University of São Paulo

“Luiz de Queiroz” College of Agriculture

Center of Nuclear Energy in Agriculture

Effects of UV-B radiation on plant litter decomposition in a tropical

ecosystem on the north coast of the State of Sao Paulo, southeast Brazil

Osmarina Alves Marinho

Dissertation presented to obtain the degree of Master in

Science. Area: Applied Ecology

Piracicaba

2017

Osmarina Alves Marinho

Bacharel’s degree in Biological Science

Effects of UV-B radiation on plant litter decomposition in a tropical ecosystem on the

north coast of the State of Sao Paulo, southeast Brazil

versão revisada de acordo com a resolução CoPGr 6018 de 2011

Advisor:

Prof. Dr. LUIZ ANTONIO MARTINELLI

Dissertation presented to obtain the degree of Master in

Science. Area: Applied Ecology

Piracicaba

2017

2

Dados Internacionais de Catalogação na Publicação

DIVISÃO DE BIBLIOTECA – DIBD/ESALQ/USP

Marinho, Osmarina Alves

Effect of UV-B radiation on plant litter decomposition in a tropical ecosystem on the north coast of the State of Sao Paulo, southeast Brazil / Osmarina Alves Marinho. - - versão revisada de acordo com a resolução CoPGr 6018 de 2011.- - Piracicaba, 2017.

35 p.

Dissertação (Mestrado) - - USP / Escola Superior de Agricultura “Luiz de Queiroz”. Centro de Energia Nuclear na Agricultura.

1. Ecossistemas tropicais 2. Fotodegradação 3. Radiação UV-B 4. Carbono 5. Decomposição da serapilheira I. Título

3

ACKNOWLEDGMENT

I express my sincere thanks to Luiz Antonio Martinelli for idealizing and building this work

with me. Having him as a mentor was a very gratifying and enriching experience.

To all my family, especially my father Osmar Candido Marinho and my mother Raimunda

Alves Marinho, my eternal gratitude for believing in my work and supporting my decisions.

To the Lafratta & Calandrelli family, especially to my life partner, Lucas Lafratta Calandrelli,

for believing in my being and helping me to be a better person on this planet.

To the friend and therapist Ana Toniolo, for welcoming me in the difficult moments and

making of me a person with more empathy.

To Professor Dr. Paulo J. Duarte Neto and his students of the Federal University of

Pernambuco, for providing technical support.

To the professors and technicians of the Laboratory of Ecology Isotopic, for all help in the

field and analysis of samples. To all the lab colleagues for all the help, thank you very much.

To Professor Humberto Rocha and his team from the Department of Atmospheric Sciences of

IAG-USP, for the data provided and guidelines.

To my second family Marikota, for helping me at all times since 2006.

To the program of Applied Ecology and to the secretary Mara, for the academic support

during those years.

To the Foundation of Amparo and Research of the State of São Paulo (FAPESP) for the

financial support granted in the form of a regular scholarship - Process 2015 / 00971-1 and by

the research grant abroad BEPE - Process 2016 / 09698-9, provided even in times “fearful”.

Without this grant this project could not became a reality.

To the University of California, Santa Barbara campus and to Professor Jennifer King for the

reception and support during my internship.

To Henrique, Fernanda and Gabriel for keeping me safe and happy during my time in

California.

To all who participated in my life and contributed to the development of this master's project.

Thank you so much.

4

SUMMARY

RESUMO .................................................................................................................................................................5

ABSTRACT .............................................................................................................................................................6

1.INTRODUCTION .................................................................................................................................................7

2.MATERIAL AND METHODS ............................................................................................................................9

2.1 STUDY SITE AND SAMPLE COLLECTION…………..…..……………………………………………….9

2.2 EXPERIMENTAL DESIGN .............................................................................................................................9

2.2.1 RADIATION AND BIOCIDE TREATMENTS ………………………………………..…………….........9

2.3 MASS LOSS AND LITTER CHEMISTRY ANALYSIS ……………………………………………….......11

2.4 DECOMPOSITION MODEL ..........................................................................................................................12

2.5 DATA ANALYSIS METHOD ........................................................................................................................12

3.RESULTS ............................................................................................................................................................15

3.1 ENVIRONMENTAL VARIABLES ................................................................................................................15

3.2 SOIL MICROBIAL BIOMASS C ...................................................................................................................18

3.3 DECOMPOSITION RATE AND REMAINING MASS ................................................................................18

3.4 LITTER CHEMISTRY ....................................................................................................................................22

