Soil nitrogen transformations and the role of light fraction organic matter in forest soils

Depletion of soil organic matter through management activities can alter substrate availability for microbes, potentially shifting the dynamic balance between nitrogen (N) immobilization and mineralization. Soil light fraction (LF) organic matter is a reactive pool that decreases upon cultivation of forest soils, and recovers slowly after agricultural abandonment. Here we examine the hypothesis that rapid incorporation of N into the relatively young and reactive LF organic matter is an important mechanism for the retention of added inorganic N in forest soils. We determined the incorporation of 15N-ammonium and 15N-nitrate into the LF (<1.75 g cm−3) and heavy fraction (HF) after 18 h, after removing KCl-extractable ammonium and nitrate. We also measured gross N transformations in forest soils with different agricultural land-use history and present-day vegetation at Harvard Forest, USA. The LF was a strong short-term sink for N, incorporating an average of 39% of added ammonium and 17% of added nitrate after an 18 h incubation. The HF was a small sink (<5%) for added N. Less than 15% of added 15N was recovered in the exchangeable pools after 18 h. Tracer recovery was only 40–60% of the 15N added, suggesting a rapid conversion into soluble organic compounds not measured in the KCl or metatungstate solution. The LF incorporated more 15N than the HF per unit of carbon, indicating that not simply the amount but the composition of organic matter controls its function as a site for N incorporation. Gross production was 1–8 mg ammonium-N kg−1 soil d−1 and <0.1–1 mg nitrate-N kg−1 soil d−1 and was unrelated to soil moisture. In contrast, low soil moisture was associated with strongly reduced gross microbial immobilization. The LF organic matter was the most important measured short-term sink for inorganic N, and may play a key role in the rapid immobilization of N in forest soils. Changes in the size and nature of the soil LF organic matter could have important implications for N dynamics in many soils.