Short-term response on the quantity and quality of rhizo-deposited carbon from Norway spruce exposed to low and high N inputs

The amount and composition of rhizo-deposition as an important component of dissolved organic carbon (DOC) in forest soils is largely unknown. The objective of our study was to examine one component of DOC production in forest soils, namely rhizo-deposition, under conditions of low, moderate, and high inorganic-nitrogen (N) input. Two-year-old Picea abies saplings were incubated in hydroponic solution for a period of 2 weeks, comprising an adaptation and an experimental period of each 1 week. Treatments included 0 (zero N), 1, 10 (moderate N), and 100 (high N) mg N L-1 nitrogen addition split equally between nitrate and ammonium. Quantity and quality of dissolved-organic-carbon (DOC) production were investigated. Cumulative DOC production ranged between 3.7 and 8.8 mg C (g root dry weight)(-1) (7 d)(-1) and was significantly reduced in zero- and high-N treatments. In turn, the specific UV absorbance as a measure of the aromaticity increased significantly in zero- and high-N treatments, indicating a reduced contribution of microbial products to rhizodeposited C. Mineralization of rhizo-deposited DOC was not significantly different between N treatments at day 1 of the experimental period, but by day 7, 69% of rhizo-deposition in zero-N hydroponic solution was mineralizable compared to 37%-49% for the moderate- and high-N treatments. Increasing N input tended to result in decreased degradability. We conclude from this short-term experiment that rhizo-deposited DOC from Norway spruce displays a fast response to changes in inorganic N.