Perubahan Pola Curah Hujan dan Dampaknya Terhadap Periode Masa Tanam

Rainfall Pattern Change and Its Impact on Length of Growing Period

E. RUNTUNUWU1 DAN H. SYAHBUDDIN2

ABSTRAKInformasi mengenai dampak perubahan iklim global terhadap sektor pertanian sangat diperlukan untuk perencanaan strategi adaptasi. Studi ini bertujuan untuk menganalisis perubahan pola hujan (rainfall pattern), serta dampaknya terhadap periode masa tanam. Data yang digunakan adalah data curah hujan bulanan selama periode 1879-2006 dari Stasiun Manonjaya, Kabupaten Tasikmalaya, Provinsi Jawa Barat. Pola hujan dianalisis dengan menggunakan metode Oldeman, yang sekaligus dapat menghitung periode masa tanam. Pola hujan telah ditentukan berdasarkan tahun basah, tahun normal, tahun kering pada masing-masing periode tiga puluh tahunan: 1879-1910, 1911-1940, 1941-1970, dan 1971-2006. Hasil studi menunjuk-kan bahwa telah terjadi perubahan pola hujan selama periode 128 tahun di Tasikmalaya,

dengan rincian sebagai berikut: pada tahun basah pola hujan tetap A, tetapi bulan basah berkurang dua bulan; pada tahun normal, pola hujan berubah dari B1 menjadi B2, dan pada tahun kering dari C2 menjadi D3. Terjadinya perubahan pola hujan tersebut telah mengakibatkan penurunan periode  masa tanam. Pada tahun basah, lahan yang awalnya dapat ditanami padi tiga kali, telah berkurang menjadi dua kali setahun. Pada tahun normal, terutama pada masa tanam yang kedua perlu teknologi irigasi untuk tetap mempertahankan periode tanam dua kali setahun. Pada tahun kering, pengaruhnya lebih serius lagi, karena yang pada awalnya dapat ditanami padi sekali setahun, menjadi tidak mungkin. Implikasi hasil penelitian ini terhadap pertanian, bahwa kegiatan adaptasi perlu dilakukan untuk mengurangi dampak negatif, bahkan sekaligus juga berusaha mencari manfaat dari perubahan tersebut.

Kata kunci : Perubahan iklim, Pola hujan, Periode masa tanam

 

ABSTRACTInformation of global climate change impact on agriculture sector is needed for planning agricultural adaptation strategy. The objective of the study was to analyze the climate change in Indonesia, especially rainfall pattern change, and its impact on the length of growing period. The data used for analysis was collected during a period of 1879-2006 from Manonjaya station in Tasikmalaya District, West Java Province. The rainfall pattern was analyzed using Oldeman method, which is used to compute the length of growing

period. Rainfall pattern was determined based on three types of rainfall characteristic, i.e., wet year, normal year, and dry year for each period of 1879-1910, 1911-1940, 1941-1970, and 1971-2006. The result of this research showed that the rainfall pattern had been changed over the past 128 years, with the following descriptions: on the wet year, the type of rainfall pattern has no changed, but the wet month has decreased about two months; on normal year, the rainfall pattern has changed from B1 to B2, and on dry year, it was from C2 to D3. The length of growing period was becoming shorter due to this changes. During wet year, three times cropping has changed to twice a year. During normal year, especially for enduring of the second growth period, the irrigation technology was necessary due to expand of water storage. During the dry year, due to the impact of dry spell, once crop a year will not be possible. The study provides insight into a strategy to adapt agriculture to climate change and to gain benefit of its change for suitable agriculture practices.

