The thesis was conducted to investigate soil fertility changes and assess the sustainability of agroecological management in different land-use systems of the southern Ecuadorian Andes using quantitative and qualitative methods. Ecuador still holds the highest deforestation rate of all Latin American countries which also has a large impact in the research area by forest conversion into agricultural land. Agricultural land-use systems in the research area are multifaceted due to heterogeneous biophysical and socio-economic conditions. To map this diversity, land-use systems were investigated in Yantzaza (low-external-input), El Tambo (irrigated cash crops) and San Lucas (integrated nutrient management). Yet, management effects on soil fertility have not been assessed systematically in Ecuadorian farming systems which hampers the evaluation whether agroecological management is sustainable. Therefore, the present study used a set of quantitative and qualitative approaches to assess soil fertility changes at plot and farm scale with a nutrient balance/chronosequence approach and local expert knowledge. Nutrient balances were modeled with Nutmon after adaptation of difficult-to-quantify flows to the local conditions facilitating area and land-use specific calculation. Soil nutrient balances in the research area were diverse and varied between −151 to 66 kg ha-1 a-1, −4 to 33 kg ha-1 a-1 and −346 to 39 kg ha-1 a-1 for NPK, respectively. The evaluation of socio-economic and soil fertility explanatory variables revealed that up to 70% of the balances’ variability could be explained. Land-uses with a strong market orientation such as annual crops in El Tambo received large amounts of external inputs which were often focused on mineral N fertilization causing strongly negative PK balances. In contrast, P balances were mainly positive after the application of organic fertilizers and nutrient recycling as was found in perennial crops of San Lucas. NP balances in annual crops of Yantzaza were most negative due to the low-external-input system with nonexistent fertilization as well as leaching and burning of crop residues. Highest soil nutrient stocks were found in land-uses benefiting from a surplus of within-farm flows. The quantification of soil nutrient stocks and their temporal changes were carried out with a chronosequence approach in Yantzaza. SOC stocks in annual/perennial crops and pastures decreased between 14% and 19% after forest conversion by slash-and-burn. Annual sites were abandoned not later than five years after forest conversion due to a shortage of available N and P closely linked to low-external-input management. Stocks for TN, TP, TS and exchangeable bases increased above forest level in perennial crops and pastures 6-20 years after forest conversion. Yet a strong decrease in SOC and soil nutrient stocks was found in oldest perennial and pasture sites compared to medium aged sites. This was traced back to adverse site processes such as the decay of clay humus complexes, leaching as well as poor pasture management. To assess sustainability of the agroecological management, a set of sustainability indicators was implemented including N balances, yearly N stock change and SOC stocks as well as total (TN) and available (PO4-P) soil nutrient stocks. Sustainability assessment took place based on individual land-uses and nutrients within each pilot study since soil fertility change did not show a consistent trend within one research area. Despite mainly negative soil nutrient balances, the impact on the yearly soil nutrient stock change was often negligible due to large soil nutrient pools. Annual and perennial crops of Yantzaza and pastures of San Lucas exceeded the threshold value of 1% for yearly TN stock losses. Yet, only annual crops in Yantzaza, having the highest yearly TN stock losses of 4.9%, also showed severe TN and SOC losses between 15-25% below those of the forest reference area. Therefore, the present agroecological management of annuals in Yantzaza is not sustainable which was also indicated by the abandonment of these sites not later than 5 years after forest conversion due to soil fertility decline. Hence, it is proposed to install an integrated agricultural management in annual crops of Yantzaza using nutrient recycling and fertilization for the replenishment of soil nutrient stocks. Nutrient balance studies indicated an average N-fertilizer application of more than 200 kg ha-1 a-1 for annual crops in El Tambo and low SOC stocks in soils of the colluvial foot slopes. Therefore, a laboratory incubation experiment was conducted to investigate fertilization effects of urea and newly introduced guinea pig manure on the microbial activity in colluvial and eroded soils of El Tambo. While urea fertilization induced an acceleration of SOM mineralization, a combined fertilization (urea + GPM) increased the amount of microbial biomass and provided mineral nitrogen for immediate plant uptake. SOM stocks in colluvial soils were 40% below those of eroded soils which was partly due to the positive priming effect after urea fertilization. A participatory appraisal with local farmers resulted in the adaptation of the present harvest residue management aiming at SOM maintenance in colluvial soils. Yet, the calculation of the potential for SOM replenishment indicated that only the maize residue biomass had the potential to compensate for SOM mineralization losses. Therefore, it is recommended to support SOM replenishment by additional organic inputs since SOM has to be maintained in the long-term to enable agricultural productivity.

