Temperature effect on CARBON biomass in soils FROM TROPICAL AND TEMPERATE REGIONS

TEMPERATURE EFFECT ON CARBON BIOMASS IN SOILS FROM TROPICAL AND TEMPERATE REGIONS

C.A.Vasconcellos

CNPMS-Embrapa, C. P. 151, CEP: 35701-970 - Sete Lagoas, MG.

ABSTRACT: Four soils from various origins, (tropical and temperate regions) were amended with ¹⁴C labelled glucose (1mg C.g^-1 soil) and incubated at 15ºC and 35ºC to determine the temperature effect on the carbon turnover and on the microbial biomass. The temperature effect on the biomass increased with the glucose addition. The biomass mineralization rates were higher at 35ºC than at 15ºC and higher for Woburn and Pegwell soils (temperate region) than for Capinopolis and Janauba (tropical region). Specific respiration rate (SRR) of new biomass (from glucose) and old biomass showed different behaviors between soils. At 15ºC, the turnover C was 207, 225, 115 and 141 days for Janauba, Capinopolis, Woburn and Pegwell soil, respectively. At 35ºC, it was 92, 69, 69 and 33 days for the same soils. The residual ¹⁴C in the soil was higher at 35ºC. The final total biomasses at 15ºC and 35ºC were correlated with the initial soil carbon content. There was an average of 31 and 8 mg of biomass C.g^-1 soil organic carbon, respectively at 15ºC and 35ºC. The initial carbon content was an important factor to explain the mineralization rate at 35ºC.

Key Words: carbon biomass, organic carbon, soil, temperature, turnover

EFEITO DA TEMPERATURA SOBRE A BIOMASSA CARBONO EM SOLOS DE REGIÃO TROPICAL E TEMPERADA

RESUMO: Quatro solos de várias origens, (clima tropical e temperado), receberam glucose marcada com ¹⁴C, na proporção de 1mg de C.g^-1 de solo e foram incubados a 15ºC e a 35ºC com o objetivo de determinar o efeito da temperatura sobre a reciclagem do carbono orgânico e da biomassa microbiana do solo. O efeito da temperatura na biomassa e na taxa de mineralização foi mais evidente a 35ºC de incubação. A 35ºC, os solos Woburn e Pegwell, da região temperada, apresentaram uma taxa de mineralização maior do que a dos solos de Janaúba e Capinópolis, de região tropical. A taxa de respiração específica da biomassa formada pela adição da glucose e da biomassa antiga demonstrou comportamento diferencial entre solos. A 15ºC a reciclagem do carbono foi de 207, 225, 115 e 141 dias para Janaúba, Capinópolis, Woburn e Pegwell, respectivamente. A 35ºC, foi de 92, 69, 69 e 33 dias para os mesmos solos. O ¹⁴C residual foi maior a 35ºC. A quantidade de biomassa final a 15ºC e a 35ºC foi correlacionada com o teor de carbono inicial do solo. Houve uma média de 31 e 8 mg de biomassa C.g^-1 de carbono orgânico, respectivamente a 15ºC e a 35ºC. O teor de carbono inicial foi um fator importante para explicar a taxa de mineralização a 35ºC.

Descritores: biomassa carbono, carbono orgânico, solo, temperatura, reciclagem

INTRODUCTION

Soil microbial biomass is the living part of soil organic matter, other than living plant material and organisms greater than 500 mm³ in volume. It is the agent of organic residue breakdown and can be considered as the agent responsible for the cycling of plant nutrients. Estimation of biomass value as a percentage of total soil organic carbon varied from 0.8 and 4.0% as mentioned by Jenkinson & Ladd (1981) and Grisi (1997).

It is well known that soil organic matter is the key to successful and sustainable productivity of soils in the tropics. Soil organic matter affects positively structure, aggregation, cation exchange capacity, microbial activity and the water retention capacity. Furthermore, soil organic matter is the major nutrient storage site affecting the availability and mobility of macro- and micro-nutrients to the plants.