Keywords : Climate change, Rainfall pattern, Length of growing period

Sifat-Sifat Tanah Dominan yang Berpengaruh Terhadap K Tersedia pada Tanah-Tanah yang Didominasi Smektit

Dominant Soil Characteristics that Effect on Available K at Smectitic Soils

D. NURSYAMSI1, K. IDRIS2, S. SABIHAM3, D.A. RACHIM3, DAN A. SOFYAN4

ABSTRAKPenelitian yang bertujuan untuk mempelajari sifat-sifat tanah dominan yang berpengaruh terhadap K tersedia pada tanah-tanah yang didominasi mineral liat smektit telah dilaksanakan di Laboratorium Penelitian dan Uji Tanah, Balai Penelitian Tanah. Karakteristik tanah yang dianalisis meliputi tekstur 3 fraksi (pipet), C-organik (Kjeldahl), dan KTK (NH4OAc); mineral fraksi liat (X-Ray Diffraction); fraksionasi K: Kl ( 0.0002 M CaCl2), Kdd (NH4OAc), dan Kt (HNO3+HClO4); serta jerapan K tanah. Hasil penelitian menunjukkan bahwa kadar Kl, Kdd, Ktdd, dan Kt tanah dari tinggi ke rendah berturut-turut adalah Vertisols > Alfisols > Inceptisols. Proporsi bentuk K dari rendah ke tinggi di ketiga tanah yang diteliti mempunyai urutan yang sama, yaitu: Kl Inceptisols. Sementara itu konstanta energi ikatan tanah Vertisols hampir sama dengan Alfisols dan sekitar dua kali lipat tanah Inceptisols. Liat, C-organik, dan KTK tanah umumnya berpengaruh nyata (P > 0,95) terhadap peubah ketersediaan K tanah. Smektit berkontribusi nyata (P > 0,95) terhadap muatan negatif tanah sehingga memegang peranan penting dalam mengendalikan daya sangga dan jerapan maksimum K tanah. Untuk meningkatkan efisiensi pupuk K, tanaman yang dapat menghasilkan eksudat asam organik tinggi dapat dikembangkan di tanah-tanah yang didominasi smektit. Selain itu penambahan kation Na, NH4, dan Fe juga perlu dipertimbangkan dalam pengelolaan K tanah.

Kata kunci : Sifat tanah, Ketersediaan K, Tanah yang didominasi smektit

ABSTRACTResearch aimed to study dominant soil characteristics that effect on available K of smectitic soils were conducted in Laboratory of Research and Soil Test, Indonesian Soil Research Institute. The soil characteristics that has been analised were soil texture (pipette), organic-C (Kjeldahl), and CEC (NH4OAc); mineralogical analyses of clay fraction (X-Ray Diffraction); K fractionation: soil soluble-K (0.0002 M CaCl2), exchangeable-K (NH4OAc), and total-K (HNO3+HClO4); and potassium sorption. The results showed that the content of soil soluble, exchangeable, non-exchangeable, and total-K was in order of Vertisols > Alfisols > Inceptisols. The percentage of soil K fraction of the soils, however, was in order of soluble-K Inceptisols. The bond energy constant of Vertisols, however, was similar with that of Alfisols but it was about twice with that of Inceptisols. Soil clay, organic-C, and CEC affected the availability of soil K significantly (P > 0.95). Smectite contributed significantly (P > 0.95) to soil negative charge so that it held an important role in controlling soil K buffering capacity and maximum sorption. To increase the efficiency of K fertilizer, plant species that can produce organic acid exudated from roots in high quantity can be developed in smectitic soils. The use of Na, NH4, and Fe cations need also to be considered for K management in the soils as well.

Keywords : Soil characteristics, K availability, Smectitic soils

Perubahan Fraksi Bahan Organik Tanah Akibat Perbedaan Cara Pemberian dan Sumber Bahan Organik pada Ultisols Jasinga

Changes of Soil Organic Matter Fraction on Ultisols Jasinga by Soil Tillage and Organic Matter Practices

