decades, proportional contribution from different respiratory components has been regarded to be less ..... based on …..whether the soil moisture use theta-probe. The data were ..... Napoleon =(0.238T)-(0.006T2)+(0.044WHC)+5.261. 0.83.
Chapter 7 Belowground CO2 efflux across different E. globulus plantations: upscaling, partitioning into different sources and its contribution to plantation carbon cycle 7.1 Introduction and Objectives 7.1.1
Accounting belowground CO2 efflux at annual basis
An accurate CO2 efflux rate measurement is essential to terrestrial carbon accounting and to predict possible response toward future climate change. A continuous drastic elevation of the atmospheric CO2 especially after industrial era has already linked to the current global warming with many devastating impacts already imposed on the ecosystem. Furthermore, a further catastrophic is also predicted due to feedback effects, ie. enhanced CO2 release upon warming effect. Compared to the carbon assimilation pattern, only little knowledge has now already available on the pattern of CO2 flux from forest ecosystem, despite many claims on potential forest ecosystem to mitigate future climate change. In turn, limited information on this CO2 flux pattern doesn’t allow an accurate prediction of strength of forest as carbon sink or source. This despite recent massive trend of proposing afforestation activities as a promising investment amongst carbon offset project which is now offered commercially and globally. Therefore, an accurate accounting of CO2 flux from forested ecosystem would not only essentially contribute to our capacity to predict future climate change, but also would provide an accurate information on the carbon sink-source potential, which is compulsory before forest sector to be involved in carbon offset project. 7.1.2
Different approaches to partitioning belowground CO2 efflux
Partitioning of belowground CO2 efflux into each source is essential to better understand control of carbon efflux from each component (especially root and soil) in this complex ecosystem. Despite its difficulties, many attempts have been undertaken to resolve this challenge (see Hanson, 2000 for the reviews) each with both advantage
and disadvantage. Whether vast degrees of soil disturbance and massive work are required, no approach has been claimed to be superior. It is important to consider that more than single approach to be employed simultaneously in order to have crosscalibration and cross-comparison among the methods (Paul J Hanson – personal communication). Although CO2 flux from soil has already received considerable attention in recent decades, proportional contribution from different respiratory components has been regarded to be less known. The lack of information on this relative contribution could bias our prediction of the total belowground CO2 flux since the auto- and heterotrophic respond differently toward environmental change, ie. soil temperature and water status (Boone et al., 1998; Lee et al., 2003; Lavigne et al., 2004), and therefore the contribution of these components needs to be understood in order that the implications of environmental change for soil carbon cycling and sequestration can be evaluated (Hanson et al., 2000). Likewise, information on proportional contribution of the autotrophic component is essential to calculate belowground carbon allocation, which is now amongst the most under-represented component during accounting of terrestrial carbon productivity. Similarly, estimates on the heterotrophic component are also required for estimating the net primary productivity (NPP) of an ecosystem from eddy covariance measurement of net ecosystem exchange (NEE), ie. NPP = NEE+ heterotrophic CO2 flux (Jassal and Black, 2006). Many approaches have been investigated to account this flux partitioning, ie. girdling, trenching, root exclusion and application of stable carbon isotope based on a distinguished signatures in the autotrophic CO2 that controlled by recent weather conditions (Hanson et al., 2000; Kusyakov et al., 2005). In part of due to uncertainty in the methods used, several efforts to partitioning total belowground CO2 into its respiratory components found a large variation of the autotrophic proportional contribution ranged from 10-90% (Hanson et al., 2000; Xu et al., 2001), which were assumed to be associated with differences in ecosystem, species and developmental stages (Hanson et al., 2000; Hogberg et al., 2001). Despite many research have increased since then, lots of contradiction in the results have appeared and has been
