Warming and Labile Substrate Addition Alter Enzyme Activities and Composition of Soil Organic Carbon

Warming can increase the efflux of carbon dioxide (CO2) from soils and can potentially feedback to climate change. In addition to warming, the input of labile carbon can enhance the microbial activity by stimulating the co-metabolism of recalcitrant soil organic matter (SOM). This is particularly true with SOM under invaded ecosystems where elevated CO2 and warming may increase the biomass of invasive species resulting in higher addition of labile substrates. We hypothesized that the input of labile carbon would instigate a greater soil organic carbon (SOC) loss with warming compared to the ambient temperature. We investigated this by incubating soils collected from a native pine (Pinus taeda) forest to which labile carbon from the invasive species kudzu (Pueraria lobata) was added. We evaluated the microbial extracellular enzyme activity, molecular composition of SOC and the temperature sensitivity of soil CO2 efflux under warming and labile carbon addition. After 14 months of soil incubation, the addition of labile C through kudzu extract increased the activity of β-1,4-glucosidase compared with the control. However, the activity of N-acetyl-β-D-glucosaminidase and fungal biomass (ergosterol) decreased with labile carbon addition. The activity of peroxidase increased with warming after 14 months of soil incubation. Although the carbon content of incubated soils did not vary with substrate and temperature treatments, the molecular composition of SOC indicated a general decrease in biopolymers such as cutin, suberin, long-chain fatty acids, and phytosterol with warming and an increasing trend of microbial-derived compounds with labile substrate addition. In soils that received an addition of labile C, the macro-aggregate stability was higher while the temperature sensitivity of soil C efflux was lower compared with the control. The increase in aggregate stability could enhance the physical protection of SOC from microbial decomposition potentially contributing to the observed pattern of temperature sensitivity. Our results suggest that warming could preferentially accelerate the decomposition of recalcitrant compounds while the addition of labile substrates could enhance microbial-derived compounds that are relatively resistant to further decomposition. Our study further emphasizes that global change factors such as plant invasion and climate change can differentially alter soil microbial activity and the composition of SOC.

Profile distribution of soil organic carbon fractions under different landforms in the Meghalaya plateau of India

Assessment of organic carbon fractions in soil provides the basis to ascertain vulnerability of an ecosystem to climate change. In the present study, we assessed SOC fractions in four pedons under contrasting landforms i.e., denudational low hill, upper plateau, lower plateau and valley in the Meghalaya plateau, India. Results indicated that soils of the studied pedons were acidic in nature, low in cation exchange capacity and base saturation. Further, surface (0-30 cm) soils were high in Walkley Black C (WBC) content (0.83-1.13%) in the studied pedons located under different landforms. The density of very labile carbon (VLC) fraction up to a depth of 150 cm was highest (49.22 Mg ha?1) in pedon 2 (P2) located in the upper plateau under shifting cultivation while that of less labile carbon (LLC) was highest (50.25 Mg ha?1) in pedon 4 (P4) in the valley under paddy cultivation. Higher densities of WBC and LLC in the valley (P4) as compared to other landforms in the study area indicate higher carbon sequestration potential of valley soil.

Carbon fractions of fortified thermochemical organic fertilizers and their response on the yield of okra and tomato