N.L. NURIDA1, O. HARIDJAJA2, S. ARSYAD3, SUDARSONO3, U. KURNIA1, DAN G. DJAJAKIRANA2

 ABSTRAKSumber bahan organik berupa mukuna, flemingia, dan sisa tanaman mempunyai kualitas berbeda, terutama nisbah C/N, kandungan lignin, selulosa dan nisbah lignin/selulosa. Perbedaan kualitas bahan organik menentukan perubahan kadar bahan organik dalam tanah, khususnya fraksi labil sehingga memberikan pengaruh yang berbeda pula terhadap sifat-sifat tanah. Penelitian ini bertujuan: 1) memahami pengaruh sumber bahan organik dengan kualitas berbeda yang diberikan dengan cara berbeda terhadap fraksi-fraksi bahan organik, dan 2) mengetahui hubungan perubahan kadar bahan organik dalam bentuk C-organik, particulate organic matter (POM) dan biomassa mikroorganisme (Cmic) dengan indikator kualitas tanah berupa berat isi, porositas, permeabilitas tanah lapisan atas, indeks stabilitas agregat, P-tersedia, K-tersedia, dan N-total. Penelitian dilaksanakan di rumah kaca Pusat Penelitian dan Pengembangan Tanah dan Agroklimat, Bogor pada bulan Januari-Mei 2003. Penelitian menggunakan rancangan acak lengkap faktorial, dengan tiga ulangan. Faktor pertama adalah cara pemberian bahan organik: 1) disebar di permukaan tanah, dan 2) dicampur/diinkorporasi. Faktor kedua adalah sumber bahan

organik (takaran setara C-organik 2%) yaitu: 1) Mucuna sp., 2) Flemingia sp., 3) sisa tanaman jagung, 4) campuran Mucuna sp. dan Flemingia sp., 5) campuran Flemingia sp. dan sisa tanaman jagung, 6) campuran Mucuna sp. dan sisa tanaman jagung, dan 7) campuran Mucuna sp., Flemingia sp., dan sisa tanaman jagung. Hasil penelitian menunjukkan bahwa pada pemberian bahan organik dengan cara dicampur, nisbah C/N dan kadar lignin bahan organik berkorelasi positif dengan C-organik, Cmic dan Cmic/ Corg, namun berkorelasi negatif dengan POMp. Perubahan kualitas tanah akibat pemberian bahan organik dalam jangka pendek dapat dilihat dari perubahan fraksi bahan organik labil. Bila bahan organik dicampur dengan tanah, POMt dan POMt/Corg berkorelasi positif dengan berat isi, tetapi berkorelasi negatif dengan RPT, PDC, K-tersedia, dan N-total. Bila bahan organik disebar di permukaan tanah, Cmic berkorelasi positif dengan kadar P-tersedia. Perubahan pada fraksi labil (Cmic dan POMt) mudah dideteksi dalam waktu singkat sehingga perubahan sifat fisik dan ketersediaan unsur hara makro tersebut lebih mudah diduga.

Kata kunci : Fraksi bahan organik tanah, Sumber bahan organik, Cara pemberian, Ultisols

ABSTRACTIn situ organic matters, such as Mucuna sp., Flemingia sp., and plant residues differ in quality (in terms of lignin, cellulose and nutrients contents). Such difference determines soil organic matter content especially labile fraction that could effect on soil properties. The aims of this research were to study: 1) the effects of various sources and quality of organic matters on soil organic fraction, and 2) the relationships between the changes of particulate organic matter and C-microbes (POM and Cmic) and soil quality indicators. The experiments were conducted in greenhouse of the Center for Soil and Agroclimate Research and Development, Bogor, from January to May 2003. The factorial completely randomized design was used with three replications. The first factor was the method of organic matters application: 1) mulch on soil surface and 2) incorporated with soils. The second factor was the sources of organic matters (dosage in equvalent with 2% organic C), i.e: 1) Mucuna sp., 2) Flemingia sp., 3) maize residue, 4) mixture of Mucuna sp., and Flemingia sp., 5) mixture of Flemingia sp. and maize residue, 6) mixture of Mucuna sp. and maize residue, and 7) mixture of Mucuna sp., Flemingia sp., and maize residue. The results showed that on organic matter application with incorporated method, C/N ratio and lignin content of organic matter have positive correlation with organic C, Cmic and Cmic/Corg, but have negative correlation with POMp. The changes of soil qualities caused by short term organic matter application could be seen from the changes of labile fraction of organic matter. When organic matter was incorporated, POMt and POMt/Corg have positive correlation with bulk density, but have negative correlation with RPT, PDC, available K, and total-N. When organic matter was spreaded on the soil surface, Cmic has positive correlation with available P. The changes on labile fraction (Cmic and POMt) was easy to be detected in short period, thus the changes of physical characteristics and the availability of macro nutrients is easy to be predicted.

