Soil Rewetting Research Articles

Soil drying and rewetting and the turnover of 14C-labelled plant residues: First order decay rates of biomass and non-biomass 14C

The effects of drying and rewetting on soil organic C derived from added plant material were determined. Three soils, one silty-loam and two loamy sands, were incubated with 14C-labelled plant material for 27 days. Each soil was then subjected (1) to drying at 40°C for 3 days, remoistening and incubation at 25°C for 10 days, and (2) to storage at 4 °C for 3 days and incubation. The silty-loam soil was also treated after 7 days of incubation with 14C-labelled plant material. Residual 14C concentrations were determined at the beginning and at the end of the 10 day incubation. During drying or storage and subsequent incubation of soils, biomass C and 14C were measured, and also the amounts of CO 2 and 14CO 2 released during the 10 day incubation. Residual 14C concentrations at the end of the incubation were not significantly affected by soil desiccation and remoistening, but the percentages of residual 14C due to biomass 14C were greatly reduced in dried, rewetted and incubated soils. The effect was largest for the soil which had been incubated for the shortest time with 14C-labelled plant material. Average first-order gross decay rates were calculated for biomass 14C and non-biomass 14C, and for different efficiencies of substrate utilization (viz. 20, 40 and 60%). Two time intervals were chosen: with and without inclusion of the drying period. Drying and rewetting of soils enhanced first-order gross decay rates of the two carbon pools. The relative increases were larger for decay rates of biomass 14C than for those of non-biomass 14C. When decay rates were averaged over a time interval that did not include the drying period, observed effects of soil desiccation and remoistening were less pronounced, but still apparent. This suggests that a previous drying-rewetting cycle has an appreciable influence on decomposition processes during later incubation of soils. It was concluded that soil drying and rewetting promoted the turnover of C derived from added plant material, and that this increase in C cycling was mainly due to enhanced turnover of microbial products. This may finally result in a change of quality of the organic C pool coming from added plant residues.

Microbial biomass responses to soil drying and rewetting: The fate of fast- and slow-growing microorganisms in soils from different climates

Two soils from a temperate humid climatic region and one soil from a tropical subhumid area were incubated with 14C-labelled plant material for 27 days. Samples from each soil were then subjected to (1) drying at 40°C for 3 days, remoistening and incubation at 25°C for 10 days, or (2) storage at 4°C for 3 days and incubation. One of the soils (a silty loam from a temperate climate area) was also treated after 7 days of incubation with 14C-labelled plant material. During drying or storage and subsequent incubation of soils, biomass 12C and 14C were determined using a fumigation-extraction method. The amounts of 12CO 2 and 14CO 2 evolved and inorganic N released during 10 days' incubation were monitored. When the silty-loam soil was dried after 7 days of incubation with added plant material, biomass 14C decreased by 63% when compared to stored control soils. A smaller relative decline in biomass 14C (viz. 40%) was observed when this soil was subjected to drying 27 days after addition of plant material, suggesting that actively-growing microorganisms are more susceptible to desiccation than populations with slower growth rates. Relative decreases of biomass 12C by drying were similar for the three soils of various texture classes and carbon contents, and ranged from 14 to 30%. It was concluded that desiccation resistance was mainly determined by intrinsic properties of soil biota, presumably cell wall characteristics. For both soils from temperate climatic regions, biomass 14C declined relatively more after soil drying than biomass 12C. In contrast, similar relative decreases in biomass 14C and biomass 12C caused by drying were observed for the tropical soil. Adaptation of the microbial communities to frequent and severe desiccation of the latter soil in its natural environment, was suggested to explain those observations. Drying and rewetting of soils enhanced carbon and nitrogen mineralization during subsequent moist incubation. It was inferred that the sources of mineralization flushes were partly biomass killed by drying, and partly non-living (labelled and unlabelled) organic residues. The sizes of flushes appeared to be influenced by soil properties such as carbon content and texture.