The transition from natural vegetation to agroecosystems resulted in an universal decline in total soil organic carbon. Henrot & Robertson (1994), measured the biomass-C in two humid tropical soils. Both soils showed a similar pattern in total soil organic matter and biomass carbon decline following vegetation removal; after 3 yr, total C and N were reduced by 20%.

Research is necessary on processes of organic matter decomposition and on the biomass activity to provide clues to improve the sustainability of the soil system. Our main aim in this work was to study the effects of incubation at different temperatures on the turnover time of the microbial biomass in four soils, two from temperate (i.e. United Kingdom) and two from tropical (i.e. Brazilian) climate.

MATERIAL AND METHODS

The soils used were surface samples (0 to 15 cm deep) and their characteristics, measured as Vettori (1969), are shown in TABLE 1.

The Janauba (Dystrophic Red Yellow Latosol) and the Capinopolis soil (Dystrophic Dark Red Latosol) were sampled from native vegetation on Minas Gerais State, Brazil. The Woburn soil (Haplic Arenosol) was sampled from the permanent grass plot at Rothamsted Experimental Station, Hapenden, Herts England and the Pegwell (Hapludalf) was sampled from a native site in Pegwell Bay, Ramsgate, East Kent , England.

After sampling and sieving (< 2 mm) the soils were stored at + 5ºC until use. Twelve portions of each moist soil at 40% of water holding capacity (WHC, ca. 0.01MPa ), each containing 20 g on an oven-dried soil (24h at 105ºC) basis were weighed into 50 ml glass jars. A solution of ¹⁴C -labelled glucose containing 1 mg C g^-1 soil (sp. act. 11.1 KBq mg C) and (NH4)₂ SO₄ to give a C: N ratio 15 : 1 was added. The labelled - glucose was added into the soil with a syringe as described by Vasconcellos (1994).

The moist soils were placed separately in a brown wide - necked bottle with a vial of 20 ml of 1N NaOH and 30 ml of distilled water was added to the bottom of the bottle. The bottle was, then, closed with a rubber bung. The NaOH vials were replaced after 7-8 days.

After glucose addition the portions of soils were incubated for 25 days at 15ºC. Then, six portions of each treatment were transferred to a 35ºC room and randomly distributed within three blocks, each of which contained two replicates of each soil, one to be extracted before and one after fumigation.

Another six portions of each treatment were kept in 15ºC and were also randomly distributed as above.

Biomass C was measured by fumigation-extraction (Vance et al. 1987; Jenkinson & Powlson 1976; Jenkinson & Ladd 1981; Wu et al.1990) was determined at 0, 20, 40, 60, 82 and 90 days for the soils incubated at 15ºC and at 2, 9, 15, 30, 50 and 70 days for soils incubated at 35ºC.

CHCl₃ fumigation of the soils was done in a large desiccator containing 30 ml of alcohol-free CHCl₃. A moist filter paper was left in the bottom of the desiccator. The moist soils were exposed for 24h at 25ºC to CHCl₃, later removed by repeated evacuation. The soils biomass were then extracted by shaking for 30 min with 0.5 M K₂SO₄ (1:4 soil: solution ratio). The unfumigated control samples were extracted with 0.5 M K₂SO₄, as described before, at the time fumigation commenced.

K₂SO₄- extractable organic C was measured by an automated UV-persulphate oxidation, as described by Wu et al. (1990). Biomass C (BC), was calculated from the relationship:

BC = 2.22 Ec (Equation 1)

Where Ec = [(organic C extracted by K₂ SO₄ from fumigated soil) - (C extracted by K₂SO₄, from non fumigated soil)]

