Konversi Tutupan Lahan Gambut Tropika dan Implikasinya terhadap Sifat Fisikokimia dan Simpanan Karbon Tanah: Pengaruh Tipe dan Umur Penggunaan Lahan.

Abstract

Dalam rangka pengembangan pertanian, sebagian besar tutupan lahan gambut di Indonesia telah dikonversi menjadi perkebunan kelapa sawit, hutan tanaman industri, dan pertanian tanaman tahunan yang memerlukan konstruksi jaringan drainase dengan dampak yang luar biasa terhadap sifat dan simpanan karbon tanah. Konversi tutupan lahan gambut untuk pertanian yang terluas di Indonesia terjadi di wilayah Provinsi Riau, dimana 61% dari luasan lahan gambutnya telah dikonversi menjadi lahan pertanian. Namun, konstruksi jaringan drainase dalam rangka konversi tutupan lahan gambut menjadi lahan pertanian menyebabkan penurunan muka air tanah serta perubahan laju asimilasi dan emisi karbon. Perubahan tipe penggunaan lahan tersebut dapat mengubah lahan gambut dari sebagai penyimpan menjadi sumber emisi karbon. Oleh karena itu, upaya untuk menekan laju emisi karbon dari lahan gambut budidaya perlu terus dilakukan. Kebanyakan penelitian mengenai lahan gambut terkonversi berfokus pada pembandingan biner antara hutan dan tipe penggunaan lahan lainnya. Penelitian yang membahas dinamika proses dekomposisi material gambut dalam mempengaruhi dinamika sifat tanah dan simpanan karbon yang melibatkan umur penggunaan lahan belum banyak dilaporkan, padahal informasi terkait hal ini sangat diperlukan sebagai dasar penyusunan arah pengelolaan lahan gambut terkonversi serta penyelamatan lahan gambut terdegradasi dan rencana restorasinya. Penelitian ini bertujuan mengelaborasi dinamika sifat fisikokimia dan simpanan karbon tanah di lahan gambut terkonversi terluas di Indonesia terkait tipe dan umur penggunaan lahan yang berbeda, serta untuk menyusun rekomendasi pengelolaan lahan gambut berkelanjutan. Penelitian dilaksanakan dari Desember 2022 sampai Mei 2023. Penelitian lapang dilaksanakan dengan metode survei pada KHG Sungai Siak-Sungai Kampar, Provinsi Riau. Lokasi pengamatan lapang ditetapkan secara purposif untuk mewakili area perkebunan kelapa sawit dengan umur tanaman 2, 8, dan 14 tahun, pertanaman nanas pada umur 2 dan 8 tahun, serta area hutan sebagai pembanding. Area pertanaman nanas dengan umur 14 tahun tidak ditemukan di lokasi penelitian. Penetapan lokasi penelitian didasarkan hasil overlay peta penggunaan lahan (Landsat 8), peta satuan lahan dan tanah, peta Rupa Bumi Indonesia, dan peta spasial gambut dengan teknik Geographic Information System. Selanjutnya, lokasi pengambilan contoh tanah ditetapkan dengan kombinasi terrain technique berdasarkan keadaan fisiografi lahan untuk mewakili keragaman kematangan gambut dalam hubungannya dengan keadaan rizosfer dan fluktuasi muka air tanah, yaitu pada lapisan 0-50 cm dan lapisan 50-100 cm. Setiap lokasi pengamatan dilakukan pengambilan contoh tanah pada 4 titik sebagai ulangan. Analisis tanah dilakukan di Laboratorium Departemen Ilmu Tanah dan Sumberdaya Lahan, Institut Pertanian Bogor. Konversi hutan gambut menjadi perkebunan kelapa sawit dan pertanaman nanas mengakibatkan peningkatan kematangan gambut, kadar abu, pH H2O, N-total, P2O5-total, dan K2O-total tanah, namun sebaliknya terjadi penurunan kadar air, E4/E6, kadar serat, dan C-organik tanah. BI tanah meningkat pada perkebunan kelapa sawit, namun menurun pada pertanaman nanas. Perubahan sifat fisikokimia tanah semakin signifikan seiring dengan meningkatnya umur penggunaan lahan kelapa sawit dan nanas. Simpanan karbon tanah pada lahan gambut berkurang seiring terjadinya konversi lahan dan meningkatnya umur penggunaan lahan perkebunan kelapa sawit dan pertanaman nanas. Rerata kehilangan karbon lahan gambut pada perkebunan kelapa sawit (0,61-0,96% tahun-1) secara signifikan lebih rendah dibandingkan pada pertanaman nanas (2,61% tahun-1). Berdasarkan analisis regresi linier berganda, terdapat tiga sifat tanah yang secara signifikan mempengaruhi simpanan karbon gambut menurut fungsi berikut: Simpanan C tanah = -367,5 + 2574 BI + 7,2 C-org + 1,1 E4/E6 (Adj. R2= 0,98) Berdasarkan fungsi di atas, pengembangan pertanian di lahan gambut harus menerapkan teknik budidaya yang mampu mempertahankan BI dan C-organik tanah tetap tinggi, serta meminimalkan laju humifikasi (E4/E6) material gambut. Upaya meminimalkan risiko degradasi lahan gambut diarahkan pada pengelolaan MAT -38 cm untuk pertanaman nanas dan -44 cm untuk perkebunan kelapa sawit. Pengembangan perkebunan kelapa sawit dengan pengelolaan MAT -44 cm lebih direkomendasikan dibandingkan pertanaman nanas dalam rangka mewujudkan pertanian lahan gambut berkelanjutan di Provinsi Riau.