MAIN ROOT‐LATERAL ROOT JUNCTIONS OF TWO DESERT SUCCULENTS: CHANGES IN AXIAL AND RADIAL COMPONENTS OF HYDRAULIC CONDUCTIVITY DURING DRYING

The influence of junctions between main roots and lateral roots on water flow was investigated for the desert succulents Agave deserti and Ferocactus acanthodes during 21 d of drying in soil. Under wet conditions, the junctions did not restrict xylem water flow from lateral roots to main roots, consistent with predictions of axial conductance based on vessel diameters. Embolism caused by drying reduced such axial conductance more at the junctions than in adjoining root regions. Connective tracheary elements at the junctions were abundantly pitted and had large areas of unlignified primary wall, apparently making them more susceptible to embolism than vessels or tracheids elsewhere in the roots. Unlike the decrease in axial conductance, the overall hydraulic conductivity of the junction increased during drying because of an increase in the conductivity of the radial pathway. Despite such increases, main roots may not lose substantial amounts of water to a dry soil during drought, initially because embolism at the junctions can limit xylem flow and later because soil hydraulic conductivity decreases. Moreover, the increased root hydraulic conductivity and a potentially rapid recovery from embolism by connective tracheary elements may favor water uptake near the junctions upon soil rewetting.

Changes in Hydraulic Conductivity and Anatomy Caused by Drying and Rewetting Roots of Agave deserti (Agavaceae)

Concurrent determinations of changes in hydraulic conductivity and tissue anatomy were made for roots of Agave deserti excised during drying and following rewetting in soil. At 30 d of drought, hydraulic conductivity had declined less than twofold for older nodal roots, tenfold for young nodal roots, and more than 20-fold for lateral roots (“rain roots” occurring as branches on the nodal roots). These decreases were consistent with increases in cortical lacunae caused by cell shrinkage and collapse. Similarly, reduction of lacunae in response to rewetting after 7 d of drought corresponded to levels of recovery in hydraulic conductivity, with young nodal roots showing full recovery, lateral roots returning to only 21 % of initial conductivity, and older nodal roots changing only slightly. Increases in suberization in the exodermis, endodermis, and cortex adjacent to the endodermis in response to drying coincided with decreases in hydraulic conductivity. Measurements of axial hydraulic conductance per unit length before and after pressurization indicated that embolism caused reductions in axial conductance of 98% for lateral roots, 35% for young nodal roots, and 20% for older nodal roots at 7 d of drought. Embolism, cortical lacunae, and increasing suberization caused hydraulic conductivity to decline during drought in the three root types, thereby helping limit water loss to dry soil; the recovery in hydraulic conductivity for young nodal roots after rewetting would allow them to take up water readily once soil moisture is replenished.

CHANGES IN HYDRAULIC CONDUCTIVITY AND ANATOMY CAUSED BY DRYING AND REWETTING ROOTS OF AGAVE DESERTI (AGAVACEAE)

Concurrent determinations of changes in hydraulic conductivity and tissue anatomy were made for roots of Agave deserti excised during drying and following rewetting in soil. At 30 d of drought, hydraulic conductivity had declined less than twofold for older nodal roots, tenfold for young nodal roots, and more than 20‐fold for lateral roots (“rain roots” occurring as branches on the nodal roots). These decreases were consistent with increases in cortical lacunae caused by cell shrinkage and collapse. Similarly, reduction of lacunae in response to rewetting after 7 d of drought corresponded to levels of recovery in hydraulic conductivity, with young nodal roots showing full recovery, lateral roots returning to only 21 % of initial conductivity, and older nodal roots changing only slightly. Increases in suberization in the exodermis, endodermis, and cortex adjacent to the endodermis in response to drying coincided with decreases in hydraulic conductivity. Measurements of axial hydraulic conductance per unit length before and after pressurization indicated that embolism caused reductions in axial conductance of 98% for lateral roots, 35% for young nodal roots, and 20% for older nodal roots at 7 d of drought. Embolism, cortical lacunae, and increasing suberization caused hydraulic conductivity to decline during drought in the three root types, thereby helping limit water loss to dry soil; the recovery in hydraulic conductivity for young nodal roots after rewetting would allow them to take up water readily once soil moisture is replenished.