For biomass ¹⁴C determinations, 0.75 ml aliquots of the K₂SO₄ soil extract solutions were placed into plastic scintillation vials with 20ml of Ultima Gold - TM (Packard Inst., Groningen, The Netherlands) as scintillation cocktail, sealed and mixed until clear. Samples were counted in a liquid scintillation counter (2500 TR Liquid Scintillation Analyser Packard) to a 2 Sigma value of 1.00 % or for 5 min. The counts detected (cpm) were converted to disintegration per minute (dpm) using a quench curve in the program for ¹⁴C counting. The quench curve efficiency was 88.4%. Biomass ¹⁴C was calculated as described above. The quantity of C- labelled biomass (¹⁴C-Bc) synthesized from residual ¹⁴C-labelled metabolities was calculated from equation 2, Wu (1990),

(Bl) _s = {[(Bl)_o - (Bl)_t ]*(Bc)_t *(SA)_m }/

[ (Bl)_o * (SA)_g - (Bc)_t (SA)_m]

(Equation 2)

Where:

t = the incubatiom time (days) (from day 25 onwards);

(Bc)_t = Total biomass C measured at day t;

(Bl)_o = ¹⁴C ^- Bc measured at the start (day 25, t=0);

(Bl)_t = ¹⁴C - Bc measured at day t;

(Bl) _s = The amount of ¹⁴C - Bc which was synthesised from the residual ¹⁴C - labelled metabolites in the total¹⁴C - Bc measured at day t.

(SA)m= the ¹⁴C - specific activity of the metabolite fraction available for synthesis of new biomass. This was taken to be the same as the ¹⁴C-specific activity of the K₂SO₄-extractable organic - C in the unfumigated soil.

(SA)g = the ¹⁴C- specific activity of the glucose originally added.

The Biomass specific respiration rates were calculated from total CO₂-C evolved during the incubation within the different time intervals divided by the average of the amounts of biomass carbon measured at the first and last day of the interval and divided by the total of days in each interval period.

The mean separation was done by Duncan's Multiple test at 5% of probability.

RESULTS AND DISCUSSION

The biomass development: The biomass (¹²C, total ¹²⁺¹⁴C and ¹⁴C) in the end of incubation time at 15ºC and 35ºC and their variation during the incubation period are shown in TABLE 2 and Figure 1.

Figure 1 - Biomass C variation at 15ºC as incubation temperature. Symbols are for experimental points and complete lines to fitted points. Biomass value for each individual soil followed by different letters are significantly different, (Duncan p < 0.05).

The biomasses decreased as the temperature increased. With glucose, the total ¹²⁺¹⁴C biomass decreased by 79%, 64%, 79% and 73%, for Janauba, Capinopolis, Woburn and Pegwell soils respectively, following incubation at 35ºC . Without glucose, the ¹²C biomass decreased only 43%, 52%, 67% and 31%.

The new biomass formed with the glucose addition, represented by ¹⁴C biomass decreased 72%, 57%, 72% and 67% when the temperature increased for Janauba, Capinopolis, Woburn and Pegwell soils. Thus, the biomass which developed following glucose addition seemed more sensitive to higher temperatures than the original biomass. Joergensen et al. (1990), also, showed declining of biomasses in temperate soil, when the incubation was done at 35ºC.

The changes in the pattern of microbial biomass could affect the N mineralization and immobilization processes of each soil during the plant development.

At 15ºC, with glucose, the Janauba and Pegwell soils showed the same biomass pattern from native organic C; Capinopolis and Woburn soils decreased the biomass native organic ¹²C, (TABLE 2). The same behaviour was reported by Anderson & Domsch (1985) and Brookes et al. (1987). The corresponding decline at 25ºC was 16-45%.

Broadbent (1953) reported an increase in the rate of decomposition of native organic matter when addition of plant material was made, i.e., there was a positive priming effect.

Thus, the soil priming effect following glucose addition seemed to have different behaviour at 15ºC. After an initial fluctuation, soil microbial biomass apparently drifted slowly upward at 15ºC only for Woburn and Pegwell soil in unamended and amended soil. It could be said that no significative variation was observed after 20 days of incubation. After this period, Janauba and Capinopolis soils, from tropical regions, apparently increased in a linear pattern (Figure1) probably in consequence of the cryptic growth of the microbial population.

