Performa fotosintesis dan hasil tanaman kelapa sawit yang ditanam di tanah mineral dan gambut tebal yang didrainase

Abstract

Tanaman kelapa sawit (Elaeis guineensis Jacq.) di Indonesia banyak ditanam pada tanah mineral dan gambut. Pengelolaan lahan gambut untuk tanaman kelapa sawit menghadapi berbagai tantangan diantaranya adalah dianggap menjadi salah satu penyebab pemanasan global karena pembukaan dan pengelolaannya melepaskan emisi karbon dioksida (CO2) dalam jumlah masif. Salah satu praktik baik penanaman kelapa sawit di lahan gambut ialah dengan membuat drainase untuk mempertahankan tinggi muka air tanah. Penggunaan sekat kanal pada saluran drainase dan mempertahankan tanaman penutup tanah (under cover story) dianggap cara yang paling baik dalam menekan emisi CO2 pada perkebunan kelapa sawit di lahan gambut tebal yang didrainase. Penelitian ini bertujuan untuk menganalisis serapan CO2 oleh tanaman kelapa sawit dan tanaman penutup tanah di bawah naungan kelapa sawit melalui proses biologi yaitu fotosintesis. Penelitian terdiri atas dua percobaan, percobaan pertama adalah pengukuran fotosintesis tanaman kelapa sawit dan tanaman penutup tanah di perkebunan kelapa sawit di lahan gambut tebal yang didrainase di Kabupaten Siak Provinsi Riau. Metode yang digunakan adalah mengambil 3 contoh tanaman kelapa sawit menghasilkan (TM) umur 16 tahun dari 3 blok yang berbeda untuk dijadikan objek pengamatan, pada saat yang bersamaan dilakukan pula pengamatan fotosintesis tanaman penutup tanah di bawah naungan kelapa sawit. Pengukuran fotosintesis pada tanaman kelapa sawit dilakukan pada pukul 07.00, 08.00, 09.00, 11.00, 13.00, dan 15.00, sedangkan pengukuran fotosintesis pada tanaman penutup tanah dilakukan pada pukul 09.00 dan 13.00. Pengamatan fotosintesis dilakukan pada Blok D1, D3, dan D6 masing-masing blok diamati sebanyak 6 kali (Pebruari 2020, April 2021, Juli 2021, Nopember 2021, Januari 2022, dan Maret 2022). Percobaan kedua dilakukan pada tanaman kelapa sawit TM umur 12 tahun yang ditanam pada tanah mineral di Kebun Percobaan Cikabayan IPB University Bogor. Contoh tanaman kelapa sawit yang diamati sebanyak 3 tanaman yang berasal dari satu blok yang sama. Pengamatan fotosintesis tanaman kelapa sawit dan tanaman penutup tanah di bawah naungan kelapa sawit dilakukan pada bulan April 2025. Pengukuran fotosintesis pada tanaman kelapa sawit dan tanaman penutup tanah di bawah naungan kelapa sawit mengikuti cara percobaan pertama. Tanaman kelapa sawit yang ditanam di lahan gambut tebal yang didrainase masih menunjukkan aktivitas fisiologis yang tinggi, dengan laju fotosintesis rata-rata 18,44-43,47 µmol CO2 m?² s?¹, konduktansi stomata 0,050–0,067 mol H2O m?² s?¹, dan laju transpirasi 0,00058–0,00119 mol H2O m?² s?¹. Namun, kemampuan asimilasi karbon tersebut berlangsung pada ekosistem yang rentan kehilangan karbon, sehingga tingginya fotosintesis pada lahan gambut lebih tepat dimaknai sebagai mekanisme kompensasi biologis pada sistem yang juga berpotensi menjadi sumber emisi karbon. Hasil analisis korelasi antara laju fotosintesis, konduktansi stomata, dan transpirasi menunjukkan korelasi yang kuat antara konduktansi stomata dan transpirasi (r = 0,775; P<0,001). Hubungan antara kerapatan stomata dan konduktansi stomata menunjukkan korelasi yang lemah (r2 = 0,004; P<0.714). Hubungan antara curah hujan dan tinggi muka air terhadap produksi tandan buah segar (TBS) menunjukkan korelasi yang lemah. Hasil pengukuran serapan karbon pada tanaman kelapa sawit di tanah gambut tebal yang didrainase rata-rata 84,01 ton CO2 ha-1. Pada lahan mineral, performa fotosintesis kelapa sawit lebih mencerminkan respons terhadap mikroklimat harian. Laju fotosintesis kelapa sawit berada pada kisaran 5,23–11,15 µmol CO2 m?² s?¹ dengan rata-rata 7,97 µmol CO2 m?² s?¹. Kondisi fisiologis yang relatif stabil tersebut sejalan dengan pertumbuhan vegetatif yang seragam, yaitu tinggi tanaman 651,0–675,0 cm dengan rata-rata 659,7 cm, dari data tinggi tanaman ini dapat diduga total serapan karbon rata-rata sebesar 109,21 ton CO2 ha-1. Hal ini menunjukkan bahwa pada lahan mineral, asimilasi karbon lebih konsisten terkonversi menjadi pembentukan biomassa dan komponen produksi. Hubungan antara kerapatan stomata dan konduktansi stomata menunjukkan korelasi yang lemah dengan nilai P>0,707 dan r2 = 0,009. Vegetasi gulma juga berkontribusi nyata terhadap dinamika serapan karbon pada kedua tipe lahan. Pada lahan gambut tebal yang didrainase, laju fotosintesis rata-rata Nephrolepis biserrata mencapai 25,31–31,49 µmol CO2 m?² s?¹, sedangkan Asystasia gangetica 23,53–36,37 µmol CO2 m?² s?¹. Pada tanah mineral, N. biserrata menunjukkan performa fotosintesis lebih tinggi dibandingkan A. gangetica, dengan rata-rata laju asimilasi masing-masing 9,50 dan 6,43 µmol CO2 m?² s?¹; selain itu, N. biserrata juga memiliki klorofil total sedikit lebih tinggi, yaitu 0,039 mg cm?² dibandingkan 0,033 mg cm?² pada A. gangetica. Dengan demikian, vegetasi bawah, terutama N. biserrata, tidak hanya berperan sebagai penutup tanah, tetapi juga sebagai komponen fungsional yang mendukung asimilasi karbon pada ekosistem perkebunan kelapa sawit.

