This study examined the transport and transformation of particulate nitrogen during large storm events in forested watersheds. The key questions addressed were: 1) the potential sources of particulate nitrogen and how they vary with storm size and location; 2) how nitrogen and carbon are leached from particulate organic matter of different sizes; and 3) the potential for mineralization and transformation of nitrogen deposited in streams. The results showed that forest floor litter is a major nitrogen source. Leaching experiments found high levels of nitrogen and dissolved organic carbon from litter. Nitrogen release increased under wet-dry cycles, especially for sediments deposited during large storms. Source tracking identified litter, humus and wetland soils as important contributors to coarser particulate organic matter upstream,

1. Large runoff flux and transformation of particulate nitrogen (PN) following large storms:
A critical but unexplored component of N cycling in watersheds
Shreeram Inamdar1, Jinjun Kan2, Rodrigo Vargas1, Richard Rowland1, Erin Johnson1 and Chelsea Krieg1
1University of Delaware, Newark, DE; 2Stroud Water Research Center, Avondale, PA.
Study Site & methods
Introduction & questions
Baseflow Receding FlowStormflow
Large storms can erode, transport and deposit substantial amounts of sediment and
particulate nutrients through the fluvial network. Particulate nitrogen (PN) and
organic carbon (POC) in floodplain sediments can leach and mineralize and thus
contribute to eutrophication.
Understanding these inputs is especially important considering that climate-change
projections (Melillo et al., 2014) indicate increased intensity of hurricanes and tropical
storms (especially for the Northeast and Mid-Atlantic US).
Key questions we addressed were:
1. What are the potential sources of PN and how do they vary with storm event
size, drainage location, and sediment particle size class?
2. How is N and C leached from particulate organic matter (POM) sources and size
class?
3. What is the potential for mineralization and transformation of PN deposited in
the fluvial network?
Photos highlighting sediment and particulate matter transport and deposition
following a large storm in the Big Elk Creek in Maryland
Sediment Size Fractions
Coarse (CPOM) 2000 µm-1000 µm
Medium (MPOM) 1000 µm-250 µm
Fine (FPOM) <250 µm
• Stream sampling at 5 drainage locations
(4.5 to 79 ha) in a forested catchment
• Potential sources – forest floor litter &
humus, stream bank, bed, wetland soils, &
upland A horizons
• Storms of varying magnitude/intensity
• Sediment characterized for 3 particle size
classes
• %C and %N determined for stream
sediment & individual sources
• Sources identified using 13C and 15N
stable isotopes
• Transformation and mineralization of POM
determined for 2 moisture regimes using
laboratory incubations
Sediment and POM transport at the 79
ha stream location in February, 2016
% N and % C content in potential sources
%Carbon%Nitrogen
Nitrogen concentrations were
highest in more organic sources,
and especially in the Forest Floor
Litter source, which is composed
of fresh plant material. This
indicates that this is a large N
source in forested systems.
Carbon concentrations displayed
more variability, with increased
C in Forest Floor Humus and
Litter.
Leaching of N and C from POM sources
%Protein-likeFluorescence
DOC(ppm)TN(ppm)
Extracted solutions show high contributions of TN and DOC from the Forest Floor Litter
source. Similarly, the highest % protein-like fluorescence, an indicator of bioavailability,
came from the Forest Floor litter. This suggests that a large bioavailable pool of N could be
contributed during extreme rainfall events.
Extracted TN and DOC did differ significantly with sediment particle size class.
Wet-dry moisture cycles and N release from
sediments (lab incubation)
POM sources (stream bed,
bank, forest floor humus,
and upland mineral A) and
floodplain deposits “storm
deposits” were incubated for
2 contrasting moisture
regimes over 60 days.
Regime 1 (R1) sediments
were watered frequently
(thin blue arrows) to
maintain their original soil
moisture contents, while
Regime 2 (R2) sediments
were allowed to dry. On Day
26, a large water addition
(thick blue arrows) was
administered to all
sediments columns.
N species were measured in
sediment and pore water
samples. Microbial
communities in
sediments/soils are also
being characterized to
identify N release and
consumption processes.
Preliminary results suggest
that the storm sediments &
sources leach N. The release
was especially pronounced
under regime 2 (R2) – wet-
dry cycles.
%N and %C of sediments by size class and
drainage location (4.5 to 79 ha)
CPOM, MPOM, FPOM
Drainage area (ha)
While % C displayed a weak decreasing trend
from CPOM to FPOM, the differences were not
significant.
On the other hand, 15N isotope data (not
included here) revealed that POM was more
processed and likely more recalcitrant for the
finer versus the coarser POM fractions.
Bayesian mixing model and source contributions
The forest floor litter, humus, and wetland soils were important contributors to CPOM,
notably for the headwater stream locations which were hydrologically connected to
these sources.
POM contributions from stream bed sediment and stream banks increased from
CPOM to FPOM particle class and from headwater to downstream locations.