Dinamika Fluks CO2 dan Karbon Organik Terlarut pada Kolom Tanah Gambut Perkebunan Kelapa Sawit dengan Simulasi Tinggi Muka Air dan Curah Hujan
Abstract
Fluks karbon dioksida (CO2) dan karbon organik terlarut (dissolved organic
carbon, DOC) di lahan gambut yang didrainase sebagian besar ditentukan oleh
tinggi muka air tanah (TMAT) dan curah hujan bulanan (CHB). Tidak mudah untuk
mengatur TMAT yang stabil di lapangan dan bahkan tidak mungkin untuk CHB.
Penelitian ini bertujuan memeroleh data dasar dan mengevaluasi pengaruh
perlakuan simulasi TMAT dan CHB terhadap dinamika fluks CO2, kadar DOC dan
sifat-sifat kimia tanah pada kolom tanah gambut tropika perkebunan kelapa sawit.
Penelitian ini dilaksanakan pada Juli 2020 sampai April 2021. Pengambilan
contoh gambut tak-terganggu menggunakan pipa PVC berdiameter 12,7 cm
sepanjang 110 cm dengan ketebalan kolom gambut 100 cm dilakukan di lahan
perkebunan kelapa sawit PT Kimia Tirta Utama di Kabupaten Siak, Provinsi Riau.
Selanjutnya kolom gambut dalam pipa ditransportasikan secara hati-hati ke IPB,
Bogor. Percobaan simulasi TMAT dan CHB serta pengukuran fluks CO2 dilakukan
di rumah kaca Kebun Pendidikan Cikabayan, IPB. Analisis kadar DOC dan sifatsifat
kimia gambut dilakukan di Laboratorium Divisi Kimia dan Kesuburan Tanah,
Departemen Ilmu Tanah dan Sumberdaya Lahan, Fakultas Pertanian, IPB.
Penelitian ini menggunakan Rancangan Acak Lengkap 2 faktor, yaitu
perlakuan simulasi TMAT (cm) 3 taraf: −40 (T1), −60 (T2), dan −80 (T3) yang
didasarkan atas hasil studi pustaka serta perlakuan simulasi CHB (mm.bulan-1) 4
taraf: 30 (C1), 120 (C2), 210 (C3), dan 300 (C4) yang didasarkan atas curah hujan
bulanan di areal pengambilan kolom gambut yang direkam Automatic Weather
Station (AWS) pada tahun 2019 (terendah April 33,6 dan tertinggi November 263,5
mm.bulan-1), masing-masing dengan 3 ulangan, sehingga terdiri atas 36 satuan
percobaan. Analisis fluks CO2, kadar DOC, dan pH air gambut dilakukan setiap
bulan, sedangkan untuk pH, kadar C-organik, dan sifat-sifat kimia gambut lainnya
setiap dua bulan. Evaluasi pengaruh perlakuan didasarkan atas hasil analisis ragam
(analysis of variance) dan uji lanjut DMRT (Duncan Multiple Range Test).
Fluks CO2 diukur dengan metode sungkup yang secara langsung mengukur
akumulasi atau penipisan CO2 dalam sungkup. Kadar CO2 terukur oleh InfraRed
Gas Analyser (IRGA) setiap detik. Penurunan permukaan yang berbeda antar
kolom gambut dalam pipa akibat pemadatan selama transportasi, dengan rerata 16,2
cm, dipertimbangkan dalam analisis fluks CO2 sebagai ketinggian sungkup,
sehingga data yang diperoleh layak digunakan dalam analisis statistika. Kadar DOC
dianalisis dengan metode Walkley & Black dengan modifikasi teknik pemekatan
melalui prosedur evaporasi. Kadar C-organik gambut diukur dengan metode
pengabuan kering atau Loss on Ignition (LOI). Nilai pH gambut dan air gambut
diukur dengan pH meter. Kadar P-tersedia dianalisis menggunakan metode Bray 1.
Kadar basa-basa dapat dipertukarkan gambut dianalisis dengan pengesktrak N
NH4OAc pH 7.0.
Hasil penelitian menunjukkan bahwa interaksi perlakuan TMAT −40 dan
−80 cm pada CHB 300 mm.bulan-1 setelah 4 bulan simulasi masing-masing menghasilkan fluks CO2 terendah dan tertinggi secara signifikan, yaitu 8,82 dan
42,75 t.ha-1.tahun-1. Hal ini menunjukkan bahwa efek simulasi TMAT lebih
dominan daripada CHB terhadap fluks CO2. Setelah 6 bulan, fluks CO2 terendah
secara signifikan (11,24 t.ha-1.tahun-1) diperoleh pada perlakuan tunggal simulasi
TMAT −40 cm , sedangkan untuk DOC (57,18 ppm) diperoleh pada perlakuan
tunggal simulasi CHB 30 mm.bulan-1 . TMAT yang lebih dekat ke permukaan tanah
lebih menurunkan fluks CO2, sedangkan CHB yang lebih rendah menghasilkan
DOC yang lebih rendah.
