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Nutrient Cycling in Agroecosystems

, Volume 111, Issue 2–3, pp 203–215 | Cite as

Can litter production and litter decomposition improve soil properties in the rubber plantations of different ages in Côte d’Ivoire?

  • Julien K. N’Dri
  • Arnauth M. Guéi
  • Ettien F. Edoukou
  • Joseph G. Yéo
  • Kévin K. N’Guessan
  • Jan Lagerlöf
Original Article
  • 121 Downloads

Abstract

Litter production and litter decomposition influence the availability of nutrients in the soil. The investigation aimed at characterizing the dynamics of leaf litter decomposition, and soil physico-chemical and biological parameters in rubber plantations of different ages. During a 12-months’ period, field studies were done in 7-, 12-, and 25-year-old rubber plantations. For measuring of litter decomposition and input from aboveground, 324 litter bags and 27 litter traps (1 m × 1 m) were placed in 3 sampling areas per age class of rubber plantations. The soil parameters were also characterized. The results showed that the annual litter production and the amounts of organic carbon in leaves increased with the aging of the plantations. The annual decomposition constant (k) ranged from 0.0381 ± 0.0040 year−1 in the 25-year-old plantations to 0.0767 ± 0.0111 year−1 in the 7-year-old plantations. The annually decomposed litter mass varied between 2.7 ± 0.3 t ha−1 year−1 in the 12-year-old plantations to 4.2 ± 0.3 t ha−1 year−1 in the 25-year-old plantations. The soil of the 25-year-old plantations showed higher values of most physico-chemical and biological variables as compared to the 7-year-old plantations: annual litter production (+ 32%), annual litter mass decomposed (+ 11%), annual carbon (+ 15%) and nitrogen (+ 11%) inputs, soil organic carbon (+ 52%), total nitrogen (+ 32%), soil organic matter (+ 52%), soil water content (+ 74%), and the total density of soil invertebrates (+ 121%). The results indicate an improvement of soil properties with the aging of the rubber plantations and the importance of this agricultural system for carbon sequestration.

Keywords

Litter production Litter decomposition Nutrients in the leaves Carbon and nitrogen inputs Soil properties Rubber plantations 

Introduction

Beyond the maintenance of biodiversity, the forest plays an important role in carbon sequestration and mitigation of climate change (Dash and Behera 2013). However, the annual rate of net forest loss worldwide represents 3.3 M ha year−1 between 2010 and 2015 (Keenan et al. 2015). The deforestation and land conversion have far-reaching consequences for soil CO2 emissions and climate change. According to Mande et al. (2014), tropical forests store almost 30% of global forest carbon. Unfortunately, the development of agriculture in the tropics is linked to the destruction of these forests, altering highly the biogeochemical cycles (Hamilton et al. 2016; Campbell et al. 2017).

Rubber has become a major perennial crop in the southern part of Côte d’Ivoire, where it was introduced in the 1950s (Ruf 2009). Rubber plantations have boomed, particularly in the 2000s when the purchase price of rubber increased dramatically. The number of farmers growing rubber has increased from 8000 in 2000 to 31,192 in 2007 (Brindoumi 2015), and the area cultivated with rubber increased from 200,000 ha in 2008 (Ruf 2009) to more than 300,000 ha in 2012 (CNRA 2013). The rubber production in tropical and intertropical zones could be perceived as one of the main cause of deforestation (Allen et al. 2015). The recent studies have revealed a strong modification of soil nutrients (Chiti et al. 2014) and biological characteristics (Krashevska et al. 2015, 2016) along the different land use types. The abundance of soil organisms and the soil fertility vary according to soil types and habitat characteristics in agrosystem (Allen et al. 2015; Krashevska et al. 2015, 2016). The decrease in soil quality is partly due to soil erosion and herbicide application during the planting of rubber tree seedlings (Liu et al. 2016). However, with aging, rubber plantations can provide favorable niches and conditions for soil biodiversity (Dash and Behera 2013), and can perform the ecological functions similar to those of a forest (Martius et al. 2004).

The principal route through which plants modify soil food webs is through their influence on the quality and quantity of organic matter that is returned to the soil, in the form of plant litter and root exudates (Bardgett 2005). Land use system and season, and their interaction have been found to influence aboveground litterfall (Kotowska et al. 2016). The litter contributes to maintenance of productivity of the agrosystems by providing a source of energy for soil decomposers (Martius et al. 2004). Litter decomposition, through the fragmentation and mineralization is considered as the main key of ecological processes (Aerts 2006), and therefore, the mechanisms of litter decomposition, translocation and soil stabilization deserve better analysis. A hierarchy of abiotic and biotic factors controls decomposition. At the highest level of the hierarchy, climate is the primary determinant of soil moisture and temperature, which in turn together affect rates of main physical, chemical, and microbiological reactions that control decomposition (Berg 2000; Bardgett 2005). The ecological traits of leaves and their quality do also influence the decomposition rate and nutrient inputs into soil (Berg 2000). The litter decomposition remains complete under the combined action of mesofauna and macrofauna (Yang and Chen 2009). A study performed by Li et al. (2016) reports that about 50% of leaf nutrients and 21% of soil nutrients could be redistributed to the rubber tree body during the leaf senescence and withering stages.

Except the investigation made on litter decomposition from a 20-year-old rubber plantation (Tié Bi and Ornont 1987) and on soil macroinvertebrates diversity in Côte d’Ivoire (Gilot et al. 1995), no studies have referred to litter production and decomposition as well as soil nutrient change in rubber plantations of different ages. The main objective of the present study was to characterize the dynamics of the leaf litter decomposition and soil physico-chemical and biological parameters in rubber plantations of different ages. We hypothesized that soil properties would be improved with the aging of the rubber plantations.