Remineralization of nitrogen immobilized by soil microorganisms and effect of drying and rewetting of soils

Abstract Microbial biomass is considered to act as one of the major sources of inorganic N pool in soil. Many investigators indicated that significant portions of the flux of inorganic N in soil originated from the mineralization of dead microbial biomass (Ladd et al. 1977; Powlson 1980; Marumoto et al. 1982a, b). However, their findings were based in experiments in which the nitrogen flux from the microbial biomass increased by the addition of dead microbial biomass and/or by the pre-treatment of soils under artificial conditions such as oven-drying, grinding, fumigation. Thus, to determine the nitrogen flux from the microbial biomass in natural soil ecosystems and to analyze the quantitative role of microbial biomass in the nitrogen cycle, it is necessary to determine the flux of inorganic N from microbial biomass under natural field conditions.

Carbon and nitrogen mineralization from two soils of contrasting texture and microaggregate stability: Influence of sequential fumigation, drying and storage

Two top soils (an Alfisol and a Vertisol) of contrasting cation exchange capacity, micro-porosityand microaggregate stability were sampled from the same climatic region and were incubated for 4 weeks with 14C-labelled plant material. Each soil was then subjected to combinations of two of the following treatments: (1) drying (40°C), remoistening and incubation for 10 days at 25°C, (2) fumigation with chloroform vapour and incubationand (3) storage (4°C) and incubation. The amounts of CO 2 and 14CO 2 evolved and inorganic N released during each incubation period were measured. Also, during drying and after remoistening of soils, concentrations of biomass C and 14C were monitored using a fumigation extraction technique. Biomass C and 14C decreased by 26–30% during desiccation and increased to 77–84% of untreated control soils during incubation after rewetting. The relative decline in biomass during soil drying was of similar magnitude for both soils and for 14C-labelled and total biomass C, indicating that factors other than soil properties had determined the extent of decline. The evolution of microbial populations exposed frequently to high temperatures and to extreme desiccation in the natural field environment has been proposed to explain the similar responses of biomass C to the imposed drying regime. Previous fumigation-incubation of soils to destroy the majority of the microbial biomass had little effect on the sizes of the C mineralization flushes obtained when the soils were subsequently dried, rewetted and incubated. The specific activities of the CO 2-C flushes after drying were much lower than those from fumigated or stored control soils respectively. This was especially evident for CO 2-C flushes from the well-aggregated Vertisol. From the magnitude of the flushes of CO 2 and 4CO 2 after the various combinations of treatmentsand from their specific activities, we have deduced that microbial cells killed by soil desiccation had made only a minor contribution to the C and N mineralization flushes after soil rewetting and incubation. The larger contribution had come from other sources, the relative importance of which appears to be influenced by soil characteristics, possibly cation exchange capacity and microporosity.

Variations in chemical composition of the soil solution over a four-year period at an upland site in southwest Scotland

Soil solution chemical composition was monitored over a period of almost four years in a peaty soil in the Loch Dee catchment, southwest Scotland. Solutions were acidic with mean pH of 4.2 in the surface 20 cm and of 4.7 at 60 cm. The increase in pH was associated with increased Ca concentrations but no increase in Al. Annual cycles in solution concentrations were identified, with winter peaks in Cl, Na and Mg, and a late summer peak in dissolved organic carbon (DOC) and Fe. Variations in DOC were related to temperature and rewetting of dry soil. Maximum DOC and Fe followed the drought summer of 1984 with concentrations of 60 and 13 mg 1 −1 respectively at one sampler. Variations in Al concentrations were partly related to DOC and partly to pH. Inorganic monomeric forms of Al were predominant. DOC also contributed significantly to the overall ionic balance.

