Showing papers by "Weixin Cheng published in 2004"

Photosynthesis controls of CO2 efflux from maize rhizosphere

Abstract: The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a 13 C natural abundance and 14 C pulse labeling simultaneously. 13 C was a tracer for total C assimilated by maize during the whole growth period, and 14 C was a tracer for recently assimilated C. CO2 efflux from bare soil was 4 times less than the total CO2 efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO2 efflux from soil, respectively. The analysis of 13 C natural abundance showed that root-derived CO2 efflux accounted for 82%, 68% and 56% of total CO2 efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO2 efflux from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO2 efflux from soil. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO2 efflux, and thereby, total CO2 efflux from soils. 14 C labeling of plants before the first light treatment did not show any significant differences in the 14 CO2 respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C ( 14 C) to the root-derived CO2 efflux by shaded plants. Results from 13 C natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.

Effects of elevated CO2 on nutrient cycling in a sweetgum plantation

Abstract: The effects of elevated CO2 on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO2 enrichment (FACE) ex- periment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO2 would be mitigated primarily by reduced tissue concentrations (resulting in increased biomass production per unit uptake) rather than increased uptake. Conversely, we hypothesized that the constraints of other nutrients to growth response to elevated CO2 would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO2 caused reduced foliar N concentrations, it also resulted in increased uptake and require- ment of N, primarily because of greater root turnover. The additional N uptake with elevated CO2 constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO2 had no significant effects on tissue concentrations of P, K, Ca, or Mg and caused significantly increased uptake and requirement of K, Ca, and Mg. Soil exchangeable pools of these nutrients are large and should pose no constraint to continued growth responses. Elevated CO2 also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution NH þ ,S O 2� 4 , and Ca 2þ concentrations, and in- creased soil solution pH. There were no statistically significant treatment effects on soil nutrient availability as measured by resin capsules, resin stakes, or in situ incubations. Despite significantly lower litterfall N concentrations in the elevated CO2 treatment, there were no significant treatment effects on translocation or forest floor biomass or nutrient contents. There were also no significant treatment effects on the rate of decomposition of fine roots. In general, the effects of elevated CO2 on nutrient cycling in this study were not large; future constraints on growth responses imposed by N limitations will depend on changes in N demand, atmospheric N deposition, and soil mineralization rates.

Defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter under a sunflower species: Helianthus annuus

Abstract: In the present study, ‘natural 13 C tracer method’ was used to partition the belowground respiration into rhizosphere respiration and soil microbial respiration to test the hypothesis that defoliation affects rhizosphere respiration and rhizosphere priming effect on decomposition of soil organic matter (SOM). A C3 plant species, Helianthus annuus (sunflower), was grown in ‘C4’ soil in microcosms so that the CO2 evolved from plant-soil system can be partitioned. Four levels of defoliation intensities were established by manual clipping. CO2 evolved from plant-soil system was trapped during 0–4 h after defoliation (HAD), 5–22 HAD and 23–46 HAD using a closed circulating system, respectively. We found that both rhizosphere respiration and soil microbial respiration of the clipped plants were either unchanged or significantly enhanced compared to unclipped plants at 45% defoliation level during all sampling intervals. Soil microbial respiration increased significantly at all defoliation levels during 0–4 HAD, however, both rhizosphere respiration and soil microbial respiration decreased significantly during 5–22 HAD or 23–46 HAD when 20% or 74 clearly demonstrated that the defoliation treatments modified the rhizosphere priming effect on SOM decomposition. The total cumulative rhizosphere priming effects on SOM decomposition during 0–46 HAD were 146%, 241%, 204% and 205% when 0%, 20%, 45% and <74%.