Gordon L. Hutchinson

Atmospheric Ammonia: Absorption by Plant Leaves

By monitoring the disappearance of ammonia from an airstream flowing through a small growth chamber containing a single plant seedling, it was discovered that plant leaves absorb significant quantities of ammonia from the air, even at naturally occurring low atmospheric concentrations. The measured absorption rates of ammonia showed large diurnal fluctuations and varied somewhat among species, but differed little with the nitrogen fertility level of plants within a species.

Nitrogen Enrichment of Surface Water by Absorption of Ammonia Volatilized from Cattle Feedlots

Apparatus designed to measure absorption of ammonia from the air by aqueous surfaces was installed near several cattle feedlots and in appropriate control areas. Ammonia absorption rates measured near feedlots were as much as 20 times greater than near the control. Their magnitudes indicate that absorption of ammonia volatilized from cattle feedlots contributes significantly to the nitrogen enrichment of surface water in the vicinity of feedlots.

Chamber measurement of surface-atmosphere trace gas exchange: Numerical evaluation of dependence on soil, interfacial layer, and source/sink properties

We employed a three-dimensional finite difference gas diffusion model to simulate the performance of chambers used to measure surface-atmosphere trace gas exchange. We found that systematic errors often result from conventional chamber design and deployment protocols, as well as key assumptions behind the estimation of trace gas exchange rates from observed concentration data. Specifically, our simulations showed that (1) when a chamber significantly alters atmospheric mixing processes operating near the soil surface, it also nearly instantaneously enhances or suppresses the postdeployment gas exchange rate, (2) any change resulting in greater soil gas diffusivity, or greater partitioning of the diffusing gas to solid or liquid soil fractions, increases the potential for chamber-induced measurement error, and (3) all such errors are independent of the magnitude, kinetics, and/or distribution of trace gas sources, but greater for trace gas sinks with the same initial absolute flux. Finally, and most importantly, we found that our results apply to steady state as well as non-steady-state chambers, because the slow rate of gas diffusion in soil inhibits recovery of the former from their initial non-steady-state condition. Over a range of representative conditions, the error in steady state chamber estimates of the trace gas flux varied from −30 to +32%, while estimates computed by linear regression from non-steady-state chamber concentrations were 2 to 31% too small. Although such errors are relatively small in comparison to the temporal and spatial variability characteristic of trace gas exchange, they bias the summary statistics for each experiment as well as larger scale trace gas flux estimates based on them.

Air Containing Nitrogen-15 Ammonia: Foliar Absorption by Corn Seedlings

Thirty-day-old corn seedlings, grown in the greenhouse with different concentrations of supplemental nitrate nitrogen, were moved to a constant-temperature growth chamber and sealed in a 560-liter tent made of polyvinyl chloride. The plants were exposed to air containing ammonia labeled with nitrogen-15 (1, 10, and 20 parts per million) for 24 hours and then harvested. The nitrogen-15 content of the tops and roots showed that at 1 part per million 43 percent of the ammonia was absorbed, whereas at 10 and 20 parts per million, 30 percent of the ammonia was absorbed. The results demonstrate that growing plants may be a natural sink for atmospheric ammonia.

Plant functional type effects on trace gas fluxes in the shortgrass steppe

Plant community structure is expected to regulate the microbial processes of nitrification and denitrification by controlling the availability of inorganic N substrates. Thus it could also be a factor in the concomitant release of NO and N2O from soils as a result of these processes. C3 and C4 plants differ in several attributes related to the cycling of nitrogen and were hypothesized to yield differences in trace gas exchange between soil and atmosphere. In this study we estimated fluxes of NO, N2O and CH4 from soils of shortgrass steppe communities dominated by either C3 plants, C4 plants or mixtures of the two types. We collected gas samples weekly from two sites, a sandy clay loam and a clay, throughout the growing seasons of 1995 and 1996. Plant functional type effects on gas fluxes at the clay site were not apparent, however we found several differences among plant communities on the sandy clay loam. CH4 uptake from atmosphere to soil was significantly greater on C4 plots than C3 plots in both years. NO fluxes were significantly greater from C4 plots than from C3 plots in 1995. NO fluxes from C3 and mixed plots were not significantly different between 1995 and 1996, however fluxes from C4 plots were significantly greater in 1995 compared to 1996. Results indicate that under certain environmental conditions, particularly when factors such as moisture and temperature are not limiting, plant community composition can play an important role in regulating trace gas exchange.

Nitrate and other water pollutants under fields and feedlots

Agriculture’s effect on nitrate pollution of ground water was investigated in the South Platte valley of Colorado. The valley is intensively farmed and contains many concentrated livestock feeding operations. A water table, generally between 3 and 20 meters below the surface, underlies much of the area. The average total nitrate-nitrogen to a depth of 6.7 meters in the profiles for the various kinds of land use was: alfalfa (Medicago safiua) (13 cores), 70; native grassland (17 cores), 81 ; cultivated dryland (21 cores), 233; irrigated fields not in alfalfa (28 cores), 452; and feedlots (47 cores), 1282 kg. per hectare. Ground water samples often contained high concentrations of nitrate, and those obtained beneath feedlots contained ammonium-nitrogen and organic carbon.