Comparing local vs. global visible and near-infrared (VisNIR) diffuse reflectance spectroscopy (DRS) calibrations for the prediction of soil clay, organic C and inorganic C

Local, field-scale, VisNIR-DRS soil calibrations generally yield the most accurate predictions but require a substantial number of local calibration samples at every application site. Global to regional calibrations are more economically efficient, but don't provide sufficient accuracy for many applications. In this study, we quantified the value of augmenting a large global spectral library with relatively few local calibration samples for VisNIR-DRS predictions of soil clay content (clay), organic carbon content (SOC), and inorganic carbon content (IC). VisNIR models were constructed with boosted regression trees employing global, local + global, and local spectral data, using local samples from two low-relief, sedimentary bedrock controlled, semiarid grassland sites, and one granitic, montane, subalpine forest site, in Montana, USA. The local + global calibration yielded the most accurate SOC predictions for all three sites [Standard Error of Prediction (SEP) = 3.8, 6.7, and 26.2 g kg^− 1]. This was similarly true for clay (SEP = 95.3 and 102.5 g kg^− 1) and IC (SEP = 5.5 and 6.0 g kg^− 1) predictions at the two semiarid grassland sites. A purely local calibration produced the best validation results for soil clay content at the subalpine forest site (SEP = 49.2 g kg^− 1), which also had the largest number of local calibration samples (N = 210). Using only samples from calcareous soils in the global spectral library combined with local samples produced the best SOC and IC results at the more arid of the two semiarid sites. Global samples alone never achieved more accurate predictions than the best local + global calibrations. For the temperate soils used in this study, the augmentation of a large global spectral library with relatively few local samples generally improved the prediction of soil clay, SOC, and IC relative to global or local samples alone.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K₂SO₄-extractable organic C and 33.3 mM KMnO₄-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO₂ production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation-extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation-incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO₂-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO₂ production rate, relative C mineralization rate (CO₂-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.     

Relationship between extractable Al and organic C in volcanic soils of Chile

In previous studies, Al extracted by acid ammonium acetate (Al_a) or Na-pyrophosphate (Al_p), rather than silt or clay content and climate conditions, was the most important factor that controls organic matter (OM) levels in volcanic soils. Here, the hypothesis was tested that Al_a is a comparable method (as much as CuCl₂) to quantify the proportion of Al bound to OM in allophanic soils. As far as we know, there are no previous antecedents in which selective dissolution method has been compared with this extractant. Secondly, we examine the effects of (a) Al, (b) silt plus clay content (particles size 0-53 µm) and (c) clay mineralogy on the control of organic carbon (OC) level in Chilean volcanic soils. This was achieved by sampling 16 soils series (11 Andisols, one Alfisol and four Ultisols, USDA classification) including 48 soil pedons up to 0.4 m depth. Soils were analyzed for Al_a, Al_p, oxalate (Al_o, Si_o and Fe_o), cold NaOH (Al_n) and un-buffered salts, CuCl₂ (Al_Cu), LaCl₃ (Al_La) and KCl (Al_k). We also measured the Al-humus as soluble C fraction after pyrophosphate extraction and the C associated to the silt plus clay fraction after sonication and gravity decantation. The statistical package (S)MATR was used to examine bivariate linear regressions among soil properties by computing the standardized major axis (SMA). Our results indicate that Al_a had a good correspondence with Al_p (R² = 0.76) in the top soil with Al_a/Al_p ratio of 0.19 and both extractans presented significant and positively relationship with soil OC (R² > 0.62). Acid ammonium acetate was as effective as Al_Cu to determine the Al-OM in allophanic soils. It is cheaper than Al_Cu and Al_p and 0.5 h shaking is required compared to 2 h of Al_Cu and 16 h of Al_p. The efficiency of the extraction was: Al_n ≥ Al_o > Al_p > Al_Cu ≥ Al_a > Al_La > Al_k. We also found that allophane content (estimated by Al/Si ratio) was strongly correlated (R² = 0.82) with the OC in the fine silt plus clay and that Al-humus together with C in the finest particles explained (R² > 0.60) the largest proportion of variation of soil OC across studied soils.     

