Soil penetrating quality in cultivated and forested coastal plain sands of Southern Nigeria

Conversion of natural forest to intensive cultivation makes to soil susceptible to flooding, declining fertility and loss of organic matter (OM) and reduced water movement into and within the soil. We studied infiltration rates and related soil penetrating indicators of forested and cultivated soils in humid tropical coastal plain sands in Southern Nigeria. Results showed that mean-weight diameter (MWD) and water stability of aggregates were higher in forested than cultivated soils. Stable aggregates > 1.00 mm were 16.5% and 31.1% respectively, for cultivated and forested soils at 0–15 cm depth, indicating formation of more macro-aggregates in forested soil. Soil disturbance through cultivation decreased hydraulic conductivity and increased bulk density of the soil. Infiltration rate attained after 2 hours was higher in forested soil. Temporary infiltration rate of 178 mm hr^?1 at initial time in cultivated soil was followed by very low infiltration rate of 7 mm hr^?1 after 2 hours. Soil organic matter (SOM), saturated hydraulic conductivity, MWD and total sand correlated positively with infiltration rates are r = 0.76, 0.61, 0.57 and 0.51 respectively. Changes in these parameters are dependent on surface soil disturbance by cultivation. Cultivation of forest decreased infiltration rates and water transmission properties of the soil.     

Nationally Coordinated Evaluation of Soil Nitrogen Mineralization Rate using a Standardized Aerobic Incubation Protocol

Abstract

Aerobic incubation methods have been widely used to assess soil nitrogen (N) mineralization, but standardized protocols are lacking. A single silt loam soil (Catlin silt loam; fine‐silty, mixed, superactive, mesic, Oxyaquic Arguidoll) was subjected to aerobic incubation at six USDA‐ARS locations using a standardized protocol. Incubations were conducted at multiple temperatures, which were combined based on degree days (DD). Soil water was maintained at 60% water‐filled pore space (WFPS; constant) or allowed to fluctuate between 60 and 30% WFPS (cycle). Soil subsamples were removed periodically and extracted in 2 M potassium chloride (KCl); nitrate (NO₃) and ammonium (NH₄) concentrations in extracts were determined colorimetrically. For each location, the rate of soil organic‐matter N (SOMN) mineralization was estimated by regressing soil inorganic N (N_i) concentration on DD, using a linear (zero‐order) model. When all data were included, the mineralization rate from four datasets was not statistically different, with a rate equivalent to 0.5 mg N kg^?1 soil day^?1. Soil incubated at two locations exhibited significantly higher SOMN mineralization rates. To assess whether this may have been due to pre‐incubation conditions, time‐zero data were excluded and regression analysis was conducted again. Using this data subset, SOMN mineralization from five (of six) datasets was not significantly different. Fluctuating soil water reduced N‐mineralization rate at two (of four) locations by an average of 50%; fluctuating soil water content also substantially increased variability. This composite dataset demonstrates that standardization of aerobic incubation methodology is possible.     

Carbon Mineralization and Water-Extractable Organic Carbon and Nitrogen as Predictors of Soil Health and Nitrogen Mineralization Potential

ABSTRACT

The United States Department of Agriculture Natural Resources Conservation Service (NRCS) launched a national “Soil health initiative” in 2012; as a part of that effort, a soil health index (SHI) has been developed. The SHI is calculated using results of three soil tests: 24-h carbon mineralization following rewetting of air-dried soil (C_min, by the “Solvita” proprietary method) and water-extractable organic carbon (C) and nitrogen (N). These tests are being promoted both as the inputs into the SHI calculation and as predictors of soil N mineralization potential. Soil was collected from 35 California fields in annual crop rotations; 20 fields were under certified organic management and the other 15 under conventional management, to provide a range of soil properties and management effects. Carbon mineralization was determined by the Solvita method, and by a comparison method utilizing head space carbon dioxide (CO₂) monitoring by infrared gas analyzer (IRGA); additionally, two soil wetting protocols were compared, capillary wetting (the Solvita method) and wetting to 50% water-filled pore space (WFPS). Both water-extractable C (WEOC) and N (WEON) were determined using NRCS-recommended protocols. Net N mineralization (N_min) was also determined after a 28-day aerobic incubation at 25°C. Solvita C_min was highly correlated with the IRGA method using capillary wetting (R² = 0.81). However, capillary soil wetting resulted in high gravimetric water content that significantly suppressed C_min compared to the 50% WFPS method. N_min was correlated with Solvita C_min (r = 0.54) and with WEOC and WEON (r = 0.62 for each comparison); combining these three measurements into the SHI slightly improved the correlation with N_min. The organically managed soils scored higher than the conventional soils on the SHI, with a minority of organic soils and the majority of conventional soils scoring below the NRCS target threshold. SHI and soil organic matter were correlated, suggesting an inherent bias that would complicate the application of a national SHI standard.     

Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils

After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root‐cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra‐microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation‐resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF‐res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N/ha) from undisturbed soil and coarse fPOM N (kg N/ha), silt and clay N (kg N/ha) and its C:N ratio (R² = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF‐res OC (% of soil organic carbon) and its C:N ratio (R² = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio‐available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils.     

Changes in soil pH and phosphorus availability during decomposition of cover crop residues

The aim of this study was to determine the effect of winter cover crop (CC) residues on soil pH and phosphorus (P) availability. Three incubation assays were performed in pots using two CC: vetch (V) (Vicia villosa Roth.) and oats (Oa) (Avena sativa L.). Soil samples were taken from 10 sites at 0–20-cm depth. The rate of residues were 0 (D0), 10 (D1), 20 (D2), 30 (D3), and 40 (D4) g dry matter kg^?1 soil and the soil sampling was after 10, 20, 30, 60, 90, and 120 days of incubation. Soil pH, extractable P (Pe), and soil organic matter (SOM) and its fractions were determined. The pH increase was correlated with the rate applied (D1 < D2 < D3 < D4). No differences were found for pH comparing V and Oa residues with low residue rates. Soil pH changes were dependent from initial pH and SOM fractions in different soils across the incubation period. The multiple regression models showed that the pH changes were dependent on initial pH level and SOM fractions with a high R² (0.81). CC residues and its quantities produced different changes on pH – especially at the beginning of the incubation – which influenced the P availability.     

Potential net soil N mineralization and decomposition of glycine-C in forest soils along an elevation gradient

The objective of this research was to better understand patterns of soil nitrogen (N) availability and soil organic matter (SOM) decomposition in forest soils across an elevation gradient (235-1670 m) in the southern Appalachian Mountains. Laboratory studies were used to determine the potential rate of net soil N mineralization and in situ studies of ¹³C-labelled glycine were used to infer differences in decomposition rates. Nitrogen stocks, surface soil (0-5 cm) N concentrations, and the pool of potentially mineralizable surface soil N tended to increase from low to high elevations. Rates of potential net soil N mineralization were not significantly correlated with elevation. Increasing soil N availability with elevation is primarily due to greater soil N stocks and lower substrate C-to-N ratios, rather than differences in potential net soil N mineralization rates. The loss rate of ¹³C from labelled soils (0-20 cm) was inversely related to study site elevation (r=−0.85; P<0.05) and directly related to mean annual temperature (+0.86; P<0.05). The results indicated different patterns of potential net soil N mineralization and ¹³C loss along the elevation gradient. The different patterns can be explained within a framework of climate, substrate chemistry, and coupled soil C and N stocks. Although less SOM decomposition is indicated at cool, high-elevation sites, low substrate C-to-N ratios in these N-rich systems result in more N release (N mineralization) for each unit of C converted to CO₂ by soil microorganisms.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Humic Substance Fractions and Attributes of Histosols and Related High‐Organic‐Matter Soils from Brazil

Abstract

Knowledge of the distribution of soil organic matter (SOM) fractions is important in managing soils toward a sustainable agricultural system in a tropical environment. However, data on Histosols is limited. This study developed 19 profiles of Histosols and soils with high organic-matter content from different regions of Brazil. Soil organic matter was fractionated into fulvic acids (FAF), humic acids (HAF), and humin (HUM). The ratios HAF/FAF and AE (alkaline extract)/HUM were calculated. The objectives were to evaluate the method for SOM fractionating in Histosols and related soils and to correlate the distribution of organic fractions with other soil attributes. The humic fractions presented significant correlations with other soil attributes, the best being the correlation between FAF and nutrient level. The HAF and HUM presented high correlation with cationic exchange capacity, active acidity (H⁺) and pH. Humin and the alkaline extract absorbance measured at 380 nm and 465 nm and presented good correlation with total organic carbon.     

