Investigation of Post-Fire Peatland Natural Recovery, South Kalimantan, Indonesia

Peatlands play a critical role in global habitats since are composed of heterogeneous materials and chemical reactions. Peatland fires significantly change the chemical characteristics of its soil, such as carbon (C), nitrogen (N), phosphorus (P) and potassium (K) content. This study aimed to measure peatland recovery only on those soil chemical characteristics based on two different times of sampling that are five years (Yr+5) and seven years (Yr+7) after the fires in 2015 (taken in 2020 and 2022). This study was conducted in the Balangan River - Batangalai River peat hydrological unit, South Kalimantan, Indonesia. Soil samples were collected at nine different locations, including are six locations in the areas that experienced fires in 2015 and three locations in the areas that did not experience fires. Those soil samples were taken with excavated pits at each sample location at each depth of 10, 20, 30, 40 and 50 cm. This study found that the carbon content in the post-fire area increased by 22.00% and in the natural area by 9.90%. The nitrogen content in the post-fire area increased by 1.94% and in the natural area by 1.17%. The potas - sium content in the post-fire areas increased by 16.33% and in the natural areas by 4.44%. The phosphorus content in the post-fire area increased by 3.18% and in the natural area by 5.11%. C/N ratio increased by 19.68% and C/P ratio increased by 18.24%. Overall, the increase in carbon, nitrogen, potassium, phosphorus, C/N ratio and C/P ratio in post-fire and natural peatlands indicates an improved condition. This study can provide supporting information for the regulator, management or expertise of the land and forest rehabilitation to speed up the recovery process.


INTRODUCTION
Peatlands play a critical role in global habitats (IUCN 2017, Williams-Mounsey et al. 2021).Peatlands are composed of heterogeneous materials and reactions of nitrification, denitrification, sulphate reduction, oxidation of reduced inorganic sulphides, etc.In peatland processes, sulphate reduction occurs due to changes in Eh and pH, the formation of alkanes, C transformation, mobility and the reduction of N, P and C (Porowski et al. 2019).Peatlands, based on geospatial data, cover approximately 423 million ha or 2.84% of the global land area (Xu et al. 2018, Anda et al. 2021, Yuwati et al. 2021).The largest peatland area is located in Southeast Asia, totalling 25 million ha, most of which is in Indonesia (Kiely et al. 2021, Atkinson andAlibašić 2023).In total, peatlands in Indonesia cover an area of 13.43 million ha, spread across Sumatra 5.85; Kalimantan 4.54; Papua 3.01 and Sulawesi 0.024 million ha (Anda et al. 2021).More than 80% of peatlands in Southeast Asia are degraded (Mishra et al. 2021) and peatlands in Indonesia in 2015 experienced fires covering 2.6 million ha and in 2019 covering 1.65 million ha (Saharjo and Novita 2022).Peatland degradation is largely caused by deforestation, agriculture, plantations, housing and fires (Harrison et  ) showed that the C, N and P concentrations will increase.Those previous studies showed that the time of natural peatland recovery after fire in particular chemical properties are still very limited.This research provided information on the recovery rate of post-fire peatlands in the Balangan River -Batangalai River KHG, South Kalimantan.Since the study on the natural recovery time of tropical peatlands, particularly Indonesian peatland after fires is still very limited, this study provided information on the recovery rate of soil chemical characteristics in the post-fire peatlands in the Balangan River-Batangalai River, South Kalimantan.This study aimed to measure peatland recovery of those soil chemical characteristics based on two different time of sampling after the fires in 2015 (taken in 2020 and 2022).Soil chemical properties and characteristics is one of the key-factor of the peatland recovery after burning and it is required in the natural or artificial rehabilitation process.Through the results of its natural recovery, this study can assist the land and forest rehabilitation and/or recovery expertise to speed up the recovery process.

