Introduction
Forest degradation resulting from chronic anthropogenic modification of ecosystem structure, composition or function is gaining increasing attention as a major issue affecting human societies and biodiversity (Ghazoul and Chazdon, 2017; Grantham et al., 2020). Degradation compromises many of the benefits that forests provide (IPBES, 2018), and evidence suggests that the environmental consequences of degradation are potentially as significant as those from deforestation (Walker et al., 2020). The effect of forest degradation on soil dynamics and function is of particular concern given the fundamental role soils play in the provision of multiple ecosystem services (Smith et al., 2015). Given the scale and impacts of forest degradation resulting from chronic disturbance, it is urgent to understand its effects on soil functionality and the implications for ecosystem resilience.
A critical global ecosystem that has experienced widespread degradation is seasonally dry tropical forest (SDTF) (Miles et al., 2006). Representing 42% of tropical ecosystems worldwide and recognized for their high levels of endemicity, species richness, and functional diversity (Dirzo, 2011; Banda-Rodríguez et al., 2016), SDTFs are considered to be one of the most threatened ecosystems globally due to centuries of land use conversion and chronic disturbance (Dimson and Gillespie, 2020; Portillo-Quintero and Sánchez-Azofeifa, 2010). As in tropical rain forests, the study of degradation has experienced a recent upsurge in SDTFs due to a growing recognition of its widespread impacts on biodiversity and ecosystem function (Ribeiro-Neto et al., 2016; Sfair et al., 2018; Souza et al., 2019). The focus of most studies of degradation in SDTFs has been on its effects on taxonomic and functional plant diversity (Arnan et al., 2022; Jara-Guerrero et al., 2021; Ribeiro et al., 2019; Sagar et al., 2003). However, how degradation of SDTFs affects the conditions and ecosystem functions underpinning this diversity, particularly those related to the soil, remains poorly understood (Mora et al., 2018; Schulz et al. 2016).
Two important indicators of soil quality and function are soil organic matter (SOM) content and enzyme activity. As with deforestation and land use conversion, forest degradation drives SOM loss (Jiang et al., 2023, Jiménez et al., 2011), which has major repercussions for the capacity of forests to store C, cycle nutrients, and self-regenerate (Zhou et al., 2018). However, persistent knowledge gaps exist pertaining to the accrual, decomposition and transformation pathways of SOM (Angst et al., 2021; Sokol et al., 2022). These knowledge gaps can be partially addressed by comparing the different components of SOM. Recent approaches distinguish particulate organic matter (POM) from mineral-associated organic matter (MAOM) as two operational physical fractions of SOM with different origins and rates of turnover (Lavallee et al., 2020; Wiesmeier et al., 2019; Yu et al., 2022). Considered to be primarily plant derived, POM contains many structural C compounds with low N content and is preserved in soil via biochemical recalcitrance, physical protection in micro aggregates, and microbial inhibition. MAOM originates largely from soil microbes, contains higher N, and has a longer residence time due to chemical bonding with minerals and physical protection in small aggregates (Cotrufo et al., 2019). It is generally assumed that POM is more accessible but its quality for decomposers varies, whereas MAOM provides labile C and nutrients to microbiota and plants only following destabilization. Thus, POM is considered to be less persistent and more sensitive to disturbance than MAOM (Cambardella and Elliott, 1992; Cotrufo et al., 2019; Poeplau et al., 2018). These functional differences underscore the utility of quantifying POM and MAOM separately in order to better understand the effect of forest degradation on SOM (Lavallee et al., 2020).
Soil enzyme activity is related to SOM fluxes and can provide a useful indicator of soil function in response to disturbance (Barbosa et al., 2023). Among the myriad biotic (e.g., microbial community structure and abundance, plant inputs) and abiotic (e.g., organo-mineral interactions, soil and climatic conditions, chemical quality of C inputs) factors influencing SOM dynamics, soil enzyme activity plays a prominent role (Panettieri et al., 2022). For example, Bernard et al. (2022) demonstrated that turnover rates of SOM integrated by different pools depend on its biochemical composition and accessibility to enzymes. Kandeler et al. (1999, 2019) identified higher rates of enzyme activity in the MAOM fraction due to a greater contribution of microbial compounds. Decomposition of compounds in POM depends on processes of microbial and enzymatic inhibition, while MAOM is more subject to spatial constraints such as mineral association (Lavallee et al., 2020). While studies have demonstrated that enzyme activity decreases with the conversion of STDF to agriculture (e.g., Medeiros et al., 2015; Oliveira Silva et al., 2019), few have addressed how this crucial aspect of soil function is affected by SDTF degradation.
In this study, we examined soil quality and function along a gradient of ecosystem degradation in the STDF of Ecuador in the dry and rainy season. Our objectives were to: (1) compare soil physical-chemical properties under different levels of forest degradation between seasons; (2) evaluate the response of dehydrogenase, β-glucosidase and urease to degradation to understand how enzyme activity affects soil carbon (C) and nitrogen (N) across the degradation gradient in each season; and (3) compare particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) along the degradation gradient to gain insights into the underlying mechanisms of accrual, transformation and persistence of these components of SOM. We discuss the results in the context of the impact of forest degradation on soil functionality in SDTFs, the relationship between this functionality and vegetative structure and diversity, and how extreme degradation could lead to loss of ecosystem resilience.