4. DISCUSSION ....................................................................................................................................................29

4.1 LITTER DECOMPOSITION RATE ...............................................................................................................29

4.2 LITTER CHEMISTRY ....................................................................................................................................30

REFERENCES .......................................................................................................................................................32

5

RESUMO

Efeito da radiação UV-B na decomposição da serapilheira em um ecossistema tropical no

litoral norte do Estado de São Paulo

A radiação solar em geral e a radiação ultravioleta (UV) em particular têm

sido reconhecida por estimular a decomposição da serapilheira através da

mineralização fotoquímica de moléculas fotossensíveis, como a lignina,

facilitando a decomposição microbiana, com um papel de grande relevância em

ecossistemas áridos onde a atividade microbiana é baixa, no entanto pouco se sabe

como a fotodegradação pode influenciar outros ecossistemas como por exemplo

os mais úmidos e sob quais condições a fotodegradação é favorecida, portanto os

mecanismos ainda não foram estabelecidos. Decomposição em ecossistemas

tropicais é um processo complexo e pode ser influenciado por vários fatores

ambientais e com certas diferenças quando comparada com ecossistemas áridos e

semiáridos. Para avaliar os mecanismos subjacentes à fotodegradação via radiação

UV-B, um experimento de campo de 300 dias foi estabelecido em um ecossistema

tropical com alto índice de precipitação anual onde a serapilheira foi exposta a três

níveis de radiações diferentes, combinada com um tratamento com biocida.

Resultados mostram que a remoção da radiação UV-B desacelerou a

decomposição da serapilheira durante o último estágio do experimento comparado

com a serapilheira exposta a radiação ambiente, no entanto a serapilheira quando

sombreada teve perda de massa similar à exposta a radiação ambiente. Além

disso, diferenças na taxa de decaimento entre os tratamentos de radiação devido

ao efeito da radiação UV-B foram independentes da perda de lignina. No geral,

nosso estudo sugere que a radiação UV-B contribui com a decomposição da

serapilheira através da perda de carbono, no entanto não teve efeito na perda de

massa de nitrogênio, lignina e celulose. Portanto, mais estudos são necessários

para investigar o efeito positivo e negativo da exposição à radiação UV-B na

atividade microbiana e na decomposição da serapilheira em ecossistemas

tropicais.

Palavras-chave: Ecossistemas tropicais; Fotodegradação; Radiação UV-B; Carbono;

Decomposição da serapilheira

6

ABSTRACT

Effects of UV-B radiation on plant litter decomposition in a tropical ecosystem on the

north coast of the State of Sao Paulo

The solar radiation in general and UV radiation in particular has been

recognized to stimulate plant litter decomposition through photochemical

mineralization of photosensitive organic molecules, such as lignin, facilitating

microbial decomposition, with great relevance role in dryland ecosystems where

microbial activity is low, however little is known about how photodegradation

could influence other ecosystems without moisture limitations and under what

conditions may be favored, therefore the mechanisms has not yet been

established. Decomposition in tropical ecosystem is a complex process and can

be induced by a number of environmental factors with certain differences when

compared to arid and semi-arid ecosystems. To assess the mechanisms underlying

UV-B photodegradation, we designed a 300 days field experiment at a tropical

ecosystem with high levels of annual precipitation and exposure litter to three

levels of radiation combined with a biocide treatment. Results show that the

removal of UV-B radiation decelerated plant litter decomposition during the later

stage compared to litter exposure to full sun, however shaded litter had similar

mass loss compared to litter exposed to full sun. Furthermore, differences in the

decay constant among radiation treatments due to UV-B effect is independent of

lignin loss. Overall, our study suggest that UV-B contributes to the plant litter

decomposition through carbon losses, however, had no effect on nitrogen, neither

lignin nor cellulose loss. However, more studies are needed in order to investigate

the positive and negative effects of UV exposure on microbial activity in tropical

ecosystems.

Keywords: Tropical ecosystem; Photodegradation; UV-B radiation; Carbon; Plant litter

decomposition

7

1. INTRODUCTION

Annually, plant litter and soil organic matter decomposition releases to the

atmosphere ten times more carbon than fossil fuels combustion (IPCC, 2007; Lee et al.,

2012.). On the soil, plant litter can be decomposed either by microorganisms or by solar

radiation. Pauli (1964) was the first one to raise the possibility of organic matter

photodegradation, however only in the last decade photodegradation on terrestrial ecosystems

has become more investigated (King et al., 2012). Consequently, much less is known about

the role of solar radiation on plant litter decomposition, than the role of microorganisms on

this process (Austin et al., 2016).