Kadar P-Bray gambut meningkat setiap bulan selama 6 bulan periode
simulasi dan tergolong sangat tinggi. Hal ini disebabkan oleh peningkatan
pelepasan P-organik sebagai hasil dekomposisi bahan gambut. Kadar basa-basa
dapat dipertukarkan gambut dan air gambut tergolong rendah hingga sedang.
Berdasarkan hasil percobaan simulatif ini dapat direkomendasikan agar TMAT di
lapangan penelitian selama musim kemarau diatur pada sekitar −40 cm untuk
mendapatkan kontrol terbaik terhadap fluks CO2 dan DOC. Carbon dioxide (CO2) flux and dissolved organic carbon (DOC) in drained
peatlands are determined mostly by ground water level (GWL) and monthly rainfall
(MRF). It is not easy to set up a stable GWL in the field and even impossible for
MRF. This research was aimed to obtain basic data and evaluate the effects of GWL
and MRF simulation treatments on dynamics of CO2 flux, DOC level, and soil
chemical properties in an oil-palm plantation tropical peat soil column.
This research was conducted from July 2020 to April 2021. Undisturbed
peat soil columns were sampled using PVC pipes with diameter of 12.7 cm and
length of 110 cm for peat soil layer thickness of 100 cm at an oil palm plantation
area of PT Kimia Tirta Utama in Siak Regency, Riau Province. The peat columns
were then transported and handled with care to IPB University, Bogor. GWL and
MRF distribution simulation experiment as well as CO2 flux measurements were
carried out in the greenhouse of Cikabayan Educational Farm, IPB University.
Determination of DOC level and other peat chemical properties were carried out at
Laboratory of the Division of Soil Chemistry and Fertility, Department of Soil
Sciences and Land Resource, Faculty of Agriculture, IPB University.
This experiment applied Completely Randomized Design with 2 factors that
consisted of GWL (cm) simulation treatment with 3 levels, i.e. −40 (T1), −60 (T2),
and −80 (T3) that based on the results of literature study and MRF (mm.month-1)
simulation treatment with 4 levels, i.e. 30 (C1), 120 (C2), 210 (C3), and 300 (C4)
that based on the monthly rainfall recorded using Automatic Weather Station
(AWS) at the peat column sampling area in 2019 (the lowest was in April: 33.6 and
the highest was in November: 263.5 mm.month-1), each with 3 replications or 36
experimental units. Analysis of CO2 flux, DOC level, and pH of peat water were
carried out every month, while for pH, organic-C, and other peat chemical
properties were every two months. Evaluation of the treatment effects was based on
the results of Analysis of Variance and DMRT (Duncan Multiple Range Test). The
flux of CO2 was measured by applying the closed chamber method that directly
measured the accumulation or depletion of CO2 inside the chamber. The CO2 levels
were measured by Infra-Red Gas Analyzer (IRGA) for every second. Due to
compaction during transportation, the peat column surfaces decreased differently
among the columns, with an average of 16,2 cm. These were valuated in the analysis
of CO2 flux as the chamber head so that the data obtained were reliable for statistical
analysis. The DOC level was analyzed by modified Walkley & Black method by
applying concentrating technique using evaporation procedure. The organic-C
content of the peat was measured by Loss on Ignition (LOI) method. The pH values
of peat and peat water were measured with pH-meter. The available-P levels were
analyzed using Bray 1 method. The peat exchangeable-bases were determined using
N NH4OAc pH 7.0 extractant.
The results showed that interaction effects of GWL −40 and −80 cm at
MRF 300 mm.month-1 after 4 months simulation resulted the significant lowest and
highest CO2 flux of respectively 8.82 and 42.75 t.ha-1.year-1, indicating that the
effects of GWL were dominant over MRF simulation on the CO2 flux. After 6
months experimentation, the significant lowest CO2 flux (11.24 t.ha-1.year-1) was
measured due to the single effect of GWL −40 cm simulation, while for DOC
(57.18 ppm) was obtained from the single effect of MRF 30 mm. month- simulation.
A shallower GWL reduced more CO2 flux, while a lower MRF resulted in a lower
DOC level.
Peat P-Bray content increased every month during the 6 months period of
simulation and classified as very high. This was because of the increased in organic-
P release as a result of decomposition of the peat material. Peat and peat-water
exchangeable bases were low to moderate. Based on the results of this simulation
experimentation, it is recommended that the GWL at the research field during the
dry season should be set up at around −40 cm for the best control of CO2 and DOC
fluxes.
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