Materials and methods

Site description

The investigation was conducted in the Department of Grand-Lahou (5°13′N; 5°03′W) situated in southern Côte d’Ivoire at 140 km from Abidjan. The climate of the locality is equatorial with four seasons—a long dry season from December to March, a long wet season from April to July, a short dry season from August to September, and a short wet season from October to November (Ettian et al. 2009; Konan et al. 2013). The annual rainfall during the field work period (September 2013–August 2014) was 1085 mm. The average monthly temperature ranged from 24.8 in August to 28.6 °C in February–March. Farmers suggested that most of the area we studied was secondary forest since 1980 (33 years before the field works) and was converted to rubber plantations in 1988. A portion of the previous vegetation of rubber plantations, less than 10 years old, was used for cocoa, coffee or oil palm plantations in the former rotation. Hevea brasiliensis is the most cultivated crop in the study area. Its increased area in rural domains and marginal forest areas could be explained by the attractive price of latex during the past two decades. Today, 140,000 ha of mature rubber plantations are recorded in Côte d’Ivoire (CNRA 2013). During the management of plantations, fertilizers and pesticides are applied once or twice per year during the first 4 years after establishment. The age of 7 years marks the beginning of the latex harvest. At 12 years, the plantations reach their maximum production level, whereas at 25 years, the latex production begins to decrease. The soils in the studied rubber plantations are of ferralitic type.

Sampling design

The study was undertaken using a randomized design with three age classes: 7-, 12-, and 25-year-old rubber plantations. Three replicate sampling areas, each measuring approximately 1–2 ha, were randomly established in each of the selected age classes, whether a total of 9 sampling areas. In each sampling area, 3 sampling points were established along a 40 m transect with 20 m intervals between two consecutive points.

Litterfall collection

The litterfall was collected monthly for one year at 0.5 m above ground level using three litter traps (1 m × 1 m) per sampling area. The traps were placed every 20 m interval along the 40 m transect in sampling area. This gave a total of 27 traps. Every month during the 12-month study period, the litterfall accumulated material was collected and sorted into different fractions, where only the leaves were taken into account. The accumulated leaves were oven dried at 80 °C for 24–48 h and then weighed (Podong and Poolsiri 2012; Podong et al. 2013). The monthly values of litter production for each sampling area were summed to obtain the annual litterfall. A subsample of 100 g of leaves from each sampling area accumulated at the end of the long dry season was chosen for the analysis of organic carbon and total nitrogen contents.

Litter decomposition

The data of litter decomposition were estimated by using a 2 mm mesh vinyl litter bags (0.20 m × 0.25 m). On each selected sampling area, rubber tree leaves in a state of senescence were collected from the trees. 5 g of air-dried leaves of Hevea brasiliensis (Euphorbiaceae) were put into each bag. The bags were closed by sewing. Before laying the litter bags on the ground, the soil was cleared of old leaves and dead woods, which allowed contact between the soil and the bags. The bags were fixed to the soil using two clamps. 36 bags were applied on each sampling area, whence a total of 324 litter bags for the 9 sampling areas. Three bags were retrieved randomly from each sampling area at 1-month intervals for 1 year. After the cleaning of the bags, the leaves were oven dried at 80 °C for 24–48 h and weighed. A single exponential equation according to an exponential decay model was used to calculate the decomposition constant (k) (Podong et al. 2013),
$$Ln\left( {X_{t} /X_{0} } \right) = - kt$$
(1)
where X0, the original weight of litter; Xt, the final weight of litter; k, the decomposition constant; t, the time (in days); Ln, natural logarithm.

Soil physico-chemical characteristics

The soils for chemical analysis were sampled according to the Tropical Soil Biology and Fertility method recommended by Anderson and Ingram (1993). Composite soil samples were obtained from five cores taken in quadrats (0.5 m × 0.5 m × 0.1 m) and at 0.5 m from each sampling point. Along the same transect, three soil cores (non-composite samples) were taken using the cylinder method (Assié et al. 2008), for physical measurements. During the two campaigns, a total of 54 composite soil samples and 54 non-composite soil samples were taken for the physico-chemical measurements.

Soil invertebrates

In each of the 9 sampling areas, soil cores for mite (Acari) extraction were taken during two campaigns with a steel corer (Ø 0.05 m) at 0.1 m soil depth (Bedano and Ruf 2007) following a 40 m transect. Five sampling points were allocated along each transect with a 10 m intervals between two consecutive sampling points. At each sampling point, two adjacent soil cores including litter thickness were taken. Overall, 180 soil cores were taken on the 9 sampling areas. In laboratory, soil mites were extracted for 10 days with a Berlese–Tullgren funnels and counted under a stereo microscope. Soil monoliths were taken for collection of macroinvertebrates by the Tropical Soil Biology and Fertility method (Anderson and Ingram 1993). Three monoliths (0.5 m × 0.5 m × 0.1 m) were taken along each transect. The soil of the monoliths was manually sorted in trays. The Earthworms were preserved in 4% formalin, while the other macroinvertebrates were fixed in 70% alcohol. During the two campaigns, a total of 54 monoliths were sampled.

