Systems Theory in Soil Agroecology

Abstract

Systems theory is increasingly important for understanding the recently globalized Anthropocene period. Popular ideas from complex systems in physical and social sciences appear in some news media, including ‘fractals’ and ‘six degrees of separation’. One key example complex system is soil, which is both the namesake for humans (i.e., “humus”) and a fundamental basis for sustainable agriculture, yet also experiencing drastic erosion, degradation, and loss, and in need of conservation. Soils are a highly heterogeneous medium and habitat, which could be better understood by applying tools from the emerging field of complex systems, especially in human- centric contexts such as small-scale tropical and urban agriculture. The first study in this dissertation is a transdisciplinary synthesis of how soils can be studied using analytical tools from complexity theory, highlighting important phenomena to be explored, namely: memory or time lags; temporal and/or spatial oscillations highlighting relevance of reporting non-linear dynamics; and critical tipping points and hysteresis curves that may describe future longer-term changes in key soil properties such as soil organic matter, bulk density, and possibly microbial diversity, as well as robust scaling laws for soil aggregates, soil pores, microbial diversity, and/or regional soil carbon concentrations. The second study tests an analog to traditional island biogeography theory at the micro-scale of soil aggregates, and finds support for soil aggregates as reservoirs of soil biodiversity, which increases or scales following long-tailed distribution families observed in other complex physical and social systems. Soil aggregate-associated microbial community assembly also involved relatively more ecological drift, as communities in larger soil aggregates were more similar in composition to randomly simulated communities. This second study overall ties the fine-scale physical soil habitat with higher-level ecological processes and patterns. Expanding on higher-order ecological interactions, the third study links above-belowground interactions by examining the impact of an ant that nests in trees and soil properties, testing the extent near the tree. In this third study, we test the extent of food web cascades by keystone tropical species, and find evidence of more consistent accumulation of relatively larger soil aggregates, supported by consistent power function parameters that describe soil aggregate size-frequency distributions, as well as changes in soil aggregate size and variance under similar distribution families. Related results also show faster soil water infiltration under ant nests, and tendencies toward lower soil carbon and nitrogen stocks, depending on host tree species. Finally, the fourth study analyzes charred wood (biochar) as a potentially regenerative soil amendment in re-purposed urban agricultural soils, testing the mechanistic hypothesis about how biochar effects on soils are mediated by biochar particle size, as well as addressing practitioner-motivated questions about biochar application amount and comparison to current practices of separate- or co-additions with compost. We found detectable benefits, including lower soil compaction and increased soil organic matter along with bacterial diversity, but minor negative effects on fungal and invertebrate diversity. These studies together may broadly support more attention and a new sense of hope toward soil and ecosystem regeneration. Finally, this research supports the perspective that soil should be studied with a transdisciplinary approach, integrating multiple sub-disciplinary tools from microbial to mathematical ecology, as well as incorporating theories from entirely other disciplines like complex systems and non-linear dynamics, to test new hypotheses in ecology that are more general in scope.