N dynamics of leaf and root litter decomposition

Litter nitrogen (N) release during decomposition crucially influences ecosystem N cycling and the amount of N available to plants and other soil organisms. However, the role of initial litter traits in affecting patterns of temporal N release from litters and determining the fate of N in the soil is still poorly understood.

Here, we measured litter N release during the 12-month decomposition of 15N-labelled leaf and root litters of 12 common Mediterranean herbaceous species. We further traced the fate of N during decomposition of the litters into the soil and measured the inorganic N content in the soil solution as well as total N in the particulate (POM) and mineral-associated organic matter (MAOM) fractions at the end of the incubation.

While most litters exhibited a very rapid initial N release, they were also quite variable in their N dynamics. As such they differed in the quantity and temporal patterns of N release, the accumulation of inorganic N in the soil solution, as well as in the incorporation of N in stable soil organic matter (SOM) fractions during decomposition. This variability was driven by differences in initial litter chemistry, particularly N, but also P and Mn, and litter C leachate concentrations. These and other traits (including water soluble compounds and lignin concentrations) explained up to 81% of the variance in N release rate, 48% for N accumulation in the soil solution, 57% for N entering the soil POM and 55% for MAOM fraction.

Synthesis:

We identified litter traits and litter types (root versus leaf) with afterlife effects on N cycling in soils and important implications for ecosystem functioning. Highly decomposable litters (typically high-N; including most leaf litters) generally support a fast N release through mineralisation, thereby the immediate supply of N to plants, but also potentially higher losses from the system, and long-term storage as MAOM-N, likely within microbial resynthesis products. In contrast, poorly decomposable litters (typically low-N; including most root litters) promote longer and weaker N release and foster N retention via lower N losses and increased POM-N formation.