The ultraviolet radiation B (UV-B) is considered to have the largest impact on

ecological processes, affecting several regulatory process on organisms such as growth and

reproduction (Caldwell et al., 1998; Brandt et al., 2009; Ballaré, 2011; Day et al., 2015).

Atmospheric conditions modulates the intensity of UV-B radiation; changes in ozone layer,

cloud cover, aerosol concentration are likely to change UV-B radiation (Ballaré, 2011). In

turn, such atmospheric properties are also heavily influenced by changes in land use, such as

deforestation.

UV-B radiation is especially important for decomposition in arid and semiarid

ecosystems, where microbial activity is low (Austin & Vivanco, 2006; Day et al., 2007; Smith

et al., 2010). In these regions, UV-B not only cause direct photolysis, but also cause indirect

photolysis, by breaking recalcitrant compounds, which consequently enhances the

susceptibility of litter tissue to microbial decomposition (Austin & Ballaré, 2010; King et al.,

2012; Austin et al., 2016, Wang et al.,2015; Wang et al., 2017). For instance, it has been

suggested that lignin is susceptible to photodegradation due to UV radiation absorption by

aromatic rings (Austin & Ballaré, 2010). However, studies have shown contradictory results

up to now (Baker et al., 2015).

As already mentioned, the effects of solar radiation on plant litter decomposition is

much less investigated than microbial degradation (Pancotto et al., 2003; Austin et al., 2016;

Wang et al., 2017). This is especially true in wetter ecosystems of the subtropical and tropical

regions, since most studies has been conducted in arid zones of the globe. Furthermore, even

though the predicted increase of UV radiation is small in the tropics, this region receives more

solar radiation across the year than mid and high latitudes (Bias et al., 2015). Therefore,

understanding how solar radiation affects plant litter decomposition in other regions of the

8

globe is important to better understand changes in biogeochemistry processes and how global

changes would impact it (Almagro et al., 2015).

We conducted here a series of litterbags experiments to investigate the effect of

photodegradation in a tropical region where the mean annual precipitation is approximately

2000 mm. Our main questions are if photodegradation would accelerate decomposition rates

(k) and would alter litter quality during litter decomposition.

9

2. MATERIAL AND METHODS

2.1. Study site and sample collection

This study was conducted in the State Park of Serra do Mar at the Santa Virginia

research station, São Paulo, southeast Brazil (23°24’S; and 45°11’W). The Serra do Mar is a

mountain rift system located Dense Montane Forest (evergreen rainforest) and soils were

classified cambisols haplics dystrophics at the end of the Atlantic plateau forming coastal

scarps and features a humid tropical climate. The vegetation in Santa Virginia was classified

according to the Brazilian classification system as Ombrophylus (Martins et al., 2015). The

mean altitude is 1000-1100 m elevation above sea level, and the mean annual precipitation

and temperature are 1990 mm, and 16 ºC, respectively (Rocha, H; nonpublished data). The

site of this experiment, known as Puruba, is located in an abandoned pasture under natural

regeneration process nearby the forest edge, but not shaded by this forest. Senescent leaves of

Tibouchina sellowiana, a dominant pioneer tree species in the forest, were collected in August

2015 and the initial litter chemistry was determined on four replicates.

2.2. Experimental design

2.2.1. Radiation and biocide treatments

The litter decomposition experiment was initiated in 17th

September 2015, early

rainy season. We placed 10 g of senescent leaves in each litterbag (nylon with mesh 2 mm).

To manipulate radiation, we placed 1 m2-PVC frames at 40 cm from the soil surface, covered

with plastics films with different transmittance according to the radiation treatments (Austin

& Vivanco 2006). We established three of them: (1) full sun (Aclar films which transmits

>90% of total solar radiation); (2) UVB[-]

(Mylar films, 125 mm thickness (Mylar® DuPont)

which blocks UV-B spectrum (280 -315 nm)), and (3) shade (Mylar films covered with

aerosol paint, which blocks > 90% of solar radiation). Each treatment had four replicates (i.e.

four PVC frames), totaling 12 frames (3 treatments x 4 replicates). We set two blocks

containing 12 frames approximately 200 m apart from each other (Figure 2).