Keywords : Soil organic matter fraction, Organic matter sources, Method of organic

matter application, Ultisols

Peningkatan Serapan N pada Kedelai yang Diinokulasi Bakteri Diazotrof Endofit di Medium Vermiculit

Increasing of N-uptake by Inoculation of Diazotroph Endophytic Bacteria in Vermiculite Media

DWI N. SUSILOWATI1, R. SARASWATI2, R.D. HASTUTI2, DAN E. YUNIARTI2

ABSTRAKDari hasil seleksi intensif 15 isolat terpilih bakteri diazotrof endofit asal tanaman kedelai terhadap kemampuannya menambat N2 dan menghasilkan auksin didapatkan lima isolat terpilih, yaitu KACP12 (0,2569 ?mol jam-1 kultur-1), KACP13 (0,3026 ?mol jam-1 kultur-1, KACP21 (0,4592 ?mol jam-1 kultur-1), KACP32 (0,3131 ?mol jam-1 kultur-1), dan KAMG2 (0,4843 ?mol jam-1 kultur-1). Inokulasi lima isolat terpilih pada benih kedelai di medium vermiculit menunjukkan bahwa tanaman kedelai yang diinokulasi KAMG2 memiliki aktivitas spesifik nitrogenase tertinggi yaitu 2,54 ? 1,2 ?mol jam-1 tanaman-1 dibandingkan lainnya dan kontrol (tanpa inokulasi). Namun inokulasi bakteri diazotrof endofit KACM dan KACP32 tanaman kedelai meningkatkan serapan N tanaman kedelai lebih tinggi. Meskipun penelitian ini masih pada fase awal pertumbuhan tanaman kedelai, ternyata inokulasi bakteri diazotrof endofit pada tanaman kedelai di media vermiculit tampaknya dapat menjadi metode yang bagus untuk mengintroduksikan strain-strain terpilih yang dapat memacu pertumbuhan dan penambatan N2.

Kata kunci : Diazotrof, Endofit, Tanaman kedelai

ABSTRACTIntensive selection of selected 15 isolates on N2-fixing activities and auxin production to diazotroph endophytic bacteria showed that five isolates were superior, that is KACP12 (0.2569 ?mol hour-1 culture-1), KACP13 (0.3026 ?mol hour-1 culture-1), KACP21 (0.4592 ?mol hour-1 culture-1), KACP32 (0.3131 ?mol hour-1 culture-1), and KAMG2 (0.4843 ?mol hour-1 culture-1). Inoculated five superior isolates into soybean seeds in vermiculite media showed that soybean plant inoculated by KAMG2 has the highest nitrogenase specific activity compared to others and control, that is 2,54 ? 1,2 ?mol hour-1 plant-1. However inoculation with KACM and KACP32 showed higher N-uptake of soybean plant. Although this research has conducted within the  early stage of soybean plant growth, it is obvious that inoculated diazotroph endophytic bacteria in vermiculite media seem to be a good method to introduce selected strain envisaging growth promoting and nitrogen fixation.

