Hill slope effects on the vertical fluxes of photosynthetically fixed 14C in a grazed pasture

Abstract

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.