In order to investigate if microbial activity could mask the effect of UV-B radiation

on decomposition, half of the PVC frames in each block were sprayed with an aqueous

10

combination containing a dose equivalent of 15 g m-2

of fungicide (Ortocide, N-

(trichloromethyltio phthalimide) and 15 g m-2

of bactericide (Casugamicina) (Figure 2).

Usage of biocide compound was made five times over the course of the experiment.

Afterwards, as suggested by the literature, we also incorporated naftaline pallets into the soil

to help control microbial activity (Austin & Vivanco, 2006; García-Palacios et al., 2013).

Distilled water was sprayed in the other half of PVC frames.

Soil microbial biomass C were determined according to the fumigation-extraction

method described by Vance et al. (1987) in all PVC frames at the end of the experiment in

order to evaluate the potential of biocide in suppressing microbial biomass and the effects of

UV-B radiation on this population. Initially, the plastics transmission properties were tested

using a spectrometer (USB4000, Ocean Optics). Under each PVC frame, all above-ground

vegetation was removed to avoid shading. Perforations were made in the plastics to allow

precipitation and plastics were periodically evaluated for damage and replaced when it was

the case. During the day, three repeated measures of soil temperature under the PVC frames

was performed using a thermometer buried at 5 cm depth. Soil moisture was also measured

using the gravimetric method in each collecting date. Total solar radiation and mean annual

precipitation during the experiment time were obtained from an eddy-covariance flux tower

located near the study area (Rocha, H; non-published results).

Figure 1. PVC frames covered with plastic films on the top according to the treatments (full sun, UVB[-

] and shade).

11

Figure 2. Illustration of the experimental design representing the litterbags above the PVC frames placed on the

area for each treatment. Radiation treatments are represented by different colors: yellow - Full sun treatment,

green - UVB[-]

treatment, grey – shade treatment. The biocide treatment is represented by the red PVC frames

and control by the grey color.

2.3. Mass loss and litter chemistry analysis

In order to estimate mass loss during litter decomposition, one litterbag from each

PVC frame (four litterbags per treatment) was randomly collected at 14, 35, 66, 217 and 300

days after the beginning of the experiment (Figure 2). Litterbags were cleaned and oven dried

at 50 º C for 48 h before weighing. Subsamples of litter for each litter bag were ashed at 600 º

C to express remaining mass on an ash-free dry mass basis. For determination of carbon and

nitrogen concentrations in the litter, a subsample of it was ground to power and then

encapsulated for analysis using an elemental analyzer (Carlo Elba model 1110, Milan, Italy).

For chemical analyses of lignin, cellulose and total polyphenols; acid detergent fiber (ADF)

and lignin were determined following the methodology proposed by Van Soest et al. (1991);

cellulose was calculated by the difference between ADF and lignin; and total polyphenols was

evaluated according to Makkar (2000).

12

2.4. Decomposition model

Three decomposition models were previously tested: single exponential equation

(1E) (i) proposed by Oslon (1963); double exponential (2E) (ii) proposed by Lousier &

Parkson (1976); and exponential deceleration (D) (iii) proposed by Rovira & Rovira (2010).

X = X0𝑒−𝑘𝑡 ............................................ (i)

X = 𝑎𝑒(−𝑘1t) + (1 − 𝑎)𝑒(−𝑘2t) ......... (ii)

X = X0 𝑒−(𝑎t−

𝑏

𝑚(𝑒

−𝑚t

−1)).......................(iii)

For equation (i), t is for time of decomposition expressed in years; Xt is the litter

mass at time t; and X0 is the initial litter mass. The double exponential model (equation ii)

considers two constant decomposition rates, where a is the initial amount of the labile pool; k1

is the first decomposition rate, 1-a is the initial amount of recalcitrant pool; and k2 is the

second decomposition rate. The exponential deceleration (equation iii) also considers a single

compartment and describe a decrease of decomposition rate along the time with changes in

the substrate from a labile to more recalcitrant organic matter compounds, where m represents

decreasing rate of k; a is the basal rate; and b is the range of rate variation.

2.5. Data analysis method

A set of possible models were developed in order to determine the effect of the

treatments and their interactions on decomposition. Models were evaluated using Aikake’s

Information Criterion (AICc) in which the model that had the best fit with the lowest number

of variables would had the lowest AIC value and differences were considered when ΔAIC

was more than two units (Burnham & Anderson 2002; Hobbs & Hilborn, 2006).