Keywords : Diazotroph, Endophytic, Soybean plant

Analisis Perubahan Kualitas Lingkungan Daerah Aliran Sungai Citarum Jawa Barat dan Pengaruhnya Terhadap Biaya Produksi PLTA dan PDAM (Studi Kasus

PLTA Saguling, PLTA Cirata, PLTA Jatiluhur, PDAM Purwakarta, dan PDAM DKI Jakarta)

Analysis of Environmental Quality Changes of Citarum Watershed of West Java and their Effects on Operational Costs of Hydroelectric Power Plans and the Regional Drinking Water Companies (Case Study at Saguling, Cirata, and Jatiluhur Hydroelectric Power Plans and Purwakarta and Jakarta Drinking Water Companies)

R. TAMPUBOLON1, B. SANIM2, M. SRI SAENI3, DAN R. BOER4

ABSTRAKTujuan penelitian ini adalah untuk menganalisis perubahan tutupan lahan, perubahan karakteristik hidrologis dan pengaruhnya terhadap nilai ekonomi sumberdaya air bagi pengguna jasa lingkungan (PLTA dan PDAM). Penelitian dilakukan selama tahun 2006 di DAS Citarum Wilayah Hulu yang meliputi Sub DAS Saguling, Sub DAS Cirata, dan Sub DAS Jatiluhur dengan luas 486.237 ha. Aktor-aktor ekonomi yang menjadi objek penelitian adalah PLTA Saguling, PLTA Cirata, dan PLTA Jatiluhur serta PDAM Purwakarta (Tirta Dharma) dan PDAM DKI Jakarta (PT. Thames Jaya) di DAS Citarum Wilayah Hilir sebagai pengguna jasa lingkungan (sumberdaya air) DAS Citarum. Metode dan teknik analisis yang digunakan dalam penelitian ini adalah : a) supervised classification analysis untuk mengetahui perubahan tutupan lahan; b) model GR4J untuk menduga debit, volume air, dan sedimentasi; dan c) analisis kimia air dan replacement cost sebagai teknik valuasi ekonomi jasa lingkungan bagi pengguna. Hasil penelitian menunjukkan bahwa selama periode 1992-2003 telah terjadi penurunan tutupan lahan hutan (pohon) dengan laju 2,23% (3.804,2 ha) per tahun. Penurunan luas tutupan lahan tersebut terutama disebabkan oleh kenaikan pembukaan lahan untuk memenuhi kebutuhan pemukiman dan sarana sosial lainnya, yang tumbuh 9,81% (2.404,5 ha) per tahun. Perubahan tutupan lahan tersebut menyebabkan perubahan pada karakteristik hidrologis DAS Citarum Wilayah Hulu berupa penurunan debit air masuk lokal (DAML) dengan laju 1,49% (3,14 m3 dt-1) dan volume air masuk lokal (VAML) dan 4,20% (275,26 juta m3), peningkatan rasio Q max-min 5,99% (rata-rata 131,94), peningkatan laju sedimentasi rata-rata 10,20 juta m3-12,86 juta m3 (total tiga waduk) setiap tahun yang membahayakan terutama Waduk Saguling dan Cirata, dan penurunan kualitas kimiawi air di Sungai Citarum. Perubahan tutupan lahan dan karakteristik hidrologis tersebut telah menyebabkan kerugian ekonomi (œkeuntungan yang hilang) bagi PLTA dan PDAM. Besarnya keuntungan yang hilang akibat penurunan kualitas lingkungan DAS Citarum Wilayah Hulu bagi PLTA adalah sebesar Rp 43,44 miliar (Rp 9.538,- MWh-1 energi listrik atau Rp 3,29,- m-3 air yang digunakan PLTA), sedangkan bagi PDAM adalah Rp 212,43,- m-3 (PDAM Purwakarta) dan Rp 821,48,- m-3 (PDAM DKI Jakarta). Berkaitan dengan besarnya kerugian ekonomi yang ditimbulkan oleh penurunan kualitas lingkungan DAS Citarum Wilayah Hulu terhadap PLTA dan PDAM maka upaya pengendalian pemanfaatan (tataguna) lahan dan tindakan-tindakan konservasi sangat diperlukan dengan tetap mempertahankan luas minimal hutan. “Keuntungan yang hilang“ tersebut dapat digunakan sebagai investasi lingkungan untuk perbaikan kualitas lingkungan (replacement cost) di DAS Citarum Wilayah Hulu.

