Soil organic matter turnover as a function of the soil clay content: consequences for model applications

Based on a literature review including 201 surface soils from wet, mild, mid-latitude climates and 290 soils from the Lower Saxony soil monitoring programme (Germany), we investigated the relationship between soil clay content and soil organic matter turnover. The relationship was then used to evaluate the clay modifier for microbial decomposition in the organic matter module of the soil-plant-atmosphere model DAISY. A positive relationship was found between soil clay content and soil microbial biomass (SMB) C. Furthermore, a negative relationship was found between soil clay content and metabolic quotient (qCO₂) as an indicator of specific microbial activity. Both findings support the hypothesis of a clay dependent capacity of soils to protect microbial biomass. Under the differing conditions of practical agriculture and forestry, no or only very weak relationships were found between soil clay content and non-living soil organic matter C (humus C). It is concluded that the stabilising effect of clay is much stronger for SMB than for humus. This is in contrast to the DAISY clay modifier assuming the same negative relationship between soil clay content, on the one hand, and turnover of SMB and turnover of soil humus on the other. There is a positive relationship between SMB and microbial decomposition activity under steady-state conditions (microbial growth≈microbial death). The original concept of a biomass-independent simulation of organic matter turnover in the DAISY model must therefore be rejected. In addition to the original modifiers of organic matter turnover, a modifier based on the pool size of decomposing organisms is suggested. Priming effects can be simulated by applying this modifier. When using this approach, the original modifiers are related to specific microbial activity. The DAISY clay modifier is a useful approximation of the relationship between the metabolic quotient (qCO₂) as an indicator of specific microbial activity and soil clay content.     

The turnover of organic carbon in subsoils. Part 2. Modelling carbon turnover

A new model, RothPC‐1, is described for the turnover of organic C in the top metre of soil. RothPC‐1 is a version of RothC‐26.3, an earlier model for the turnover of C in topsoils. In RothPC‐1 two extra parameters are used to model turnover in the top metre of soil: one, p, which moves organic C down the profile by an advective process, and the other, s, which slows decomposition with depth. RothPC‐1 is parameterized and tested using measurements (described in Part 1, this issue) of total organic C and radiocarbon on soil profiles from the Rothamsted long‐term field experiments, collected over a period of more than 100 years. RothPC‐1 gives fits to measurements of organic C and radiocarbon in the 0–23, 23–46, 46–69 and 69–92 cm layers of soil that are almost all within (or close to) measurement error in two areas of regenerating woodland (Geescroft and Broadbalk Wildernesses) and an area of cultivated land from the Broadbalk Continuous Wheat Experiment. The fits to old grassland (the Park Grass Experiment) are less close. Two other sites that provide the requisite pre‐ and post‐bomb data are also fitted; a prairie Chernozem from Russia and an annual grassland from California. Roth‐PC‐1 gives a close fit to measurements of organic C and radiocarbon down the Chernozem profile, provided that allowance is made for soil age; with the annual grassland the fit is acceptable in the upper part of the profile, but not in the clay‐rich Bt horizon below. Calculations suggest that treating the top metre of soil as a homogeneous unit will greatly overestimate the effects of global warming in accelerating the decomposition of soil C and hence on the enhanced release of CO₂ from soil organic matter; more realistic estimates will be obtained from multi‐layer models such as RothPC‐1.     