Regional and local factors affecting diversity, abundance and activity of free-living, N2-fixing bacteria in Australian agricultural soils

N₂-fixation by free-living (diazotrophic) microorganism is a key process affecting ecosystem functioning in soils. Understanding drivers affecting diazotrophic community assemblages and activities may lead to management practices to increase primary production and/or environmental sustainability. We used PCR-DGGE to determine the fundamental relationships between diazotrophic community structure and in a wide range of soils across southern Australia. In addition qPCR, RT-qPCR and N₂-fixation (acetylene reduction) were used to investigate factors influencing gene abundance, expression and processes in similar soils with different agricultural inputs. Across 22 soils, the structural composition of the nifH community was significantly influenced by site (ANOSIM R = 0.876; P = 0.001). The effects of management practices were evident, and often larger than between-soil differences, but were only present at some sites. Differences in nifH communities between sites correlated to particulate organic carbon (POC; measured by mid-infrared spectroscopy) content of the soils (BIO-ENV test; ρ = 0.502; P = 0.001), but not other factors including total soil C. In 3 soils from the Murrumbidgee irrigation region of NSW, intensification of the farming systems was associated with increasing N₂-fixation (P < 0.05), except where rice was cultivated. N₂-fixation correlated either with nifH abundance or gene expression in soils, but not both. Our data shows that soil C is closely linked to diazotrophic ecology. Principally, the amount of C entering the soil system is directly related to the abundance and N₂-fixation activity of free-living bacteria. However, we also show that C in the POC pool has associative links to the genetic diversity of the soil diazotroph community. Given the importance of diversity and abundance of functional organisms in supporting ecosystem processes, we suggest that soil C inputs should be considered for both qualitative and quantitative properties when considering impacts on diazotrophic bacterial ecology.     

Organic carbon accumulation in a 2000-year chronosequence of paddy soil evolution

Considerable amounts of soil organic matter (SOM) are stabilized in paddy soils, and thus a large proportion of the terrestrial carbon is conserved in wetland rice soils. Nonetheless, the mechanisms for stabilization of organic carbon (OC) in paddy soils are largely unknown. Based on a chronosequence derived from marine sediments, the objectives of this study are to investigate the accumulation of OC and the concurrent loss of inorganic carbon (IC) and to identify the role of the soil fractions for the stabilization of OC with increasing duration of paddy soil management. A chronosequence of six age groups of paddy soil formation was chosen in the Zhejiang Province (PR China), ranging from 50 to 2000 years (yrs) of paddy management. Soil samples obtained from horizontal sampling of three soil profiles within each age group were analyzed for bulk density (BD), OC as well as IC concentrations, OC stocks of bulk soil and the OC contributions to the bulk soil of the particle size fractions. Paddy soils are characterized by relatively low bulk densities in the puddled topsoil horizons (1.0 and 1.2 g cm^− 3) and high values in the plow pan (1.6 g cm^− 3). Our results demonstrate a substantial loss of carbonates during soil formation, as the upper 20 cm were free of carbonates in 100-year-old paddy soils, but carbonate removal from the entire soil profile required almost 700 yrs of rice cultivation. We observed an increase of topsoil OC stocks from 2.5 to 4.4 kg m^− 2 during 50 to 2000 yrs of paddy management. The OC accumulation in the bulk soil was dominated by the silt- and clay-sized fractions. The silt fraction showed a high accretion of OC and seems to be an important long-term OC sink during soil evolution. Fine clay in the puddled topsoil horizon was already saturated and the highest storage capacity for OC was calculated for coarse clay. With longer paddy management, the fractions < 20 μm showed an increasing actual OC saturation level, but did not reach the calculated potential storage capacity.     