For remaining mass and quality litter parameters (C, N, lignin, cellulose and total

polyphenols) the results were presented as percent of their initial value against time of

decomposition. In order to investigate temporal changes in litter quality, a preliminary three-

ways analysis of variance (ANOVA) found significant interaction between the treatments and

time of decomposition, indicating that data should be checked for each period of time

independently ( Lin & King, 2014; Lin et al., 2015). For differences in soil temperature, soil

moisture and soil microbial biomass we also used analysis of variance (ANOVA). Data were

tested for normality using Shapiro-Wilk test, log transformations was used for cellulose and

13

total polyphenols. The post-hoc Tukey’s HSD (Honestly Significant Differences) test was

used to compare means. The effects were considered statistically significant at p < 0.05.

Statistical analysis were conducted using R (version 3.3.2).

14

15

3. RESULTS

3.1. Environmental variables

In general, there were not significant differences on soil temperatures among

radiation treatment nor soil moisture (Figure 3a and b). Solar radiation, monthly precipitation

and monthly mean air temperature followed the typical pattern found for the latitude and

longitude of the experiment location (Figure 4a and b).

16

Figure 3. Effect of the radiation treatments on soil. (a) Soil temperature (ºC) and (b) gravimetric soil moisture

(%) along the experiment. Measurements were taken during the collecting dates. Values are mean ±SE (n=8).

15

17

19

21

23

25

27

29

31

14 35 66 217 300

So

il t

emp

erat

ure

( °

C)

Time (days)

Full Sun UVB[-] Shade

b

a

(a)

0

10

20

30

40

50

0 35 217 300

So

il M

ois

ture

(%

)

Time ( days)

Full Sun UVB[-] Shade(b)

a

17

Figure 4. Solar radiation condition and climate conditions of the study area. (a) Mean monthly total solar

radiation for the experimental time and (b) mean monthly temperature and accumulated precipitation for the

experimental time. * Numbers represent the collecting dates after the beginning of experiment.

5

7

9

11

13

15

17T

ota

l so

lar

rad

iati

on

( M

J m

-2 d

ay-1

)

(a)

0

50

100

150

200

250

300

350

400

0

5

10

15

20

25

Acc

um

ula

ted

pre

cip

itat

ion

(m

m)

Air

tem

per

atu

re (

ºC

)

66

217

300

35

(b)

18

3.2. Soil microbial biomass C

The use of biocide was ineffective or ceased to have affect before the end of the

experiment, since no statistical difference (p > 0.05) in the soil carbon biomass (SCMB) was

found between control and biocide treatment plots (Figure 5). Additionally, SCMB was not

affected by any radiation treatment (Figure 5; p>0.05).

Figure 5. Soil microbial biomass C (mg C g-1

) for biocide and radiation treatments: Full sun, UVB[-]

and shade

treatment at the last sampling of the experiment. Values are mean ±SE (n=4).

3.3. Decomposition rate and remaining mass.

Litter mass showed a typical rapid loss during the first month with no UV-B impact

on mass loss during this stage. After 60 days, mass loss proceeded at a lower rate in the UVB[-

] treatment compared to full sun and shade treatments (Figure 6).

The D model had a better fit compared to 1E and 2E models, therefore, the D model

was considered the most adequate to describe the decomposition process (Table. 1). Under

this model, the best fit includes all main effects (radiation and biocide treatments) without

interaction between them. However, by independently excluding radiation and biocide

0.00

0.50

1.00

1.50

2.00

2.50

3.00

Full sun UVB[-] Shade

So

il m

icro

bia

l b

iom

ass C

(m

g C

g-1

)

Control Biocide

19

treatments, the AIC increased 9.7 and 2.4 in magnitude, respectively. This indicates that

despite the fact that the decomposition process has been affected by both factors, radiation

treatments was more important in affecting decomposition rate than biocide treatment (Table

2), while use of biocide showed little effect on litter decomposition (Figure 10f). Therefore,

we opted to use the D model that only includes radiation treatment (DR). Additionally, due to

the wide use of the 1E model in studies of plant litter decomposition process, we also showed

results generated by this model.

In the DR model, the m coefficient (k deceleration rate) of the UVB[-]

treatment was

significantly higher than of full sun and shade treatments, suggesting the lack of UV-B may

retard litter decomposition (Figure 7a). Compared to full sun, UVB[-]

treatment decelerated

the litter decomposition rate almost 35% (ratio of full sun to UVB[-]

treatments) (Figure 7a).

