| dc.description.abstracteng | Land-use change, particularly the conversion of natural forest to agriculture to sustain the growing global population, has severe environmental impacts, including emission of greenhouse gases, diminished biodiversity, and altered soil functions. Agriculture intensification further reduces the fertility of soil, negatively impacting the sustainability of agriculture production and increasing the loss of soil organic carbon, which contributes to climate change. This thesis aims to assess the impacts of land-use systems on soil fertility and carbon sequestration.
Due to its agricultural based economy, land is the most valuable resource in Nepal. The study site ‘‘Chitwan district’’ lies in the Terai region, a plain in southern Nepal. Known as the grain house of Nepal, the Terai region covers 17% of the country’s total land area. Forests are dominated by Shorea robusta Roth and possess high economic value and biological diversity. People are extremely dependent on forests for timber and non-timber forest products. After eradication of malaria in the 1950s, the government introduced a resettlement and migration scheme from the Middle Mountain region to different parts of the Terai region, resulting in, massive deforestation to support cultivation and new settlements which continues to this day. Hence, the forest cover has been continuously declining. Agricultural intensification through conventional farming practices is also being implemented to feed the growing population.
Soil samples were collected from three major land-use systems: forest, organic and conventional farming in Chitwan district, Nepal. The content of soil organic carbon (C), total nitrogen (N), microbial biomass (C and N) and six enzyme activities (β-glucosidase, cellobiohydrolase, chitinase, leucine aminopeptidase, tyrosine aminopeptidase, and sulfatase) were significantly higher under organic farming than conventional farming and forest, especially in topsoil layer. However, acid phosphatase activity was significantly higher (up to 6 fold) under conventional farming than forest and organic farming. The pools varying in P availability were estimated by P sequential fractionation approach (Hedley, 1982). The concentration of microbial biomass P, easily-available P, moderately available P, non-available P, and total P were much higher under organic farming than conventional farming and forest. However, the ratio of C to organic P was greater (>100) under conventional farming and forest than under organic farming, indicating the limitation of P in the former two land use systems. Indeed, higher acid phosphatase activity under conventional farming and forest is responsible for hydrolyzing organic P to be made available for plant growth. Various organic based management practices, i.e., application of farmyard manure and vermicompost, incorporation of crop residues, and cropping system under organic farming, contributed to increases in soil organic matter (SOM) and microbial properties, which play significant roles in maintaining soil fertility status.
The decomposition of native SOM is regulated by availability of nutrients under different land use systems. Microbial-necromass, formed by fast growing r-strategist microorganisms under starvation conditions, contributed to increased decomposition of SOM (i.e. positive priming effect (PE)) following addition of 14C labelled glucose without nutrients to soil under organic farming. Conversely, K (slow growing) and L- (stress tolerant) microbial strategists in soil under conventional farming and forest, respectively, were responsible for the relatively low decomposition process. Addition of either a single nutrient (N or P) or multiple nutrients (N and P) with C showed opposing effects on decomposition processes in soil under different land use systems. Microorganisms utilized the added N and C under conventional farming and forest, which suppressed the decomposition process and caused a negative PE in these soils. Conversely, the microorganisms activated after P and C addition mined SOM to meet their demand for N, resulting in a positive PE in all land use systems. The decomposition of SOM was suppressed in soil under conventional farming and forest, however, microbial biomass was stable after addition of multiple nutrients. This could be due to reduction in active microbial biomass, which contributes to respiration in soil, instead of total microbial biomass. Additionally, bacterial community structure may be modified by protozoan infiltration following N addition, decelerating the decomposition process in these two land use systems. Microbial biomass increased by 18% in soil under organic farming after addition of multiple nutrients. Thus, the decomposition process increased to fulfil the metabolic requirements of an increased microbial population, resulting in a strong positive PE. The dominance of fast growing r-strategists in organic farming showed that microorganisms will utilize available C and nutrients for their growth, thus, higher incorporation of C into their biomass. Furthermore, microbial immobilization of N or P will be higher, which can be released and taken up by plants during turnover of microbial biomass or microbial death. Hence, organic farming has a great potential to promote soil fertility and C sequestration.
In conclusion, the land-use change to organic farming positively affected soil and microbial properties, resulting in improved soil fertility and enhanced carbon sequestration. Farming, which aims at enhancing soil carbon pools and microbial activity, can address the challenge of sustaining food security while protecting the environment. | de |