Electron donors and acceptors influence anaerobic soil organic matter mineralization in tidal marshes

Anaerobic decomposition in wetland soils is carried out by several interacting microbial processes that influence carbon storage and greenhouse gas emissions. To understand the role of wetlands in the global carbon cycle, it is critical to understand how differences in both electron donor (i.e., organic carbon) and terminal electron acceptor (TEA) availability influence anaerobic mineralization of soil organic matter. In this study we manipulated electron donors and acceptors to examine how these factors influence total rates of carbon mineralization and the pathways of microbial respiration (e.g., sulfate reduction versus methanogenesis). Using a field-based reciprocal transplant of soils from brackish and freshwater tidal marshes, in conjunction with laboratory amendments of TEAs, we examined how rates of organic carbon mineralization changed when soils with different carbon contents were exposed to different TEAs. Total mineralization (the sum of CO₂ + CH₄ produced) on a per gram soil basis was greater in the brackish marsh soils, which had higher soil organic matter content; however, on a per gram carbon basis, mineralization was greater in the freshwater soils, suggesting that the quality of carbon inputs from the freshwater plants was higher. Overall anaerobic metabolism was higher for both soil types incubated at the brackish site where SO₄²⁻ was the dominant TEA. When soils were amended with TEAs in the laboratory, more thermodynamically favorable respiration pathways typically resulted in greater organic matter mineralization (Fe(III) respiration > SO₄²⁻ reduction > methanogenesis). These results suggest that both electron donors and acceptors play important roles in regulating anaerobic microbial mineralization of soil organic matter.     

Water dispersible clay in calcareous soils of southwestern Spain

Water dispersible clay (WDC) is a good indicator of the risk of soil erosion by water and the consequent losses of nutrients and contaminants in overland flow. We measured the content and studied the properties of WDC in 26 samples of calcareous Xeralfs, Xerepts and Xererts of southwestern Spain collected from fields under different crop and tillage management; the soils ranged widely in total clay content (60–455 g kg^− 1), calcium carbonate equivalent (CCE) (< 1–559 g kg^− 1) and active calcium carbonate equivalent (ACCE; 2–135 g kg^− 1), and were poor in organic carbon and soluble salts. The WDC content was determined by shaking 10 g of soil in 1 L of simulated rainwater for 4 hours. Non-carbonate WDC contents were found to be strongly correlated with the total clay content of the soils and ranged from 1 to 92 g kg^− 1 soil (mean = 29 g kg^− 1), the non-carbonate WDC/total clay ratio ranging from 0.01 to 0.29 (mean = 0.12). Based on regression analyses, illite was more dispersible than smectite and iron oxides decreased dispersion of clay. Carbonate WDC contents ranged from 1 to 27 g kg^− 1 (mean = 8 g kg^− 1) and were averaged one third the non-carbonate WDC contents; also, they were strongly correlated with the soil ACCE. WDC was rich in phosphorus (P) relative to the bulk soil. The enrichment ratio (ER) for total P (i.e. the ratio of total P in WDC to total P in soil) ranged from 0.2 to 29 (mean = 5) and was inversely related to the total clay content. On average, about one tenth of the soil total P was exported in the WDC and about one fifth of the total P in WDC was in the form of bicarbonate-extractable P (i.e. relatively soluble or ‘labile’ P). Part of the P in WDC seemingly occurred as metal phosphate particles formed by reaction of P fertilizers with soil. In summary, significant amounts of P can be exported via WDC, even though the proportion of total clay that is water dispersible is substantially lower in these soils than in cultivated soils of other semiarid regions.     

Relationships between particle size distribution and VNIR reflectance spectra are weaker for soils formed from bedrock compared to transported parent materials