Stabilization of carbon from maize in a sandy soil in a long-term experiment

The sequestration of carbon (C) in soil is not completely understood, and quantitative information about the amounts of organic carbon in the various fractions and their rates of turnover could improve understanding. We aimed (i) to quantify the amounts of C derived from maize at various depths in the soil in a long‐term field experiment with and without fertilization using ¹³C/¹²C analysis, (ii) to model changes in the organic C, and (iii) to compare measured and modelled pools of C. The organic C derived from the maize was measured in soil samples collected to a depth of 65 cm from four plots, two of which had been under continuous maize and two under continuous rye during long‐term field experiments with NPK and without fertilization. The fractionation procedures included particle‐size fractionation and extractions in water and in pyrophosphate solution. We used the Rothamsted Carbon Model to model the dynamics of the carbon from ¹³C data. The amounts of C derived from maize in the Ap horizon after 39 years of continuous maize cropping were 9.5% of the total organic C (where unfertilized) and 14.0% where NPK had been applied. Fertilization did not affect the residence time of carbon in the soil. The amounts of C derived from maize in water extracts were 21% of the total organic C (where unfertilized) and 22% where NPK had been applied. The extracts that were soluble in pyrophosphate and insoluble in acid were depleted in C from maize (the amounts were 5% and 7% of the total organic C, respectively). The results of the ¹³C natural abundance technique were used to model the dynamics of the organic C. Both the total organic C and the C derived from maize in the particle‐size fraction 0–63 μm agreed well with the total and maize‐derived sums of the model pools ‘inert organic matter’, ‘humified organic matter’ and ‘microbial biomass’. The model suggested that 64% (unfertilized) or 53% (NPK) of the organic C in the Ap horizon were inert. Only one of three published equations to determine the size of the inert pool agreed well with these model results.     

Labile,recalcitrant, microbial carbon and nitrogen and the microbial community composition at two Abies faxoniana forest elevations under elevated temperatures

We investigated the interactions of altitude and artificial warming on the soil microbial community structure in a subalpine Abies faxoniana forest in southwestern China after four years of warming. Open top chambers (OTCs) at two elevations (3000 m and 3500 m) were established, and their soil microbial characteristics, organic carbon (C) and nitrogen (N) were measured. The microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis. A two-step sulfuric acid hydrolysis was used to quantify the labile and recalcitrant C fractions in the soil organic matter. The results showed that bacterial PLFAs and gram-negative bacterial PLFAs increased and the fungal PLFAs and the fungi/bacteria ratio decreased with warming at the high altitude. By contrast, the warming effects on those parameters at low altitude were small. The higher proportion of labile easily decomposable soil C may explain the different responses of the microbial community composition at the two altitudes. An RDA analysis confirmed that the variations in the soil community structure were significantly associated with soil organic matter properties such as the sizes of the soil labile N pool (LP-N), the recalcitrant N pool (RP-N), and the labile C pool as well as dissolved organic C (DOC) and dissolved organic N concentrations (DON). Our results also showed that labile C and N pools increased with the altitude, but the microbial biomass C as measured with chloroform fumigation techniques decreased. Warming increased only the recalcitrant C pools at the high altitude. Given the longer mean residence time for recalcitrant C and the much greater size of this soil organic carbon pool, the results indicated that a rise in temperature in our case increased soil C pools at higher altitudes, at least during the early stages of experimental soil warming. Warming could also cause changes in the composition of the microbial community and enzyme activities, consequently leading to functional changes in soil ecosystem processes at the high altitude.     

C and N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is ¹³C, and especially ¹⁵N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was ¹⁵N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and ¹³C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also ¹³C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but ¹³C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.     

Movement and turnover of soil organic matter as indicated by carbon isotope measurements

The distribution in a New Zealand pasture soil of total organic carbon, of ¹³C/¹²C and ¹⁴C/¹²C ratios, up to a depth of l m and over a period of 15 yr, has been analysed to provide models for radiocarbon enrichment and organic matter turnover. For the enrichment model, the parameters of C input, rates of decomposition and diffusion down the soil profile, and turnover rates have been determined. Input rates are compatible with herbage productivity measurements; the turnover period is estimated as 63 yr, and the downward diffusivity of C is estimated at 13cm² yr^?1. The steady state model indicates that a small fraction of the soil organic C, about 16%, is relatively very old and uniformly distributed throughout the soil profile. Most of the remaining fraction of modern C, other than “bomb” C, is less than 100 yr old and decreases exponentially with increasing depth. The models provide a rational integration of empirical measurements of C input, rates of organic matter decomposition, diffusivity and turnover, enabling an unknown to be calculated from measured parameters. They also provide a rational and precise method for determining the age of soil organic matter from radiocarbon measurements and have been used to examine alternative hypotheses for the distribution of UC with depth.     

