Dampak kegiatan pertanian dan kebakaran atas watak Gambut Ombrogen

KURNAIN, Ahmad, Promotor Prof.Dr.Ir. KPH. Tejoyuwono Notohadikusumo

2005 | Disertasi | S3 Ilmu Pertanian

Penelitian bertujuan untuk mengkaji dampak kegiatan pertanian dan kebakaran hutan gambut atas beberapa sifat fisika dan kimia tanah gambut, serta dinamika kemasaman dan nitrogen di lahan gambut ombrogen. Kajian atas sifat-sifat fisika dan kimia tanah gambut dilakukan pada enam tipe penggunaan lahan: (1) hutan gambut tebang pilih, (2) hutan gambut terbakar, (3) lahan jagung, (4) lahan nenas, (5) lahan karet, dan (6) lahan terbuka. Kajian atas dinamika kemasaman dan N dilakukan pada tiga tipe penggunaan lahan yang disebutkan pertama selama periode 18 Desember 2002–1 Maret 2003. Kajian dinamika kemasaman meliputi pengamatan komposisi ion dalam cuplikan air hujan, larutan tanah, dan serapan tanaman. Sedang kajian dinamika N meliputi pengamatan proses alihragam N dalam sistem tanah, dan proses alihtempat N antara komponen air hujan, pemupukan, serapan tanaman, pelindian, dan proses dakhil tanah. Akibat kegiatan pertanian dan kebakaran bahan gambut menjadi lebih matang, terutama di lapisan 0–15 cm. Hal ini selanjutnya meningkatkan berat volume dan bagian pori mikro, dan menurunkan daya menyimpan lengas maksimum, volume spesifik, kerutan, porositas total, dan bagian pori makro. Kadar lengas pada potensial air di bawah -1 kPa menurun, dan yang di atas -1 kPa meningkat dengan pematangan bahan gambut. Dalam hal sifat kimia, dampaknya juga tergantung atas masukan luaran, selain karena adanya pematangan gambut. Jika tanpa ada masukan luaran, pH (CaCl2), N-total, P-total, N-NH4, dan nisbah Nmineral/ N-total tanah gambut menurun; dan nisbah C/N, nisbah C/P, Chumus, dan nisbah C-humus/C-organik tanah gambut meningkat. Sebaliknya, jika ada masukan luaran seperti di lahan jagung, dampaknya agak berlainan. Jatuhan air hujan hanya kecil peranannya dalam pemasaman tanah gambut, kecuali di lahan jagung peranannya cukup berarti. Proses yang memasamkan gambut di tiga plot yang diamati dirajai oleh disosiasi asam-asam organik. Keterlibatan alihragam N lebih tinggi di lahan yang terusik, terutama di lahan jagung. Neraca proton di tiga plot menunjukkan adanya pemasaman lapisan gambut di plot hutan gambut tebang pilih, yang meningkat dengan kedalaman. Begitu pun di hutan gambut terbakar, tetapi pemasamannya lebih rendah daripada di hutan gambut tebang pilih. Sedang di lahan jagung, pembasaan terjadi di lapisan 0–10 cm, tetapi terjadi pemasaman di lapisan 10–30 cm. Alihragam N di dalam sistem tanah menunjukkan mineralisasi bersih N positif, kecuali di lapisan 10–30 cm plot hutan gambut tebang pilih. Mineralisasi bersih N terutama berasal dari amonifikasi, dan masukan luaran dari pupuk urea di plot lahan jagung yang menambah pasokan N-mineral. Nitrifikasi bersih di semua plot bernilai negatif, dan semakin negatif di lahan yang terusik. Aliran masuk N di plot hutan gambut tebang pilih terutama bersumber dari hasil mineralisasi lapisan serasah. Di plot lahan jagung, aliran masuk N terutama dari pemupukan urea. Sedang air hujan memasok hanya sedikit N di tiga plot yang diamati. Neraca N menunjukkan bahwa plot hutan gambut tebang pilih memanfaatkan N secara lebih efisien, sehingga meninggalkan sisa nisbi kecil, yakni 2,02 kg N ha-1. Sedang di plot lahan jagung dan hutan gambut terbakar masing-masing meninggalkan sisa 174,38 dan -8,92 kg N ha-1. Hal ini dapat berarti terjadi pemiskinan N di hutan gambut terbakar, dan pengayaan N dalam tanah gambut di lahan jagung. Hanya saja pengayaan N di lahan jagung berpotensi mencemari lingkungan perairan, karena N-mineral mudah bergerak dan terlindi.

The study aims to investigate impact of agricultural practices and fire on some physical and chemical properties of peat soils, and acidity and N dynamics in the ombrogenous peatland. The study on the impact on some physical and chemical properties of peat soils was taken place on: (1) selectively logged peat forest, (2) burnt peat forest, (3) maize-cultivated peatland, (4) pineapple-cultivated peatland, (5) rubber-cultivated peatland, and (6) clear cuts. The study on the acidity and N dynamics was taken place on the first three plots as mentioned above, during 18 December 2002–1 March 2003. The acidity dynamics includes measurement of ionic composition in rainwater, soil solution, and plant uptake. While the N dynamics includes measurement of N transformation in soil system, and of N translocation between components of rainwater, fertilizer, plant uptake, leaching, and internal processes in the soil system. Agricultural practices and fire in peatland enhanced peat decomposition, particularly in the layer of 0–15 cm. This, in turn, increased bulk density and micro pore fraction, and decreased maximum moisture holding capacity, specific volume, shrinkage, total porosity, and macro pore fraction. Moisture content at water potential below -1 kPa decreased, but at above -1 kPa it increased with increasing peat ripeness. In term of chemical properties, the impact also depended on external input, as well as due to peat ripeness. If without the external input, pH (CaCl2), total N and P, mineral N/total N ration of the peat soil decreased; C/N and C/P ratios, and humic C and its ratio to organic C increased. If the external input was added, such as in the plot of maize-cultivated peatland, its change was rather different. Deposition of rainwater had a little role in peat acidification, except in the plot of maize-cultivated peatland its role was rather significant. Acidifying processes in all plots were dominated by dissociation of organic acids. Involvement of N transformation was more significant in damaged plots, especially in the maize-cultivated peatland. Proton balance showed acidification of peat layer in the selectively logged peat forest and the burnt peat forest, and its acidification increased with depth. However, the top peat layer of the burnt peat forest was less acidified. While in the maizecultivated peatland, alkalinization occured in the layer of 0–10 cm, but in the contrary with the layer of 10–30 cm. Nitrogen transformation in the soil system showed positive net N mineralization, except in the layer of 0–10 cm of the selectively logged peat forest. Net N mineralization mainly comes from ammonification, as well as from external input of urea fertilizer in plot of the maize-cultivated peatland. Net nitrification in all plot was negative, and more negative in damaged plots. Nitrogen inflow in the selectively logged peat forest comes especially from litter mineralization. In the maize-cultivated, the inflow comes mostly from urea fertilization. However, deposition of rainwater supplied only a little of inorganic N in all plots. Nitrogen balance showed that the plot of selectively logged peat forest used efficiently inorganic N, such that its net balance was relatively low (2.02 kg N ha-1). While in plots of the maize-cultivated peatland and the burnt peat forest, its residues were 174.38 and -8.92 kg N ha-1, respectively. This implies that declining inorganic N might occur in the burnt peat forest, but either enrichment of inorganic N in the peat soil, or declining water quality in the surrounding environment might occur in the maize-cultivated peatland

Kata Kunci : Pertanian,Kebakaran Tanah Gambut,Kimia Tanah