The cost of determining particle size distribution (psd) of the soil can be significantly reduced by using statistical relationships between visible and near infra red diffuse reflectance spectra (VNIR-DRS) and the proportions of the three size fractions (sand, silt and clay). The spectra contain information on the quantities of soil minerals which occur in these fractions. Statistical models for estimating psd based on a set of soil samples from common parent materials (PM)—with similar mineralogy—may provide more accurate predictions than more comprehensive, global models. The aim of this paper is to compare the performance of statistical models for the prediction of psd from VNIR-DRS for soils with differing types of parent material; specifically soils derived directly from bedrock (coal-bearing and mudstone-bearing strata) or from transported parent materials (glacial till, glacio-lacustrine deposits and alluvium) across eastern England. We assessed the accuracy of psd predictions using partial least squares regression (PLSR) models between two additive log ratios of the three size fractions and VNIR-DRS. We also formed a global PLSR model from all five soil groups. We used mean residual prediction deviation (RPD) from repeated (n = 100) cross-validation to compare the performance of the models because it accounts for the magnitude of variation in the sample data. The most accurate models for the clay (RPD range 1.82-2.33) and sand fractions (RPD range 1.71-1.94) were for soils developed over the transported PM; the models for soils developed over bedrock were substantially poorer (clay RPD range 1.33-1.68; sand RPD range 1.34-1.39). The RPD values for the silt fraction models were smaller, but the same distinction between transported (better; RPD range 1.4-1.88) and bedrock-derived soils (poorer; RPD range 1.15-1.25) was observed. The global model had intermediate RPD values for the three size fractions (clay = 1.75, silt = 1.76 and sand = 1.74). Of the five groups, the soils developed from glacio-lacustrine deposits had the largest mean sand size fraction (58%), but also the most accurate models for estimation of clay and sand size fractions. Due to sedimentary transport and deposition, the mineralogy of the soils developed from Quaternary substrates may be more homogeneous than the bedrock-derived soils, which may in part account for the more accurate models developed for the former. To date we do not have sufficient evidence to demonstrate this unequivocally.     

Effects of physical and chemical properties of soils on adsorption of the insecticidal protein (Cry1Ab) from Bacillus thuringiensis at Cry1Ab protein concentrations relevant for experimental field sites

The adsorption of the insecticidal Cry1Ab protein of Bacillus thuringiensis (Bt) on Na-montmorillonite (M-Na) and soil clay fractions was studied. The aim of this study was not to find the adsorption capacity of the soils from the experimental field site, where Bt corn (MON810) was cultivated, but rather to characterize the adsorption behavior of the Cry1Ab protein at concentrations typically found at experimental field sites. In kinetic experiments, the Cry1Ab protein adsorbed rapidly (<60 min) on M-Na. As the concentration of M-Na was varied and the added Cry1Ab protein concentration was kept constant (20 and 45 ng ml⁻¹), the adsorption per unit weight of Cry1Ab protein decreased with increasing concentrations of M-Na. Adsorption of Cry1Ab protein on M-Na decreased as the pH value of the suspension increased. All adsorption isotherms could be described mathematically by a linear regression with the parameter k, the distribution coefficient, being the slope of the regression line. Although their mineralogical composition was nearly identical, the soil clay fractions showed different k values. The different k values were correlated with the physical and chemical properties of the soil clay fractions, such as the organic carbon content, the specific external surface area, and the electrokinetic charge of the external surfaces of the clays, as well as with the external surface charge density. An increase in the amount of soil organic matter, as well as an increase in the electrokinetic external surface charge of the soil clays, decreased the distribution coefficient k. An increase of the specific external surface areas of the soil clays resulted in a higher distribution coefficient k.Less than 10% of adsorbed Cry1Ab protein was reversibly adsorbed on the soil clays and, thus, desorbed. The desorption efficiency of distilled water was higher than that of a solution of CaCl₂ (2.25 mmol) and of dissolved organic carbon (50 mg C l⁻¹).     

Complexed organic matter controls soil physical properties

It is shown that, for mineral soils, it is not the total amount of organic carbon (or organic matter) that controls soil physical behaviour but the amount of complexed organic carbon (COC). We assume that this complex is formed by the association of unit mass (i.e. 1 g) of organic carbon with n grams of clay. Analysis of data from two French and two Polish databases shows that, for these soils, n = 10. A consequence of this is that in soils with small contents of organic carbon (OC), such as arable soils, COC is proportional to OC. However, in soils with large contents of organic carbon, such as pasture soils, COC is proportional to the clay content. This explains why we find that soil bulk density is significantly correlated with OC in French arable soils but with the clay content in French pasture soils. The use of COC instead of OC enables the arable and pasture soils to be considered on the same scale.