For a given cohort of biomass, the ¹⁴C decay can be expressed by the first-order equation:

BCt = BCo* e^-k't, ( Equation 3)

Where

BCo = the initial amount of biomass ¹⁴C at time = 0;

BCt = the amount remaining after time t;

k' = the decay constant rate per day of initial biomass;

For this system,

T ( turnover) = 1/k' ( Equation 4)

These equations, as mentioned by Wu (1990) are true only for a biomass cohort which do no replenished as the organism die and it is necessary to correct the decay constant rate (k value).

At 15ºC there was a decline in ¹⁴C biomass as described by the equations:

R² % Half-live(t_1/2) Janauba B- ¹⁴C=126.5 *
e^{-(0.0047 ± 0.0008)*t} 89 *** 147 days CapinopolisB- ¹⁴C= 122.7 *
e^{-(0.0042±0.0008)*t} 87*** 165 days Woburn B- ¹⁴C= 175.9 *
e^{-(0.0008±0.0007)*t} 98*** 87 days Pegwell B- ¹⁴C= 270.4*
e ^-(0.007 ± ^0.0006*t)), 97 *** 97 days

At 35ºC the ¹⁴C biomass were fitted to the following equations:

R² % Half-live(t_1/2) Janauba B- ¹⁴C = 90.01*
e^-(0.0305 ± ^0.0036)*t, 83^*** 23 days Capinopolis B- ¹⁴C = 104.6*
e^-(0.0241 ± ^0.002)*t, 96^*** 29 day Woburn B- ¹⁴C = 148.4*
e^-(0.0409 ± ^0.0019)*t, 99^*** 17 days Pegwell B- ¹⁴C = 196.4*
e^-(0.0485 ± ^0.006)*t, 92^*** 14 days

At 35ºC the total amended biomass (BCt) showed a decline as described by:

R² % Half-live(t_1/2) Janauba BCt = 159.07*
e ^{-(0.018 ± 0.003)*t} 84^*** 39days Capinopolis BCt = 320.05*
e^{-(0.0149 ± 0.0149)*t} 86^*** 47 days Woburn BCt = 361.40*
e^{-(0.028 ± 0.0045)*t} 88^*** 25 days Pegwell BCt = 749.95*
e^{-(0.017 ± 0.003)*t} 90^*** 41 days

From these equations it is possible to make some observations as the temperature effect on biomass. The k'values were higher at 35ºC than at 15ºC. For the ¹⁴C biomass, the k values indicate higher mineralization rates for temperate Woburn and Pegwell soils, and smaller ones for tropical Janauba and Capinopolis. However, the Co values (initial ¹⁴C biomass) were higher for Woburn and Pegwell. It means that the reserve of microbial carbon and available energy was greater in English soils than in the Brazilian ones, emphasising the observation of the sensibility of biomass from temperate soils to high temperatures (Jenkinson et al. 1991, Grisi 1997).

The half-life was calculated from t(1/2) = ln2/k'

When an energy source is added to the soils, normally the micro-organisms multiply rapidly until the substrate is nearly exhausted and then decrease again. The CO₂ evolution must have the same pattern. However, the pattern seen in these experiments was different as shown by the specific respiration rate - (mg CO₂ evolved. * g^-1 * biomass * day^-1), TABLES 3 and 4. The data for CO₂ evolved are in Vasconcellos (1994).

Either for unamended soils at 15ºC, the specific respiration rate (SRR) had decreased - not in a linear pattern, throughout of incubation time, - from 43 to 12, 8 to 3, 34 to 15 and 15 to 6 mg CO₂/g of biomass per day for Janauba, Capinopolis, Woburn and Pegwell soil, respectively.

The Janauba and Capinopolis biomass showed an increasing pattern; Woburn and Pegwell a constant biomass pattern, (Figure 1).

