Soil and sample analysis
We used a Shimadzu TOC-VCN to determine the DOC and
total dissolved nitrogen (TDN) concentrations of the filtered
leachate. Sample replicates indicate a 0.08 mg L21 and
0.04 mg L21 precision for DOC and TDN, respectively. Mass
loss of C and N (dissolved flux) were determined by multiplying the
DOC and TDN concentrations by the water volume of the
sample, respectively. In contrast, water throughput was calculated
using the evaporation-corrected water volumes. The aromaticity of
the dissolved C was determined by calculating the specific UV
absorbance at 254 nm (SUVA254; [50]). The UV absorption
(m21) of filtered leachate was measured on a Cary UV-Vis
spectrophotometer and divided by the DOC concentration (mg C
L21) following the protocol developed by Weishaar et al. [50] to
calculate SUVA254 (L mg C21 m21).
After six flushing events, soil mixtures were removed from the
columns, weighed, dried at 100uC for at least 24 hours, and
reweighed to determine water content at field capacity by mass.
We measured C and N content on original soils, biochar, and
dried, post-experiment soils using a Costech 4010 CHNS/O
Elemental Analyzer. Replicate analysis shows a precision of 0.6%
and 0.02% for C and N measurements, respectively.