Data analysis

The carbon content of the rubber tree leaves was analyzed by the loss on ignition method (Gallardo et al. 1987), whereas the nitrogen content was measured by the Kjeldahl method (Waneukem and Ganry 1992). The soil physico-chemical and biological data from the two campaigns were pooled before the analysis. The abundance of soil invertebrates was expressed as mean number of individuals per square meter. The taxonomic richness and Shannon index were used to assess the soil invertebrates’ diversity. The soil bulk density was estimated using the cylinder method (Assié et al. 2008) while the soil water content was calculated after drying at 105 °C for 48 h. The soil organic carbon was measured by the Walkley and Black (1934) method and total nitrogen using the Kjeldahl method (Waneukem and Ganry 1992). Soil pH-H2O was determined by means of a glass electrode in 1:2.5 soil:water (Tondoh et al. 2015). The amount of litter annually degraded (LAD) was calculated by the formula below:
$$LAD = \frac{A \times P}{5} \times 0.01$$
(2)
with LAD, the amount of litter annually degraded and expressed in t ha−1 year−1; A, the amount of annual litter production and expressed in g m−2 year−1; P, the amount of litter annually degraded on 5 g input, 0.01 being the conversion factor.
$$AC = (LAD \times B) \times 0.001$$
(3)
with AC, the amount of annual carbon input and expressed in t ha−1 year−1; LAD, the litter annually degraded and expressed in t ha−1 year−1; B, the amount of organic carbon (g) contained in 1 kg of rubber leaves, 0.001 being the conversion factor. The same approach was used for AN (amount of annual nitrogen input) calculation. The amount of carbon and nitrogen accumulated in soil per year (Kongsager et al. 2013), the soil carbon and nitrogen stock (Tondoh et al. 2015), and soil carbon sequestration (Nelson and Sommers 1982) were estimated for each plantation.

Statistical analysis

The effect of the age of the plantations on monthly and annual litter production, litter decomposition and the decomposition constant k was measured using a one-way ANOVA test. The same analysis was applied to the chemical parameters of leaves and the soil physico-chemical and biological variables. The average values of the soil physico-chemical and biological parameters were compared by using the Tukey’s multiple-comparison test. All tests mentioned above were carried out by using the R software after verification of the normality (Shapiro–Wilk test). However, the software Statistica 7.1 (StatSoft Inc., Tulsa, USA) was used to perform the Spearman correlation between soil biological variables, litter characteristics and the soil physico-chemical parameters.

Results

Litter production

The litter production per month during the 12 months study period (Fig. 1) peaked during the months of March and April. The monthly litter production did not vary significantly (one-way ANOVA, F = 0.25; p > 0.05) across the rubber plantations of different age. The annual litter production was the highest in the 25-year-old rubber plantations (5.1 ± 0.6 t ha−1 year−1), whereas the 7-year-old (3.9 ± 0.1 t ha−1 year−1) and 12-year-old rubber plantations (3.9 ± 0.7 t ha−1 year−1) presented similar quantities. The annual litter production did not differ significantly (one-way ANOVA, F = 1.73; p > 0.05) between the rubber plantations of different age, but the mean values were + 32% in the 25-year-old plantations compared to the 7-year-old plantations.
Fig. 1

Monthly litter production (mean and SE) of 7-, 12- and 25-year-old rubber plantations. S September, O October, N November, D December, J January, F February, M March, A April, M May, J June, J July, A August. N = 36, one-way ANOVA test, p > 0.05

Variation of the litter decomposition

Whatever the age of plantations, the dynamics of accumulated litter mass losses presented a rising trend (Fig. 2), but the rate of mass loss did not differ significantly (one-way ANOVA, F = 0.30; p > 0.05) due to the age of the rubber plantations. After 180 days of experimentation, the litter bags had lost on average 48, 26 and 31% of their initial litter mass, respectively, in the 7-, 12-, and 25-year-old rubber plantations. The litter mass loss was the highest in the 7-year-old rubber plantations (4.9 ± 0.1 g) after 360 days of experimentation. It was followed by the 25-year-old rubber plantations (4.1 ± 0.2 g) and the 12-year-old rubber plantations (3.7 ± 0.4 g). Whatever the age of plantations, over 75% of the initial litter mass was decomposed after 360 days of experimentation. The decomposition half time was 4.1, 7.2, and 7.5 months, respectively, in the 7-, 12-, and 25-year-old rubber plantations. The annual litter mass decomposed varied significantly (one-way ANOVA, F = 7.54; p < 0.01) across the rubber plantations of different age, and represented 3.8 ± 0.1, 2.7 ± 0.3, and 4.2 ± 0.3 t ha−1 year−1, respectively, in the 7-, 12-, and 25-year-old rubber plantations. In other words, the annual litter mass decomposed changed by + 11% in the 25-year-old plantations compared to the 7-year-old plantations.
Fig. 2

Accumulated mass loss (mean and SE) of rubber leaf litter in litterbags put on the ground of 7-, 12- and 25-year-old rubber plantations. N = 36, one-way ANOVA test, p > 0.05

Decomposition constant (k)

The monthly decomposition constant (k) differed significantly (one-way ANOVA, F = 6.40; p < 0.01) between the rubber plantations of different age. The average of monthly decomposition constant (k) ranged from 0.0031 ± 0.0003 year−1 in the 25-year-old rubber plantations to 0.0056 ± 0.0007 year−1 in the 7-year-old rubber plantations. As expected (Fig. 3), the values of the decomposition constant (k) were greater in the wet period (June–September) and lower in the dry period (October–May). The average of annual decomposition constant (k) varied between 0.0381 ± 0.0040 year−1 in the 25-year-old rubber plantations to 0.0767 ± 0.0111 year−1 in the 7-year-old rubber plantations.
Fig. 3

