『Abstract
There are few records of long-term trends in soil C and N in
grazed pasture systems but recent measurements have demonstrated
unexplained losses on New Zealand lowlands. To determine whether
losses were also occurring in hill country pastures, we analyzed
archived soil samples collected between 1983 and 2006 from two
slope classes (steep and easy) at the Whatawhata Research Centre.
Soils were Ultic Hapludand and Typic haplohumult on the easy slopes
(10-20゜), and Typic Haplohumult on the steeper slopes (30-40゜).
Soil samples (0-75 mm) had been collected from paddocks that were
fertilized with six different loading rates of P (ranging from
0 to 100 kg P ha-1 - year-1 since 1985).
This range of P loadings allowed us to determine whether P inputs
would regulate trends in soil C and N. While there were significant
temporal trends in C and N (P＜0.05), these were not unidirectional
and trends were not dependent on P loading rate. On average, soil
C initially increased during the first 6 years of the trial at
0.270％ C year-1 (1.56 t ha-1 year-1)
and 0.156％ C year-1 (1.06 t ha-1 year-1)
on easy and steep slopes, respectively. Subsequently, there was
no significant trend in soil C on the easy slopes but soil C declined
at -0.066％ year-1 (0.45 t ha-1 year-1)
on the steep slopes. Similarly, soil N increased between 1983
and 1989 at 0.025％ N year-1 (144 kg ha-1
year-1) and 0.012％ N year-1 (82 kg ha-1
year-1) on easy and steep slopes, respectively. Post-1989,
small but significant losses of total N were measured on the steep
slopes of 0.004％ year-1 (27 kg N ha-1 year-1)
(P＜0.05) with no trend on the easy slopes. Two potential causal
factors for these decadal-scale patterns were identified, operating
via changes in primary productivity. These were lower S inputs
from 1989 due to a change in fertilizer type, and a series of
relatively dry summers during the 1990s. These significant inter-annual
trends in soil C and N complicate attempts to measure long-term
changes in soil organic matter associated with land use change
and management practices. This study has demonstrated the potential
error associated with infrequent soil sampling to determine long-term
trends in soil C and N; large gains or losses could have been
detected at Whatawhata depending on when sampling started and
finished. Understanding these long-term trends in soil organic
matter dynamics and driving factors requires more long-term sampling
trials.
Keywords: Soil carbon; Soil nitrogen; Phosphorus fertilizer; Pasture
Abbreviation: SU=Stocking units』
Introduction
Materials and methods
Site description and management
Fertilizer application
Grazing management
Soil sampling
Soil analysis
Climate data and modeling
Data analysis
Results
Changes in Olsen P
Bulk density
Changes in soil carbon and nitrogen
Climate data and modeled plant transpiration
Discussion
Changes in total carbon and nitrogen
C:N ratio
Slope class effects
Implications for measuring changes in C and N
Acknowledgments
References