MATERIALS AND METHODS
This study was conducted in the Balangan River -Batangalai River peat hydrological unit, South Kalimantan, Indonesia, which covers a geographical range of 2.34°-2.56°S and 115.22°-115.41°E (Figure 1).The peat soil samples tested came from fire peatlands in 2015 and those that did not experience fires.This study aimed to measure peatland recovery of those soil chemical characteristics based on two different time of sampling that are 5 years (Yr+5) and 7 years (Yr+7) after the fires in 2015.The level of peatland recovery is based on soil chemical characteristics, including carbon (C), nitrogen (N), phosphorus (P) and potassium (K) contents.Soil samples were collected at nine different locations that included six locations in the areas that experienced fires in 2015 (P-1, P-2, P-3, P-4, P-5 and P-6) and three locations in the areas that did not experience fires (P-7, P-8 and P-9).Those soil samples were taken in 2020 and 2022 with excavated pits at each sample location at each depth of 10, 20, 30, 40 and 50 cm.A total of 45 samples were tested for carbon, nitrogen, phosphorus and potassium contents.
Figure 2 shows the process of sample collection, preparation and testing.In the sample collection and preparation process, the soil samples were then air-dried and sieved to 2 mm after the fine plant roots were carefully removed.Total carbon content was measured using a spectrophotometer using the following formula (Agus et where: ppm curve -sample content obtained from the relationship curve between content standard series with its reading after correction of the blank, f k -moisture content correction factor = 100/(100 -% moisture content).
Total nitrogen content determined using colorimeter with the calculation of the percentage of N-total content in the soil is as follows ( ( where: V c -volume of H 2 SO 4 sample titration result, N -normality H 2 SO 4 (0.05 N), V bvolume of H 2 SO 4 titration result of blank, KBK -dry matter correction.
Phosphorus content using colorimeter and using the formula is as follows (Sulaeman et al. 2005): where: ppm curve -sample content obtained from the relationship curve between the level of the standard series with its reading after corrected blanks, 142/90 -conversion factor of the form PO 4 to P 2 O 5 .Total potassium content using a flame photometer with the calculation of the percentage of potassium content using the formula (Sulaeman et al. 2005):