The SRR was constant, during the incubation period, for amended soils at 15ºC. The SRR for Janauba were 39 and 16 mg CO₂/g of total and ¹⁴C biomass per day, respectively; for Capinopolis, 47 and 22 mg of CO₂/g per day, for Woburn, 48 and 13 mg of CO₂/g per day and for Pegwell 39 and 8 mg of CO₂/g of biomass per day. In a sharp contrast, biomass ¹⁴C felt during the incubation period. The biomass at 35ºC did not show a specific SRR pattern in unamended soils. For amended soil without the first point as initial temperature adaptation, the SRR showed constant values from 15 to 70 days. For Capinopolis it was 57 and 41 mg of CO₂ g^-1 total biomass and ¹⁴Cbiomass per day.

Janauba, (88 mg.g^-1.day^-1), Woburn (95 mg.g^-1.day^-1) and Pegwell (57 mg.g^-1.day^-1) showed constant values for total biomass and a significant linear increase for ¹⁴C biomass (new biomass). The slope b values from fitted linear equations were 0.313 (± 0.08), 0,95 (± 0.155) and 1,57 (±1.57) mg of CO₂.g^-1C biomass.day^-1, respectively.

As pointed out by Santruckova & Straskraba (1991) the increase of specific respiration could be due to effect of stress. But, the water content of the soils at 15ºC varied from 50 to 46% of WHC; at 35ºC the range was 50% to 31% of WHC. However, during the last week of incubation period the water content was corrected by weight. By another way it is possible that energy for biomass maintenance and CO₂ evolved is coming from various sources (carbon inputs, organic matter, dead microbial cells, endocellular reserve, etc) as showed by Grisi (1997).

As point out by Jenkinson et al. (1991) changes in the climate pattern can alter the total carbon storage in soil. In this way, there is a characterisation of vicious cycle, global warming will accelerate the decomposition of soil organic matter, the CO₂ thus formed further increasing global warming and decreasing the soil biomass microbiology.

These results also emphasise the importance of management practice on soil organic carbon and with the possibility to quantify the organic matter quality, mainly knowing that continuous crop (and plowing processes) cause losses in the natural reserves of organic matter (Zadorin &Yumagulova, 1983).

The biomass turnover: As point out by Jenkinson & Parry (1989) the internal recycling of biomass metabolites can affect the estimate of turnover time. In consequence the uncorrect rate (k') or the degra-dation of ¹⁴C-Biomass C was first obtained by fitting a first- order regression equation to the residual ¹⁴C-Biomass C as shown in the equations above.

The true rate constant of ¹⁴C-Biomass C degradation (k) in the soils can be obtained from measured ¹⁴C-Biomass C subtracted from the ¹⁴C-Biomass C synthesised from metabolities as shown in TABLE 5. The ¹⁴C-biomass synthesised was calculated by the equation 2 with the specific activities data in TABLE 6.

The declines in total biomass C during the incubation period was faster in amended soils than in unamended control soils. As Wu (1990) has pointed out, it means that with new energy source the turnover of biomass C would be faster than in unamended soil. The faster decline of total biomass C with glucose is due to the enhanced turnover of biomass due to the residual effects of glucose incorporation. The rate of biomass C turnover in unamended soils can be calculated from the following formula:

D

P = e^{- kt} - e^-kut

Where

D

k and ku are the rate constants of biomass turnover in amended and unamended soils, respectively at a specific time (t) in days. The ku value is determined by solving the equation above.

Unfortunately, at 15ºC, the results observed for the biomass C in unamended treatments were not consistent. During the incubation period the microbial population could use the soil organic matter as substrate for its development (Grisi, 1997).

In the amended treatments, after a decline from time 0 to time 20, the total biomass showed an increasing linear pattern for all soils, except for Capinopolis, which showed no specific biomass variation.

