Ecosystem services provided by soils in a Mexican agro-forest landscape

Samenvatting

Tropical agro-forest landscapes are dynamic mosaics of natural vegetation, semi-natural habitats, and farming systems. These landscapes provide essential ecosystem services, benefits that people obtain from ecosystems, which are not restricted to ‘natural’ ecosystems but also occur in human-modified, managed systems. Ecosystem services are co-produced in social-ecological systems, which imply the active role and influence of management decisions and practices on ecosystem processes. Thus, land use intensification affects a range of ecosystem processes and services with a potential disconnect between supporting and regulating services which form the basis for provisioning services in systems with little external inputs. I investigated the ecosystem services provided by soils in a landscape consisting of (managed) forests, rangelands and croplands with a special focus on the effect of land use intensification on the forest – rangeland frontier. I carried out my research in La Sepultura Man and the Biosphere Reserve, located in the Sierra Madre de Chiapas, Mexico, specifically in two communities or ejidos. I aimed to understand how land use intensification affects the linkages between aboveground components (vegetation) and belowground components (soil system and soil microbiota) of these human-modified agro-forest frontier (forest and rangeland) landscape and their impact on two groups of ecosystem services, supporting and forage-provisioning services. Through a general soil characterisation in both ejidos and interviews on cattle management to farmers, I evaluated the impact of landscape and land use on supporting and forage-provisioning ecosystem services and their linkages. Then, I assessed land use intensification on the soil system and litter decomposition through a decomposition field experiment. Finally, I investigated the effects of land use intensification on above and belowground components (plants, soil fertility, and soil microbiota). I evaluated whether and how these above and belowground components of the plant – soil feedback were linked. Results showed that indirect effects of land use (through some soil physicochemical characteristics) were more important than direct effects on supporting ecosystem services. Linkages between supporting and forage-provisioning ecosystem services were complex associations, sometimes with unexpected trade-offs. Although land use intensification contributed to differences in soil microbial characteristics, it had little cascading effects on litter decomposition as differences in litter quality of the two dominant litter types in this rangeland system (exotic grasses and a mixture of pine needles and oak leaves) were the main factor regulating decomposition. Furthermore, land use intensification had both positive (forage biomass and digestibility, nutrient pools in vegetation, soil microbial biomass and diversity) and negative (tree cover, plant diversity, and soil available K plus enhancement of N limitation) effects on different attributes of the plant – soil feedback in this ecosystem. The three components of the plant – soil feedback were partly coupled and plants had the strongest effect in the plant – soil feedback and, in turn, vegetation productivity was positively influenced by nutrient availability and soil microbiota. With these results I address the question: Does land use intensification inevitably have mostly negative impacts on the soil system and ecosystem processes? Many studies suggested that land use intensification leads to a decline of soil quality indicators and a loss of functional diversity, whereas other studies emphasised a trade-off between increasing short-term productivity versus maintaining long-term sustainability. I argue that alternative pathways leading to more efficient systems are needed. Ecological intensification which is defined as a process that improves performance and efficiency of managed systems through optimal management of their ecological functions and biodiversity, while providing farmers’ livelihoods, may be one of such alternative pathways. I recommend general practices of ecological intensification at the landscape level to improve overall soil quality and plant nutrient uptake and to support increasing water-holding capacity, which is a key ecosystem service, especially during the dry season. Next to the recommendations on the landscape scale I also list recommendations on farmer level for two categories of farmers: livestock specialists and livestock generalists, and include general recommendations for forest frontier landscapes.