Comparison of in situ incubation methods to estimate N mineralization rates in a northern hardwood forest

Background/Question/Methods

Nitrogen mineralization is a critical ecosystem process that is difficult to measure.  Several laboratory indices are available to estimate nitrogen (N) mineralization rates, but extrapolating results obtained in the laboratory to field conditions is challenging.  As a result, a variety of in situ incubation methods are used to calculate N mineralization rates in soils.  Among them the buried-bag and covered-cylinder methods are two of the most widely used techniques. While they provide helpful estimates of soil N mineralization, N losses due to damage caused by roots and animals on the bags, modified N transformation rates due to high gas concentrations, or soil disturbance during the installation of bags and cylinders may reduce the ability of both methods to provide accurate measurements. In addition, several observations suggest that the diameter of the core used to extract the soil and install the bag or the cylinder can affect the results. However, there are very few studies in the literature that compare different in situ N mineralization methods. The objective of this study is to compare the buried-bag and the covered-cylinder techniques, and two different core diameters, to determine which method produces the most accurate estimates of N mineralization rates in forest soils.

Results/Conclusions

Results show that differences in core diameter significantly affects the amount and variation of organic matter included in the incubated samples, primarily due to the effect of core size on inclusion of rocks in the sample. Larger diameter core samples had higher amount of organic matter and water content, as well as more variability in the amount of sample incubated, in the water content, and in the organic matter content than smaller diameter core samples. However, there were no significant differences between the buried-bag and the covered-cylinder methods. Ongoing laboratory analyses will determine whether the detected differences caused by core diameter are reflected in N transformation rates. Those results will aid in our understanding of which in situ incubation provides the most accurate estimates of N mineralization in forest soils.