Water retention data were fitted to a double-exponential equation which allows both the matrix and structural porosities to be estimated. It is shown that in soils with low contents of organic carbon, the carbon content is positively correlated with the matrix porosity. In contrast, in soils with high contents of organic carbon, the matrix porosity is constant at its maximum value and the structural porosity is not significantly correlated with either the total organic carbon or the non-complexed organic carbon (NCOC). It is suggested that the complexed organic carbon can be considered as being sequestered. The soil clay content can similarly be partitioned between clay that is complexed with organic carbon and clay that is not complexed. It is shown that non-complexed clay is more easily dispersed in water than clay that is complexed with organic carbon. These findings indicate how improved pedo-transfer functions for the prediction of soil physical properties may be produced. Such functions need to use the values of complexed and non-complexed organic carbon and clay which must be determined by algorithms. The values produced by the algorithms may then be used in the improved pedo-transfer functions.     

CO2 emissions from subtropical arable soils of China

CO₂ efflux plays a key role in carbon exchange between the biosphere and atmosphere, but our understanding of the mechanism controlling its temporal and spatial variations is limited. The purpose of this study is to determine annual soil CO₂ flux and assess its variations in arable subtropical soils of China in relation to soil temperature, moisture, rainfall, microbial biomass carbon (MBC) and dissolved organic carbon (DOC) using the closed chamber method. Soils were derived from three parent materials including granite (G), tertiary red sandstone (T) and quaternary red clay (Q). The experiment was conducted at the Ecological Station of Red Soil, The Chinese Academy of Sciences, in a subtropical region of China. The results showed that soil CO₂ flux had clear seasonal fluctuations with the maximum value in summer, the minimum in winter and intermediate in spring and autumn. Further, significant differences in soil CO₂ flux were found among the three red soils, generally in the order of G>T>Q. The average annual fluxes were estimated as 2.84, 2.13 and 1.41 kg CO₂ m⁻² year⁻¹ for red soils derived from G, T and Q, respectively. Soil temperature strongly affects the seasonal variability of soil CO₂ flux (85.0-88.5% of the variability), followed by DOC (55.8-84.4%) and rainfall (43.0-55.8%). The differences in soil CO₂ flux among the three red soils were partly explained by MBC (33.7-58.9% of the variability) and DOC (23.8-33.6%).     

Evaluation of the soil carbon budget under different upland cropping systems in central Hokkaido,Japan

Abstract

To evaluate the carbon budget in soils under different cropping systems, the carbon dioxide (CO₂) flux from soils was measured in a total of 11 upland crop fields within a small watershed in central Hokkaido over the no snow cover months for 3 years. The CO₂ flux was measured using a closed chamber method at bare plots established in each field to estimate soil organic matter decomposition. Temporal variation in instantaneous soil CO₂ fluxes within the sites was mainly controlled by soil temperature and moisture. Annual mean CO₂ fluxes and cumulative CO₂ emissions had no significant relationship with soil temperature and moisture (P > 0.2). However, there was a significant quadratic relationship between annual mean CO₂ flux or cumulative CO₂ emission and soil clay plus silt content (%) (R² = 0.72～0.74, P < 0.0003). According to this relationship, the optimum condition for soil CO₂ emission is at a clay plus silt content of 63%. The cumulative CO₂ emission during the no snow cover season within each year varied from 1,159 to 7,349 kg C ha^?1 at the different sites. The amount of crop residue carbon retained in the soils following a cropping season was not enough to offset the CO₂ emission from soil organic matter decomposition at all sites. As a consequence, the calculation of the soil carbon budget (i.e. the difference between the carbon added as crop residues and compost and the carbon lost as CO₂ from organic matter decomposition) ranged from –7,349 to –785 kg C ha^?1, except for a wheat site where a positive value of 4,901 kg C ha^?1 was observed because of a large input of organic carbon with compost. The negative values of the soil carbon budget indicate that these cropping systems were net sources of atmospheric CO₂.     

