New Zealand hill-country farms consist of an amalgamation of land units in
different slope and aspect categories, each with unique production potentials.
Information on the influence these slope categories have on carbon (C)
partitioning is imperative for more accurate and complete understanding of C
inputs and fluxes through a grazed hill pasture ecosystem. The effects of 3
slope categories [representing 1–12°, 13–25°, and
≥26° microtopographical units corresponding to low (L), medium (M), and
steep (S) slopes] on the vertical translocation of photosynthetically
fixed C was studied by using a 14
C-CO2 pulse-labelling chamber technique.
Pasture and soil samples were taken after 4-h, 7-day, and 35-day chase
periods, to examine the fluxes of 14C in the pasture
plant{root–soil system. Total C and 14C were
determined in the pasture shoot, root, and soil components. Microbial biomass
C and 14C contents in each soil were also determined
using the chloroform fumigation{extraction technique. Pasture composition
and growth varied with slope category. High fertility grasses (90%)
were dominant on the L slope while low fertility grasses (≥60%) were
dominant on the M and S slopes. Shoot growth over 35 days amounted to 4470,
2045, and 1308 kg/ha at the L, M, and S slopes, respectively. The standing
root biomass did not differ signifficantly among the slopes. Allocation of the
14C-labelled assimilate below-ground was rapid, with
23–35% detected in the roots within 4 h of pulse-labelling. The
above- and below-ground partitioning of 14C varied with
the length of the chase period, and was strongly influenced by slope. Pasture
plants allocated more C below-ground in the M and S slope categories. During
the study period, 173 kg C/ha was assimilated daily at the L slope site,
with 73 kg being respired, 50 kg remaining above-ground in the shoot, and 43
kg being partitioned into the root. In comparison, at the S slope, of the 56
kg/ha C assimilated daily, 22 kg was respired, 14 kg remained in the
shoot, and 18 kg was partitioned into the root, and the daily input to the
soil varied between 2 and 7 kg C/ha. By using annual growth measurements
from adjacent areas, the amounts of C translocated annually to roots and soil
at each slope category were also estimated from the 14C
distribution of spring growth. At the L slope site, 9340 kg C/ha was
respired, 6375 kg remained above-ground in the shoot, and 5510 kg was
translocated to roots and 930 kg to soil. At the S slope site, 5710 kg
C/ha was respired, 3490 kg remained in the shoot, and 4490 kg was
translocated to the roots and 555 kg to soil.