Forest conversion to agricultural lands: impact on soil physical characteristics

Forest conversion to other land uses, particularly for agriculture, has been prevalent worldwide, with Indonesia being particularly affected. The removal of forest cover, which exposes the organic matter to more severe climatic conditions, leads to a deterioration in the physical characteristics of the soil. This paper investigates the alterations in various soil physical properties when forests are converted into agricultural land uses, including fallow areas with shrubs, wetland rice cultivation, cacao plantations, durian plantations, and avocado plantations. Both undisturbed and disturbed soil samples from each land use were analyzed to assess hydraulic conductivity, bulk density, and soil organic carbon (C) content. With the exception of wetland rice, all land uses exhibited a sandy loam texture. The forests contained the highest amount of SOC (4.70%), while the other samples contained less than half (2.01%) of this quantity.


Introduction
Indonesia had one of the highest rates of forest removal throughout the 1990s ranks second only to Brazil documented by the United Nations Food and Agriculture Organization [1].The conversion of forest into other land uses has great impact on soil e.g., erosion, soil C decrease, and soil physical degradation.Deforestation has increased the frequency and mobility of mass erosion processes in many places (e.g [2,3]) primarily due to loss of organic matter and vegetal cover.Forest soils store over half of the organic carbon present in terrestrial ecosystems [4].
Removal of the forest cover will expose soil C to harsh climatic condition leads to its rapid decomposition and losses.In South America, pasture accounted for the majority of forest area (71.2%) followed by commercial agriculture (14%) and the resulting carbon loss were 71.6% and 12.1%, respectively [5].The dynamics of forest edges in the Amazonia cause a 37% correspond to loss of carbon as a result of deforestation [6].Deforestation in Padang Indonesia for 319.92 hectares lessen its carbon by 81,724.7 ton [7].
The rapid loss in soil carbon due to deforestation can lead to soil physical properties to decline.Using the visual evaluation of soil structure (VESS), Amazon Forest conversion to low-input pasture severely deteriorated the physical properties of the soil [8].Converting natural deciduous woods into meadow and arable soils reduce carbon concentration in the top 15 m soil layer, increase soil's susceptibility to compaction, particle density and bulk density [9].Saturated soil hydraulic conductivity was found to decrease when dry land farming was created by clearing sparse forest in Baneh West Iran [10].Deforestation has also been happening for years in different regions of Indonesia, notably Labuan, a region in Donggala district on Sulawesi Island Indonesia.The forest in this area had been cleared, for agricultural and other purposes or was left fallow after profitless cultivation (shrubs).Some of them that are frequently found in this region, including wetland rice cultivation, cacao plantations, durian plantations, and avocado plantations.This study therefore was carried out to compare the soil physical characteristics between those under forest and the other five different land uses.

Materials and methods
Disturbed and undisturbed soil samples were collected from the forest, shrubs, wetland rice cultivation, cacao plantation, durian plantation, and avocado plantation (top soil 20 cm).The number of soil sample collected was consisted of three samples for each kind of sample as well as three samples for each land use, thus there were 18 samples in total.The disturbed soil samples of 0.5 kg from each land use were taken using a spade and packed into a plastic container.The disturbed soil samples then were air dried to constant moisture content and passed through a sieve with mesh opening of 0.5 mm and analyzed for texture [11] and organic C [12].
The undisturbed soil samples were collected in metal cylinder with a diameter of 5 cm and a height of 5 cm using sampling equipment.The cylinder was pushed into the soil to a depth of 15 cm.The undisturbed soil samples then were brought to the Soil Laboratory at Agricultural Faculty of Tadulako University for bulk density and hydraulic conductivity analysis.The bulk density was estimated by dividing the dry weight (oven dried at 105 o C) of the soil sample by the volume of the ring.The hydraulic conductivity (HC) was measured by constant water head permeameter technique.Before the HC was measured, the soil was saturated for ±24 hours (h) and then placed in the HC equipment.The amount of water passing through the soil was recorded hourly over the course of three hours.The HC then was calculated using Darcy's equation as shown in Equation 1.
where Khydraulic conductivity (cm/h), Qvolume of water (cm 3 ), ttime (h), Lsoil thickness (cm), hwater head (cm) and Aarea of soil surface (cm 2 ) One-way Analysis of variance was used for all data to determine the difference of organic C, bulk density and water hydraulic conductivity between the land uses, and the means were separated by Tukey's HSD test (P = 0.05).

