Beyond total carbon: conversion of amazon forest to pasture alters indicators of soil C cycling

Abstract

It is well established that land use change (LUC) can impact soil organic carbon (SOC) in tropical regions, but the long-term effects of LUC on soil quality and C cycling remain unclear. Here, we evaluated how LUC affects soil C cycling in the Amazon region using a 100-year observational chronosequence spanning primary forest-to-pasture conversion and subsequent secondary forest succession. We found a surprising increase in topsoil SOC concentrations 60 years following conversion, despite major losses ([ 85%) of forest-derived SOC within the first 25 years. Shifts in molecular composition of SOC, identified with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, occurred in tandem with a significant decline in permanganateoxidizable C (POXC) and b-glucosidase activity (per unit SOC), interpreted as a deceleration of soil C cycling after pasture grasses became the dominant source of C inputs to soil. Secondary forest succession caused rapid reversal to conditions observed under primary forest for b-glucosidase activity but not for SOC molecular composition (DRIFT spectroscopy), reflecting a long-lasting effect of LUC on soil C cycling. Our results show that rapid changes in the origin of SOC occur following deforestation with legacy effects on some indicators of C cycling (e.g. enzyme activity) but not others (e.g. molecular composition). This approach offers mechanistic

Impacts of land-use change on soil microbial communities and their function in the Amazon Rainforest

“…One study across a land-use chronosequence at ARMO, State of Rondônia, Brazil, analyzed C dynamics in 13-18-year-old secondary forests that were abandoned after 7-10 years of pasture use. The study found that the organic C profiles (quantity and chemical composition) of secondary forest soil closely resemble that of primary forests, with concomitant increases in C-cycling enzyme (β-glucosidase) activity (Durrer et al, 2021); this pattern is perhaps intuitive because the plant community and detrital substrate of secondary forest is much more similar to original forests than pastures filled with exotic grasses. The interrelationship of CH 4 -C emissions and methanecycling microbial functional groups with forest recovery is of distinct interest.…”

“…Across pasture chronosequences in Rondônia, Brazil, soil C concentration consistently shows an increase with pasture age, and isotopic δ 13 C values become significantly less negative, indicating gradual turnover of forest-derived C and replacement with pasture-derived C (Durrer et al, 2021;Neill et al, 1996). Yet, the change in δ 13 C value of microbial respiration greatly outpaces that of soil C stocks; in one study, for example, pasture-derived C of 3-year-old pastures constituted 17% of soil stocks, but 69% of microbial-respired C (Neill et al, 1996).…”

“…However the net impact of land-use conversion on soil C is far less clear as compared to impacts on plant biomass-stored. Various studies have found that soil C increases (de Durrer et al, 2021;Neill et al, 1997a), decreases (Fearnside, 1997;Maia et al, 2010), or does not change appreciably (Durigan et al, 2017;Rittl et al, 2017) following land-use conversion. This suggests that the impact of land-use change on soil C storage is dependent either on pre-existing conditions, such as initial C stocks, soil texture, and nutrient status, or management factors such as the frequency of burns or grazing intensity.…”

“…Further, the chemical-structural composition of soil organic matter (i.e., substrate) changes with forest-to-pasture conversion, resulting in an increased concentration of hemicellulose and a decreased concentration of lignin in pastures compared to primary forest, even if total pool size is unchanged (Lammel et al, 2015b). Additionally, pasture soils contain a significantly higher concentration of permanganate-oxidizable C, which serves as an indicate of easily-catabolized C (Durrer et al, 2021). Several other metagenomic-based studies reporting bulk annotation of genes related to degradation of aromatic compounds did not find differences between land-use types (Mendes et al, 2015b;Pedrinho et al, 2019).…”

Soil geochemistry as a driver of soil organic matter composition: insights from a soil chronosequence

“…lignin, polysaccharides, lipids) found in the terrestrial C pool (Paul, 2016;Hall et al, 2020). Hence, SOM composition (the prevalence of certain biochemical compounds) may serve as a proxy for ecosystem properties and soil functioning, as it responds to land use change (Durrer et al, 2021) and affects C cycling by the amount of energy provided to soil microorganisms (Nunan et al, 2015;Barré et al, 2016). Furthermore, the peak areas related to biochemical SOM compounds has been successfully used to parameterize fast and slow-cycling SOM pools (Todd-Brown et al, 2013;Bailey et al, 2018;Laub et al, 2020;Baldock et al, 2021).…”