Decomposition constant k (mean and SE) of rubber leaves in litter bags put on the ground in 7-, 12- and 25-year-old rubber plantations. S September, O October, N November, D December, J January, F February, M March, A April, M May, J June, J July, A August. N = 36, one-way ANOVA test, p < 0.01

Carbon and nitrogen content in leaf litter

The organic carbon (one-way ANOVA, F = 18.10; p < 0.001) and total nitrogen (one-way ANOVA, F = 10.22; p < 0.001) contents in leaf litter were significantly different in the rubber plantations of different age (Table 1). The highest amounts of organic carbon (552.1 ± 1.2 g kg−1 leaves) were found in leaf litter of 25-year-old rubber plantations whereas the highest contents of total nitrogen (24.5 ± 0.5 g kg−1 leaves) were measured in leaf litter from 7-year-old rubber plantations. In contrast to nitrogen, the amount of organic carbon increased with the aging of the rubber plantations. The carbon/nitrogen ratio varied significantly (one-way ANOVA, F = 12.15; p < 0.001) across the rubber plantations of different age.
Table 1

Chemical characteristics and means values ± SE measured in rubber leaf litter of 7-, 12- and 25-year-old rubber plantations

 

Organic carbon (g kg−1 leaves)

Total nitrogen (g kg−1 leaves)

Carbon/nitrogen ratio

R7

536.8 ± 2.4a

24.5 ± 0.5a

22.0 ± 0.5a

R12

544.7 ± 1.1b

21.6 ± 0.4b

25.3 ± 0.5b

R25

552.1 ± 1.2c

23.1 ± 0.3ab

24.0 ± 0.3b

p value

0.0001***

0.0006***

0.0001***

R7 7-year-old rubber plantations, R12 12-year-old rubber plantations, R25 25-year-old rubber plantations. N = 27, one-way ANOVA test, p < 0.05

***p < 0.001; means values followed by the same superscript lowercase letter within column are not significantly different at the 0.05 level (Tukey’s multiple-comparison test)

Soil invertebrates

In all, 17 taxa were recorded along the rubber plantations of different age. The total density of soil invertebrates significantly increased (one-way ANOVA, F = 8.24; p < 0.01) with the aging of the rubber plantations. However, the taxonomic richness (one-way ANOVA, F = 0.51; p > 0.05) and Shannon index (one-way ANOVA, F = 0.68; p > 0.05) did not vary significantly (Table 2). Only the density of Earthworms (one-way ANOVA, F = 3.89; p < 0.05), Acari (one-way ANOVA, F = 7.10; p < 0.01), Mollusca (one-way ANOVA, F = 3.68; p < 0.05) and Diplura (one-way ANOVA, F = 4.40; p < 0.05) had changed significantly across the rubber plantations of different age. The density of Earthworms (7-year-old rubber plantations: 32.4 ± 4.9 ind m−2, 12-year-old rubber plantations: 136.7 ± 20.8 ind m−2, 25-year-old rubber plantations: 151.3 ± 52.7 ind m−2), Termites (7-year-old rubber plantations: 46.2 ± 17.9 ind m−2, 12-year-old rubber plantations: 92.4 ± 32.5 ind m−2, 25-year-old rubber plantations: 139.3 ± 100.4 ind m−2), Acari (7-year-old rubber plantations: 2712.3 ± 397.5 ind m−2, 12-year-old rubber plantations: 4333.8 ± 581.7 ind m−2, 25-year-old rubber plantations: 5955.3 ± 783.1 ind m−2) and Araneae (7-year-old rubber plantations: 8.9 ± 2.2 ind m−2, 12-year-old rubber plantations: 9.8 ± 2.2 ind m−2, 25-year-old rubber plantations: 11.8 ± 2.6 ind m−2) increased with the aging of the rubber plantations. The Collembola and Pauropoda were both recorded under the 7-year-old rubber plantations. The total density of soil invertebrates varied by + 121% in the 25-year-old plantations compared to the 7-year-old plantations.
Table 2

Density (means values ± SE individuals per square meter) and diversity of soil invertebrates observed across the rubber chronosequence

