Keanekaragaman Jenis Azospirillum Penambat Nitrogen Pada lahan pasir Besi

OEDJIJONO, Prof. Dr. A. Endang Sutariningsih Soetarto, M.Sc.; Prof. Sukarti Moeljopawiro, M.App.Sc., Ph.D.; Dr. Ir. Heru Adi Djatmiko, M.P.

2015 | Disertasi | S3 Ilmu Biologi

Penelitian mengenai keanekaragaman jenis Azospirillum penambat nitrogen pada lahan pasir besi telah dilakukan. Penelitian ini bertujuan untuk menganalisis keanekaragaman jenis Azospirillum penambat nitrogen pada lahan pasir besi dengan pendekatan sistematika polifasik, menseleksi kemampuan isolat Azospirillum asal rizosfer tumbuhan dan tanah pasir besi dalam menambat nitrogen, menguji pengaruh berbagai konsentrasi besi terhadap kemampuan Azospirillum dalam penambatan nitrogen, dan menguji kemampuan isolat Azospirillum penambat nitrogen terpilih dalam pelarutan fosfat dan pembentukan IAA. Karakteristik tanah pasir besi merupakan lingkungan ekstrim yang memiliki kesuburan rendah ditunjukkan dengan variasi suhu cukup tinggi (29 - 39 derajat C), kandungan air rendah (3,22-6,14%), rasio C/N rendah (5,57), dan kandungan besi tinggi (10,02 - 14,99 %). Bakteri Azospirillum diisolasi dari rizosfer 15 jenis tumbuhan (D. ischaemum; S.littorius, C. gigantea, Cp. mucunoides, P.serratifolia, S. chamaelea, Pandanus sp., Cr.pumila, T.cordifolia, H.ovalifolium, M.chamaelea, R.scabra, A.maritima, Al. monilifer, I. pres-caprae) dan tanah pasir besi menggunakan medium Caceres. Keanekaragaman jenis Azospirillum pada lahan pasir besi bervariasi dengan jumlah isolat pada rizosfer (87 isolat) ditemukan lebih banyak dibandingkan pada tanah pasir besi (31 isolat). Kepadatan populasi Azospirillum pada rizosfer juga lebih tinggi daripada tanah pasir besi yaitu berturut-turut berkisar antara 0,04 - 8,0 x 10^7 CFU g-1 dan 0,01 - 6,0 x 10^5 CFU g-1. Penambatan nitrogen oleh isolat Azospirillum ditunjukkan dengan konsentrasi nitrogen berkisar antara 5,73 - 94,54 mikro g mL-1. Isolat Azospirillum sp. KR66 yang diisolasi dari rizosfer S.chamaelea menunjukkan kemampuan tertinggi (94,54 mikro g mL-1). Aktivitas penambatan nitrogen isolat tersebut lebih tinggi daripada kemampuan penambatan nitrogen tiga strain acuan (A.lipoferum DSM 1840T, A.brasilense DSM 1690T, dan A.halopraeferens DSM 3675T). Semakin tinggi konsentrasi besi terutama di atas 25 mikro M, kemampuan Azospirillum spp. dalam menambat nitrogen semakin menurun. Isolat Azospirillum penambat nitrogen asal rizosfer dan tanah pasir besi mampu melarutkan kalsium fosfat, besi fosfat, dan aluminium fosfat dengan kisaran nilai E masing-masing berkisar antara 100,76 - 140,74; 118,52 - 140,60; dan 100 - 119,40. Isolat Azospirillum sp. KP11 dan Azospirillum sp. HR11 menunjukkan kemampuan yang tinggi dalam pelarutan fosfat. Isolat Azospirillum penambat nitrogen asal rizosfer dan tanah pasir besi juga mampu menghasilkan IAA berkisar antara 32,20 - 58,84 mikro g mL-1; dan kemampuan tertinggi ditunjukkan oleh Azospirillum sp. HR11. Berdasarkan kemampuan penambatan nitrogen, pelarutan fosfat, dan produksi IAA, isolat Azospirillum spp. terpilih berpotensi sebagai rizobakteri penginduksi pertumbuhan tanaman. Identitas keempat isolat Azospirillum terpilih, yaitu Azospirillum sp. KP11, Azospirillum sp. KP35, Azospirillum sp. HR11, dan Azospirillum sp. KR66 diduga kuat sebagai spesies baru anggota genus Azospirillum berdasarkan metode pendekatan sistematika polifasik.

A study regarding species diversity of nitrogen fixing Azospirillum on iron sand land has been conducted. The objectives of this study were to analyze species diversity of nitrogen fixing Azospirillum on iron-sandy land using systematical polyphasic approach, select Azospirillum isolates originated from plant rhizospheres and iron-sandy soils based on its ability to fix nitrogen, assay the effect of various concentrations of iron on the ability of Azospirillum isolates to fix nitrogen, and assay the selected nitrogen fixing Azospirillum isolates in phosphate solubilization and IAA production. Iron sand land characteristically is extreme environment which has low fertility with high temperature variation (29 - 39 degree C), low water content (3.22-6.14 %), low C/N ratio (5.57), and high iron content (10,02 - 14,99 %). Bacterium of Azospirillum was isolated from 15 plant species (D. ischaemum; S. littorius, C. gigantea, Cp. mucunoides, P. serratifolia, S. chamaelea, Pandanus sp., Cr. Pumila, T. cordifolia, H. ovalifolium, M.chamaelea, R. scabra, A. maritima, Al. monilifer, I. pres-caprae) and iron sand soils using a Caceres medium. Species diversity of Azospirillum in iron sand land varied with numbers of isolate in plant rhizospheres (87 isolates) were found higher than in iron sand soils (31 isolates). Population density of Azospirillum in the rhizospheres was also higher than in iron sand soils in amount of 0.04 - 8.0 x 10^7 CFU g-1 and 0.01 - 6.0 x 10^5 CFU g-1. Fixation of atmospheric nitrogen by Azospirillum isolates was shown with concentrations of nitrogen ranging from 5.73 - 94.54 micro g mL-1. Azospirillum sp. KR66 isolated from S.chamaelea rhizosphere showed the highest ability (94.54 micro g mL-1). Nitrogen fixing activity of those isolate was higher than fixation ability of the three reference strains (A.lipoferum DSM 1840T, A.brasilense DSM 1690T, and A. halopraeferens DSM 3675T). At iron concentration especially that is over 25 micro M, the ability of Azospirillum in fixing nitrogen was lower. Nitrogen fixing Azospirillum which originated from either plant rhizospheres or iron sand soils was capable of solubilizing calcium phosphate, iron phosphate and aluminum phosphate with E values ranging from 100.76 - 140.74, 118.52 - 140.60, and 100 - 119.40, respectively. Isolates of Azospirillum sp. KP11 and Azospirillum sp. HR11 demonstrated higher ability in solubilizing phosphates. Nitrogen-fixing Azospirillum isolated from plant rhizospheres and iron sand soils were also capable of producing IAA with concentrations ranging from 32.20 - 58.84 micro g mL-1; and the highest ability was shown by Azospirillum sp. HR11 isolate. Based on the ability of nitrogen fixation, phosphates solubilization, and IAA production, selected isolates of Azospirillum sp. were potential as plant growth promoting rhizobacteria. Identity of the four selected Azospirillum isolates i.e. Azospirillum sp. KP11, Azospirillum sp. KP35, Azospirillum sp. HR11, and Azospirillum sp. KR66 were very likely to be a new species belong to Azospirillum genus based on polyphasic systematic approach method.

Kata Kunci : Azospirillum, penambatan nitrogen, pelarutan fosfat, IAA, polifasik