The thesis was conducted to investigate soil fertility changes and assess the sustainability of agroecological management in different land-use systems of the southern Ecuadorian Andes using quantitative and qualitative methods. Ecuador still holds the highest deforestation rate of all Latin American countries which also has a large impact in the research area by forest conversion into agricultural land. Agricultural land-use systems in the research area are multifaceted due to heterogeneous biophysical and socio-economic conditions. To map this diversity, land-use systems were investigated in Yantzaza (low-external-input), El Tambo (irrigated cash crops) and San Lucas (integrated nutrient management). Yet, management effects on soil fertility have not been assessed systematically in Ecuadorian farming systems which hampers the evaluation whether agroecological management is sustainable. Therefore, the present study used a set of quantitative and qualitative approaches to assess soil fertility changes at plot and farm scale with a nutrient balance/chronosequence approach and local expert knowledge. Nutrient balances were modeled with Nutmon after adaptation of difficult-to-quantify flows to the local conditions facilitating area and land-use specific calculation. Soil nutrient balances in the research area were diverse and varied between −151 to 66 kg ha-1 a-1, −4 to 33 kg ha-1 a-1 and −346 to 39 kg ha-1 a-1 for NPK, respectively. The evaluation of socio-economic and soil fertility explanatory variables revealed that up to 70% of the balances’ variability could be explained. Land-uses with a strong market orientation such as annual crops in El Tambo received large amounts of external inputs which were often focused on mineral N fertilization causing strongly negative PK balances. In contrast, P balances were mainly positive after the application of organic fertilizers and nutrient recycling as was found in perennial crops of San Lucas. NP balances in annual crops of Yantzaza were most negative due to the low-external-input system with nonexistent fertilization as well as leaching and burning of crop residues. Highest soil nutrient stocks were found in land-uses benefiting from a surplus of within-farm flows. The quantification of soil nutrient stocks and their temporal changes were carried out with a chronosequence approach in Yantzaza. SOC stocks in annual/perennial crops and pastures decreased between 14% and 19% after forest conversion by slash-and-burn. Annual sites were abandoned not later than five years after forest conversion due to a shortage of available N and P closely linked to low-external-input management. Stocks for TN, TP, TS and exchangeable bases increased above forest level in perennial crops and pastures 6-20 years after forest conversion. Yet a strong decrease in SOC and soil nutrient stocks was found in oldest perennial and pasture sites compared to medium aged sites. This was traced back to adverse site processes such as the decay of clay humus complexes, leaching as well as poor pasture management. To assess sustainability of the agroecological management, a set of sustainability indicators was implemented including N balances, yearly N stock change and SOC stocks as well as total (TN) and available (PO4-P) soil nutrient stocks. Sustainability assessment took place based on individual land-uses and nutrients within each pilot study since soil fertility change did not show a consistent trend within one research area. Despite mainly negative soil nutrient balances, the impact on the yearly soil nutrient stock change was often negligible due to large soil nutrient pools. Annual and perennial crops of Yantzaza and pastures of San Lucas exceeded the threshold value of 1% for yearly TN stock losses. Yet, only annual crops in Yantzaza, having the highest yearly TN stock losses of 4.9%, also showed severe TN and SOC losses between 15-25% below those of the forest reference area. Therefore, the present agroecological management of annuals in Yantzaza is not sustainable which was also indicated by the abandonment of these sites not later than 5 years after forest conversion due to soil fertility decline. Hence, it is proposed to install an integrated agricultural management in annual crops of Yantzaza using nutrient recycling and fertilization for the replenishment of soil nutrient stocks. Nutrient balance studies indicated an average N-fertilizer application of more than 200 kg ha-1 a-1 for annual crops in El Tambo and low SOC stocks in soils of the colluvial foot slopes. Therefore, a laboratory incubation experiment was conducted to investigate fertilization effects of urea and newly introduced guinea pig manure on the microbial activity in colluvial and eroded soils of El Tambo. While urea fertilization induced an acceleration of SOM mineralization, a combined fertilization (urea + GPM) increased the amount of microbial biomass and provided mineral nitrogen for immediate plant uptake. SOM stocks in colluvial soils were 40% below those of eroded soils which was partly due to the positive priming effect after urea fertilization. A participatory appraisal with local farmers resulted in the adaptation of the present harvest residue management aiming at SOM maintenance in colluvial soils. Yet, the calculation of the potential for SOM replenishment indicated that only the maize residue biomass had the potential to compensate for SOM mineralization losses. Therefore, it is recommended to support SOM replenishment by additional organic inputs since SOM has to be maintained in the long-term to enable agricultural productivity.