Influences of root distribution and growth on predicted water uptake and interspecific competition.

Root distribution and growth measured in the field were incorporated into a water uptake model for the CAM succulent Agave deserti and its nurse plant Hilaria rigida, a common desert bunchgrass. Agave deserti responds to the infrequent rainfalls of the Sonoran Desert by extending its existing established roots and by producing new roots. Most of such root growth was completed within one month after soil rewetting, total root length of A. deserti increasing by 84% for a seedling and by 58% for a mediumsized plant in the summer. Root growth in the winter with its lower soil temperatures was approximately half as much as in the summer. For a 15-year period, predicted annual root growth of A. deserti varied more than 18-fold because of annual variations in rainfall amount and pattern as well as seasonal variation in soil temperature. Predicted annual water uptake varied 47-fold over the same period. The nurse plant, which is crucial for establishment of A. deserti seedlings, reduced seedling water uptake by 38% during an average rainfall year. Lowering the location of the root system of a medium-sized A. deserti by 0.24 m reduced its simulated annual water uptake by about 25%, reflecting the importance of shallow roots for this desert succulent. Lowering the root system of a medium-sized H. rigida by 0.28 m increased the simulated annual water uptake of an associated A. deserti seedling by 17%, further indicating the influence of root overlap on competition for water.

Simultaneous field measurements of biogenic emissions of nitric oxide and nitrous oxide

Seasonal and diurnal emissions of nitric oxide (NO) and nitrous oxide (N2O) from agricultural sites in Virginia and Colorado were simultaneously determined as a function of soil temperature, percent moisture, and exchangeable nitrate, nitrite, and ammonium concentrations. Nitric oxide fluxes at the Virginia site were significantly correlated (P < 0.01) with nitrate concentration, temperature, and percent moisture. At the Colorado site, NO fluxes were both positively and significantly correlated (P < 0.01) with temperature and moisture. Nitrous oxide emissions were only observed when percent moisture approached or exceeded field capacity of the soil. Nitric oxide emissions at the Virginia site were observed throughout the entire year with 76% of the annual flux produced between May and October and 24% between November and April. Wintertime fluxes of NO have not previously been reported in the literature. Annual NO emissions at the Virginia site ranged from 0.53 kg(N) ha−1 yr−1 from unfertilized land to 2.08 kg(N) ha−1 yr−1 from fertilized land. Of the 196.4 kg ha−1 of fertilizer added to the soil site being studied, 0.79% was lost as NO(N), and 1.2% was lost as N2O(N). A series of diurnal studies demonstrated that variations in NO flux throughout the day were correlated with changes in soil temperature. Nitric oxide was emitted over a broad range of soil moisture conditions, provided that percent moisture did not exceed field capacity of the soil. When field capacity was exceeded, NO fluxes declined whereas N2O emissions increased. Rewetting of dry soils at the Colorado site resulted in dramatic increases in emissions of both NO and N2O. Our data suggest that NO is produced primarily by nitrification in aerobic soils whereas N2O is formed by denitrification in an aerobic soils.

The Nitrogen Supplying Capacities, Transformation of Fixed Ammonium and Chemical Nature of Decomposable Organic Nitrogen through the Successively Air-drying or Oven-drying, Rewetting and Incubation of Residual Soils Amended with 15^N-Labelled Rice Straw

The Nitrogen Supplying Capacities, Transformation of Fixed Ammonium and Chemical Nature of Decomposable Organic Nitrogen through the Successively Air-drying or Oven-drying, Rewetting and Incubation of Residual Soils Amended with 15^N-Labelled Rice Straw

A soil water balance model for sloping land

Abstract Simulation models of soil water balance for flat land are not applicable to hill soils principally because on the latter sites significant runoff can occur before the soil reaches field capacity. Part of the data from regular measurements of topsoil (0-75 mm depth) moisture content over a 3-year period on the north aspect of a steepland yellow-brown earth soil was used to construct a simulation model which described changes soil moisture to 150 mm depth during the year. SImIlar data collected on a south aspect of the soil and also on north and south aspects of a yellowbrown loam hill soil were used to evaluate the model. A 4-layer model was developed in which the rate of soil rewetting was empirically limited according to soil moisture content and evapotranspiration rate was primarily soil-controlled. Predicted topsoil moisture levels provided similar, but generally lower values compared to actual levels especially during the summer to late winter period: The greater discrepancy during early spri...