The 1E model also indicated again the importance of radiation in affecting

decomposition process where the model that only includes radiation treatment (1ER ) showed

the lowest AICc , however the 1ER model showed a slightly distance (ΔAICc < 1) from Null

model (1EN ), indicating that there is no clarity about the importance of the radiation treatment

on decomposition process (Table 2). Additionally, evaluating the k coefficient obtained from

1E model and based on confidence intervals, there was a trend toward lower decay constant

for the UVB[-]

treatment. Compared to full sun, UVB[-]

treatment decreased the litter

decomposition rate by 15%, however effects were not significant (Figure 7b).

Figure 6. Effects of the radiation treatments on remaining mass during the decomposition process. Values

are mean ±SE (n=8).

20

Table1. Model results for the three candidate decomposition models.

Model Description Df AICc ΔAICc

Single

exponential (1E)

X = X0𝑒−𝑘𝑡 12 952.7 316.7

Double

exponential (2E)

X = 𝑎𝑒(−𝑘1t) + (1 − 𝑎)𝑒(−𝑘2t) 14 824.9 189.0

Exponential

deceleration (D) X = X0 𝑒−(𝑎t−

𝑏𝑚

(𝑒−𝑚t

−1)) 14 635.9 0.0

For comparisons, we used full models (biocide treatment, radiation treatment and their interactions). Akaike

Information Criterion corrected for small samples size (AICc). Degrees of freedom and the differences between

each ranked model with best model are represented by Df and ΔAICc.

Table.2 Model results for the competing models based on single exponential and exponential deceleration model

of plant litter decomposition.

Decomposition

model model

Terms included

AIC Δ AIC df Biocide

treat.

Radiation

Biocide

treat:

Radiation

Single

exponential

model (1E)

Interation * * * 950,3 16,3 12

No interaction * *

936,2 2,2 6

Biocide

treatment *

934,9 1 3

Radiation

*

933 0 4

Null

934,8 0,8 2

Exponential

deceleration

model (D)

Interation * * * 635,9 17,2 14

No interaction * *

618,7 0 8

Biocide

treatment *

628,5 9,7 5

Radiation

*

621,1 2,4 6

Null 633,2 14,4 4

21

Figure 7. Effects of the radiation treatments on decomposition rate, using exponential deceleration model

(Rovira & Rovira, 2010) and the single exponential model (Oslon, 1963). (a) k- decreasing rate (m, yr-1

),

calculated from parameter estimates made by the DR treatment model and (b) decay constant (k, yr-1

),

calculated from parameter estimates made by the 1ER model. Values are mean ±SE (n = 8). Parameters

estimated using AICc analysis to be significantly different from zero at an α=0.05 significance level are

indicated with letters.

0

20

40

60

80

Full sun UVB[-] Shade

Ex

po

nen

tiia

l d

ecel

erat

ion

(m,y

r-1

)

b

a

a

0

0,1

0,2

0,3

0,4

0,5

0,6

Full sun UVB[-] Shade

Dec

ay c

on

stan

t(k,

yr-1

)

(a)

(b)

22

Table.3 Initial nutrients concentration of Tibouchina sellowiana.

Parameters T. sellowiana

N (mg g-¹) 9,08 ± 0,07

C (mg g-¹) 429,21 ± 3,78

Lignin (mg g-¹) 126,56 ± 6,23

P (mg g-¹) 0,415 ± 0,05

Cellulose (mg g-¹) 224,97 ± 2,34

Polyphenols (mg g-¹) 27,83 ± 3,69

C: N 47,30 ± 0,60

* mean and standard errors shown (n = 4).

3.4. Litter chemistry

After 300 days of decomposition, there was a distinct temporal pattern in litter

chemistry among radiation treatment. A major decrease in relative concentrations of most

elements and compounds was observed in the first two weeks of decomposition, while a

decrease in carbon was observed for every radiation treatment during the whole experiment

(Figure 8a). Differences among radiation treatments were only significant after 66 days of

decomposition. Loss of C was lower in the UVB[-]

than under full sun and shade treatments,

suggesting that the lack of UV-B retard carbon loss (Figure 8a).

Generally, N loss was faster during the initial stage of decomposition without

significant differences among treatments (Figure 8b). Finally, C:N ratio became significantly

higher in the UVB[-]

treatment only in the end of the experiment (Figure 8c).

23

(a)

(b)

24

Figure 8. Effects of radiation treatments on litter chemistry. (a) Carbon remaining (%), (b) Nitrogen remaining

(%) and (c) C:N ratio over the 300 days of decomposition. Values are mean ±SE (n=8).