Soil texture
Except for wetland rice farming, where the soil texture is loamy, the other land uses were found to have sandy loam texture.This may imply that farmers select fine soil texture when specifying the soil for wetland rice cultivation.Similarities in soil texture also suggest that the altered land uses were located close to the forest's edge.The sand content varies from 50% to 70% suggesting that inherently these soils are less fertile.Mismanagement of this kind of soil could lead to severe chemical and physical soil deterioration.Survey of this kind of soil showed that poor farmer usually abandon their farming commonly corn after two crop planting seasons particularly when the soil was in the slope land and leave the soil to fallow changing into shrubs.

Soil organic carbon
Organic matter levels in forest soils are often much higher than in agricultural soils.Organic matter has grown by 6.7±2.2Tg carbon annually over 20 years which is mostly attributable to an increase in forest area [13].Figure 1 shows the presence of organic matter as indicated by the organic C levels of the forest, bushes, wetland rice cultivation, cacao plantation, durian plantation, and avocado plantation.The forest land produced the highest organic C level of 4.70% which is significantly higher than the other land uses.There is no significant difference among the agricultural lands and shrubs land use.The organic C in the agricultural plantations and the shrubs declined rapidly ranging from 2.01% in the shrubs to only 0.80% in the durian plantation.The levels of organic C were decreased by more than six times in the durian plantation down to less than three times in the shrubs land.A sharp decline in organic C from 5.21% to 1.14% was also occurred when forest was converted into dry arable land in Tawaeli watershed Indonesia [14].These decreases are much higher than that found by [9].According to their findings, conversion of forest to meadows leads to soil organic matter loss by 26% and to arable land by 32%.The reducing number of vegetation caused by the conversion of the forest to agricultural uses altered the microclimate of the soil as more soil surface was exposed to sunlight.This lead to the forest's stored organic matter to withstand more severe climatic conditions which accelerated the rate of organic matter decomposition and left the forest with considerably less organic matter than it had before.

Soil bulk density
When the forest was transformed into shrubs and various forms of plantations, the decline in organic C was immediately followed by changes in physical soil characteristics.The average values of soil bulk density under the forest, shrubs, wetland rice cultivation, cacao plantation, durian plantation, and avocado plantation are depicted in Figure 2. When the forest is changed to the other land uses, the soil bulk density significantly increases, yet the differences are not significance among the cultivated lands.The average value of the bulk density under the forest is 0.97 g cm -3 whereas that under the converted land uses ranges between 1.53 g cm -3 in the wetland rice cultivation up to 1.75 g cm -3 in the shrubs implying that more compacted soils are created.This drastically changes in the value of bulk density can be related to such factors as the excessive loss of organic matter and anthropogenic activities.The impact of soil compaction due to machinery traffics can be diminished effectively by adding organic matter [15] and soil containing higher organic matter seems to be more resilience to compaction [16].The soil becomes more prone to compaction when a forest is converted into a plant-cultivated area.The continuing activities of plants rearing and regular harvesting usually involve equipment or many labors that lead also to more intensive soil compaction.

Soil hydraulic conductivity
It is normal for soil bulk density to significantly increase when forest is converted to agricultural fields, which lowers the soil's capacity to transmit water because of reduced porosity.The soil hydraulic conductivity of the forest, shrubs, wetland rice, cacao, durian, and avocado plantation is depicted in Figure 3.The hydraulic conductivity of the forest was found significantly higher than the other land uses but there is no significant different between the remaining land uses.The hydraulic conductivity was declined from 9.46 cm h -1 in the forest to only 2.36 cm h -1 in the shrubs land, 2.47 cm h -1 in the wetland rice cultivation, 2.11 cm h -1 in the cacao plantation, 2.03 cm h -1 in the durian plantation and the least 0.40 cm h -1 in the avocado plantation.The forest conversion causes the stored organic matter to be loss triggering the soil to be more prone to compaction decreasing the soil bulk density as shown in the Figure 2. In most cases, compacted soil results in the loss of soil macro-pores, which serve as water-conducting pores and air, which leaves behind micro-pores that impede water movement in the soil.

Conclusions
Forest conversion to various agricultural lands declined the soil physical characteristics significantly.
a. Soil organic carbon reduces from 4.70% to only 0.80% in durian plantation.b.Soil bulk density increases from 0.90 g cm -3 up to 1.75 g cm -3 in shrubs land.c.Soil hydraulic conductivity lowers from 9.46 cm h -1 to only 0.40 cm h -1 in avocado plantations.

Figure 1 .
Figure 1.Soil organic carbon content under forest, shrubs, wetland rice cultivation, cacao plantation, durian plantation and avocado plantation.*Different letter shows significant different at P <0.5%