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.