Estimating soil labile organic carbon and potential turnover rates using a sequential fumigation-incubation procedure

Labile carbon is the fraction of soil organic carbon with most rapid turnover times and its oxidation drives the flux of CO₂ between soils and atmosphere. Available chemical and physical fractionation methods for estimating soil labile organic carbon are indirect and lack a clear biological definition. We have modified the well-established Jenkinson and Powlson's fumigation-incubation technique to estimate soil labile organic carbon using a sequential fumigation-incubation procedure. We define soil labile organic carbon as the fraction of soil organic carbon degradable during microbial growth, assuming that labile organic carbon oxidizes according to a simple negative exponential model. We used five mineral soils and a forest Oa horizon to represent a wide range of organic carbon levels. Soil labile organic carbon varied from 0.8 mg/g in an Entisol to 17.3 mg/g in the Oa materials. Potential turnover time ranged from 24 days in an Alfisol to 102 days in an Ultisol. Soil labile organic carbon contributed from 4.8% in the Alfisol to 11.1% in the Ultisol to the total organic carbon. This new procedure is a relatively easy and simple method for obtaining indices for both the pool sizes and potential turnover rates of soil labile organic carbon and provides a new approach to studying soil organic carbon.     

Predicting and mitigating the net greenhouse gas emissions of crop rotations in Western Europe

Nitrous oxide, carbon dioxide and methane are the main biogenic greenhouse gases (GHGs) contributing to net greenhouse gas balance of agro-ecosystems. Evaluating the impact of agriculture on climate thus requires capacity to predict the net exchanges of these gases in a systemic approach, as related to environmental conditions and crop management. Here, we used experimental data sets from intensively monitored cropping systems in France and Germany to calibrate and evaluate the ability of the biophysical crop model CERES-EGC to simulate GHG exchanges at the plot-scale. The experiments involved major crop types (maize-wheat-barley-rapeseed) on loam and rendzina soils. The model was subsequently extrapolated to predict CO₂ and N₂O fluxes over entire crop rotations. Indirect emissions (IE) arising from the production of agricultural inputs and from use of farm machinery were also added to the final greenhouse gas balance. One experimental site (involving a maize-wheat-barley-mustard rotation on a loamy soil) was a net source of GHG with a net GHG balance of 670 kg CO₂-C eq ha⁻¹ yr⁻¹, of which half were due to IE and half to direct N₂O emissions. The other site (involving a rapeseed-wheat-barley rotation on a rendzina) was a net sink of GHG for −650 kg CO₂-C eq ha⁻¹ yr⁻¹, mainly due to high C returns to soil from crop residues. A selection of mitigation options were tested at one experimental site, of which straw return to soils emerged as the most efficient to reduce the net GHG balance of the crop rotation, with a 35% abatement. Halving the rate of N inputs only allowed a 27% reduction in net GHG balance. Removing the organic fertilizer application led to a substantial loss of C for the entire crop rotation that was not compensated by a significant decrease of N₂O emissions due to a lower N supply in the system. Agro-ecosystem modeling and scenario analysis may therefore contribute to design productive cropping systems with low GHG emissions.     

Temperature, water content and wet-dry cycle effects on DOC production and carbon mineralization in agricultural peat soils

Agricultural peat soils in the Sacramento-San Joaquin Delta, California have been identified as an important source of dissolved organic carbon (DOC) and trihalomethane precursors in waters exported for drinking. The objectives of this study were to examine the primary sources of DOC from soil profiles (surface vs. subsurface), factors (temperature, soil water content and wet-dry cycles) controlling DOC production, and the relationship between C mineralization and DOC concentration in cultivated peat soils. Surface and subsurface peat soils were incubated for 60 d under a range of temperature (10, 20, and 30 °C) and soil water contents (0.3-10.0 g-water g-soil⁻¹). Both CO₂-C and DOC were monitored during the incubation period. Results showed that significant amount of DOC was produced only in the surface soil under constantly flooded conditions or flooding/non-flooding cycles. The DOC production was independent of temperature and soil water content under non-flooded condition, although CO₂ evolution was highly correlated with these parameters. Aromatic carbon and hydrophobic acid contents in surface DOC were increased with wetter incubation treatments. In addition, positive linear correlations (r²=0.87) between CO₂-C mineralization rate and DOC concentration were observed in the surface soil, but negative linear correlations (r²=0.70) were observed in the subsurface soil. Results imply that mineralization of soil organic carbon by microbes prevailed in the subsurface soil. A conceptual model using a kinetic approach is proposed to describe the relationships between CO₂-C mineralization rate and DOC concentration in these soils.     