Kata kunci : Perubahan tataguna lahan dan hutan, Daerah aliran sungai, Hidrologi, Jasa lingkungan, Valuasi ekonomi

ABSTRACTThe objectives of this research were to analyze land cover changes, hydrological characteristics changes, and their effects on the economic value of water resources for environmental services beneficiaries (Hydroelectric Power Plan, HEPP and Drinking Water Companies, DWC). This research was conducted from January to December 2006 in the upper Citarum watershed, covering the Saguling, Cirata, and Jatiluhur catchments, with total area of 486,237 ha. The economic actors under this study are Saguling HEPP, Cirata HEPP, and Jatiluhur HEPP as well as Tirta Dharma DWC (Purwakarta) and PT Thames Jaya DWC (Jakarta) which are located downstream of Citarum watershed as the beneficiaries of environmental services of Citarum watershed. The methodology and analysis technique used in this research were : a) supervised classification analysis to find out land cover changes; b) GR4J model to predict water discharge, water volume, and sedimentation; and c) water chemical analysis and replacement cost method for economic valuation of environmental services among the beneficiaries (HEPP, DWC). The results showed that during the 1992 to 2002 period, there has been a reduction of forest (tree) area at the rate of 2.23% (3,804.2 ha) annually. The reduction of forest area was mainly caused by increasing of land clearing for settlement and other infrastructure which grew at the rate of 9.81% (2,404.5 ha) annually. This landuse change has influenced on hydrological characteristics of the upper Citarum watershed as shown by the reduction of local water discharge as high as 1.49% (3.14 m3 sec-1) and the volume of local water input as high as 4.20% (275.26 million m3), the increasing ratio of Qmax/Qmin as high as 5.99% (at the average of 131.94), the increasing of sediment yield as high as 10.20 to 12.86 million m3 annually (for the three dams) which is very dangerous, especially for Saguling and Cirata dams, as well as the decreasing of chemical water quality of Citarum River. Landuse and hydrological characteristics changes has caused an economic loss (opportunity cost) among the HEPPs and DWCs. The amount of the opportunity cost due to environmental degradation of upper Citarum watershed suffered by the HEPPs was as high as Rp 43.44 billion (equivalent with Rp 9,538,- MWh-1 electricity or Rp 3.29 m-3 water used by HEPP).  Whereas economic lost suffered by DWC were Rp 212.43,- m-3 (Purwakarta DWC) and Rp 821.48 m-3 (Jakarta DWC) respectively. Based on significant economic loss caused by environmental degradation of the upper Citarum watershed on HEPP and DWC, the efforts for controlling landuse allocation and soil conservation seem very necessarily, by assigning permanent forest cover area. The amount as reflected by opportunity cost could be used as environmental investment for improving environmental quality (replacement cost) in the upper Citarum watershed.

Keywords : Landuse change and forestry, Watershed, Hydrology, Environmental services, Economic valuation

Transformasi Nitrogen dalam Tanah Tergenang : Aplikasi Jerami Padi dan Kompos Jerami Padi serta Pengaruhnya Terhadap Serapan Nitrogen dan Aktivitas Penambatan N2di Daerah Perakaran Tanaman Padi

Nitrogen Transformation in Flooded Soil : Application of Rice Straw and Rice Straw Compostsand Its Effect on Nitrogen Uptake and Acetylene Reduction Activity in Rice Plant Rhizosphere

