Terra Preta Properties in Northwestern Amazonia (Colombia)

Whereas many researchers still approach Terra Preta (TP) as a soil category, new evidence suggests that TP refers to a directional grading of soil property changes (i.e., color, pH, nutrients, etc.) within human-made soils, originating from human activities in pre-Columbian times. Currently, most TP research focuses on the Brazilian part of the Amazon basin, but only little information is available on TP soils in the Colombian Amazon. Here, we sampled four TP and surrounding soils in the Colombian Amazon region at different soil depths and analyzed them for (i) general soil properties such as color, pH and texture, (ii) soil organic carbon and black carbon (BC) contents, the latter using benzene polycarboxylic acids as molecular marker, (iii) phosphorus availability based on sequential fractionation, and (iv) microbial residue contents using amino sugars. Our data from Colombia’s middle Caquetá River and Leticia confirmed that SOC, BC, and total P were present in significantly higher concentrations in the TP areas than the surrounding soils, while pH values and microbial residue contents were unchanged. The enrichment of P forms comprised both easily extractable and stable P pools, which both dominated to a different degree, both in TP and adjacent soils. The different degree of SOC, BC and P enrichment suggests different amounts of waste disposal by the ancient populations at different TP sites, now warranting further research for reconstructing ancient population sizes from TP chemical analyses.

Threshold conditions for the accumulation of microbial residues in terrestrial ecosystems

Microbial residues play important roles in the formation and stability of soil carbon pools; however, the factors affecting large-scale accumulation of microbial residues remain unclear. Here, we collected data of 268 amino sugar levels (biomarkers of microbial residues) from previous field studies and found that soil organic carbon (SOC), soil C:N ratio, and aridity index mainly determine the accumulation of microbial residual carbon. Moreover, we found that the threshold of the aridity index where microbial residue starts decreasing is in the range of humid climate type, while the threshold of soil C:N ratio represents a point of sharp decrease in fungal abundance. Although SOC and aridity index were important in all cases, the dominant factors for predicting microbial residues varied across different ecosystems and climate zones, with pH being particularly important. Hence, climate and soil environment play important roles in the process of microbial residue accumulation.

Effect of temperature on microbial residue dynamics in a temperate farmland soil

Microorganisms mediate soil organic carbon (SOC) turnover, and microbial residues contribute a significant portion to SOC storage in temperate agroecosystems. However, little is known about the direct effect of temperature on microbial residues associated with SOC sequestration/decomposition. We assessed microbial residue dynamics in a 28 d incubation conducted at four temperatures (5, 15, 25, and 35 °C). Microbial residues did not change with time from 5 to 25 °C. However, at 35 °C, fungal residues decomposed significantly with time, and the decomposition rate was higher than SOC. Considering the important contribution of fungal residues to stable-C pool, our findings indicated warming may be detrimental to C stability in this temperate soil.

Contribution of Microbial Residues Obtained from Lignin and Cellulose on Humus Formation

The contribution of microbial residues formed on lignin and cellulose to the formation of humus (HS) was investigated. The microbial residues formed by Aspergillus niger (A. niger) in the cultures of cellulose and lignin in a fluid medium were structurally characterized by elemental analysis, differential thermal analysis (DTA), FTIR spectroscopy and CP/MAS 13C NMR spectroscopy. Compared to cellulose itself, the microbial residue from cellulose contains more aromatic compounds and N-containing compounds and fewer carbohydrates and carboxylic compounds. A. niger improved the thermal stability and aromaticity of the cellulose. However, compared with that on lignin, more N-containing compounds, carbohydrates and carboxylic acid derivatives and less aromatic material were found in the microbial residue from lignin. Regardless of whether the carbon source was cellulose or lignin, A. niger utilized the N in the fluid medium to synthesize its own cells, and eventually, they could transfer the N into the microbial residue; in addition, the O-alkyl species dominated over the alkyl and aromatic compounds in the microbial residue. Although the molecular structures of the components of the microbial residue from lignin tended to be simpler, they were more alkylated, more hydrophobic and less aliphatic than those from cellulose. During culture with A. niger, the cellulose underwent degradation and then a polymerization, which led to an increased degree of condensation but a lower degree of oxidation, providing essential precursor substances for HSs formation. However, lignin underwent oxidative degradation. The microbial residue from lignin had a lower degree of condensation and a higher degree of oxidation.

Conversion of Natural Evergreen Broadleaved Forests Decreases Soil Organic Carbon but Increases the Relative Contribution of Microbial Residue in Subtropical China

It has been recognized that land use change affects soil organic carbon (SOC) dynamics and the associated microbial turnover. However, the contribution of microbial residue to SOC storage remains largely unknown in land use change processes. To this end, we adopted a “space for time” approach to examine the dynamics of SOC and amino sugars, which was a biomarker of microbial residue C, in different natural forest conversions. Three typical converted forests were selected: an assisted natural regeneration (ANR) and two coniferous plantations of Cunninghamia lanceolata (Lamb.) Hook (Chinese fir) and Pinus massoniana Lamb. (pine) each. All of these were developed at the same time after the harvest of an old natural forest and they were used to evaluate the effects of forest conversions with contrasting anthropogenic disturbance on SOC and microbial residue C, along with the natural forest. Natural forest conversion led to an approximately 42% decrease in SOC for ANR with low anthropogenic disturbance, 60% for the Chinese fir plantation, and 64% for the pine plantation. In contrast, the natural forest conversion led to a 32% decrease in the total amino sugars (TAS) for ANR, 43% for the Chinese fir plantation, and 54% for the pine plantation at a soil depth of 0–10 cm. The ratios of TAS to SOC were significantly increased following natural forest conversion, with the highest ratio being observed in the Chinese fir plantation, whereas the ratios of glucosamine to muramic acid (GluN/MurA) were significantly decreased in the two plantations, but not in ANR. The contents of SOC, individual amino sugar, or TAS, and GluN/MurA ratios were consistently higher at a soil depth of 0–10 cm than at 10–20 cm for all of the experimental forests. Redundancy analysis showed that microbial residue C was significantly correlated with SOC, and both were positively correlated with fine root biomass, annual litterfall, and soil available phosphorus. Taken together, our findings demonstrated that microbial residue C accumulation varied with SOC and litter input, and played a more important role in SOC storage following forest conversion to plantations with higher anthropogenic disturbance.