linked with uncertainties of the methods used and interpretation of the results. It was concluded, after reviewing many articles related to this partitioning, that more work is required to refine methods and interpretations (Hanson et al., 2000), with special emphasise to apply a multiple approaches simultaneously enable to cross-calibration (Dr. Paul J Hanson, personal communication, 2004). Amongst various approaches, root exclusion has been widely used to separating autofrom heterotrophic CO2 especially for forest ecosystem due to its simplicity and practicality to also include different replications (Bowden et al., 1993; Kelting et al., 1998; Epron et al., 1999; Hanson et al., 2000). In this method, roots are severed by digging a trench around the plot and the trenched is lined with root-impermeable plastic sheet to prevent further root introgression into the trenched plots. According to this method, autotrophic contribution could be estimated by subtracting CO2 flux for the control plot with those flux measured from the adjacent trenched plots. It was shown that artefact following this trenching may biased the results especially due to: (i) disturbance effect (Edwards, 1975; Blet-Charaudeau et al., 1990; Ewel et al., 1983; Bowden et al., 1993), (ii) residual CO2 from decomposing trenched roots (Lavigne et al., 2003), (iii) differences in soil water regimes due to absence of tree water use on the trenched plots or altered soil drainage properties following trenching (Edward, 1975; Hanson et al., 1993; Thierron and Ladelout, 1996), (iv) it usually take a while for the CO2 flux to decline following trenching, depending on the carbon pool reserve in the trenched roots. In addition to these concerns, the trenched approach has been very often gave only a low estimate of the relative autotrophic contribution since unexpected high CO2 flux had been found even following trenching. Two possible explanations on this contradictory issue has recently been available and therefore it is now possible to improve the interpretation by correcting results from the trenching experiments. First, by involving a correction factor that consists of CO2 flux estimate that derived from decomposition of the trenched roots (Nakane et al., 1993). This was done by incubation of different root classes. The uncertainties are apparent since so far our knowledge to accurately predict decomposability rate of different root classes across soil profiles has been so limited and therefore the root decomposition rate measured in artificial condition may not represent those occurring in situ. This uncertainty is
obvious since resource and environmental conditions and how they influence the root decomposability at the deeper soil profile are still rarely described (Jobbagy and Jackson, 2000; Jackson et al., 2000; Bauhus et al., 2005). Secondly, a high CO2 flux in the trenched plots was also possible due to increased contribution of CO2 from beyond the depth of the trenching (Jassal and Black, 2006). The correction of this incoming flux was recently proposed but this analytical procedure was only possible if knowledge on depth distribution of soil CO2 diffusivity and source strength were available on each trenching experiment condition (Jassal and Black, 2006), which is again difficult to conclude as it requires many accompanying analytical procedures at the field. As an alternative component integration from direct measurement of root respiration rate to estimate the autotrophic contribution was also used in this study. There are many efforts to estimate autotrophic CO2 flux through direct root measurement, however mostly have been used to investigate physiological aspect of root respiration, especially how they confront with seasonal pattern and aboveground phonological condition (references). Measuring respiration immediately following exposing root to the atmospheric conditions had been suggested to overestimate the rate since the actual CO2 concentration in the soil is far higher. However, many findings suggested that the measuring CO2 concentration has no influence to the root respiration rate (Pregitzer et al., 2000 and the articles therein). More importantly, separate measurement of root respiration in a controlled condition (within the chamber) could be used to account environmental control of root respiration rate. Likewise, it is also possible to measure roots with different diameter classes, different branching order roots and roots that occur in different soil depth to also account the uncertainties related to these issues. Moreover, uncertainty of the root respiration rate has been associated with their mortality since it is difficult to visually determine root mortality. Furthermore, combining with the results from the root distribution within the stand obtained through root excavation, this information of the root respiration has potential to be up-scaled to estimate the CO2 efflux that derived from autotrophic component at stand level.