At 35ºC, the corrected constant rate of Biomass C turnover for unamended soil are given in TABLE 7 and Figure2. The biomass in the Janauba and Capinopolis soils had turnover times (1/ku) of 92 and 69 days; Woburn and Pegwell, 69 and 33 days. Thus, biomass C in the Pegwell soil was most sensitive to temperature.

Figure 2 - Biomass C variation at 35ºC as incubation temperature. Symbols are for experimental points and complete lines to fitted points. Biomass value for each individual soil followed by different letters are significantly different, (Duncan p < 0.05).

It can be calculated from ku values in TABLE 7 that, at 35ºC, about 19% (100 * (1 - e^{-ku* 20})) of the biomass C is replaced during turnover in 20 days in Janauba unamended soil, 25% in Capinopolis, 25% in Woburn soil and 45% in Pegwell soil. Van Veen et al. (1981) showed that clay soils have a greater capacity to preserve microbial biomass than sandy soils. In our experiment, the clay content did not vary widely (16.7% to 25.4%).

At 15ºC, for amended soils, the turnover time were 207 days for Janauba, 225 day for Capinopolis, 115 days for Woburn and 141 days for Pegwell . According to Jenkinson et al. (1987), the air temperature conversion factor is 3.81 at 25ºC. If that factor is used to convert the turnover times of biomass C obtained in the laboratory to those expected under field conditions, the following values are obtained at 15ºC: Janauba, 2.2 years; Capinopolis 2.3 years; Woburn, 1.2 years and Pegwell 1.5 years. Wu (1990) showed biomass C turnover times of 1.3 years in the Woburn soil, and 1.8-2.2 years in a range of soils from Rothamsted.

TABLE 8 gives the ¹⁴C balance. Probably because biomass activity was lower at 35ºC the ¹⁴C remaining was higher at 35ºC. It is necessary to note that the total ¹⁴C evolved at 15ºC and at 35ºC was very close.

Biomass and Correlations: Final total biomass at 15ºC and 35ºC were very close correlated with initial soil carbon content, (r values were 0.982 and 0.992, respectively), but not with the clay content. There was an average of 31 and 8 mg of biomass C.g^-1 of C, respectively at 15 and 35ºC . The residual ¹⁴C was closely correlated with clay content at 15ºC. No correlation were found at 35ºC. There was correlation between the decomposition rate at 35ºC and the initial soil organic carbon.

CONCLUSIONS

- The biomass mineralization rates were higher at 35ºC than at 15ºC and higher for Woburn and Pegwell (soils from temperate region) than for Janauba and Capinopois (soils from tropical region).

- The specific respiration rate of new biomass from glucose and old biomass showed different behaviour between soils, but not between temperate and tropical region;

- At 15ºC, the turnover C was 207, 225, 115 and 141 days for Janauba, Capinopolis (soils from tropical regions), Woburn and Pegwell (soils from temperate regions) respectively. At 35ºC, it was 92, 69, 69 and 33 days for the same soils. These results emphasised the effect of global warming on soil biomass and on the stability of the soil quality.

- The residual ¹⁴C remaining in the soil was higher at 35ºC.

- The final total biomasses at 15ºC and 35ºC were correlated with the initial soil carbon content. There was an average of 31 and 8 mg of biomass C.g^-1 soil organic carbon, respectively at 15ºC and 35ºC.

- The initial carbon content was an important factor to explain the mineralization rate at 35ºC being important for soil management practice to improve the soil organic matter and the sustentability of food production

ACKNOWLEDGEMENTS

C. A. Vasconcellos would like to thank CNPq, Brazil, for its financial support; Rothamsted Experimental Station, Harpenden, Herts, England for Laboratory conditions and Dr. Phillip Charles Brookes for suggestions.

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Recebido para publicação em 11.12.96

Aceito para publicação em 13.01.98

Publication Dates

• Publication in this collection
  12 Nov 1998
• Date of issue
  Jan 1998

History

• Accepted
  13 Jan 1998
• Received
  11 Dec 1996