Organic matter in density fractions of water-stable aggregates in silty soils: Effect of land use

The location of soil organic matter (SOM) within the soil matrix is considered a major factor determining its turnover, but quantitative information about the effects of land cover and land use on the distribution of SOM at the soil aggregate level is rare. We analyzed the effect of land cover/land use (spruce forest, grassland, wheat and maize) on the distribution of free particulate organic matter (POM) with a density <1.6 g cm⁻³ (free POM_<1.6), occluded particulate organic matter with densities <1.6 g cm⁻³ (occluded POM_<1.6) and 1.6-2.0 g cm⁻³ (occluded POM_1.6-2.0) and mineral-associated SOM (>2.0 g cm⁻³) in size classes of slaking-resistant aggregates (53-250, 250-1000, 1000-2000, >2000 μm) and in the sieve fraction <53 μm from silty soils by applying a combined aggregate size and density fractionation procedure. We also determined the turnover time of soil organic carbon (SOC) fractions at the aggregate level in the soil of the maize site using the ¹³C/¹²C isotope ratio. SOM contents were higher in the grassland soil aggregates than in those of the arable soils mainly because of greater contents of mineral-associated SOM. The contribution of occluded POM to total SOC in the A horizon aggregates was greater in the spruce soil (23-44%) than in the grassland (11%) and arable soils (19%). The mass and carbon content of both the free and occluded POM fractions were greater in the forest soil than in the grassland and arable soils. In all soils, the C/N ratios of soil fractions within each aggregate size class decreased in the following order: free POM_<1.6>occluded POM_<1.6-2.0>mineral-associated SOM. The mean age of SOC associated with the <53 μm mineral fraction of water-stable aggregates in the Ap horizon of the maize site varied between 63 and 69 yr in aggregates >250 μm, 76 yr in the 53-250 μm aggregate class, and 102 yr in the sieve fraction <53 μm. The mean age of SOC in the occluded POM increased with decreasing aggregate size from 20 to 30 yr in aggregates >1000 μm to 66 yr in aggregates <53 μm. Free POM had the most rapid rates of C-turnover, with residence times ranging from 10 yr in the fraction >2000 μm to 42 yr in the fraction 53-250 μm. Results indicated that SOM in slaking-resistant aggregates was not a homogeneous pool, but consisted of size/density fractions exhibiting different composition and stability. The properties of these fractions were influenced by the aggregate size. Land cover/land use were important factors controlling the amount and composition of SOM fractions at the aggregate level.     

Organic carbon mineralization responses to temperature increases in subtropical paddy soils

Purpose

Understanding organic carbon mineralization and its temperature response in subtropical paddy soils is important for the regional carbon balance. There is a growing interest in factors controlling soil organic carbon (SOC) mineralization because of the potential for climate change. This study aims to test the hypothesis that soil clay content impedes SOC mineralization in subtropical paddy soils.

Materials and methods

A 160-day laboratory incubation at temperatures from 10 to 30 °C and 90% water content was conducted to examine the dynamics of SOC mineralization and its temperature response in three subtropical paddy soils with different clay contents (sandy loam, clay loam, and silty clay soils). A three-pool SOC model (active, slow, and resistant) was used to fit SOC mineralization.

