Nitrogen mineralization and nitrification in four Minnesota old fields

Area of Science:

• Soil Science
• Ecology
• Biogeochemistry

Background:

• Old fields represent successional ecosystems with changing soil properties.
• Nitrogen availability is a critical factor regulating ecosystem productivity and function.
• Understanding nitrogen cycling in old fields is essential for predicting ecosystem responses to environmental changes.

Purpose of the Study:

• To investigate nitrogen mineralization and nitrification rates in old fields of varying ages.
• To assess the impact of fertilizer amendments on nitrogen availability.
• To determine the relationship between field age, soil nitrogen content, and organic matter quality.

Main Methods:

• In situ soil incubation was used to measure nitrogen mineralization and nitrification.
• Four old fields aged 16 to over 100 years were studied.
• Fertilizer treatments were applied to assess their effects on nitrogen pools.

Main Results:

• Net nitrogen mineralization increased with field age, while the proportion of total nitrogen mineralized decreased.
• Nitrogen mineralization rates were positively correlated with total soil nitrogen content.
• Fertilizer additions did not significantly increase mineral nitrogen pools in the youngest or oldest fields, but did in intermediate-aged fields.

Conclusions:

• Field age significantly influences nitrogen mineralization and nitrification rates, with increasing mineralization but decreasing organic matter quality over time.
• Plant and microbial uptake, along with potential leaching, likely explain the limited stimulation of nitrogen mineralization by fertilizer additions.
• Soil nitrogen content is a key driver of nitrogen mineralization in these old fields, unlike in many regional forests.