Modelling refractory soil organic matter

Most models for the turnover of soil organic matter (SOM) include a compartment that is either considered inert, or has a very slow turnover time (refractory SOM; RSOM). The RSOM content of soils varies markedly between sites, and knowledge of its size and variability are essential for determining whether soils behave as sources or sinks of atmospheric CO₂. It has also been suggested that the accurate specification of RSOM pools is essential to modelling studies, and that uncertainty in estimates of the size of RSOM pool could be a major source of error in modelling soil organic C. In this paper, current SOM models are reviewed, and approaches to modelling RSOM and its significance are discussed. Simulations of SOM turnover for the Rothamsted Broadbalk winter wheat experiment using the Rothamsted C model and CENTURY are presented as examples. Received: 13 July 1999     

Soil organic matter, microbial biomass and enzyme activities in a tropical agroforestry system

 The effects of growing trees in combination with field crops on soil organic matter, microbial biomass C, basal respiration and dehydrogenase and alkaline phosphatase activities were studied in soils under a 12-year-old Dalbergia sissoo (a N₂-fixing tree) plantation intercropped with a wheat (Triticum aestivum) – cowpea (Vigna sinensis) cropping sequence. The inputs of organic matter through D. sissoo leaf litter increased and crop roots decreased with the increase in tree density. Higher organic C and total N, microbial biomass C, basal soil respiration and activities of dehydrogenase and alkaline phosphatase were observed in treatments with tree-crop combination than in the treatment without trees. Soil organic matter, microbial biomass C and soil enzyme activities increased with the decrease in the spacing of the D. sissoo plantation. The results indicate that adoption of the agroforestry practices led to an improved organic matter status of the soil, which is also reflected in the increased nutrient pool and microbial activities necessary for long-term productivity of the soil. However, tree spacing should be properly maintained to minimize the effects of shading on the intercrops. Received: 21 February 1997     

Micro-scale modelling of carbon turnover driven by microbial succession at a biogeochemical interface

The detritusphere is a very thin but microbiological highly active zone in soil. To trace the fate of litter carbon in the detritusphere we developed a new 1D dynamic mechanistic model. In a microcosm experiment soil cores were incubated with ¹³C labelled rye residues (δ¹³C=299‰), which were placed on the surface. Microcosms were sampled after 3, 7, 14, 28, 56 and 84 days and soil cores were separated into layers of increasing distance to the litter. Gradients in soil organic carbon (TOC), dissolved organic carbon (DOC), microbial biomass and activity were detected over a distance of 3 mm from the litter layer. The newly developed 1D model simulates both the total carbon and the ¹³C carbon pools and fluxes, so that it was possible to include the ¹³C data in model optimisation. The special feature of the model is that it operates with two decomposer populations; the first one is assumed to be dominated by bacteria (initial-stage decomposer) and second one by fungi (late-stage decomposer). Moreover, in the model the DOC pool is divided into two sub pools. Each DOC pool is consumed by one of the decomposer populations. After parameter optimisation the model was well suited to simulate the experimental data. The model explained 92% of the observed variance. The model output provides a comprehensive insight into the carbon cycling within the detritusphere. The simulation results showed among others that after 84 days about 10% of total litter C was transferred to the soil organic matter (SOM) pool. Only 3% was located in the microbial biomass. From the evolved CO₂ 71% was litter-derived and 29% was soil-derived. From the litter-derived CO₂, 69% was directly formed in the litter layer. The remaining 31% was transported to soil before mineralisation. Our study shows that a combination of experimental work and mathematical modelling is a powerful approach to provide a comprehensive insight into the small-scale carbon turnover in soil.     