城郊土壤不透水表面有土壤机碳转化及其相关性质的研究

Installation of impervious surface in urban area prevents the exchange of material and energy between soil and other environmental counterparts, thereby resulting in negative effects on soil function and urban environment. Soil samples were collected at 0-20 cm depth in Nanjing City, China, in which seven sites were selected for urban open soils, and fourteen sites with similar parent material were selected for the impervious-covered soils, to examine the effect of impervious surface on soil properties and microbial activities, and to determine the most important soil properties associated with soil organic carbon （SOC） transformation in the urban soils covered by impervious surfaces. Soil organic carbon and water-soluble organic carbon （WSOC） concentrations, potential carbon （C） and nitrogen （N） mineralization rates, basal respiration, and physicochemical properties with respect to C transformation were measured. Installation of impervious surface severely affected soil physicochemical properties and microbial activities, e.g., it significantly decreased total N contents, potential C mineralization and basal respiration rate （P 〈 0.01）, while increased pH, clay and Olsen-P concentrations. Soil organic carbon in the sealed soils at 0-20 cm was 2.35 kg m-2, which was significantly lower than the value of 4.52 kg m-2 in the open soils （P 〈 0.05）. Canonical correlation analysis showed WSOC played a major role in determining SOC transformation in the impervious-covered soil, and it was highly correlated with total N content and potential C mineralization rate. These findings demonstrate that installation of impervious surface in urban area, which will result in decreases of SOC and total N concentrations and soil microbial activities, has certain negative consequences for soil fertility and long-term storage of SOC.     

Effect of Fertilizer Nitrogen (N) on Soil Organic Carbon,Total N,and Soil pH in Long-Term Continuous Winter Wheat (Triticum Aestivum L.)

Carbon sequestration via sound agronomic practices can assist in combating global warming. Three long-term experiments (Experiment 502, Experiment 222, and The Magruder Plots) were used to evaluate the effect of fertilizer nitrogen (N) application on soil organic carbon (SOC), total nitrogen (TN), and pH in continuous winter wheat. Soil samples (0–15 cm) were obtained after harvest in 2014, analyzed, and compared to soil test results from these experiments in 1993. Soil pH decreased with increasing N fertilization, and more so at high rates. Nitrogen application significantly increased TN in Experiment 502 from 1993 to 2014, and TN tended to be high at high N rates. Fertilizer N significantly increased SOC, especially when N rates exceeded 90 kg ha^?1. The highest SOC (13.1 g kg^?1) occurred when 134 kg N ha^?1 was applied annually. Long-term N application at high rates increased TN and SOC in the surface soil.     

Evolution of soil respiration depends on biological soil crusts across a 50-year chronosequence of desert revegetation

Despite intensive study in recent decades, soil respiration rate (Rs) and its evolution accompanying vegetation succession remain perplexing. Using a 50-year chronosequence of sand-fixing revegetation in the Tengger Desert of China, we took intact soil columns of 20 cm in depth, incubated them at 12 levels of soil water content (0–0.4 m³ m^?3) and at nine temperatures (5–45°C) in a growth chamber, and measured Rs. The results showed that Rs increased rapidly 15 to 20 years following revegetation but stabilized after 25 years. Rs for soils covered with moss crusts were markedly higher than those covered with algal crusts. Further, Rs correlated significantly with sand content (negatively) and fine particle contents (positively), and increased exponentially with increased soil organic matter (SOM) and total nitrogen (TN) contents. Soil texture had a stronger influence on Rs than did SOM and TN. Also, Rs increased linearly with increased coverage and depth of biological soil crusts, which had a more pronounced influence on Rs than did soil physicochemical properties. Our results suggest that the capacity of carbon sequestration likely increases during the 50-year period after revegetation because the linear increase in SOM outweighs the limited sigmoidal increase in Rs.     

Organic matter fractions and N mineralization in vegetable‐cropped sandy soils

Soil organic nitrogen mineralization rates and possible predictors thereof were investigated for vegetable‐growing soils in Belgium. Soil organic matter (SOM) was fractionated into sand (> 53 μm) and silt+clay (< 53 μm) fractions. The latter fraction was further separated into 6%NaOCl‐oxidation labile (6%NaOCl‐ox) and resistant N and C and subsequently into 10%HF‐extractable (mineral bound) and resistant (recalcitrant) N and C. The N mineralization turnover rate (% of soil N/year) correlated with several of the investigated N or C fractions and stepwise linear regression confirmed that the 6%NaOCl‐ox N was the best predictor. However, the small R² (0.42) of the regression model suggests that soil parameters other than the soil fractions isolated here would be required to explain the significant residual variation in N mineralization rate. A next step could be to look for alternative SOM fractionations capable of isolating bioavailable N. However, it would appear that the observed relationships between N fractions and N mineralization may not be causal but indirect. The number of vegetable crops per rotation did not influence N mineralization, but it did influence 6%NaOCl‐ox N, probably as an effect of differences in crop residues returned and organic manure supply. However, the nature of this relation between management, SOM quality and N mineralization is not clear. Explanation of correlations between N mineralization and presumed bioavailable N fractions, like the 6%NaOCl‐ox N, requires further mechanistic elucidation of the N mineralization process.     

Long‐lasting impact of biowaste‐compost application in agriculture on soil‐quality parameters in three different crop‐rotation systems

Soil‐quality parameters, such as soil organic matter (SOM) and plant‐available nutrient contents, microbial properties, aggregate stability, and the amounts of heavy metals were carried out in arable soils of different rotation schedules applied with a total of 50 Mg dry mass ha^–1 biowaste compost relative to an untreated control. This was investigated during a 10 y period from 1994 to 2004. Overall, soil‐quality parameters studied appeared to be promoted by biowaste‐compost application. This was evidenced for example by a remarkable increase of SOM and total N content of ≈ 15%–20% relative to the control. Subsequently, amounts of soil microbial biomass and alkaline phosphatase activity were significantly increased as well. In addition, biowaste‐compost application revealed an increase of plant‐available P and K contents and aggregate stability in soil. There was, however, no treatment effect for net N‐mineralization rates. Moreover, in soils of maize and sugar beet rotation schedule a slight decrease was found. Heavy‐metal contents of Pb and Zn were significantly increased in all compost‐treated soils, whereas no significant increase of Cd and Cu contents was measured. However, the investigated amounts were far below of the limits of the German Biowaste Ordinance. It is finally recommended, that biowaste compost may sustain and improve soil quality in agriculture when N nutrition will be considered.     

中国农耕区土壤有机质含量及其与酸碱度和容重关系

对我国农耕区土壤有机质区域变化及其与酸碱度和容重关系进行系统分析，为耕地地力提升和改善土壤结构提供支撑。基于国家级耕地长期定位监测点913个，统计分析全国及7大区域（东北NE、华北NC、西北NW、长江中游MYR、长三角YRD、华南SC、西南SW）耕层土壤有机质含量、酸碱度及容重变化特征。结果表明，全国农耕区耕层土壤有机质含量平均值为22.4~24.8 g/kg。其中有机质含量中等偏低的监测点位占比达72.5%。不同区域耕层土壤有机质含量差异显著（p<0.05），MYR耕层土壤有机质含量显著高于其他6个区域。全国农耕区耕层土壤pH和容重平均分别为（6.90±1.20），（1.30±0.15） g/cm³。不同土壤利用方式对土壤有机质、酸碱度及容重产生影响。水田耕层土壤有机质含量显著高于旱地，旱地耕层土壤pH和容重则显著高于水田。亚当斯方程和指数函数分别推荐拟合土壤容重对有机质含量响应关系（R²=0.09，RMSE=0.17，n=759），以及土壤pH对土壤有机质含量响应（R²=0.16，RMSE=1.24，n=886）。全国农耕区耕层土壤有机质含量总体中等偏低，呈现出东南向西北依次降低趋势。土壤pH及容重与土壤有机质呈现显著的负相关关系。亚当斯模型及指数方程能较好地拟合土壤容重及pH对有机质的响应关系，可用于非线性插值法补充土壤容重及pH缺失值。     