Aim: To study the carbon fractional status of the growing media and to find out the best organic nitrogen source for fortification of thermochemical organic fertilizer, the manurial constituent of growing media for container cultivation of okra and tomato. Methodology: Container cultivation of okra and tomato were done in completely randomised design. Treatments included fortification with farmyard manure, neem cake, groundnut cake, poultry manure, vermicompost, coir pith compost, hatchery waste organic fertilizer, urea and unfortified thermochemical organic fertilizer. Carbon fractions, viz. total organic carbon, permanganate oxidisable labile carbon, microbial biomass carbon and soil respiration of the growing media were analysed. The yield and yield attributes of the crops were determined. Results: Irrespective of the organic source of nitrogen used, the fortified thermochemical organic fertilizer imparted a high status of total organic carbon to the growing media. Fortification with farmyard manure enhanced labile carbon, soil microbial biomass carbon and soil respiration over those fortified with other organic and inorganic sources. Container grown okra in a growing media with thermochemical organic fertilizer fortified with farmyard manure out yielded urea based fortification by 55.96%. Tomato grown in coir pith compost fortified growing media enhanced yield by 27.37% over the groundnut cake fortified growing media. Linear regression models of labile carbon with microbial biomass carbon (R2 = 0.8946) and with soil respiration (R2 = 0.9053) were significant and with a good fit. Interpretation: Fortification of thermochemical organic fertilizer with various organic sources of nitrogen imparted a high total soil organic carbon status. Synergic effect of the farmyard manure fortification was evident in labile carbon, microbial biomass carbon and soil respiration. Growing media fortified with farmyard manure was ideal for container cultivated okra whereas that with coir pith proved to be ideal for tomato, a solanaceous vegetable crop.

Fine roots stimulate nutrient release during early stages of leaf litter decomposition in a Central Amazon rainforest

Purpose Large parts of the Amazon rainforest grow on weathered soils depleted in phosphorus and rock-derived cations. We tested the hypothesis that in this ecosystem, fine roots stimulate decomposition and nutrient release from leaf litter biochemically by releasing enzymes, and by exuding labile carbon stimulating microbial decomposers. Methods We monitored leaf litter decomposition in a Central Amazon tropical rainforest, where fine roots were either present or excluded, over 188 days and added labile carbon substrates (glucose and citric acid) in a fully factorial design. We tracked litter mass loss, remaining carbon, nitrogen, phosphorus and cation concentrations, extracellular enzyme activity and microbial carbon and nutrient concentrations. Results Fine root presence did not affect litter mass loss but significantly increased the loss of phosphorus and cations from leaf litter. In the presence of fine roots, acid phosphatase activity was 43.2% higher, while neither microbial stoichiometry, nor extracellular enzyme activities targeting carbon- and nitrogen-containing compounds changed. Glucose additions increased phosphorus loss from litter when fine roots were present, and enhanced phosphatase activity in root exclusions. Citric acid additions reduced litter mass loss, microbial biomass nitrogen and phosphorus, regardless of fine root presence or exclusion. Conclusions We conclude that plant roots release significant amounts of acid phosphatases into the litter layer and mobilize phosphorus without affecting litter mass loss. Our results further indicate that added labile carbon inputs (i.e. glucose) can stimulate acid phosphatase production by microbial decomposers, highlighting the potential importance of plant-microbial feedbacks in tropical forest ecosystems.

Long-Term Fertilization Alters the Chemical Composition of Dissolved Organic Carbon via Regulating Soil Physicochemical Factors

Soil dissolved organic carbon (DOC) plays a key role in fundamental biogeochemical processes; however, the influences of different types of fertilization on the chemical composition and properties of DOC molecules in soils are poorly understood. In this study, DOC samples were extracted from gray desert soils treated with five fertilization treatments 1) chemical nitrogen, phosphorus and potassium fertilizers; 2) pig and cattle manure; 3) 50% nitrogen from manure and 50% nitrogen from chemical fertilizer; 4) total chemical fertilizers and total manure; and 5) no fertilization (control). Data were compared by a combination of Fourier-transform infrared spectroscopy, 13C nuclear magnetic resonance, and statistical analyses. Results showed that manure application increased the aliphatic carbon and decreased the aromatic carbon levels, implying manure application promotes the transformation of labile carbon to stable carbon structures. Redundancy analysis indicated that available nutrients and available forms of magnesium were positively associated with the labile carbon groups, and the available forms of calcium were positively associated with the stable carbon groups. These results demonstrated that the availability of soil nutrients and minerals are influential factors on DOC turnover. Moreover, our results showed that fertilization method had negative direct effects on DOC transformation and positive indirect effects on DOC turnover via soil physicochemical factors, and various forms of mineral ions were the strongest explanatory factors of DOC variation. These findings indicated that soil physicochemical factors play an important role as mediators in the influence of fertilization practices on the chemical composition of DOC and turnover of carbon-containing functional groups in DOC.