Abstract:
In the present thesis, implications of pasture establishment, fertilization and abandonment on soil C and nutrient dynamics were investigated for the mountain rainforest region of southern Ecuador. Over the past decades the natural forest of the study area has been threatened by conversion to cattle pastures. However, the soil fertility of these extensively grazed pastures (active pastures) declines continuously during pasture use. The invasion of bracken fern (Pteridium arachnoideum) leads to pasture abandonment when bracken becomes dominant. In order to reveal the mechanisms behind the deterioration of soil fertility, biotic and abiotic soil properties and their interaction were analyzed along a land?use gradient (natural forest – active pasture – abandoned pasture).
The ecosystem disturbance of the mountain rainforest through pasture use changed the microbial function and structure, and affected soil CO2?C fluxes. Annually, 2 Mg soil CO2?C ha?1 were additionally emitted from the pasture land. This acceleration in soil respiration rates was related to accelerated rates of microbial C mineralization and fine?root respiration. The high?quality, N?rich above? and belowground residues of the pasture grass (S. sphacelata, C4?plant), especially the huge fine?root biomass, provided a high C and N availability for soil microbes. Compared to the forest, increased soil pH and accelerated base saturation were further factors beneficial for soil microbial growth and metabolism of the upper mineral soil at active pastures. Three times higher amounts of microbial biomass C and a significant shift in the microbial community structure towards a higher relative abundance of Gram(?)? bacteria and fungi were observed.
Long?term pasture use and the invasion of bracken (C3?plant) diminished beneficial effects for microbes, causing a significant decrease in the C, net, and gross N mineralization rates as well as a two?third reduction in the microbial biomass. A preferential substrate utilization of grass?derived C4 by the soil microbes resulted in a rapid decline of the C4?pool. As a consequence, the less available C3?pool from bracken and former forest increased its dominance in the SOC?pool, further decreasing pasture productivity and finally causing pasture abandonment. The lower quality and quantity of above? and belowground residues of the bracken (high lignin content, C/N) resulted in resource?limited conditions that influenced the microbial function to greater extent than their structure. The microbial structure seemed to be sensitive mainly to soil pH along the land?use gradient. Thus, a disconnection between microbial structure and function was identified.
Fertilization experiments were conducted both in the lab and in the field to evaluate the impact of urea and/or rock phosphate amendment on SOM dynamics and on pasture productivity of active pastures. After combined fertilization the pasture yield was most efficiently increased by 2 Mg ha?1 a?1, indicating a NP?limitation of grass growth. Furthermore, the fodder quality was improved by a higher content of P and Ca in the grass biomass. The microorganisms of the active pasture soil responded with an adaptation of their structure to the increased substrate availability in the short term, but did not change their initial functions in the long term. After urea/ rock phosphate addition a significant increase in the relative fungal abundance was detected, but neither a microbial limitation of energy nor of N or P was observed. However, urea addition accelerated gaseous losses of soil CO2?C in the short term.
In the study area, pronounced alterations in ecosystem functioning due to land?use changes were detected, especially in soil C and N cycling rates. For a sustainable land?use in this region it is crucial to prevent pasture degradation and to rehabilitate degraded pastures in order to protect the prevailing mountain rainforest ecosystem. It is of crucial importance for active pasture soils to maintain or even increase resource availability, being one indicator of soil fertility. In this context, the soil organic matter has to be retained in the long?term to maintain high microbial activity and biomass, and thus pasture productivity. A moderate fertilization with urea and rock phosphate can be a first step to provide continuous nutrient supply for grass growth and to strengthen livestock health through increased fodder quality. However, the risk of further additional emissions of soil CO2?C due to increased loads of urea fertilizer application has to be kept in mind. Overall, for the establishment of a sustainable land?use management the control of bracken invasion and an adjusted nutrient management are needed. Further investigations on the reduction of soil nutrient losses and increased nutrient use efficiencies of plants, such as combined planting with legumes or the usage of cultivars with special nutrient acquisition strategies, should be in the focus of future work.
