Summary To understand the effect of land use changes on the composition of humus in tropical soils, samples from land under primary forest, secondary forest, coffee plantation, and arable crops were investigated at three sites in south Sumatra, Indonesia. Total carbon and total nitrogen contents were 1.7 to 4.3 times and 1.1 to 2.8 times greater in the topsoil under primary forest than under the other types of land use. Following change from primary forest to other uses, the proportion of humic acids in the organic matter of the topsoils decreased while that of the fulvic acid fraction increased. Within the range of land uses, differences in the yields of humic acids and fulvic acid fractions were, respectively, larger and smaller than those in total carbon content. The humic acids were classified into the low and middle classes in the degree of humification. Absorption due to the green fraction of humic acids, Pg, was detected in the UV‐visible spectra of almost all the humic acids. No relation was observed between the degree of humification of humic acids or the strength of Pg absorption in their spectra and land use change. The fulvic acid fractions were fractionated on insoluble polyvinylpyrrolidone (PVP) into the adsorbed fractions consisting of humic substances and the non‐adsorbed fractions consisting of non‐humic substances. A positive correlation between the amount of the fulvic acid fraction and the percentage of the PVP‐adsorbed fraction within it indicated that the variation in the amount of the fulvic acid fraction was attributable to acid‐soluble humic substances. The ionization difference spectra of solutions between pH 12 and pH 7 suggested that the chemical structures of the PVP‐adsorbed fulvic acids have been altered by land use change.

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We evaluated the effect of soil conservation by weeds on the degradation and generation of humic acids, fulvic acids, and water-soluble non-humic substances (WS-NHS) in a red-acid soil (Vertic Dystrudept) (Indonesia) from the changes in humus composition and stable carbon isotopic ratio (d 13 C). Three plots, a weeded plot (T-1; the common practice), a plot covered with Paspalum conjugatum Berg., a C 4 plant (T-2), and a plot in which native weeds were allowed to grow (T-3), were prepared. An incubation experiment determined the d 13 C values of the humus fractions generated from Paspalum in soil. Based on the increase in d 13 C value, the proportion of total C that originated from Paspalum C after 4 years under coffee was 16 AE 4% in the T-2 topsoil (0-10 cm). Humic and fulvic acids in the T-1 topsoil decreased to 46 and 84%, respectively, whilst both increased or remained constant in the T-2 and T-3 soils. The WS-NHS content varied little and was independent of land management. The preferential loss of the humic acids with a smaller degree of humification as assessed by their darkness in colour was shown in T-1. The decrease in the degree of humification suggested the accumulation of the weedderived humic acids in T-2 and T-3. In the T-2 topsoil, 36 AE 2%, 13 AE 3% and 15 AE 2% of C in the humic acids, fulvic acids and WS-NHS, respectively, were estimated to be Paspalum-derived after 4 years. The estimated initial C loss during the same period was 17 AE 3%, 14 AE 2% and 7 AE 2%, respectively, for those fractions, which suggests the fastest turnover rate for the humic acids and significant retardation of their degradation in soil colonized by weeds.

Variabilitas iklim seperti musim kering, defisit air, dan “dry spell” sangat mempengaruhi pertumbuhan dan hasil kelapa sawit. Penelitian ini bertujuan mempelajari keragaan agronomi tanaman kelapa sawit akibat kekeringan periodik khususnya pada periode 2011-2015 di perkebunan swasta di Lampung yang mengelola 4900 ha kebun sawit. Pengamatan dilakukan pada Juni-September 2016 (setahun setelah kemarau panjang 2015) untuk mengumpulkan data agronomi dari pohon sawit yang dipilih secara random dari blok kebun yang ditentukan secara purposif mewakili tanaman sawit muda (umur 3-8 tahun), remaja (umur 9-13 tahun), dewasa (umur 14-20 tahun), dan tua (umur 21-24 tahun). Data sekunder produksi dan iklim dari perusahaan dan Stasiun Pengamatan Iklim Radin Intan. Hasilnya menunjukkan bahwa perkebunan kelapa sawit di Lampung menghadapi kekeringan periodik yang mempengaruhi pertumbuhan dan produksi. Pada periode 2011-2015 ada 3-5 bulan kering dengan defisit air 190 mm (stadium I), 285-359 mm (stadium II) dan  406 mm (stadium III), dengan dry spell maksimum mencapai 45-51 hari (kategori kekeringan sangat  panjang) sampai 126 (kategori kekeringan ekstrim). Keragaan agronomi ditunjukkan oleh rendahnya tegakan pohon per hektar (SPH) dengan model penurunan SPH tertinggi y = -2,8649x + 150,99 (nilai R2 = 0,89, x = umur kelapa sawit). Kebun kelapa sawit tua memiliki SPH yang rendah. Setahun setelah kemarau pohon tua yang memiliki lebih banyak pelepah patah dan lebih sedikit bunga betina yang mengindikasikan pengaruh kekeringan yang berat. Produksi tandan tidak berbeda nyata antar kategori umur kelapa sawit.