Soil Quality Indicators as Affected by Shallow Tillage in a Vineyard Grown in a Semiarid Mediterranean Environment

Within the Mediterranean basin, soil tillage enhances the mineralisation of soil organic matter. We assessed the short‐term impact of shallow tillage [field cultivator (FC), rotary tiller (RT) and spading machine (SM)] on some soil quality indicators [bulk density, water‐stable aggregates, total and labile organic C pools (microbial biomass and extractable organic C), soil respiration and related eco‐physiological indexes] in a Sicilian vineyard. Also no tillage was included. We hypothesized that (i) RT and FC worsened soil quality indicators more than SM, and (ii) within the same tillage system, labile C pools, soil respiration and eco‐physiological indexes will respond more efficiently than chemical and physical soil properties since the tillage starts. The experiment started at March 2009, and each tillage type was applied three times per year (March or April, May and June), with soil tilled up to 15‐cm depth. Soil was sampled (0–15 and 15–30‐cm depth) in March 2009, April 2010, May 2012 and June 2014. SM was very effective in preserving soil organic matter pool and in improving any monitored soil quality indicator, similarly to no tillage. By contrast, RT was the most deleterious machine as it worsened most investigated indicators. Such deleterious effects were due to drastic disruption of soil aggregates and consequent exposition of protected soil organic matter to further microbial mineralization. Labile organic C pools and microbial quotients were the most responsive soil parameters for assessing the impact of shallow tillage on soil quality, even in the short term (<5 years). Copyright © 2016 John Wiley & Sons, Ltd.     

Soil organic matter,microbial properties,and aggregate stability under annual and perennial pastures

The use of annually sown pastures to provide winter forage is common in dairy farming in many regions of the world. Loss of organic matter and soil structural stability due to annual tillage under this management may be contributing to soil degradation. The comparative effects of annual ryegrass pastures (conventionally tilled and resown each year), permanent kikuyu pastures and undisturbed native vegetation on soil organic matter content, microbial size and activity, and aggregate stability were investigated on commercial dairy farms in the Tsitsikamma region of the Eastern Cape, South Africa. In comparison with soils under sparse, native grassy vegetation, those under both annual ryegrass and permanent kikuyu pasture had higher soil organic matter content on the very sandy soils of the eastern end of the region. By contrast, in the higher rainfall, western side, where the native vegetation was coastal forest, there was a loss of organic matter under both types of pasture. Nonetheless, soil organic C, K₂SO₄-extractable C, microbial biomass C, basal respiration, arginine ammonification and fluorescein diacetate hydrolysis rates and aggregate stability were less under annual than permanent pastures at all the sites. These results reflect the degrading effect of annual tillage on soil organic matter and the positive effect of grazed permanent pasture on soil microbial activity and aggregation. Soil organic C, microbial biomass C, K₂SO₄-extractable C, basal respiration and aggregate stability were significantly correlated with each other. The metabolic quotient and percentage of organic C present as microbial biomass C were generally poorly correlated with other measured properties but negatively correlated with one another. It was concluded that annual pasture involving conventional tillage results in a substantial loss of soil organic matter, soil microbial activity and soil physical condition under dairy pastures and that a system that avoids tillage needs to be developed.     

Feedback on plant productivity can be constrained by SOM in N-limited grasslands

In many terrestrial ecosystems plant productivity is limited by the availability of mineral nitrogen, which is produced by soil microbial transformations of organic N in soil organic matter (SOM-N). Mineral N availability results from two opposing processes, 1) gross mineral N production (gross ammonification/gross nitrification) and 2) microbial N immobilization. These processes can be influenced by the availability of plant-derived C (PDC) inputs to the microbes, SOM-N pool size, and the size of the microbial community (microbial biomass). We considered how changes in PDC inputs and SOM-N pool size together may alter microbial biomass, mineral N availability, and feedbacks on plant productivity. In areas dominated by one of six tallgrass prairie species along a natural gradient of PDC inputs and SOM-N pool size, we conducted a field survey of microbial biomass and gross ammonification. We also performed greenhouse manipulations of SOM-N pool size and PDC inputs on two species in our study area (Poa pratensis and Schizachyrium scoparium). Structural equation modeling of the field data showed that gross ammonification was both positively and directly related to microbial biomass and SOM-N pool size. Gross ammonification was positively and indirectly related to SOM-N pool size and belowground PDC inputs, via microbial biomass. In the short-term greenhouse study, PDC inputs and SOM-N pool size positively affected gross mineral N production, although only at high SOM-N pool size. If the patterns in the greenhouse can be applied to field conditions, this suggests that SOM-N pool size may constrain plant driven feedbacks on plant productivity by limiting gross mineral N production.     

