wAbstract
@Although erosion occurs under pastures in New Zealand North Island
hill country, previous work shows that the soils on mid-slopes
(19K) have large and significant average gains of soil carbon
(C) and nitrogen (N) over a period of about 30 years. Both sheetwash
erosion (from patches of bare soil) and soil deposition (from
upslope) could have occurred in the intervening 30 years, and
this could have led to a loss or gain of soil C. The fission-derived
radionuclides 137Cs is a unique tracer for studying
erosion and sedimentation because it is strongly adsorbed onto
soil particles, and its movement in the environment is predominantly
due to physical processes. We hypothesized that accumulation and
loss of 137Cs and C would be correlated if erosion
and deposition processes were responsible for the observed changes
in soil C. We therefore measured 137Cs in archived
samples, previously used to assess changes in C and N, together
with 137Cs in the re-sampled soil horizons, to an average
depth of 37 cm. The data show that, on average, that has been
a small gain in total profile 137Cs over this time,
and complex patterns within individual soil profiles over 30 years.
There was only a weak correlation between 137Cs and
C accumulation above 15 cm soil depth, but, in contrast, a much
stronger relationship was found below 15 cm. A possible explanation
for the observed pattern is soil thickening due to deposition,
but the deposition may occur with variable 137Cs contents.
Generally deposition does not account for all of the increase
in C, and regression analysis between Cs and C for all layers
suggested that erosion/deposition accounted for 20-30 of the
total change in C. In a detailed case study, the sampling of erosion
scars caused by landslides showed that soil C stocks (0-20 cm)
can increase from 10 to 80 tonne (Mg) C ha-1 within
70 years. Use of the CENTURY model indicated that input of N from
legumes in the pasture enabled N and C to accumulate in the surface
soil. This suggest that gains in soil C and N under hill country
pasture tat occur in these facets of the landscape are probably
largely due to the ongoing input from C and N in the grass-legume
pastures rather than erosion or deposition of soils.
Keywords: Carbon accumulation; Pasture production; Soil recovery;
storm sedimentx
1. Introduction
2. Material and methods
@2.1. Site selection and sampling
@2.2. Cs analysis
@2.3. Te Whanga case study and modelling using CENTURY
@2.4. Data analysis
3. Results
@3.1. 137Cs data
@3.2. Te Whanga case study
4. Discussion
@4.1. Soil C recovery after sheetwash erosion
@4.2. Soil C recovery after landslide erosion
5. Conclusions
Acknowledgments
References