At the watershed scale, soil moisture is the major control for rainfall–runoff response, especially where saturation excess runoff processes dominate. From the ecological point of view, the pools of soil moisture are fundamental ecosystem resources providing the transpirable water for plants. In drylands particularly, soil moisture is one of the major controls on the structure, function, and diversity in ecosystems. In terms of the global hydrological cycle, the overall quantity of soil moisture is small, ∼0.05%; however, its importance to the global energy balance and the distribution of precipitation far outweighs its physical amount. In soils it governs microbial activity that affects important biogeochemical processes such as nitrification and CO2 production via respiration. During the past 20 years, technology has advanced considerably, with the development of different electrical sensors for determining soil moisture at a point. However, modeling of watersheds requires areal averages. As a result, point measurements and modeling grid cell data requirements are generally incommensurate. We review advances in sensor technology, particularly emerging geophysical methods and distributed sensors, aimed at bridging this gap. We consider some of the data analysis methods for upscaling from a point to give an areal average. Finally, we conclude by offering a vision for future research, listing many of the current scientific and technical challenges.

/pdf/soil-moisture-measurement-for-ecological-and-hydrological-1jak2uhu52.pdf

Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review

Effect of Water Potential on Microbial Growth and Activity

A dynamic simulation model for the soil plant system is described. The model includes a number of main modules, viz., a hydrological model including a submodel for soil water dynamics, a soil temperature model, a soil nitrogen model including a submodel for soil organic matter dynamics, and a crop model including a submodel for nitrogen uptake. The soil part of the model has a one-dimensional vertical structure. The soil profile is divided into layers on the basis of physical and chemical soil characteristics. The simulation model was used to simulate soil nitrogen dynamics and biomass production in winter wheat grown at two locations at various levels of nitrogen fertilization. The simulated results were compared to experimental data including concentration of inorganic nitrogen in soil, crop yield, and nitrogen accumulated in the aboveground part of the crop. Based on this validation it is concluded that the overall performance of the model is satisfactory although some minor adjustments of the model may prove to be necessary.

Simulation of nitrogen dynamics and biomass production in winter wheat using the Danish simulation model DAISY

Modelling temperature and moisture effects on C-N transformations in soils: comparison of nine models

Nitrification, the microbial oxidation of NH $^+\_4$ -N, plays a key role in the cycling of N in forested and other terrestrial ecosystems. Solution losses of nitrate and gaseous losses of N $\_2$ and nitrous oxides are important vectors of N loss from many forested systems and are directly or indirectly controlled by the activity of the nitrifiers. These losses can also have important consequences for downstream ecosystems, groundwater quality, and atmospheric concentrations of ozone. Relative nitrification (the proportion of the total mineral N that is nitrate at the end of an incubation period) provides an independent means of evaluating the general importance of site factors thought to regulate nitrification in situ. Regression of relative nitrification against soil pH, C:N, and percentage N, with the use of data from previously published studies, suggest that although these factor may be important regulators of nitrification in particular sites, they are not good predictors of nitrification across a wide range of sites. Reasons for their low predictive ability may include limitations of current measurement techniques or the capacity of nitrifiers to adapt to relatively extreme conditions.

Nitrification in forested ecosystems

Samples of a Chestnut and Chernozem soil, with and without additions of (NH₄)SO₄ and K₂HPO₄, were incubated at constant water contents ranging from suctions of 0.2 to about 100 bars, and the exchangeable NH₄- and soluble NO₂-N measured periodically. NO₂-N in excess of 1/3 ppm was never detected. Addition of P had no effect on N transformations. Ammonification and nitrification of soil N were almost directly proportional to soil water content at suctions between 0.2 and 15 bars. Measurable ammonification and nitrification occurred at a suction of 15 bars.

Effect of Soil Mositure on Ammonification and Nitrification in Two Northern Plains Soils1

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Effect of Soil Mositure on Ammonification and Nitrification in Two Northern Plains Soils1