Labile pools of organic matter and microbial biomass in the surface soils under shifting cultivation in northern Thailand

In order to analyze the N mineralization process under shifting cultivation in northern Thailand, labile pools of soil organic matter were studied, which were considered to be the factors contributing to the N mineralization process. Organic C, (organic + NH₄ ⁺)-N, and hexose-C were extracted from fresh soils in the surface 0–5 cm layers with a 0.5 M K₂S₀. solution at 110°C in an autoclave (fraction A) or at room temperature with a reciprocal shaker (fraction B), and analyzed as labile pools of organic matter. In the traditional shifting cultivation system, the content of organic C in fraction A in the fallow fields for 8 to 15 y was 3,710 mg kg^-1 while that in the fallow fields for 1 y and 3 to 5 y was 2,640 and 2,600 mg kg^-1, respectively. A high correlation was observed between the contents of the labile pool in fraction A and total soil organic matter. The ratio of the pool in fraction A to total soil organic matter apparently remained constant through the input-output balance in the pool. The content of the labile pool in fraction B was the highest among the fields cultivated for 1 y after the slash and burn practice and it decreased in the course of the fallow period. The content of organic C was 548 mg kg^-1 in the fields cultivated for 1 y and 235 mg kg^-1 in the fallow fields for 8-15 y, respectively. There was a reverse relation between the contents of the pool in fraction B and microbial biomass. Therefore, the origin of the pool in fraction B was attributed to the microbial debris associated mainly with a decrease in the soil moisture content in the dry season. On the other hand, in the relatively intensive cultivation system, there was no significant difference in the contents of the labile pools both in fractions A and B among the land use stages, suggesting that the preservation mechanism of these pools, which was observed in the traditional cultivation system, did not operate well in the intensive system. In alternative farming systems in future, it will be essential to apply organic materials to soils to supply organic matter and to maintain the microbial biomass.     

Carbon turnover in the rhizosphere under continuous plant labeling with <Superscript>13</Superscript>CO<Subscript>2</Subscript>: Partitioning of root,microbial, and rhizomicrobial respiration

The input of labeled C into the pool of soil organic matter, the CO₂ fluxes from the soil, and the contribution of root and microbial respiration to the CO₂ emission were studied in a greenhouse experiment with continuous labeling of oat plants with ¹³CO₂ using the method of the natural ¹³C abundance in the air. The carbon of the microbial biomass composed 56 and 39% of the total amounts of ¹³C photoassimilates in the rhizosphere and in the bulk soil, respectively. The contribution of root respiration to the CO₂ emission from the soil reached 61–92%, including 4–23% of the rhizomicrobial respiration. The contribution of the microbial respiration to the total CO₂ emission from the soil varied from 8 to 39%. The soil organic matter served as the major carbon-containing substrate for microorganisms in the bulk soil and in the rhizosphere: 81–91% of the total amount of carbon involved in the microbial metabolism was derived from the soil organic matter.     

Modelling nitrogen mineralization from manures: representing quality aspects by varying C:N ratio of sub-pools

The mineralization/immobilization of nitrogen when organic sources are added to soil is represented in many simulation models as the outcome of decomposition of the added material and synthesis of soil organic matter. These models are able to capture the pattern of N release that is attributable to the N concentration of plant materials, or more generally the C:N ratio of the organic input. However, the models are unable to simulate the more complex pattern of N release that has been observed for some animal manures, notably materials that exhibit initial immobilization of N even when the C:N of the material suggests it should mineralize N. The APSIM SoilN module was modified so that the three pools that constitute added organic matter could be specified in terms of both the fraction of carbon in each pool and also their C:N ratios (previously it has been assumed that all pools have the same C:N ratio). It is shown that the revised model is better able to simulate the general patterns on N mineralized that has been reported for various organic sources. By associating the model parameters with measured properties (the pool that decomposes most rapidly equates with water-soluble C and N; the pool that decomposes slowest equates with lignin-C) the model performed better than the unmodified model in simulating the N mineralization from a range of feeds and faecal materials measured in an incubation experiment.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.     