Distribution, salinity, kinetic and thermodynamic characteristics of urease activity in a vertisol profile

The distribution, and the effects of temperature, moisture and organic C substrate on urease activity of a vertisol up to 120 cm depths were studied. The kinetic and thermodynamic characteristics of urease activity in the soil were also studied. Urease activity (measured at the natural soil pH) was found to be higher in soil at 20-60 cm than in the top soil (0-10 cm). It remained relatively constant for up to 900 days, when the soil was kept either field moist at 4�C or air-dried at 25�C. However, partial drying of the soil or rewetting of air-dried soil, especially at elevated temperatures (40�C), and glucose addition, affected substantially but differentially the urease activity of soil obtained from different depths. The distribution in soil urease activity assayed at an optimum pH (using tris-HCl buffer, pH 9.0) became relatively uniform at all depths. From the effect of different buffer pH's on soil urease and from kinetic and thermodynamic characteristics, it is suggested that lower urease activity in the top soil was primarily a consequence of suboptimal pH, resulting in greater configurational or steric hindrance in formation and/or breakdown of urea-urease complex in the soil. Differences in seasonal variation in temperature, moisture, organic substrate and microbial activity may also affect distribution and stability of urease at different depths.

Spatial and temporal variations in concentrations of three ions in solutions extracted from a woodland soil

Concentrations of Cl, Mg and K in soil solutions extracted from a gleyic brown earth under mixed oak woodland were measured at weekly intervals over a 1 year period. Spatial variability of ion concentrations was large, with coefficients of variation of 30–90% in a 0.1 ha plot. Seasonal variability in Mg and K was effectively buffered by the soil exchange complex, but Cl displayed a distinct pattern with a maximum concentration in late autumn. Flushing of Cl during soil rewetting is the most probable explanation.

Spatial and temporal variations in concentrations of three ions in solutions extracted from a woodland soil

Concentrations of Cl, Mg and K in soil solutions extracted from a gleyic brown earth under mixed oak woodland were measured at weekly intervals over a 1 year period. Spatial variability of ion concentrations was large, with coefficients of variation of 30–90% in a 0.1 ha plot. Seasonal variability in Mg and K was effectively buffered by the soil exchange complex, but Cl displayed a distinct pattern with a maximum concentration in late autumn. Flushing of Cl during soil rewetting is the most probable explanation.

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

15NO − 3 was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic- 15N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of 15N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized 15N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period. Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic- 15N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic 15N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic- 15N of the silt and fine clay fractions. Drying and rewetting of soils hastened or magnified changes occurring in the organic- 15N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions. The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.

Disappearance of nitrate under transient conditions in columns of soil

Previously a rapid reduction of NO 3 − was obtained in soil columns with continuous flow, before the establishment of steady state conditions. This initial behavior was not a function of flow rates. In order to determine what influence NO 3 − concentrations might have on transient and steady state rates, of NO 3 − loss, small soil columns (2·5 cm i.diam × 10 cm length) were infiltrated with 0, 100 and 1000 μg ml −1 NO 3 −N and the effluents were analyzed daily for NO 3 − and NO 2 −. Several days after an apparent steady state had been obtained, as determined by the effluent NO 3 − concentration, the influent NO 3 − concentrations were changed to 100 μg ml −1 NO −N in some of the columns. All columns showed transitions to new and identical steady state of NO 3 − loss from flowing solutions. Evidently the initial transient kinetics are primarily determined by some solubilization of organic matter during rewetting of air dry soil and subsequent steady states are dependent on lesser available organic matter.