Taxa

Rubber chronosequence

R7

R12

R25

p value

Acari

2712.3 ± 397.5b

4333.8 ± 581.7ab

5955.3 ± 783.1a

0.003**

Ants

13.1 ± 5.4a

12.7 ± 3.2a

23.8 ± 11.2a

0.495

Araneae

8.9 ± 2.2a

9.8 ± 2.2a

11.8 ± 2.6a

0.675

Chilopoda

11.1 ± 4.0a

17.8 ± 5.1a

16.2 ± 5.9a

0.625

Coleoptera

6.2 ± 1.2a

8.0 ± 1.4a

6.2 ± 1.1a

0.383

Collembola

0.2 ± 0.2a

0.0 ± 0.0a

0.0 ± 0.0a

0.382

Diplopoda

23.3 ± 5.9a

12.7 ± 3.3a

14.4 ± 3.5a

0.207

Diplura

0.7 ± 0.5b

7.1 ± 2.3a

2.4 ± 1.4ab

0.023*

Diptera

0.9 ± 0.4a

1.1 ± 0.5a

1.1 ± 0.5a

0.920

Earthworms

32.4 ± 4.9b

136.7 ± 20.8ab

151.3 ± 52.7a

0.034*

Hemiptera

0.7 ± 0.7a

0.9 ± 0.7a

0.4 ± 0.3a

0.861

Homoptera

0.7 ± 0.7a

3.3 ± 3.1a

0.0 ± 0.0a

0.407

Isopoda

2.9 ± 1.5a

6.4 ± 2.9a

2.7 ± 1.2a

0.347

Mollusca

7.6 ± 1.8a

1.1 ± 0.5b

5.1 ± 2.1ab

0.040*

Orthoptera

3.3 ± 1.2a

0.7 ± 0.5a

1.3 ± 0.5a

0.065

Pauropoda

0.2 ± 0.2a

0.0 ± 0.0a

0.0 ± 0.0a

0.382

Termites

46.2 ± 17.9a

92.4 ± 32.5a

139.3 ± 100.4a

0.574

Total density

2870.7 ± 402.7b

4644.4 ± 578ab

6331.5 ± 770a

0.0018**

Taxonomic richness

10.8 ± 0.5a

10.9 ± 0.5a

10.2 ± 0.5a

0.606

Shannon index

0.3 ± 0.1a

0.4 ± 0.1a

0.3 ± 0.0a

0.512

R7 7-year-old rubber plantations, R12 12-year-old rubber plantations, R25 25-year-old rubber plantations. Soil depth: 0.1 m, Acari: N = 180, other invertebrates: N = 54, one-way ANOVA test, p < 0.05

*p < 0.05, **p < 0.01; means values followed by the same superscript lowercase letter within row are not significantly different at the 0.05 level (Tukey’s multiple-comparison test)

Soil physico-chemical parameters

The soil physico-chemical parameters varied significantly across the rubber plantations of different age (Table 3). The average bulk density was the greatest in the 7-year-old rubber plantations (1.3 ± 0.0 g cm−3), whereas those from 12- year-old rubber plantations (1.2 ± 0.0 g cm−3), and 25-year-old rubber plantations (1.2 ± 0.0 g cm−3) had lower but similar values. The amount of soil water content and carbon/nitrogen ratio significantly increased with the aging of the rubber plantations. The soil organic carbon (11.6 ± 0.7 g kg−1 soil), total nitrogen (1.1 ± 0.1 g kg−1 soil) and soil organic matter (19.7 ± 1.2 g kg−1 soil) quantities were the highest in the 12-year-old rubber plantations. Whatever the age of plantations, the soil pH was acid. The soil water content, organic carbon, total nitrogen, and organic matter were + 74, + 52, + 32, and + 52% higher, respectively, in the 25-year-old plantations compared to the 7-year-old plantations.
Table 3

Soil physico-chemical characteristics and means values ± SE measured in 7-, 12- and 25-year-old rubber plantations

 

R7

R12

R25

p value

Bulk density (g cm−3)

1.3 ± 0.0b

1.2 ± 0.0a

1.2 ± 0.0a

0.0061**

Water content (%)

9.8 ± 0.8b

14.3 ± 0.8a

16.9 ± 1.9a

0.0021**

Organic carbon (g kg−1 soil)

7.3 ± 0.2b

11.6 ± 0.7a

11.1 ± 1.1a

0.0004***

Total nitrogen (g kg−1 soil)

0.7 ± 0.0b

1.1 ± 0.1a

1.0 ± 0.1a

0.0003***

Carbon/nitrogen ratio

9.7 ± 0.2b

10.2 ± 0.1b

11.0 ± 0.2a

0.0002***

Organic matter (g kg−1 soil)

12.4 ± 0.4b

19.7 ± 1.2a

18.8 ± 1.9a

0.0004***

pH-H2O

5.9 ± 0.1a

4.7 ± 0.0c

5.0 ± 0.1b

0.0008***

R7 7-year-old rubber plantations, R12 12-year-old rubber plantations, R25 25-year-old rubber plantations. Soil depth: 0.1 m, N = 54, one-way ANOVA test, p < 0.05

**p < 0.01, ***p < 0.001; means values followed by the same superscript lowercase letter within row are not significantly different at the 0.05 level (Tukey’s multiple-comparison test)

Carbon and nitrogen inputs and accumulation in soil

The annual carbon and nitrogen inputs varied significantly between the rubber plantations of different age (Table 4). The amounts of carbon input (2.3 ± 0.2 t ha−1 year−1) and nitrogen input (0.10 ± 0.01 t ha−1 year−1) into the soil were higher in the 25-year-old rubber plantations compared to others. The accumulation rate of carbon and nitrogen significantly differed between the rubber plantations of different age. The accumulation rate of carbon (1.3 ± 0.0 t ha−1 year−1) and nitrogen (0.13 ± 0.00 t ha−1 year−1) was the highest in the 7-year-old rubber plantations compared to others. The annual carbon and nitrogen inputs were + 15 and + 11% higher, respectively, in the 25-year-old plantations than in the 7-year-old plantations.
Table 4

Amount (means values ± SE) of carbon and nitrogen inputs and accumulated in soil of 7-, 12- and 25-year-old rubber plantations

 

C input (t ha−1 year−1)

N input (t ha−1 year−1)

C accumulation (t ha−1 year−1)

N accumulation (t ha−1 year−1)

R7

2.0 ± 0.0a

0.09 ± 0.00a

1.3 ± 0.0b

0.13 ± 0.00b

R12

1.5 ± 0.2b

0.06 ± 0.01b

1.1 ± 0.1a

0.11 ± 0.01ab

R25

2.3 ± 0.2a

0.10 ± 0.01a

0.5 ± 0.0a

0.05 ± 0.00a

p value

0.0023**

0.0009***

0.0046**

0.0050**

R7 7-year-old rubber plantations, R12 12-year-old rubber plantations, R25 25-year-old rubber plantations, C Carbon, N Nitrogen. Soil depth: 0.1 m, N = 27, one-way ANOVA test, p < 0.05

