; and van Veen, 1977). In general, the denitrification component of the various mathematical models has not had adequate input data especially for the rates of denitrification. Total denitrification of applied fertilizer is used quite frequently such as 10-15% of the fertilizer N applied (Fried et al., 1976).

Very few experiments have evaluated the absolute amounts and rates of denitrification in the field. Rolston et al. (1976) demonstrated that the volatile gases from denitrification could be measured in a field profile. Total denitrification from gas fluxes compared reasonably with denitrification determined by difference for a small, intensely-instrumented, field plot. Total denitrification was determined by integrating with time the flux of the gaseous denitrification products as determined from measured soil gaseous diffusion coefficients and concentration gradients. However, these studies only evaluated the amount of denitrification under one cropping or carbon

input system and one soil-water content near saturation.

The objectives of the research reported here were:

a. To measure denitrification in a field soil directly from the fluxes of N2 and N20 at the soil surface.

b.

c.

To compare denitrification obtained from N2 and N20 gas fluxes with denitrification obtained by difference.

To measure the amount of denitrification from an applied inorganic fertilizer source as affected by an actively growing crop or manure amendment, soil-water content, and temperature of a field soil.

The research was conducted on small 1-m2 field plots because of the large 15 cost of NO fertilizer tagged with high enrichments of the stable isotope 1N. The use of 15N tagged NO is presently the only positive means of measuring N2 gas resulting from denitrification in the field. The experiments were conducted at a high and low soil temperature, at two water contents near saturation, and at three levels of carbon input (cropped, uncropped, and manure).

2

SECTION 2

CONCLUSIONS

The results of this research demonstrate that denitrification can occur at very large rates with the largest rates occurring soon after application of NO fertilizer to soil maintained constantly wet. The rate of denitrification generally decreases rapidly after the initially large rate due to a decrease in NO concentration and to the movement of the NO3 pulse out of the zone of high carbon content and low oxygen concentrations. For soils in which the profile development is such that large bulk densities occur near the surface, denitrification is limited to the top 30 to 60 cm of the soil profile inasmuch as that is the zone where water content is greatest, soilair content is smallest, and biological activity is greatest due to generally high carbon values. Thus, denitrification would be greatest in the surface zone. This is not an uncommon situation for many alluvial, irrigated soils of the West.

Denitrification occurs over a very narrow range of soil-water content on Yolo loam soil. Very little denitrification occurred in plots at which the soil-water pressure head was maintained at -70 cm of water. This soil-water pressure head corresponds to a soil-air content of 9-10%. It would be expec

ted that for alluvial, loam soils such as Yolo, that very little denitrification would occur if soil-water pressures were maintained smaller than approximately 100 cm of water.

The presence of the crop root system has a large influence on denitrification. For the Yolo loam soil, very little denitrification occurred even at the water content nearest water saturation if a crop was not growing on the soil. The influence of the root system in providing carbon and consuming 02 results in anoxic development and provides the carbon necessary for denitrification. As expected, the addition of manure to the soil greatly increased denitrification over that of the cropped plots. It would be expected that the addition of other organic materials such as crop residues along with the NO3 fertilizer would also have the effect of substantially increasing denitrification. With manure added to the soil and water contents maintained very close to saturation, approximately 73% of the fertilizer was lost by denitrification at 23°C. Only 14% of the fertilizer was lost in the cropped, wet treatment of the summer experiment. Only 3% of the NO3 was lost for the wet treatment of the summer experiment without a crop growing on the soil.

At a soil temperature of between 8 and 10°C, very little denitrification occurred. This was primarily due to limited microbial activity as reflected in relatively constant 02 concentrations with soil depth. The largest denitrification loss at the low soil temperature occurred in the wet, manure treatment with 11% of the fertilizer denitrified. The uncropped plots lost