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.