7.1.3
Prediction of belowground CO2 efflux pattern from mixture sources (soil and root)
Belowground CO2 efflux is composed partly from autotrophic components and therefore incorporation of the site productivity (LAI, standing litter biomass, root density) and environmental factors that control the canopy activity could improve their predictive model at regional basis. 7.1.4
Contribution of each respiratory sources toward the total plantation carbon cycle
Belowground CO2 flux (from soil) is a major component of CO2 exchange between terrestrial ecosystem and the atmosphere, accounted about two-third of the so called ecosystem respiration and also accounted about one half of the CO2 assimilated through gross ecosystem photosynthesis (Pg) (Black et al., 2005; Janssens et al., 2001; Valentini et al., 2000). To accurately estimate carbon sink capacity amongst the three E. globulus plantation sites, this Chapter summarizes efforts to upscale annual belowground CO2 efflux at the three key E. globulus plantation sites also summarize effort to predict CO2 efflux from the mixture components (soil+root with or without litters). The objectives of this Chapter were : 1. to estimate contribution of belowground carbon stock toward total ecosystem carbon in different E. globulus plantations across Victoria 2. to compare estimates of belowground autotrophic CO2 flux using different approaches (direct root respiration measurement vs. component integration = total soil CO2 efflux minus no root ) and to estimate annual belowground CO2 efflux from different source components (autotrophic and heterotrophic) at the three E. globulus plantation sites with different productivity 3. to predict pattern of belowground CO2 efflux from different E. globulus plantations across Victoria 4. to estimate contribution of each belowground CO2 sources toward the total plantation carbon cycle and how site productivity may affect this contribution
The annual belowground CO2 efflux at the three key E. globulus plantation sites was estimated by the respiratory models for each individual components (soil or root) (previously described in Chapter 3 and 5) as well as for the mixture models as obtained from soil respiration (soil+root with or without litters) measurement. Those models were developed mainly from temperature and soil water status as predictors and therefore the CO2 efflux could be estimated on daily basis based on the meteorological data which were logged hourly for each site. The annual CO2 efflux as predicted from the different models was compared. Likewise, outcomes derived from the component integration were compared with those derived from the mixture components. To develop model which are capable of predicting CO2 efflux from the mixture components (soil+root with or without litters), this Chapter also summarize effort based on results achieved from the models for each individual components. In addition to temperature and soil water status as predictor, prediction of CO2 efflux pattern from the mixture components also included factors that directly determine its canopy activity. Finally this Chapter also quantify proportional contribution of carbon released from different respiratory components toward carbon cycle at plantation scale. Most of the carbon cycle components were measured for each site.
7.2 Methods To be able to account at annual scale, the CO2 efflux was estimated for each daily basis using meteorological parameters obtained from the data-logger installed for each site. The daily basis calculation was considered since inter-daily variation of the meteorological condition are considerable and thus the calculation which only based on monthly basis approach (references) would certainly unable to capture such interdaily basis dynamics. The uncertainty in the efflux rate due to diurnal pattern was not considered, but recent studies have demonstrated that such diurnal variability was due to time-lag of temperature dynamics between soil – atmosphere interface and therefore could be simply resolved by using the daily average temperature (references) and expressing the efflux per daily basis. The temperature independent of diurnal variation of root respiration was recently reported by (Dryer et al., 2006) but their study was based on potted plants.
7.2.1 Determining total root density in mass and in surface area basis
Structural roots biomass of E.globulus was determined following root excavation at Dohle and Napoleons sites. The aim of root excavation was to provide an allometric equation of total root mass/surface area based on the tree diameter which then could be used to estimate total root at stand scale for each site. Ten trees vary in the dbh (11.5-21.9 cm) for the Dohle site and (10.7-18.35 cm) for the Napoleons were randomly selected for total biomass harvest. The trees were cut then were sampled to provide alometric equation for stem, branches and foliages mass based on tree dbh. Structural roots were excavated for each trees, cleaned by a high pressure air and then were classified according to their diameter classes (1-2 cm, 2-5 cm, > 5cm and root bole) and were oven dried. Virtually roots were free from soils and therefore correction to account soil impurity on the root mass was not made. For accounting total fine and coarse roots, five soil pits (1 m depth x 1.5 m width x 2 m length) randomly located across the location of the were dug for each Dohle and Napoleons site. Each soil pit layered across the tree-rows. Soils were sampled using a
metal soil monoblock (0.1 x 0.15 x 0.2 m =total volume 3000 cm3) with a 0.2 m depth increment down to 80 cm by considering the row position (mount-furrow-interrow). In total, 60 soil samples were collected from each site. Soils were then brought to the lab and were stored at 4oC until processing. The soils were placed into bucket filled with a running tap water and the roots were collected by using sieve. The collected roots were grouped into: 10 mm (structuralroots), rinsed thoroughly and then were root surface area were determined (see Chapter 5) and then were oven-dried. Due to hard-properties of the soil for Ullina site, such similar approach to account root biomass was prohibited. As an alternative, roots collected during the no-root treatment (Chapter 3) were used to estimate coarse- and fine-roots. The differences were : (i) 3 soil pits were excavated, (ii) the soil pits dimension were smaller (0.4 m depth x 0.5 width x 0.75 m length) and (iii) 0.1 m depth increment were used. The root mass down to 0.8 m was estimated by plotting the root distribution for each 0.1 m depth increment. Considering its less-sandy soil properties, an alometric equation for the Napoleons site based on the stem dbh was used as a rough estimate of structural roots for Ullina site.