Cellulose decreased slightly during decomposition without a clear effect of UVB[-]

treatment (Table.4, Figure 9a). Lignin was also not affected by the UVB[-]

in relation to full

sun treatment (Table.4). However, in the shade treatment loss of lignin was slower during

decomposition than in the two other treatments (Figure 9b). The polyphenols also had a faster

loss during the initial stages of decomposition (Figure 9c). However, polyphenol loss was

significantly lower in the UVB[-]

toward end of the experiment; suggesting that the lack of

UV-B retard polyphenol breakdown (Table.4).

(c)

25

(b)

(a)

26

Figure 9. Effects of radiation treatments on litter chemistry. (a) Cellulose remaining (%) , (b) Lignin

remaining (%) and (c) Total polyphenols remaining (%) over the 300 days of decomposition. Values are

mean ±SE (n=8).

(c)

27

Table 4. Results of ANOVA indicating the effects of the variables during stages of decomposition. Bold values indicate significant effects (0<0.05). The analysis

considered the biocide treatment, the radiation treatment (full sun, UVB[-]

and shade) and their interactions.

Source (P value) 0-66 days 217 days 300 days

Biocide

treat.

Radiation

Biocide

treat.*

Radiation

Biocide

treat. Radiation

Biocide

treat.*

Radiation

Biocide

treat. Radiation

Biocide

treat.*

Radiation

Litter remaining mass (%) 0.24 0.09 0.61 0.08 0.001 0.69 0.003 0.001 0.97

N remaining (%) 0.001 0.75 0.87 0.30 0.14 0.79 0.000 0.04 0.34

C remaining (%) 0.04 0.40 0.27 0.07 0.001 0.80 0.09 0.001 0.56

Lignin remaining (%) 0.001 0.001 0.11 0.001 0.001 0.001 0.001 0.86 0.78

Cellulose remaining (%) 0.001 0.001 0.001 0.78 0.001 0.67 0.11 0.11 0.83

Total polyphenols remaining

(%)

0.001 0.01 0.38 0.001 0.001 0.18 0.001 0.001 0.47

C:N 0.03 0.86 0.61 0.98 0.85 0.62 0.19 0.04 0.41

28

Figure 10. Effect of the biocide treatment on litter chemistry. (a) Carbon remaining (%), (b) Nitrogen remaining

(%), (c) Cellulose remaining (%), (d) Lignin remaining (%), (e) Total polyphenols remaining (%) and (f)

Remaining mass (%). Values are mean ±SE (n=12).

29

4. DISCUSSION

4.1. Litter decomposition rate

UV-B radiation blocking was enough to decelerate litter mass loss rate at the same

magnitude observed in arid climates (Austin & Vivanco, 2006; Brandt et al., 2007; Day et al.,

2007; Day et al., 2015; Lin & King, 2014; Wang et al., 2015; King et al., 2012; Day et al.,

2015; Song et al., 2013). As soil temperature and soil moisture did not show significant

differences among radiation treatments, we excluded a possible microclimate effect created

by the use of different plastics (Figure 3a and b), therefore, our results suggested that UV-B

radiation can play a role in plant litter decomposition in wetter regions. Our experimental area

receives around 2000 mm of precipitation per year and despite this high precipitation, the

effect of UV-B radiation on decomposition was notable, suggesting that this effect can occurs

in any ecosystem where litter is exposure to UV-B radiation (Day et al., 2007; Smith et al.,

2010; Austin et al., 2016).

On the other hand, another finding of our study differed from arid regions, since

there was no difference between decomposition rates between full sun and shade treatments

(Figure 7a). Decomposition rates seems to increase under full sun light than in shade

conditions in arid environments (Austin & Vivanco, 2006). Additionally, radiation had no

evident suppression effect on soil microbial biomass in our study (Figure 5). Similar findings

have also been described by Wang et al. (2015) and Baker et al.( 2015) where UV-B radiation

was responsible for increasing the efficiency of extracellular enzymes for microbial activity.

On the other hand, there are several examples where UV-B radiation had a detrimental effect

on microbial communities, suppressing microbial growth, and damaging microbial DNA

(Rohwer & Azam, 2000; Caldwell et al., 2007).

In our study, we expected that photodegradation would become more evident after

reducing microbial activity. However, we found that biocide had no significant effect at the

end of the experiment (Figure 5). We speculated here that this lack of biocide effect was due

to the high microbial activity found in tropical soils (Chapin et al., 2011). The fast cycle of

recolonization and regrowth in the tropics would probably require a much larger application

of biocides. Therefore, it remains unclear how biocide can affect decomposition by

photodegradation through microbial suppression in areas with high microbial activity.