RESULTS AND DISCUSSION
Chemical characteristics measurement of post-fire peatlands in 2015.Carbon content tests are shown in Figure 3a-c in post-fire and natural/ non-fire areas.Figure 4a-c illustrate the nitrogen content of the soil in post-fire and non-fire areas.Figure 5a-c show the results of testing the phosphorus content of the soil in post-fire and non-fire areas.In turn, Figure 6a-c illustrate the results of testing the potassium content of the soil in postfire and non-fire areas.
Figure 3a shows that the carbon content values in the areas that experienced fires in the five-year post-fire period (2015-2020) are lower than the carbon content values in the areas that did not experience fires.The average value of carbon content at several locations and sample depths in post-fire areas was 15.83% and in the areas that did not experience fires was 22.27%.Figure 3(b) shows that the carbon content value in the burned area within seven years after the fire (2015-2022) is lower than the carbon content value in the non-fired area.The average value of carbon content at several locations and sample depths in post-fire areas was 19.32% and in the areas that did not experience fires was 24.48%.Compared to previous studies (Dikici andYilmaz 2006, Ismawi et al. 2012), it has the same result which is the value of carbon content in post-fire areas is smaller than in the areas that did not experience fires.Figure 3(c) illustrates the increase in carbon content value from the fifth year to the seventh year after the fire, for the post-fire area there was an increase of 22.00% and in the area that did not experience the fire by 9.90%.
Figure 4a shows that the nitrogen content of the soil in the fifth year after the fire was lower than that in the area without fire.The average value of nitrogen content at several locations and sample depths in post-fire areas within five years was 0.69% and in the areas that did not experience fires was 0.80%.Figure 4b shows that the nitrogen content of the soil in the seventh year after the fire was lower than the nitrogen content in the area without fire.The average value of nitrogen content at several locations and sample depths in post-fire areas within seven years was 0.70% and in the areas that did not experience fires was 0.81%.This result has the same tendency with previous studies (Ismawi et al. 2012) that is the value of nitrogen content in post-fire areas is smaller than in the areas that did not experience fires.Figure 4c illustrates the increase in nitrogen content from the fifth year to the seventh year after the fire, for the post-fire area there was an increase of 1.94% and in the area that did not experience the fire by 1.17%.
Figure 5a shows that the phosphorus content of the soil in the fifth year after the fire was lower than the phosphorus content in the area without fire.The average value of phosphorus content at several locations and sample depths in post-fire areas within five years was 0.0075% and in the areas that did not experience fires was 0.0092%.Figure 5b shows that the phosphorus content of the soil in the seventh year after the fire was lower than the phosphorus content in the area without fire.The average value of phosphorus content at several locations and sample depths in post-fire areas within seven years was 0.0078% and in the areas that did not experience fires was 0.0097%.This result has the same trend with previous studies (Ismawi et al. 2012) in which the value of phosphorus content in post-fire areas is smaller than in the areas that did not experience fires.Figure 5c illustrates the increase in phosphorus content from the fifth year to the seventh year after the fire, for the post-fire area there was an increase of 3.19% and in the area that did not experience the fire by 5.11%.
Figure 6a shows that the potassium content of the soil in the fifth year after the fire was lower than the potassium content in the area without fire.The average value of potassium content at several locations and sample depths in post-fire areas within five years was 10.39% and in the areas that did not experience fires was 14.16%.Figure 6(b) shows that the phosphorus content of the soil in the seventh year after the fire was lower than the potassium content in the area without fire.The average value of potassium content at  several locations and sample depths in post-fire areas within seven years was 12.09% and in the areas that did not experience fires was 14.78%.This result is similar with previous studies (Ismawi et al. 2012) that is the value of potassium content in post-fire areas is smaller than in the areas that did not experience fires.Figure 6c illustrates the increase in potassium content from the fifth year to the seventh year after the fire, for post-fire areas there was an increase of 16.33% and in the areas that did not experience the fire by 4.44%.Figure 7a illustrates the increase in the ratio of carbon content to nitrogen from the fifth year to the seventh year after the fire, for post-fire areas there was an increase of 9.68% and in the areas that did not experience the fire by 8.63%.Figure 7b illustrates the increase in the comparison of carbon content values with phosphorus from the fifth year to the seventh year after the fire, for post-fire areas there was an increase of 18.24% and in the areas that did not experience the fire by 4.56%.
The C/N value is one measure of the condition of peatland degradation (Ismawi et al.The increase in C/P value is one of the benchmarks for the improvement of peatland conditions from land degradation (Ismawi et al. 2012).The increase in C/P value on the burned land Yr+5 was 2098.62 and Yr+7 was 2481.32 and on the natural land Yr+5 was 2411.93 and Yr+7 was 2521.84.According to the test results, there was an increase in the C/P value, which illustrates that the condition of the peatland has an improvement.

CONCLUSIONS
This study showed chemical properties recovery rate of post-fire peatlands in the Balangan River-Batangalai River, South Kalimantan.The peat soil samples tested came from peatlands after the 2015 fires and those that had not experienced fires in the timeline of five years (Yr+5) and seven years (T+7) afterwards.This study found that the direction of all measured chemical properties of the investigated post-fire peat soil are increasing.In general, the increase in carbon, nitrogen, phosphorus, potassium, C/N ratio and C/P ratio in post-fire peatlands indicates an enhanced condition.This study can provide supporting information for the regulator, management or expertise of the land and forest rehabilitation to speed up the recovery process.

Figure 2 .
Figure 2. Process sample collection, preparation and testing

Figure 3 .
Figure 3. (a) The carbon content of each location at Yr+5, (b) the carbon content of each location at Yr+7, (c) changes in carbon content values at Yr+5 and Yr+9

Figure 6 .
Figure 6.(a) The potassium content of each location at Yr+5, (b) the potassium content of each location at Yr+7, (c) changes in potassium content values at Yr+5 and Yr+9

Table 1 .
Test results and sample calculations