L.T. INDRIYATI1, S. SABIHAM2, L.K. DARUSMAN3, R. SITUMORANG2, SUDARSONO2, DAN W.H. SISWORO4

ABSTRAKPenggunaan bahan organik yang ditujukan untuk mengurangi ketergantungan terhadap pupuk kimia diharapkan akan dapat meningkatkan efisiensi N pupuk yang rendah pada tanaman padi sawah. Oleh karena itu, diperlukan pemahaman yang lebih baik tentang transformasi N dalam tanah tergenang, terutama transformasi N yang berasal dari bahan organik yang diberikan ke dalam tanah menjadi N tersedia bagi tanaman dan bentuk-bentuk gas nitrogen yang hilang melalui proses denitrifikasi. Percobaan dilakukan di laboratorium dan rumah kaca Departemen Ilmu Tanah dan Sumber Daya Lahan, Fakultas Pertanian, Institut Pertanian Bogor. Percobaan inkubasi dilakukan di laboratorium pada suhu ruang selama 120 hari untuk mengamati ketersediaan N-NH4+ dalam tanah tergenang yang diberi jerami padi, kompos jerami padi 4 bulan, kompos jerami padi 8 bulan, dan kombinasi masing-masing dengan urea. Setelah 21 hari dari masa inkubasi, tanah yang diberi jerami padi menunjukkan pelepasan N-NH4+ yang lebih besar dibandingkan dengan perlakuan lainnya. Kompos jerami padi 4 dan 8 bulan memperlihatkan laju mineralisasi N dan ketersediaan N yang rendah daripada jerami padi. Hasil percobaan pot di rumah kaca menunjukkan bahwa pengaruh perlakuan pemberian jerami padi tidak berbeda dengan perlakuan urea dalam hal bobot kering tanaman padi, jumlah anakan padi, serapan N dan efisiensi penggunaan N oleh tanaman padi. Tetapi pemberian jerami padi ke dalam tanah mendorong aktivitas enzim nitrogenase yang lebih tinggi daripada pemberian kompos jerami padi. Pengaruh jerami padi terhadap parameter-parameter yang diamati jauh lebih baik dibandingkan dengan kompos jerami padi.

Kata kunci : Transformasi N, Bahan organik, Jerami padi

ABSTRACTThe use of organic materials aimed in reducing the dependence on inputs such as chemical fertilizers can contribute to sustainability and improving the low N fertilizer efficiency of rice plants in paddy soils. Therefore, better understanding of N transformation in flooded soils, particularly the microbial transformation of N-organic amendments to plant-available N, is needed for most efficient use of soil and organic materials N for aiding in the selection of N management practices for sustainable agriculture. The experiments were conducted at the laboratory and in the greenhouse of Soil Department, Faculty of Agriculture, Bogor Agricultural University. Incubation experiment was conducted at room temperature for 120 days to observe the availability of N-NH4+ in soil amended rice straw or rice straw composts and its combination with urea. After 21 days of incubation period, the rice straw amended soils showed that N-NH4+ release was higher than the other treatments. The 4-month- and 8-month-compost of rice straw showed that N mineralization rate was lower and the N availability was lower than

rice straw. The results of pot experiment in greenhouse revealed that the added rice straw was comparable with urea in its effect on the increase of dry matter weight of rice plants, number of tillers, N uptake of rice, and recovery N efficiency. The application of rice straw, however, promoted the activity of nitrogenase enzyme higher than rice straw composts. The effect of rice straw on observed parameters were much better than its composts.

Keywords : N transformation, Organic materials, Rice straw

Pengaruh Tekstur Tanah Terhadap Karakteristik Unit Hidrograf dan Model Pendugaan Banjir (Studi Kasus di DAS Separi, Kutai Kartanegara, Kalimantan Timur)

Effects of Soil Texture on Characteristics of Hydrograph Unit and Modelling Flood Prediction(Case Study in Separi Watershed, Kutai Kartanegara, East Kalimantan)