**p < 0.01, ***p < 0.001; means values followed by the same superscript lowercase letter within column are not significantly different at the 0.05 level (Tukey’s multiple-comparison test)

Interaction between soil biological parameters and environmental characteristics

The soil biological parameters were differently influenced by soil and litter characteristics across the 7–25-year-old rubber plantations (Table 5). Diversity and the abundance of soil invertebrates were, respectively, correlated to the bulk density (Shannon index: r = 0.49, p < 0.01; density: r = − 0.51, p < 0.01) and soil water content (Shannon index: r = − 0.61, p < 0.001; density: r = 0.48, p < 0.05). The taxonomic richness of the soil invertebrates was impacted by the water content (r = − 0.38, p < 0.05), whereas the density of the soil invertebrates was affected by organic carbon (r = 0.66, p < 0.001) and total nitrogen (r = 0.59, p < 0.01).
Table 5

Spearman correlations performed at the landscape scale between soil biological parameters and environmental variables in 7–25-year-old rubber plantations

 

Density

Taxonomic richness

Shannon index

r

p

r

p

r

p

Bulk density

− 0.51

0.006**

− 0.01

0.973

0.49

0.009**

Water content

0.48

0.010*

− 0.38

0.047*

− 0.61

0.0007***

Soil organic carbon

0.66

0.0001***

0.11

0.583

− 0.17

0.389

Total nitrogen

0.59

0.0012**

0.13

0.514

− 0.10

0.614

Litter decomposition

0.05

0.789

0.11

0.578

0.19

0.340

Soil depth: 0.1 m, N = 54, p < 0.05

*p < 0.05; **p < 0.01; ***p < 0.001

Relationship between litter characteristics and soil physico-chemical parameters

The litter production and decomposition rate differently influenced the soil physico-chemical parameters (Table 6). Nitrogen inputs in soil were positively impacted by the decomposition rate of litter in the 7-, 12-, and 25-year-old rubber plantations (r = 0.99, p < 0.001). Carbon inputs in soil were affected by the decomposition rate of litter in the 12-year-old rubber plantations (r = 0.99, p < 0.001). The litter production positively controlled the nitrogen inputs in the 25-year-old rubber plantations (r = 0.99, p < 0.001). Except for the bulk density in the 7-year-old rubber plantations (r = 0.76, p < 0.05), the soil physical parameters did not significantly impact the decomposition rate of the litter. The litter characteristics influenced, respectively, the carbon inputs (litter production: r = 0.99, p < 0.001; litter decomposition rate: r = 0.99, p < 0.001) and nitrogen inputs (litter production: r = 0.98, p < 0.001; litter decomposition rate: r = 0.99, p < 0.001) through the 7–25-year-old rubber plantations.
Table 6

Spearman correlations performed between soil physico-chemical parameters and litter characteristics in 7-, 12- and 25-year-old rubber plantations

 

R7

R12

R25

R7–R25

r

p

r

p

r

p

r

p

Litter production

 Carbon input

0.99

0.999

0.99

0.999

0.99

0.999

0.99

0.001***

 Nitrogen input

0.99

0.999

0.99

0.999

0.99

0.001***

0.98

0.0001***

Litter decomposition rate

 Bulk density

0.76

0.017*

− 0.19

0.626

0.45

0.217

0.33

0.095

 Water content

− 0.10

0.802

− 0.17

0.656

− 0.28

0.457

− 0.08

0.672

 Carbon input

0.99

0.999

0.99

0.001***

0.99

0.999

0.99

0.001***

 Nitrogen input

0.99

0.001***

0.99

0.001***

0.99

0.001***

0.99

0.001***

R7 7-year-old rubber plantations, R12 12-year-old rubber plantations, R25 25-year-old rubber plantations, R7-R25 7–25-year-old rubber plantations. Soil depth: 0.1 m, plantation scale: N = 18, landscape scale: N = 54, p < 0.05

*p < 0.05; ***p < 0.001

Discussion

Environmental factors influencing the litter production

Several research works devoted to tropical ecosystems pointed out that litter production varies according to tree species, soil quality, season, microclimate and the successional stage of ecosystems (Bernhard-Reversat and Loumeto 2002; Martius et al. 2004; Ukonmaanaho et al. 2008; Podong and Poolsiri 2012; Kotowska et al. 2016). In monospecific ecosystems, the age effect was perceived as an essential variable affecting the litter production (Bernhard-Reversat and Loumeto 2002). In contrast to this assertion, the results showed that the annual and monthly litter production did not vary significantly across the rubber plantations of different age. Nevertheless, the peak of litter production was observed at the end of dry season and particularly during the months of March and April. Aboveground litterfall estimated in 7- to 16-year-old rubber plantations was negatively correlated with precipitation showing a distinct peak during the drier season (Kotowska et al. 2016). Indeed, monthly total aboveground litterfall was significantly higher in the drier season (514 ± 18 kg ha−1 month−1) than in the wet season (181 ± 19 kg ha−1 month−1) in rubber monoculture systems (Kotowska et al. 2016). The litterfall increases through drought severity (Brando et al. 2008). This trend was confirmed by the investigation made by Podong and Poolsiri (2012), and where an increasing litter production was recorded during the dry period in northern Thailand. The same observation was made by Martius et al. (2004) through the forest ecosystems and agrosystems based on polyculture in Manaus, Brazil. The annual litter production increased with the aging of the rubber plantations. This result is similar to the research performed by Chaudhuri et al. (2013) in western of Tripura, India, and where the authors highlighted an increase of litter production with the aging of the rubber plantations (3 year: 0.03 t ha−1, 10 year: 2–7 t ha−1, 14 year: 7.5–14 t ha−1, 25 year: 8–15 t ha−1). In consequence, the high litter production favored an increase of soil water content over time (Chaudhuri et al. 2013).