Results and discussion

Total CO₂ evolved during incubation following the order of clay loam > silty clay > sandy loam. The temperature response coefficients (Q ₁₀) were 1.92?±?0.39, 2.36?±?0.22, and 2.10?±?0.70, respectively, for the sandy loam soil, clay loam soil, and silty clay soil. But the soil clay content followed the order of silty clay > clay loam > sandy loam. The sandy loam soil neither released larger amounts of CO₂ nor showed higher temperature sensitivity, as expected, even though it contains lower soil clay content among the three soils. It seems that soil clay content did not have a dominant effect which results in the difference in SOC mineralization and its temperature response in the selected three paddy soils. However, dissolved organic carbon (DOC; representing substrate availability) had a great effect. The size of the active C pool ranged from 0.11 to 3.55% of initial SOC, and it increased with increasing temperature. The silty clay soil had the smallest active C pool (1.40%) and the largest Q ₁₀ value (6.33) in the active C pool as compared with the other two soils. The mineralizable SOC protected in the silty clay soil, therefore, had even greater temperature sensitivity than the other two soils that had less SOC stabilization.

Conclusions

Our study suggests that SOC mineralization and its temperature response in subtropical paddy soils were probably not dominantly controlled by soil clay content, but the substrate availability (represented as DOC) and the specific stabilization mechanisms of SOC may have great effects.     

Methane flux and regulatory variables in soils of three equal-aged Japanese cypress (Chamaecyparis obtusa) forests in central Japan

To compare factors that control methane flux in forest soils, we studied three equal-aged Japanese cypress (Chamaecyparis obtusa) forests in Chubu district, central Japan. The three sites are located at different altitudes: 630 m (SET), 1010 m (INB), and 1350 m (OSK). Methane was absorbed at every site. The highest uptake rate was observed in the middle-altitude soil (INB, 5.89 mg CH₄ m⁻² d⁻¹), which was the only site where methane uptake rate was correlated with air and soil surface temperatures. Methane flux in the field was not correlated with water content, inorganic N content, or water-soluble organic carbon. C/N ratio was correlated with methane flux (r=0.64,p<0.001). The results suggest that some organic inhibitors might be produced through decomposition of organic matter. There was a negative correlation between methane uptake rate and water-soluble Al (r=−0.63,p<0.001). Inhibition of methane consumption by 1 and 5 mM Al solutions was observed in laboratory incubation. This result suggests that water-soluble Al may be a factor controlling methane uptake. Multiple regression with a backward-elimination procedure identified three variables that were significantly associated with methane flux in the field (p<0.05): air temperature, C/N ratio, and the concentration of water-soluble Al.     

Atypical soil carbon distribution across a tropical steepland forest catena

Soil organic carbon (SOC) in a humid subtropical forest in Puerto Rico is higher at ridge locations compared to valleys, and therefore opposite to what is commonly observed in other forested hillslope catenas. To better understand the spatial distribution of SOC in this system, plots previously characterized by topographic position, vegetation type and stand age were related to soil depth and SOC. Additional factors were also investigated, including topographically-related differences in litter dynamics and soil chemistry. To investigate the influence of litter dynamics, the Century soil organic model was parameterized to simulate the effect of substituting valley species for ridge species. Soil chemical controls on C concentrations were investigated with multiple linear regression models using iron, aluminum and clay variables. Deeper soils were associated with indicators of higher landscape stability (older tabonuco stands established on ridges and slopes), while shallower soils persisted in more disturbed areas (younger non-tabonuco stands in valleys and on slopes). Soil depth alone accounted for 77% of the observed difference in the mean 0 to 60 cm SOC between ridge soils (deeper) and valley soils (shallower). The remaining differences in SOC were due to additional factors that lowered C concentrations at valley locations in the 0 to 10 cm pool. Model simulations showed a slight decrease in SOC when lower litter C:N was substituted for higher litter C:N, but the effects of different woody inputs on SOC were unclear. Multiple linear regression models with ammonium oxalate extractable iron and aluminum, dithionite–citrate-extractable iron and aluminum, and clay contents explained as much as 74% of the variation in C concentrations, and indicated that organo-mineral complexation may be more limited in poorly developed valley soils. Thus, topography both directly and indirectly affects SOC pools through a variety of inter-related processes that are often not quantified or captured in terrestrial carbon models.