Kata kunci: A. gangetica, emisi CO2, N. biserrata, serapan karbon, tinggi muka air

Oil palm (Elaeis guineensis Jacq.) in Indonesia is widely cultivated on mineral soil and peatland. The management of peatland for oil palm cultivation faces various challenges, including the fact that it is considered one of the causes of global warming because its clearing and management release massive amounts of carbon dioxide (CO2) emissions. One best practice for oil palm cultivation on peatlands is to install drainage systems to maintain the water table. The use of canal barriers in drainage channels and the maintenance of a ground cover (understory) are considered the most effective ways to reduce CO2 emissions in oil palm plantations on drained thick peatlands. This study aims to analyze CO2 uptake by oil palm trees and groundcover plants under oil palm canopies through the biological process of photosynthesis. The study consisted of two experiments. The first experiment involved measuring photosynthesis in oil palm trees and groundcover plants at an oil palm plantation on drained thick peatland in Siak Regency, Riau Province. The method used involved selecting three samples of 16-year-old productive oil palm trees (mature plant) from three different blocks as observation subjects; simultaneously, photosynthesis measurements were conducted on the groundcover plants under the shade of the oil palms. Photosynthesis measurements on oil palm trees were taken at 7:00 a.m., 8:00 a.m., 9:00 a.m., 11:00 a.m., 1:00 p.m., and 3:00 p.m., while photosynthesis measurements on groundcover plants were taken at 9:00 a.m. and 1:00 p.m. Photosynthesis observations were conducted in Blocks D1, D3, and D6, with each block observed six times (February 2020, April 2021, July 2021, November 2021, January 2022, and March 2022). The second experiment was conducted on 12-year-old (mature plant) oil palm trees planted in mineral soil at the Cikabayan Experimental Station of IPB University in Bogor. A total of three oil palm trees from the same block were observed. Observations of photosynthesis in oil palm trees and groundcover plants under the shade of oil palms were conducted in April 2025. Photosynthesis measurements on oil palm trees and groundcover plants under the shade of oil palms followed the same procedure as in the first experiment. Oil palm plants grown on drained thick peatlands still exhibit high physiological activity, with an average photosynthesis rate of 18.44–43.47 µmol CO2 m?² s?¹, stomatal conductance of 0.050–0.067 mol H2O m?² s?¹, and a transpiration rate of 0.00058–0.00119 mol H2O m?² s?¹. However, this carbon assimilation capacity occurs within an ecosystem prone to carbon loss; therefore, the high photosynthesis rates on peatlands are more accurately interpreted as a biological compensation mechanism in a system that also has the potential to be a source of carbon emissions. The results of the correlation analysis between photosynthesis rate, stomatal conductance, and transpiration showed a strong correlation between stomatal conductance and transpiration (r = 0.775; P < 0.001). The relationship between stomatal density and stomatal conductance showed a weak correlation (r² = 0.004; P < 0.714). The relationship between rainfall and water table height and fresh fruit bunch (FFB) production showed a weak correlation. Carbon uptake measurements in oil palm plants on thick peat soil that had been drained averaged 84.01 metric tons of CO2 ha-1. On mineral soils, oil palm photosynthetic performance more closely reflects its response to the daily microclimate. The net assimilation rate of oil palm ranged from 5.23 to 11.15 µmol CO2 m?² s?¹ with an average of 7.97 µmol CO2 m?² s?¹. These relatively stable physiological conditions are consistent with uniform vegetative growth, specifically plant heights of 651.0–675.0 cm with an average of 659.7 cm; based on these plant height data, the average total carbon uptake is estimated at 109.21 metric tons CO2 ha?¹. This indicates that on mineral soils, carbon assimilation is more consistently converted into biomass formation and production components. The relationship between stomatal density and stomatal conductance showed a weak correlation with a p-value > 0.707 and r² = 0.009. Weed vegetation also made a significant contribution to carbon uptake dynamics on both land types. On drained thick peatlands, the average photosynthetic rate of Nephrolepis biserrata reached 25.31–31.49 µmol CO2 m?² s?¹, while that of Asystasia gangetica was 23.53–36.37 µmol CO2 m?² s?¹. On mineral soil, N. biserrata exhibited higher photosynthetic performance than A. gangetica, with average assimilation rates of 9.50 and 6.43 µmol CO2 m?² s?¹, respectively; in addition, N. biserrata also had a slightly higher total chlorophyll content, at 0.039 mg cm?² compared to 0.033 mg cm?² in A. gangetica. Thus, understory vegetation, particularly N. biserrata, serves not only as ground cover but also as a functional component that supports carbon assimilation in oil palm plantation ecosystems.

Keywords: A. gangetica, CO2 emission, N. biserrata, carbon sequestration, water table depth