The litter production is also perceived as an indicator of soil fertility (Ukonmaanaho et al. 2008). Indeed, trees stand on fertile sites produce more litter than those growing on infertile sites. Whatever the age of rubber plantations, the annual litter production remained larger to those (1.4 ± 0.2 t ha−1 year−1) estimated by Podong and Poolsiri (2012) and smaller to those (6 t ha−1 year−1) provided by Tié Bi and Ornont (1987). It is admitted that forest systems produce more litterfall than rubber tree plantations, according to results provided by Martius et al. (2004) on secondary (7.2–7.6 t ha−1 year−1) and primary forests (7.9–9.5 t ha−1 year−1). However, at an advanced age, the agrosystems (e.g. rubber tree plantations) can perform ecological functions similar to those of a forest (Martius et al. 2004). This could explain the higher annual litter production in the 25-year-old rubber plantations compared to those (4.7 ± 0.2 t ha−1 year−1) from the secondary forest (Podong and Poolsiri 2012; Podong et al. 2013). Additionally, the nutrient use efficiency (NUE) of N, P and K in natural forest and in the rubber systems showed comparably values (Kotowska et al. 2016).

Dynamic of the litter decomposition

The litter decomposition contributes to the sustainable management of agriculture, since the nutrients released by the microbial activity remain determinant for the net productivity of agrosystems (Gréggio et al. 2008). So, litter decomposition is an important aspect in mineral cycling as it determines the rate at which nutrients in the litter become available for recycling as well as the store of inorganic elements still remaining in the litter component (Kurzatkowski et al. 2004; Gréggio et al. 2008). However, the rate of litter decomposition is controlled by three main types of factors: the physical environment, the quality of the substrate available to decomposers and the characteristics of soil organisms (Bernhard-Reversat and Loumeto 2002; Kurzatkowski et al. 2004; Coleman et al. 2004; Bardgett 2005; Gréggio et al. 2008; Podong and Poolsiri 2012). At the highest level of the factors classification, the climate is perceived as the primary determinant of soil moisture and temperature, which together affect the rates of the main physical, chemical, and microbiological reactions that control decomposition (Bardgett 2005). The use of litter bags for estimating the rate of litter decomposition is accepted by several researches (Bernhard-Reversat and Loumeto 2002; Kurzatkowski et al. 2004; Gréggio et al. 2008; Yang and Chen 2009; Podong and Poolsiri 2012; Podong et al. 2013).

The results showed that the amounts of litter decomposition during 360 days did not differ significantly through the rubber plantations of different age. Whatever the age of plantations, over 75% of the initial litter mass was decomposed. The leaves of 7-year-old rubber plantations decomposed faster than those from 12 and 25-year-old rubber plantations. Indeed, the monthly (0.0056 ± 0.0007 year−1) and annual (0.0767 ± 0.0111 year−1) decomposition coefficients (k) was greater in the 7-year-old rubber plantations than in the 25-year-old rubber plantations (month: 0.0031 ± 0.0003 year−1, annual: 0.0381 ± 0.0040 year−1).

The leaf inputs to the soil varied greatly in terms of the relative proportions of different carbon constituents that they contained, which degrading at different rates reflect the leaves’ resource value to the decomposer organisms (Bardgett 2005). At carbon/nitrogen ratios of leaves higher than 30–40, microbial activity is significantly reduced, leading to N-immobilisation and slowed decomposition (Torreta and Takeda, 1999). This is not the case in the present study; because whatever the age of plantations, the carbon/nitrogen ratios were lower than 26. The high amount of litter annually decomposed (4.2 ± 0.3 t ha−1 year−1) in the 25-year-old rubber plantations could be explained by the greater annual litter production (5.1 ± 0.6 t ha−1 year−1). Whatever the age of plantations, the rate of litter decomposition remained inferior to those observed by Podong and Poolsiri (2012). In contrast to our data, Tié Bi and Ornont (1987) recorded a decomposition coefficient k = 5 year−1 in a 20-year-old rubber plantation in Côte d’Ivoire. This variation could be explained by the higher rainfall, which might favor an increase in rate of decomposition (Gréggio et al. 2008; Podong and Poolsiri 2012). The decomposition coefficient k was greater during the humid months (June–September) and lower during the dry months (October–May). The litter decomposition is further enhanced under the combined action of soil invertebrates (Yang and Chen 2009).

State of soil properties

Soil organisms, particularly decomposers are essential for the functioning of terrestrial ecosystems, largely because they decompose dead organic material in soil, converting this into carbon dioxide and other soluble nutrient forms that provide resources for other biota and primary production (Bardgett 2005). The species and community structure of soil animals differ markedly in their response to litter quantity and quality change during the successional stage and agrosystem types, management systems, and abiotic factors (Coleman et al. 2004; Yang and Chen 2009). The results of our investigation indicated that the total density of soil invertebrates increased significantly with the aging of the rubber plantations. The same trend was observed with for densities of earthworms, termites and ants. These taxa are considered as ecosystem engineers (Lavelle et al. 1997). They affect soil properties, substantially modifying the physical structure of the soil profile, activities of other organisms and the passage of materials across the soils. The increase of soil water content and the decrease of bulk density following the aging of the rubber plantations were probably due to the increasing proportion of macropores and channels in the soil, indicating improvement of soil quality, and resulting from the activity of ecosystem engineers. The investigation made by Chaudhuri et al. (2013) revealed an increase in the density of earthworms and biomass with the aging of rubber plantations. The comparative study of Chaudhuri and Nath (2011) between a mixed forest and a 15- to 25-year-old rubber plantation describes an increase in the density and a reduction of the diversity of earthworms under the rubber plantations. The soil microbes and microfauna also undergo a change in abundance. In the litter of rubber plantations, the concentrations of certain phospholipid-derived fatty acids, including the gram-negative bacteria marker cy17:0 and the gram-positive bacteria marker i17:0 were reduced compared to rainforest (Krashevska et al. 2015). The same variation was observed with gram-negative bacteria in soil (Krashevska et al. 2015). Recent works by Krashevska et al. (2016) on the communities of protists have shown a reduction in density, species richness and biomass in 13- to 29-year-old rubber plantations compared to rainforest.

