wAbstract
@Rates of organic carbon accumulation and changes in C:N ratio
are reported for 10 New Zealand soils converted to pastures from
scrub.. The data were derived from archive papers originally published
in 1964, but which did not report on changes in the C contents
of the soils. The soils had been sampled to 0-7.5, 7.5-15, and
15-30 cm depths and chronosequences of up to 66 years obtained
by selecting sites with differing times since pasture establishment.
We calculated changes in the mass of C and N in the 0-7.5 cm depth
and compared that to the mass in the 0-30 cm depth of soil. The
shortest time over which organic matter change was assessed was
18 years and the longest was 66 years. Nine of the ten soils showed
increases in the C contents of the 0-7.5 cm depth soil, and a
natural logarithmic curve generally gave a better fit to the time
course data than a linear fit. however, when the full 0-30 cm
depth was considered, only two soils showed a significant increase
in total C, changes in the C contents of other soils were non-significant,
and two soils showed a decline in total C. The rates of change
in the C contents were averaged over 0-5 years, 5-25 years and
25-50 years. Across all 10 soils, the mean rates of accumulation
of C in the 0-7.5 cm depth were 1.07 (between 0 and 5 years),
0.27 (between 5 and 25 years) and 0.09 Mg C ha-1 year-1
(between 25 and 50 years) and significantly (P0.05) greater than
zero. Very similar rates were obtained for the 0-30 cm depth of
soil with mean rates across all soils 1.01 (0-5 years), 0.25 (5-25
years) and 0.09 Mg C ha-1 year-1 (25-50
years), respectively. In the 0-7.5 cm depth of soil, total Kjeldahl
N (TKN) increased significantly in seven of the 10 soils. When
expressed for the 0-30 cm depth of soil, only five soils still
showed significant increases in TKN contents over time. Using
the data for the 0-7.5 cm depth, the predicted time (mean and
standard error) for the soil to reach a C:N ration of 10 was
46}17 years. The soils were originally sampled over 44 years ago,
suggesting that currently (2009), very few of them could be expected
to have capacity for further N storage in organic matter in the
surface soil unless there was an increase in soil C. Changes in
soil C and N in the shallow upper soil layers are easily masked
by the relatively small changes in C and n contents and much greater
masses of soil at lower depths.
Keywords: Soil organic C; N storage; Sequestration; Deforestation;
Pasture chronosequencex
Introduction
Materials and methods
Results
@Total C
@Total N and C:N ratio
Discussion
Acknowledgements
References