Computing the mean residence time of soil carbon fractions using stable isotopes: impacts of the model framework

Soils contain the largest carbon (C) reservoir on Earth, but the mean residence time (MRT) of soil C is often poorly estimated, despite its importance for assessing the efficiency with which soils may serve as a sink for atmospheric C. The objective of this study was to evaluate how the structure of simple models of soil C dynamics affects the MRT determined from isotope‐mixing experiments, using data from field studies with either artificial labelling (FACE) or C3/C4 vegetation change. We first highlighted theoretically how non‐steady‐state conditions and the model structure (one single, two successive, or two parallel C pools) can have an impact on the MRT assessment. We then tested these different model structures against published data on the dynamics of different soil organic matter separates and their constituents. Our findings indicated that many of the reviewed studies assumed wrongly that the system was at steady state or could be described by a single‐pool approach. To select the correct model, exact knowledge of C input rates and several data points are needed from the beginning of the experiment. For steady‐state conditions an apparent temporal change of MRT computed from a single‐pool model is thus a clear indicator that a two‐pool approach must be chosen. The errors made by the wrong choice of model varied with the length of the experiment and usually resulted in an over‐estimate of MRT by a factor of 1.15 for some data published on physical size separates, but by a factor of up to 11 for individual microbial biomarkers such as muramic acid.     

Calculating the annual input of organic matter to soil from measurements of total organic carbon and radiocarbon

Measurements of total organic C, δ¹³C and δ¹⁴C are given for topsoils taken from six experimental sites in southern England. At each site, some of the samples were collected before and some after the thermonuclear tests of the early 1960s, so that pre- and post-bomb samples could be compared for radiocarbon content. The current Rothamsted model for the turnover of organic C in soil gave an acceptable fit to the data from five of the sites, apart from one aberrant radiocarbon measurement. The annual input of C to the topsoil was calculated for the five sites from these fits; the values obtained were: 0.15 t C ha^?1 a^?l for a site on silty clay loam, kept bare by hand weeding since 1870; 0.2 for unmanured spring barley growing on a sandy loam; 2.95 for a fertilized all-arable rotation on a loamy sand; 1.9 for the same fertilized all-arable rotation on a silty loam and 2.5 from this rotation on a calcareous silty loam. The corresponding values for Net Primary Production at the five sites were 0.15, 0.76, 5.16, 5.71 and 5.46 t C ha^?1 a^?l. In fitting the model to the radiocarbon data it was necessary to postulate that all these sites contained substantial quantities of biologically-inert organic matter, ranging from 2.2 to 10.0 t C ha^?1.     

Short-term effects of tillage on mineralization of nitrogen and carbon in soil

Tillage is known to decrease soil organic nitrogen (N) and carbon (C) pools with negative consequences for soil quality. This decrease is thought partly to be caused by exposure of protected organic matter to microbial degradation by the disturbance of soil structure. Little is known, however, about the short-term effects of tillage on mineralization of N and C, and microbial activity. We studied the short-term effects of two types of tillage (conventional plough- and a non-inverting-tillage) on mineralization and microbial N and C pools in a sandy loam under organic plough-tillage management. The release of active and protected (inactive) N by tillage was further studied in the laboratory by use of ¹⁵N labelling of the active pool of soil N followed by simulation of tillage by sieving through a 2 mm sieve. Results showed that the two types of tillage as well as the simulation of tillage had very few effects on mineralization and microbial pools. The simulation of tillage caused, however, a small release of N from a pool which was otherwise protected against microbial degradation. The use of soil crushing for disruption of larger macroaggregates (>425 μm) and chloroform fumigation for perturbation of the microbial biomass increased the release from both active and protected N pools. The relative contribution from the protected N pool was, however, similar in the three treatments (22-27%), thus the pools subjected to mineralization were characterised by similar degree of protection. On the basis of isotopic composition the pools of N mineralised were indistinguishable. This suggests that the released N originated from the same pool, that is the soil microbial biomass. The study points to the microbial pool as the main source of labile N which may be released by tillage, and thus to its importance for sustained soil fertility in agricultural systems.     

Soil carbon dynamics in saline and sodic soils: a review

Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil's health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO₂ fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils.