Search

The evolution of soil terrestrial ecosystems is a subject with difficulties to define their maturity and evolutionary state. In the last century, thermodynamics was one of the options considered by ecologists for that goal. Difficulties in quantifying the thermodynamic parameters needed by the evolutionary theories caused that this subject has been practically locked since the end of the last century. Application of thermodynamics needs reactions and one of the main reactions in soil ecosystems are those involved in the decomposition of the soil organic matter. This paper aims to provide an initial step to study those reactions from a thermodynamic perspective. With that goal in mind, thermal analysis and isothermal calorespirometric measurements were made on soil samples collected at three depths in semi‐natural oak woodlands at three different sites in southwest Ireland. It is assumed that the organic matter evolves from a less to a higher mature state as soil depth increases. The maturity state could be chemically defined by the redox state. The proposed methods yield the enthalpy change, Gibbs energy change and entropy change for the microbial catabolism and combustion reactions of the soil organic matter. The degree of reduction was calculated by the enthalpy changes. Results show the soil organic matter becomes more reduced from the soil organic surface to mineral soils. The top layer is characterized by high carbon content, organic materials with low energy content per Cmole, and fast biodegradation rates. Mineral soils are characterized by low carbon content, organic materials with high energy content per Cmole, and slow biodegradation rates. Values obtained for the entropy change were sensitive to these differences among the different soil layers. These results contribute to unlock the thermodynamics of the soil reactions and to develop the bioenergetics of soil ecosystems. [ABSTRACT FROM AUTHOR]

Leaf water relations, net gas exchange and leaf and root constituent responses to 9 days of drought stress (DS) or soil flooding were studied in 6-month-old seedlings of Carrizo citrange [ Citrus sinensis (L.) Osb. × Poncirus trifoliata L.; Carr] and Cleopatra mandarin ( Citrus resnhi Hort. ex Tanaka; Cleo) growing in containers of native sand in the greenhouse. At the end of the drought period, both species had similar minimum stem water potentials but Cleo had higher leaf relative water content (RWC) and higher leaf osmotic potential at full turgor (Ψπ100) than Carr. Flooding had no effect on RWC but osmotic adjustment (OA) and Ψπ100 were higher in Cleo than in Carr. Net CO2 assimilation rate (ACO2) in leaves was decreased more by drought than by flooding in both species but especially in Carr. Leaf water-use efficiency (ACO2/transpiration) was lower in Carr and was decreased more by DS and flooding stress than in Cleo. Higher values of intercellular CO2 concentration (Ci) in stressed plants than in control plants indicated that non-stomatal factors including chlorophyll degradation and chlorophyll fluorescence [maximum quantum efficiency of PSII (Fv/Fm, where Fm is the maximum fluorescence and F0, minimum fluorescence in dark-adapted leaves)] were more important limitations on ACO2 than stomatal conductance. In both genotypes, leaf proline was increased by drought but not by flooding, whereas both stresses increased proline in roots. Soluble sugars in leaves were increased by DS, and flooding decreased leaf sugars in Cleo. In general, DS tended to increase the concentrations of Ca, K, Mg, Na and Cl in both leaves and roots, whereas flooding tended to decrease these ions with the exception of leaf Ca in Cleo. Based on water relations and net gas exchange, Cleo was more tolerant to short-term DS and flooding stress than Carr. [ABSTRACT FROM AUTHOR]