7.2.2 Meteorological data The data-logger equipped with sensors that measures air temperature and RH (relative humidity) at 50 cm aboveground, soil temperature at 5 and at 10 cm depth, soil moisture at 5 cm depth. The probes were calibrated. The temperature sensors are based on …..whether the soil moisture use theta-probe. The data were logged every 30 minutes and then were averaged into daily basis. Based on calibration of the periods when the meteorological data were available, the gap-filling of the meteorological data at daily basis was conducted by using the SILO data following the equations described in the Table Appendix 1. There are discrepancies between the values obtained from these data-logger and those from manual measurement of soil temperature and moisture during periodical field-
campaign and therefore the calibrations were made as necessary, as described in the Table Appendix 2.
7.2.3 Up-scaling CO2 efflux from root at plantation basis CO2 efflux from root was determined by periodical measurement of individual root respiration. From this, model to calculate CO2 efflux on the root mass and on the root surface area basis were developed (Chapter 5) and therefore annual CO2 efflux from root for each site could be estimated, after multiplying into total root mass or surface area for each site (7.2.1). A correction factor was provided from the previous measurement (Chapter 5) to account effect of soil depth (down to 0.8 m) to the respiration rate of coarse- and fine-roots. This correction was also assumed to applicable for the structural roots. Since the root respiration model was developed from an in situ measurement of the intact (attached to the trees) roots without separation of coarse- and fine-roots, the temperature models were derived from mixture between coarse- and fine-roots. No difference in the temperature response between coarse- and fine-roots at the detached condition as measured by using an automated alkaline trap (RespicondTM) (data not presented) and therefore both root classes were assumed to have similar temperature response. Differences in root respiration rate between coarse- and fine-roots (per root biomass or surface area) were detected for Dohle and Napoleons sites (Chapter 5), but again as the model were developed from the measurement where no separation between coarse- and fine-roots were made, thus the respiration rate per root biomass and per root surface area were assumed to be similar. This assumption should be valid as long as the exposed roots during the respiration measurement proportionally represent both the root diameter classes for each site. This was ensured by presence of at least three root-branching orders for each the measured roots, as previously suggested by Pregitzer et al. (2002, 2003). As no effort has been made to measure respiration rate of the structural roots, an assumption to estimate this root class was made by using the results obtained from the measurement of separated coarse- and fine-root respiration at Dohle and Napoleons sites. From those results, the larger root diameter classes
showed 30% respiration rate if were expressed per root biomass basis and were similar if were expressed per root surface area basis. 7.2.4 Determining belowground CO2 efflux from different E. globulus plantations Periodically (a twelve-weekly for Dohle and six weekly for the Napoleons and Ullina sites) CO2 efflux from soil under E. globulus were conducted to derive an empirical model to account annual CO2 efflux per site. Further, these database were also to be used to develop a possible generic model which could account belowground CO2 efflux under E. globulus plantations across different sites. For this purpose, recent approach to derive a multi-regional model to account soil CO2 efflux proposed by Reichstein et al. (2003) was adopted. This was done by introducing site-specific LAI (leaf area index) as a proxy value for root density. This LAI should also indicate thickness of standing-litter which partly constituted soil heterotrophic component. The LAI of each plantation was measured during 2004-2005 periods using Licor-2000. Soil carbon content was introduced as a surrogate for the soil heterotrophic respiration. The differences amongst different soil water status were minimized by using %WHC, since no effort was made to measure soil water potential for each site as a more biological meaningful index for soil water status. In addition, to account canopy control of the belowground CO2 efflux which may determine inter-daily variation (see Chapter 6), the daily radiation which obtained from the data-drill (SILO) was introduced.
