CTCD: Soil: Iain Hartley's PhD Work

Iain Hartley's PhD work - completed June 2006

Experimental Work:

Dependence of soil respiration on photosynthesis:

Biosphere 2, Arizona, USA

Study Site: The intensive forestry bays at Biosphere 2, Arizona, provide a unique facility in which to investigate the links between photosynthesis and below-ground respiration. The atmosphere CO₂ concentration and the air temperature can be regulated independently in the three bays. This allowed us to investigate how photosynthetic rate affected the below-ground and ecosystem respiration and how this relationship was modified by temperature.

Rational: Altering the atmospheric CO₂ concentration was expected to affect the rate of canopy photosynthesis. In turn, it was hypothesised that this would control the amount of substrate being allocated below-ground and hence result in a change in the amount of CO₂ evolved from the soil. Although elevated CO₂ concentrations have been shown to result in increased below-ground respiration rates ^[1], it is not clear whether most of this response is due a change in the rate of root growth and turnover or rather an increase in the specific rate of root/rhizosphere respiration. By changing the atmospheric CO₂ concentration over a relatively short period and by measuring responses to increasing and decreasing photosynthetic rates it was possible to distinguish between these two possible responses. Over the course of two months the effect of changes in the atmospheric CO₂ concentration and air temperature on the rates of canopy photosynthesis and ecosystem respiration were also investigated. Modifying the atmospheric CO₂ concentration produced a large range of photosynthetic rates whilst controlling the air temperature allowed investigation into whether the ratio of photosynthesis to respiration at the whole ecosystem level was constant or modified by temperature.

Results: Altering the atmospheric CO₂ concentration from 280 ppm to 900 ppm or vice versa altered the rate of soil respiration by ~17% over a two week period. This followed a change of ~85% in the rate of canopy photosynthesis. Over this relatively a short time period, this result is likely to be due to a response in specific rate of root/rhizosphere respiration as a result of increased photosynthetic inputs rather than a change in root biomass.

At the level of the ecosystem, a very strong relationship was identified between photosynthesis and respiration (Fig. 1). This relationship was indeed modified by temperature, with the rate of ecosystem respiration for a given rate of photosynthesis increasing with temperature.

Fig. 1

Plantago lanceolata experiment at the University of York

Rationale:

₂

^[2]

Experiment: The effect of soil temperature on the speed of the link between photosynthesis and below-ground respiration was investigated in a greenhouse study. Plantago lanceolata was grown in soils held at 3 different temperatures (ambient, +6°C and -6°C). These plants were pulse-labelled with 99 %¹³C CO₂ at 370 ppm for 3 hours and the speed at which this label was released from the soil was monitored.

Results: The study demonstrated that the speed of the link between below-ground respiration and photosynthesis increases with temperature (Fig. 2). At the higher temperatures below-ground respiration declined more over the course of a night suggesting substrates were being used up at the higher temperatures. In a future warmer world, below-ground respiration (especially root/rhizosphere respiration) may become more dependent on the rate of photosynthesis.

Fig. 2

¹³

Field Warming

Soil-warming experiment, University of York

Rationale:

^[3]

Experiment: In the walled garden at the University of York, 4 plots of wheat, maize and bare soil were set up. One half of each plot was warmed to three degrees above ambient using heating cables placed 2.5cm below the soil surface. The cables were placed in the ground prior to planting and only switched on once the plants were 4 weeks old.

Results: The temperature response of below-ground respiration was highly dependent on the time point within the growing season in the planted plots. While the plants were growing vegetatively, the Q10, calculated between warmed and unwarmed plots, was very low but after flowering, the Q₁₀ in the wheat and maize plots increased and became comparable to that observed in the bare soil plots (Fig. 3). The use of monocultures in which whole stands showed the same phenology allowed the identification of periods within the year when plant processes (photosynthesis and growth) determined the rate of belowground respiration and periods in which abiotic factors (temperature) became more important.

Fig. 3

Laboratory Incubations

Soil incubation system

Laboratory setup: A continuous-monitoring soil incubation system has been set up at the University of York. After extensive testing and modification the system is now up and running and is capable of recording the rate of soil CO₂ production in each of 24 containers once every 2 hours. The temperature of the soils can be controlled accurately to 0.1°C in range between 5°C and 25°C. The loss of moisture in the soils is limited by controlling the temperature of the incoming air resulting in a loss of less than 2% in terms of soil moisture content over a two week period. The large amount of data generated over the course of just one day can be averaged and so partly eliminates problems with outlying results. A schematic (of the) design of the system is shown below. Planned experiments: The temperature sensitivity of the decomposition of old vs. new soil organic matter will be investigated. A number of recent studies have suggested that the decomposition of the more recalcitrant soil organic matter pools is at least as temperature sensitive as the decomposition of the more labile carbon pools^{[4] [5]}. Models predict that it will be the response of these “slow” carbon pools that determines the degree to which terrestrial ecosystems will become a positive feedback to global warming^[6] . These pools represent very large carbon stores within terrestrial ecosystems. Therefore there is the potential for these pools to respond in a much less transient way to elevated temperature than the smaller more labile pools. I aim to investigate how the duration of an incubation and the addition of substrates such as glucose affects the temperature sensitivity of soil respiration. As nitrogen availability is often cited as a factor limiting soil microbial activities the effects of inorganic nitrogen additional on the rates and temperature sensitivities of soil respiration will be investigated. Soil moisture content can affect the availability of substrates to microbes as well as the diffusion of oxygen and CO₂ through a soil. With rainfall likely to become more unpredictable in a future warmer world it is important to investigate how soil respiration rate and its temperature sensitivity responds to changes in soil water content.

References:

[1] King JS, Handson PJ, Bernhardt E, Deangelis, P, Norby RJ, Pregitzer KS (2004) A multiyear synthesis of soil respiration responses to elevated atmospheric CO₂ from four forest FACE experiments. Global Change Biology, 10, 1027-1042.
[2] Ekblad A, Hogberg P(2001)Natural abundance of C-13 in CO₂ respired from forest soils reveals the speed of link between tree photosynthesis and root respiration. Oecologia, 127, 305-308.
[3] Fitter AH, Graves JD, Self GK, Brown TK, Dogie DS, Taylor K (1998). Root production, turnover and respiration under two grassland types along an altitudinal gradient: influence of temperature and solar radiation. Oecologia, 114, 20-30.
[4] Knorr W, Prentice IC, House JI, Holland EA(2005) Long-term sensitivity of soil carbon turnover to warming. Nature, 433, 298-301.
[5] Fang C, Smith P, Moncrieff JB, Smith JU (2005). Similar response of labile and resistant soil organic matter pools to changes in temperature. Nature, 433, 57-59.
[6] Eliassson PE, McMurtrie RE, Pepper DA, Stromgren M, Linder S, Agren GI (2005). The response of heterotrophic CO₂ flux to soil warming. Global Change Biology, 11, 167-185.