The change of soil nutrients in agrosystems would vary according to crop types (Chiti et al. 2014), plantations age (Chaudhuri et al. 2013; Chiti et al. 2014), and the rate of litter decomposition (Bernhard-Reversat and Loumeto 2002; Kurzatkowski et al. 2004; Gréggio et al. 2008; Yang and Chen 2009; Podong and Poolsiri 2012; Podong et al. 2013). Our results showed that the annual carbon and nitrogen inputs to the soil were higher in the 25-year-old rubber plantations compared to 7-year-old ones. This modification, consecutive to the high litter production could explain the large amounts of carbon and nitrogen stocks observed in the older plantations (Table 7).
Table 7

Amount (means values ± SE) of carbon and nitrogen stocks and carbon sequestration rates in soil of 7-, 12- and 25-year-old rubber plantations

 

Nitrogen stock (t ha−1)

Carbon stock (t ha−1)

Carbon sequestration (Mg CO2 eq ha−1)

R7

0.9 ± 0.0b

9.2 ± 0.3b

33.9 ± 1.1b

R12

1.3 ± 0.1ab

13.7 ± 0.9a

50.1 ± 3.4a

R25

1.2 ± 0.1a

13.2 ± 1.4a

48.5 ± 5.2a

p value

0.0050**

0.0046**

0.0046**

R7 7-year-old rubber plantations, R12 12-year-old rubber plantations, R25 25-year-old rubber plantations. Soil depth: 0.1 m, N = 54, one-way ANOVA test, p < 0.05

**p < 0.01; means values followed by the same superscript lowercase letter within column are not significantly different at the 0.05 level (Tukey’s multiple-comparison test)

Whatever the age of the plantations, carbon and nitrogen inputs into soil remained larger than the data recorded in rubber tree plantations (carbon input: 0.38 t ha−1 year−1, nitrogen input: 0.012 t ha−1 year−1) by Podong and Poolsiri (2012), and in 7- to 16-year-old rubber plantations (carbon input: 1.9 t ha−1 year−1, nitrogen input: 0.05 t ha−1 year−1) (Kotowska et al. 2016). However, the greater amount of carbon and nitrogen annually accumulated in the 7-year-old rubber plantations would result from the lower microbial activity. The soil organic matter was lower in the 7-year-old rubber plantations, indicating the decrease of CO2 production (Kurzatkowski et al. 2004). Indeed, the higher amounts of several variables, such as the annual litter production, the annual litter mass decomposed, the annual carbon and nitrogen inputs, the annual carbon and nitrogen stocks, the soil organic matter, the density of soil invertebrates, the soil water content, and the lower amounts of carbon and nitrogen annually accumulated in the 25-year-old rubber plantations, as compared to the younger plantations, could indicate an optimal combination of litter quality and environmental conditions (Kurzatkowski et al. 2004; Gréggio et al. 2008; Podong and Poolsiri 2012; Podong et al. 2013).

A favorable microclimate may enhance microbial decomposition and promote invertebrates, most of them detritivores, and therefore has a positive effect on litter decomposition (Attignon et al. 2004). The investigation highlighted an improvement in the soil properties with the aging of the rubber plantations over time and this was characterized by an increase of the soil physico-chemical and biological parameters. Beyond the improvement of the soil properties, the rubber plantations could contribute to carbon sequestration. In fact, the soil from the mature rubber plantations would sequester on average 44 Mg CO2 eq ha−1, whence 6,160,000 Mg CO2 eq at the country scale, thus showing its major role in climate change mitigation.

Notes

Acknowledgments

The authors gratefully acknowledge the financial support of International Foundation for Science (IFS) through the Project Ref. D/5287-1 and titled “Effect of age of rubber plantations on the soil microarthropods diversity and the recycling of the organic matter in the Grand Lahou department”, which made this study possible. Thank to farmers, SODEFOR and IDH staff for their involvement in identifying suitable plantations for this work. A big thank you goes out to Prof Jérôme E. Tondoh for his assistance during the field works and data analysis. Thanks to Dr. Martine Kah Touao GAUZE, Director of the Ecological Research Center for hosting the project in her center.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

This article does not contain any studies with human participants or animals performed by any of the authors.

Informed consent

Informed consent was obtained from all individual participants included in the study.

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Copyright information

© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  1. 1.Unité de Formation et de Recherche (UFR) des Sciences de la NatureUniversité Nangui AbrogouaAbidjan 02Côte d’Ivoire
  2. 2.Centre de Recherche en EcologieAbidjan 08Côte d’Ivoire
  3. 3.Université Jean Lorougnon GuédéDaloaCôte d’Ivoire
  4. 4.Department of EcologySwedish University of Agricultural Sciences (SLU)UppsalaSweden

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