7.2.5 Determining ecosystem carbon stock The ecosystem carbon stock and its component (carbon stored as biomass (living and dead components) above-belowground and carbon stored as soil carbon) for each plantation was reported as tonC/ha and then was compared across the 3 sites with contrasting productivity and soil types. This comparison was made by referring to the values reported in the middle of 2005 and thus all calculation of the biomass carbon (foliages, branches, stem and structural roots) that are derived from the allometric of stem diameter, was calculated based on the average of stem dbh measured (or interpolated) at mid-2005. The carbon stored as soil carbon was calculated down to
0.8 cm, by considering the percent of soil carbon and the bulk density for each soil depth. The standing litter was measured for each PSP. All of the biomass was converted into carbon mass by using conversion a constant factor of 0.5 (references). 7.2.6 Determining contribution of belowground CO2 efflux to the plantation carbon cycle The annual belowground CO2 efflux from different components was then presented in the context of plantation carbon cycle. Two different parameters of carbon cycle were used ie nett primary productivity (NPP) and gross primary productivity (GPP) at annual basis, especially during 2005 (January 1 – December 31). NPP = GPP – R where GPP is gross primary productivity and represents the total carbon input to the forest system as a result of photosynthesis, and R represents the total autotrophic respiration from woody tissues, foliage and roots. As no data was currently available to directly estimate GPP and components of respiration other than woody tissues (root and foliage) for the plantations studied, NPP was estimated from the measured carbon pools and fluxes of organic matter using: NPP = increase in biomass + litter fall + mortality + consumption
(Attiwill and
Neales 2006) In the plantations studied in this project mortality and above ground consumption was insignificant. Below ground fine root turnover (consumption) was not able to be determined. Above ground (ANPP) and below ground (BNPP) components of NPP were therefore calculated as: ANPP = ∆ woody tissue biomass + ∆ foliage biomass + litter-fall BNPP = ∆ root biomass The changes in woody tissue, foliage and structural root biomass at the Ullina and Napoleons plantations was determined using the allometric relationships developed between stem diameter and biomass (described in Appendix 1) and the measured stem diameter (dbh) growth between year 6 and 7. The changes in the non-structural (coarse- and fine-) roots was assumed to be zero as such non-structural roots density
were already peaked before the canopy closure (O’Grady et al., 2005). Fine-root turnover was estimated to be ….(references). At the time of writing a harvest of below ground biomass at the Ullina plantation had not been undertaken. The allometric relationship developed between structural root biomass and tree dbh for the Napoleons plantation was therefore also applied to the Ullina plantation. Litter collected from four traps established at each PSP of the two plantations for the year between age 6 and 7 was used to estimate annual loss of carbon from tree canopies due to litter fall.
7.3 Results Table 7.1 Carbon stocks (ton C/Ha) in the three E. globulus plantations at 7 years old and partitioning into its different components No
Parameter
Dohle
SE
Napoleons SE
Ullina
SE
4.75 12.94 na 48.84 66.53 na na 2.94 2.41 71.89
0.52 1.09 na 3.71 5.32 na na 0.53 0.20 5.35
3.97 5.63 na 19.48 31.42 na na 2.94 2.25 36.61
0.68 1.18 na 4.02 6.15 na na 0.53 0.26 6.92
2.38 2.87 na 10.07 17.02 na na
0.16 0.28 na 0.95 1.46 na na
8
Abovegrounds Tree foliages Tree branches dead branches Tree stems Aboveground live trees =(1+2+3) Dead stems Understorey Standing litter litter-fall 2004-05 Total aboveground =(4+5+6+7)
0.98 21.44
0.08 1.58
9 10 11 12 13 14
Belowground Fine-roots Coarse roots Structural roots Total roots = (9+10+11) SOC Total belowground =(12+13)
