Organic matter quality of forest floor as a driver of C and P dynamics in acacia and eucalypt plantations established on a Ferralic Arenosols, Congo

Abstract

Background: Land-use change and forest management may alter soil organic matter (SOM) and nutrient dynamics, due in part to alterations in litter input and quality. Acacia was introduced in eucalypt plantations established in the Congolese coastal plains to improve soil fertility and tree growth. Eucalypt trees were expected to benefit from N 2 fixed by acacia. However, some indicators suggest a perturbation in SOM and P dynamics might affect the sustainability of the system in the medium and long term. In tropical environments, most of the nutrient processes are determined by the high rates of organic matter (OM) mineralization. Therefore, SOM stability might play a crucial role in regulating soil-plant processes. In spite of this, the relationship between SOM quality, C and other nutrient dynamics are not well understood. In the present study, OM quality and P forms in forest floor and soil were investigated to get more insight on the C and P dynamics useful to sustainable management of forest plantations. Methods: Thermal analysis (differential scanning calorimetry (DSC) and thermogravimetry (TGA)) and nuclear magnetic resonance (solid state 13 C CPMASS and NMR and 31 P-NMR) spectroscopy have been applied to partially decomposed forest floor and soils of pure acacia and eucalypt, and mixed-species acacia-eucalypt stands. Results: Thermal analysis and 13 C NMR analysis revealed a more advanced stage of humification in forest floor of acacia-eucalypt stands, suggesting a greater microbial activity in its litter. SOM were related to the OM recalcitrance of the forest floor, indicating this higher microbial activity of the forest floor in this stand might be favouring the incorporation of C into the mineral soil.

Introduction of Dalbergia Odorifera Enhances Nitrogen Absorption on Eucalyputs Through Stimulating Microbially Mediated Soil Nitrogen-cycling

“…Faster growth of N-xing tree species and Eucalyptus due to higher N availability at the MP sites could also increase the input of plant residuals and hence enhance SOC and TN. Similar results were also found in other studies with a mixed plantation such as E. globulus and A. mearnsii in Australia (Forrester et al 2005), Eucalyptus grandis and Acacia mangium in Brazil (Bini et al 2013a(Bini et al , 2013b and in Congo (Koutika et al 2019(Koutika et al , 2020, or Eucalyptus regnans and Acacia dealbata in southeastern Australia (Pfautsch et al 2009). Taken together, the higher SOC ,TN and inorganic N at the MP sites (Table 1) indicate that mixed-species plantation not only improved soil N availability, aiming at preventing soil degradation; but also increased soil C sequestration, contributing to mitigating climate change.…”

Influence of Acacia mangium on Soil Fertility and Bacterial Community in Eucalyptus Plantations in the Congolese Coastal Plains

“…However, its status improved in all planted stands with Eucalyptus and/or Acacia compared to savannas [6]. Its significance is also shown through the great amount of extractable P in the forest floor of the Acacia stands [13] resulting from important inputs of P in organic residues and litterfall [11] relative to Eucalyptus, probably due to Acacia ability to phosphorous retranslocation [10]. The dynamics of P in the studied soils is also explained by the fact that most of the mineral soil P is in inorganic (70%) form with orthophosphate as the prevalent P form in floor forest and mineral soil (0-5cm) [13] and P cycling is dominated by biological process [12].…”

“…Its significance is also shown through the great amount of extractable P in the forest floor of the Acacia stands [13] resulting from important inputs of P in organic residues and litterfall [11] relative to Eucalyptus, probably due to Acacia ability to phosphorous retranslocation [10]. The dynamics of P in the studied soils is also explained by the fact that most of the mineral soil P is in inorganic (70%) form with orthophosphate as the prevalent P form in floor forest and mineral soil (0-5cm) [13] and P cycling is dominated by biological process [12]. Other findings outlining increased available soil P as a crucial factor that reinforce the link between plant diversity, soil attributes, and ecosystem function which ensure soil P bioavailability and alleviate soil P limitations [78].…”

“…Due to the higher SOM contents [36] and mesofauna density and richness [37] in the upper layer of the studied soil type, soil samples were collected with an auger at 0-0.05 m at 5 years into the second rotation (March 2017), as previously described [13]. In particular, the soil was sampled in 9 replicates by stand beneath monoculture of A. mangium (100 A) and E. urophylla × E. grandis (100 E) and 18 replicates mixed-species of Acacia and Eucalyptus (50 A 50 E) in 3 out of the 5 blocks.…”

“…Soil phosphorus (P) status also improves through increased soil available P in the coarse fraction of particulate organic matter POM (cPOM; 4000-250 µm) of the plantation of Acacia or/and Eucalyptus compared to tropical savannas [6,8]. Even though the well-known high P demand of A. mangium as a NFT to sustain symbiotic root nodules and atmospheric N 2 fixation processes [9,10] involves a decrease in soil available P beneath stands containing Acacia relative to Eucalyptus [5,11], P cycling in these soils is dominated by biological processes, i.e., organic mineralization [12], while forest floor and mineral soil contain most of the extractable P in inorganic form, reaching up to 70% in the mineral soil P [13].…”