M. LUTHFUL HAKIM1, O. HARIDJAJA2, SUDARSONO3, DAN G. IRIANTO4

ABSTRAKPenelitian mengenai pengaruh tekstur tanah terhadap karakteristik unit hidrograf dan model pendugaan banjir pada daerah aliran sungai (DAS) di Kalimantan Timur perlu dilakukan karena intensitas dan luasan daerah banjir semakin meningkat. Penelitian ini bertujuan untuk: 1) mengetahui pengaruh tekstur tanah terhadap karakteristik unit hidrograf, 2) menyusun model pendugaan banjir (debit puncak dan waktu menuju debit puncak) di DAS Separi, dan 3) menentukan komposisi luas penggunaan lahan secara optimal dalam rangka penanggulangan banjir, serta pengelolaan DAS Separi. Lokasi penelitian adalah DAS Separi, Kabupaten Kutai Kartanegara, Provinsi Kalimantan Timur pada koordinat 00003-00038 LS dan 117008-117031 BT. Waktu penelitian lapangan adalah bulan Januari-Juni 2006. Metode penelitian didasarkan pada beberapa tahapan, yaitu : 1) pengumpulan data, 2) pengembangan model pendugaan banjir, 3) uji akurasi model, dan 4) penerapan model. Model pendugaan banjir didasarkan pada: 1) pemodelan fungsi produksi dan 2) pemodelan fungsi transfer. Hasil analisis menunjukkan tekstur tanah berpengaruh terhadap debit puncak dan waktu menuju debit puncak, dimana tanah bertekstur lempung memiliki debit puncak yang lebih tinggi dibandingkan pada tanah bertekstur pasir dan liat, sedangkan untuk waktu menuju debit puncak dimana tanah bertekstur liat memiliki waktu menuju debit puncak yang lebih cepat dibandingkan dengan tanah bertekstur lempung dan pasir. Pendugaan banjir (debit puncak dan waktu menuju debit puncak) berbasis karakteristik lahan dan geomorfologi DAS mampu mensimulasi debit puncak dan waktu menuju debit puncak dengan hasil tidak berbeda dengan pengukuran. Analisis optimasi luas penggunaan lahan berkenaan dengan menurunnya debit puncak dan memperlambat waktu menuju debit puncak menunjukkan bahwa luas penggunaan lahan di DAS Separi adalah 54% hutan, 1,9% kebun/ladang, 0,12% pemukiman, 0,5% persawahan, 42% semak belukar, dan 1,99% tambang batubara.

Kata kunci  : Tekstur tanah, Unit hidrograf, Pendugaan banjir, Daerah aliran sungai

ABSTRACTA Research on effects of soil texture on hydrograph unit characteristic and flood prediction modelling in East Kalimantan need to be developed, because intensity and area of flood progressively increased. The objectives of this research are : 1) to study the effects of soil texture on the characteristic of hydrograph unit, 2) to develop flood prediction models (peak discharge and time to peak discharge) of Separi watershed, and 3) to determine an optimal landuse area composition in line with flood and Separi watershed management. The location of research site is on Separi watershed in Kutai Kartanegara, East Kalimantan at 00003-00038 S and 117008-117031 E coordinate position. Field research has been conducted from January to June 2006. The methodology of the research based on several steps, e.i. : 1) data collecting, 2) flood prediction models develop-ment, 3) model accuration testing, and 4) model application. The model of flood prediction based on: 1) modelling of production function and 2) modelling of transfer function. The result showed that soil texture has an effect towards peak discharge and time to peak discharge. Soil with loam texture has peak discharge higher than soil with sand and clay texture, whereas time to peak discharge of soil with clay texture is higher than soil with loam and sand texture. Flood prediction (peak discharge and time to peak discharge) based on land characteristics and watershed geomorphology was able to simulate peak discharge and time to peak discharge, which has no different result compared with result from measurement. Analysis of optimum composition landuse with decreasing peak discharge and time to peak discharge indicated that compositions of landuse in Separi watershed are 54% for forest area, 1,9% for farm/garden, 0,12% for urban, 0,5% for paddy field, 42% for shrubs/ underbrush, and 1,99% for coal-mining.

Keywords  : Soil texture, Hydrograph unit, Flood prediction, Watershed