30

4.2. Litter chemistry

In this study, carbon and total polyphenols were significantly affected by the lack of

UV-B radiation. The lack of UV-B radiation decreased the loss of carbon and polyphenols

along the decomposition process (Figures 8a and 9c). As a consequence, the C:N ratio at the

end of the experiment was significantly higher in the UVB[-]

treatment (Figure 8c).

Recent studies have found that UV contributes to the decomposition process by

facilitating microbial decomposition via the breakdown of recalcitrant compounds, an effect

call “photopriming”, which still depends on microbial activity (Wang et al., 2015; Austin et

al., 2016). Furthermore, as “photopriming” facilitates microbial decomposition, decreases of

the C:N ratio due to UV-B radiation increase substrate quality and accelerate microbial

decomposition (Chapin et al., 2011). Similar findings were also described by Wang et al.

(2017) in an experiment conducted in a Chinese semi-arid ecosystem, where UV exposure

increased litter biodegradability, changing litter chemistry while facilitating microbial

decomposition at the last stage of the process. Therefore, the increase of C:N ratio observed in

the end of the decomposition process in the UVB[-]

treatment, suggest that a similar process in

occurring in wetter regions.

It seems that the C:N ratio in the UVB[-]

treatment was affected by changes in C, but

not in N, since our results did not show any consistent UV-B effect on N concentration across

the entire experiment (Figure 8b ). This finding is in line with previous studies, which found

that N dynamics were unaffected by UV-B radiation (Brandt et al., 2010; Wang et al., 2015)

and contradicts Wang et al. (2015) where litter N was reduced due to UV radiation exposure.

These opposing trends, illustrates how the effect of UV in N dynamic is still controversial.

During this experiment, the lack of UV-B radiation not show a strong effect on the

loss of cellulose and lignin (Figure 9a and b), which are considered light-absorbing

recalcitrant compounds that may experience photodegradation (Day et al., 2007; Henry et al.,

2008, Austin & Ballaré, 2010; Austin et al., 2016). Indeed some studies has showed that UV-

B radiation caused a higher cellulose loss (Brandt et al., 2007; Lin & King, 2014) and in

some cases increased lignin loss too (Day et al., 2007; Austin & Ballaré, 2010). On the other

hand, other studies failed in show the same effect (Brandt et al., 2007, 2010). Among others,

King et al. (2012), have suggested that lignin may not be the most susceptible compound to

photodegraded by UV-B radiation. Nevertheless, when litter is shaded, notably a significant

lesser loss of lignin was observed during major part of the decomposition process compared

to the other radiation treatments (Figure 9b), which highlight the importance of sunlight in

31

lignin loss, however the mechanisms which break down this compound and which spectrum

has the more relevance on the process is yet not clear (King et al., 2012; Day et al., 2015).

Polyphenols was clearly the most affected compound by UV-B radiation (Figure 9c).

Gallo et al.(2009) have suggested that photodegradation of polyphenols can accelerate litter

mass loss. Our results also show a surprisingly higher loss of polyphenols in the shade

treatment compared to other radiation treatments (Figure 9c), which suggest that in tropical

ecosystems, the high microbial activity seems to be efficient in the breakdown of recalcitrant

compounds as polyphenols.

Our results demonstrate that the removal of UV-B decreased plant litter

decomposition rates in a wet tropical region. Therefore, it seems that UV-B radiation has a

positive effect on decomposition, especially in the late stage, where the lack of UV-B

decreased C losses, increasing litters C:N ratios. On the other hand, the lack of UV-B

radiation did not affect lignin degradation as have been seemed in some studies nor microbial

soil communities.

Finally, this study enhances the role of photodegradation extending the importance of

UV-B radiation on plant litter decomposition in other ecosystems without moisture

limitations, and under the scenario of climate change where predicts warming and long

periods of drought, photodegradation could become more important.

Obviously, the understanding of changes in litter chemistry during decomposition

remains an important gap in studies of temporal dynamics in decomposition (Parsons et al.,

2014; García-Palacios et al., 2016), and how much could be influenced by solar radiation

remains unclear. Our findings have to be interpreted with caution, since only one tree specie

was tested. Clearly, investigate the effects of UV-B radiation on other litter types in wetter

conditions is in order.

32

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