Modeling Temperature Effects on Nitrogen Mineralization

Organic fertilizer and amendment products are commonly produced from municipal biosolids. When they are applied as fertilizers, the materials must supply enough nutrients to assure plant growth, but over-application can pollute both surface and ground waters with excess nutrients. Over-application may also introduce unnecessarily high amounts of contaminants, such as heavy metals, pathogens, or toxic organic compounds, if they are present. Best management practices supply nutrients from organic fertilizers at rates just sufficient to meet plant demands. When organic amendments such as composts are used, their nutrient contribution potential should be used to adjust overall fertilization schedules which often also include inorganic fertilizers.Farmers and landscapers may choose to apply nitrogen fertilizers in either organic or inorganic form. Before the nitrogen in organic fertilizers can be used, however, it must first be mineralized by bacteria and fungi to inorganic forms. To plan appropriate land application rates, growers must estimate the rate at which the mineralization process occurs and these rates are dramatically affected by temperature. A rule of thumb is that decay rates roughly double for every 18°F (10°C) change in temperature. Mineralization rates therefore vary both seasonally at a particular place and from location to location due to climate differences.This paper describes an approach for generalizing first-order mineralization models to include temperature. Normally, when first-order decay models are adjusted to temperature effects, the decay constant is modified according to an Arrhenius or similar relationship. Modifications are required whenever temperatures vary making this an inconvenient approach for designing land application rates. This approach taken here includes the Arrhenius relationship by modifying time instead of decay rates constants. The resulting temperature- adjusted-time the replaces linear time in the first-order model and the decay rate term retains its values regardless of temperature.The approach is useful for determining mineralization rates as season change as well as for applying decay rates measured under one set of temperature conditions to others. Use of the approach will help to assure that biosolids are applied at agronomically appropriate rates.