Carbon and Nitrogen Mineralisation of Different Soil Amendments in Ultisols

Plants die and decompose through a complex process involving microorganisms such as fungi, bacteria and actinomycetes. In this process many nutrients are released to soil and plants can uptake them for their growth. It is vital to investigate the mineralisation of organic matter and the pattern of release of carbon and nitrogen to the soil. Therefore, this research study was conducted by an incubation study incorporating different soil amendments with surface Ultisols. The objective of the present study is to study the Carbon and Nitrogen mineralisation and the release pattern of nutrients with the time. The study was conducted at the soil science laboratory of Faculty of Agriculture, University of Ruhuna, Mapalana, Kamburupitiya. Two hundred and fifty glass bottles with 100 g of soil were prepared and pre incubated for 2 weeks prior to the application of treatment. Then a calculated amount of each treatment according to their application rate was incorporated and thoroughly mixed with soil. The seven treatments were arranged in a Completely Randomized Design with four replicates; 10 T/ha level of leaf litter compost+soil (CS), soil+10 T/ha level of Gliricidia sepium (GS)+soil , 100 kg/ha Urea (US)+soil, 10 T/ha level of Siam weed (Chromolaena odorata)+soil (SWS), 10 T/ha cattle manure+soil (CMS) and 10 T/ha spent poultry litter+soil (PLS) and the control soil (S). The moisture content was adjusted to 60% of the field capacity by adding distilled water. The samples were analysed for NH4+-N, NO3--N, pH, microbial activity in each mixtures in 1, 3, 5, 7, 14, 21, 28, 35, and 42 days after adding the soil amendments. The microbial activity was measured using carbon dioxide evolution. Alkali traps of 1M NaOH and distilled water tube were kept inside the bottle and after the incubation period it was titrated with 0.1M HCl. The statistical analysis was done using SPSS software. The results revealed that nitrogen mineralisation pattern was varied among the treatments. Poultry litter and the G. sepium amended soil reported the highest and rapid release of nitrogen. Cattle manure and compost amended soil had the highest value of microbial activity which showed the peak CO2 evolution. pH value of the different treatments were increased gradually over time with highest pH value in the compost amended soil. The highest phosphorus mineralisation was observed in day 3 and thereafter it decreased significantly. The cattle manure and poultry litter amended soils observed the highest available phosphorus after day 7 of the incubation. Further, it revealed that the variation of mineralisation was effected by the C:N ratio of the different soil amendments.Key words: Carbon, Incubation, Mineralisation, Nitrogen, Siam wee

Soil Carbon Storage Potential of the Home Gardens Adjoining Selected Natural Forests of the Southern Sri Lanka

Conversion of forests to other land uses is inevitable despite the environmental issues associated with forest clearing. Tropical home gardens have long been identified as a land-use system analogous to forests from various perspectives. Carbon storage in the tropical forest soils is estimated to be nearly 32% of the global soil carbon. Although few in number, research conducted in Sri Lanka has shown that the home garden soils could store relatively a large amount of carbon. Present study was conducted in the low country wet zone of Sri Lanka (WL2) using the topsoil (0-15 cm) and subsoil (15-30 cm) samples taken from two sets of home gardens (HG) adjoining two natural forests (Mulatiyana and Wilpita) where latter was used as benchmark sites. Twelve samples each was collected from the topsoil and the subsoil under each system reaching a total of 96 samples for the four systems. The objective was to assess the potential of the home garden soils to sequester carbon as an alternative to the dwindling carbon storage in forest soils. Analysis of bulk density data of 12 samples each from the HGs and forests showed no significant difference (T-test, α=0.05) between the forests and the HGs for the selected soil depths. Therefore, the assessment of the systems was done in terms of the percentages of soil organic matter (SOM). All the comparisons were done using T test with α=0.05. For the topsoil, Mulatiyana HGs and the forest had mean SOM contents of 4.14%±0.98SD and 5.50%±0.71SD, respectively and, it was significantly different (p=0.001). On the contrary, Wilpita HGs and the forest had values 3.71%±1.6SD and 3.17%±0.059SD, respectively and, they were not significantly different (p=0.296). When the two forests and the home gardens were compared separately, Mulatiyana forest had significantly higher SOM content (p=0.000) over Wilpita forest while the home gardens had no significant difference (p=0.437). When the subsoil was compared neither Mulatiyana forest and the HGs (3.62%±0.64SD, 3.18%±0.70SD; p=0.072) nor Wilpita forest and the HGs (2.60%±1.01SD, 3.10%±1.14SD; p=0.268) had significant differences in the SOM contents. When the two forests and home gardens were compared separately, Mulatiyana forest had significantly higher SOM compared to Wilpita forest (p=0.008) while HGs had no significant difference (p=0.985). These results indicate that the soils of the home gardens adjoining the forests are capable of storing carbon almost up to the natural storage limit for a given locality. Hence, it can be envisioned that the carefully structured home gardens in view of capturing and storage of carbon could be a good alternative for the dwindling forests in the humid tropical environments.Keywords: Soil carbon, Home gardens, Low country, Wet zon

Carbon Management in Agricultural Soils

global warming, soil carbon dynamics, soil carbon sequestration, soil quality, soil aggregation, humification, secondary carbonate,