For agriculture development, Indonesian peatland covers were mainly converted into oil palm, industrial forest, and annual crop farming that required drainage construction with its remarkable implications on soil properties and carbon stock. The most extensive peatland cover conversion for agriculture in Indonesia occurred in the Riau Province, where 61% of the peatland area has been converted into agricultural land. However, the construction of drainage in the context of converting peatland into agricultural land causes a decrease in groundwater levels and changes in the carbon assimilation and emissions rate. Changes in land use types can change peatlands from being a storage to a source of carbon emissions. Therefore, efforts to reduce the rate of carbon emissions from cultivated peatlands need to be continued. Most study on converted peatlands focuses on binary comparisons between forests and other land use types. The study that discusses the dynamics of the peat material decomposition process in influencing the dynamics of soil properties and carbon stock involving the age of land use has not been widely reported, even though information related to this is very much needed as a basis for developing management directions for converted peatlands as well as saving degraded peatlands and restoration plans. This study was aimed to elaborate soil properties and carbon stock dynamics in the most Indonesian converted peatland area under different land use types and ages, as well as to develop recommendations for sustainable peatland management. The study was carried out from December 2022 to May 2023. Field observation was carried out using survey methods in the Peat Hydrological Unit between Siak and Kampar Rivers, Riau Province. Soil samples were taken purposively to represent peatlands in Riau Province under oil palm plantation with land use ages of 2, 8, and 14 years, pineapple cropping at ages of 2 and 8 years, and a forest area as reference. No pineapple cropping areas aged 14 years were found in the research area. Determination of field observation was based on the results of an overlay of land use maps (Landsat 8), land and soil unit maps, Indonesian Earth maps, and peat spatial maps using the Geographic Information System technique. Next, the location for soil sampling was determined using a combination of terrain techniques based on the land physiographic conditions to represent the diversity of peat maturity in relation to the condition of the rhizosphere and fluctuations in groundwater levels, namely in the 0-50 cm layer and the layer 50-100 cm. At each observation location, soil samples were taken at 4 points as replications. Soil analysis was conducted at the Laboratory of the Department of Soil Science and Land Resources, IPB University. Conversion of peat forests into oil palm and pineapple plantations increased peat maturity, ash content, pH H2O, total N, total P2O5, and total K2O, but vice versa, there was a decrease in water content, E4/E6, fiber content, and soil organic C. Soil BD increased in oil palm plantations but decreased in pineapple plantations. Changes in soil physicochemical properties become more significant as the age of land use for oil palm and pineapple increased. Soil carbon stock in peatlands decreases as land conversion occurs and the age of land use for oil palm plantations and pineapple plantations increased. The average carbon loss in oil palm plantations (0.61-0.96% yr-1) was significantly lower than in pineapple cropping (2.61% yr-1). Based on multiple linear regression analysis, three soil properties significantly influence peat carbon stock according to the following function: Soil C stock = -367,5 + 2574 BD + 7,2 C-org + 1,1 E4/E6 (Adj. R2= 0,98) Based on the equation above, peatland development for agriculture must apply cultivation techniques that can maintain high BD and soil organic C and minimize the humification rate (E4/E6) of peat material. Efforts to minimize the risk of peatland degradation were directed at managing GWL of -38 cm for pineapple cropping and -44 cm for oil palm plantations. Converting the peatlands into oil palm plantations with managed GWL at around -44 cm was more sustainable than pineapple cropping to realize sustainable peatland agriculture in Riau Province.