1.30 0.96 10.14 12.40 86.47 98.87
0.89 0.89 1.94 1.14
2.81 2.32 6.83 11.97 112.90 124.87
1.37 1.37 4.37 5.74
0.96 1.97 3.64 6.56 236.94 243.51
0.00 0.00 0.32 0.32 3.47 3.71
15 16 17
Ecosystem Total live tree = (4+12) Total vegetation =(8+12) Ecosystem total = (8+14)
78.93 84.29 170.75
6.22 6.24 4.51
43.39 48.58 161.48
7.52 8.28 12.65
23.58 28.01 264.95
1.78 1.89 5.19
0.84
-
0.72
0.02
0.72
0.01
0.85
-
0.75
0.01
0.77
0.01
0.42
0.02
0.22
0.02
0.08
0.00
0.46
0.02
0.26
0.02
0.09
0.01
0.51
0.02
0.70
0.03
0.89
0.01
1 2 3 4 5 6 7
Ratio Aboveground live tree/total live tree = (4/15) Total aboveground/total vegetation = (8/16) Total aboveground/ecosystem total = (13/17) Total live tree/ ecosystem total =(15/17) SOC/ecosystem total =(13/17)
Most of carbons stored in the biomass are in the form of woody materials (Table 7.1). The productivity amongst each sites was clearly shown by total biomass ie. Dohle>Napoleons>Ullina (78.93>43.39>23.58 ton C/Ha). Likewise, this difference in the productivity was also clearly reflected by the total live tree biomass: total ecosystem carbons, being lower for the less productive sites; ie. 0.46>0.26>0.09 (Dohle>Napoleons>Ullina). This also means that carbon stored as soil carbon have a greater proportional contribution for the lowest productive site (Ullina). Pattern of total ecosystem carbon, however, did not correspond with the productivity, since Ullina, being as the lowest productive site, in fact, contains the highest carbon pools 264.95 ton C/Ha. The belowground carbon pools which potentially serve as the autotrophic respiratory organs (roots) is greatest for the Dohle and Napoleon (12.4 and 11.97 ton C/Ha) and about half of those for the Ullina (6.56 ton C/Ha). This pattern was in opposite for the belowground carbon pools of heterotrophic respiration (soil carbon), being the highest for Ullina soils (236.94 tonC/Ha) and less than half of those found in Dohle and Napoleons soils (86.47 and 112.9 ton C/Ha). To be able to estimate total belowground CO2 efflux from the mixture sources (soil+root with and without litter) in different E. globulus plantations, periodical measurement of CO2 efflux was conducted. In accompanying with a continuous measurement of the meteorological parameters which control those efflux pattern for each site, this study allow an upscale those efflux at annual basis, given pattern of these efflux could be explained by the meteorological records. Such approaches have been implemented for the individual respiratory components
Dohle
60
day vs datalog temp 5 cm day vs moist 5 cm datalogg day vs measured resp
25
40
40
20
30
15
20
10
10
20
5 000
000
Napoleons
60
000
000
000
000
day 40
20
30
15
20
10
10
o
25
40
20
0
60
40
5 1/1/04
1/5/04
1/9/04
Ullina
1/1/05
1/5/05
1/9/05
0 1/1/06
day
25
40
20
30
15
20
10
10
20
0
5 1/1/2004
1/5/2004
1/9/2004
1/1/2005
time
1/5/2005
1/9/2005
0 1/1/2006
CO2 efflux (g/m2/day)
000
soil temperature ( C)
soil water content (% w/w)
0
Figure 7.1 Soil temperature, water status and CO2 efflux from the three key E. globulus plantation sites
Figure 7.1 shows the meteorological and CO2 efflux pattern for the three key sites. Seasonality of the soil temperature and soil water status was clearly shown across the three sites. The higher soil temperature was co-incidence with the lower soil water status during the summer periods, and vice versa, during the winter. In addition to this strong seasonality pattern, a pronounced variation for the inter-daily pattern was noted, especially for the soil temperature throughout the year. Such pronounced interdaily variation for the soil water status mainly occurred during spring periods. In addition, episodic rainfall event during the dry soil conditions seemingly contribute to the dynamics of soil water status, as shown by evidences of several peaks. Table 7.2 Effect of soil collar placement methods on the soil respiration rate normalized at 15oC (gCO2/m2/h) Site Dohle Napoleons Ullina
24 hours 0.37 0.86 0.33
permanent 0.33 0.79 0.27
% differences 5.71 4.24 10.00
P value 0.141 0.031
