Estimating in-Situ N Mineralization Rate with a Buried Bag Method.

Accurate estimates of soil nitrogen (N) mineralization rate and timing are needed for refining N management practices. This paper reviews the use of in-situ cylindrical polyethylene buried bags as a simple method for evaluating N mineralization dynamics. We present data showing that this method can be used successfully to monitor N mineralized during the course of a summer growing season. We also discuss what aspects of the buried bag protocol may require standardization or adaptation for implementation in other regions. Bags are open cylindrical tubes that are tied at both ends after soil is added. In spring, field-moist soil samples are collected with an auger. After sieving to pass a 10-mm sieve, composited soil is packed into bags, returned to the field (in auger holes), and covered with field soil. Bags are installed just prior or after seeding the crop. At prescribed intervals, bags are destructively harvested to determine mineral N accumulation. Idaho and OR field studies have been conducted within fields planted to irrigated crops (cereals, corn, potato, sugar beet) using conventional tillage. Therefore, soil in the buried bags had similar moisture and physical properties vs. soil in the field. The coefficient of variation for net N mineralized (0-30 cm depth) over a 90+ day growing season was usually 10 to 20%. Observed field N mineralization rates suggest that denitrification loss of mineralized NO₃-N, or inhibition of the mineralization process by accumulated NO₃  are not major issues with this method. Mineral N recovered from buried bags was similar to N accumulated by sugar beet in ID trials. We conclude that this buried bag method can be used to improve estimates of N mineralization dynamics in western USA irrigated cropping systems under conventional tillage. We invite collaborators to more rigorously evaluate the precision and accuracy of the method.