Comparison of Near Infrared Reflectance Spectroscopy with Combustion and Chemical Methods for Soil Carbon Measurements in Agricultural Soils

As interest in soil organic carbon (SOC) dynamics increases, so do needs for rapid, accurate, and inexpensive methods for quantifying SOC. Objectives were to i) evaluate near infrared reflectance (NIR) spectroscopy potential to determine SOC and soil organic matter (SOM) in soils from across Tennessee, USA; and ii) evaluate potential upper limits of SOC from forest, pasture, no-tillage, and conventional tilled sites. Samples were analyzed via dry-combustion (SOC), Walkley–Black chemical SOM, and NIR. In addition, the sample particle size was classified to give five surface roughness levels to determine effects of particle size on NIR. Partial least squares regression was used to develop a model for predicting SOC as measured by NIR by comparing against SOM and SOC. Both NIR and SOM correlated well (R² > 0.9) with SOC (combustion). NIR is therefore considered a sufficiently accurate method for quantifying SOC in soils of Tennessee, with pasture and forested systems having the greatest accumulations.Abbreviations SOC, soil organic carbon; NIR, Near Infrared Reflectance Spectroscopy; MTREC, Middle Tennessee Research and Education Center; RECM, Research and Education Center at Milan; PREC, Plateau Research and Education Center; PLS, Partial least squares.     

Soil organic matter and sorption capacity under different soil management practices in a productive vineyard

The effect of soil organic matter (SOM) on sorptive parameters under different soil management practices in Rendzic Leptosol was studied. In 2006, an experiment of different management practices in a productive vineyard was established in the locality of Nitra-Dra?ovce (Slovakia). The following treatments were established: (1) control (grass without fertilization), (2) T (tillage), (3) T + FYM (tillage + farmyard manure), (4) G + NPK3 (grass + 3rd intensity of fertilization for vineyards), (5) G + NPK1 (grass + 1st intensity of fertilization for vineyards). Soil samples were collected every month during the year 2010. Obtained results showed increased hydrolytic acidity (by 23%), sum of basic cations (by 37%) and decreased total cation exchange capacity (CEC) (by 36%) with higher doses of fertilization in comparison to control. Application of farmyard manure had a positive effect on the increase in the SOM cation sorption capacity. Positive correlations between pH and sum of basic cations (SBC) (r = 0.493, p ≤ 0.001), CEC (r = 0.498, p ≤ 0.001) and cation sorption capacity of SOM (r = 0.391, p ≤ 0.01) were observed. Higher values of labile carbon:potentially mineralizable nitrogen (C_L:N_pot) ratio corresponded with lesser CEC, SBC and base saturation values in the soil. With increased humus quality (higher values of humic acid:fulvic acid (C_HA:C_FA) ratio), cation sorption capacity of SOM significantly increased (r = 0.329, p ≤ 0.01). The results of this study proved that the application of farmyard manure had a positive effect on the increase of SOM sorption capacity, but higher doses of mineral fertilizers added to soil had a negative effect.     

Temperature sensitivity of nitrogen mineralization in agricultural soils

As concerns continue to mount over nitrogen (N) use efficiency and the fate of excess N lost to the environment, the need for accurate assessments of the contribution of non-fertilizer sources is increasing. Nitrogen mineralization from soil organic matter (SOM) can supply large quantities of plant-available N through a growing season and is affected by numerous factors including soil temperature and soil properties. The objective of this study was to determine the temperature sensitivity of N mineralization in soils under annual crops in California and to determine whether growing region and soil properties affect the temperature sensitivity of N mineralization. Nitrogen mineralization was assessed for 55 soils from different regions in central and northern California by incubating undisturbed soil cores at 5, 15, and 25 °C for 10 weeks. Modeling was done by fitting the data to both an S-shaped curve and a Q₁₀ function. Q₁₀ values averaged 2.87 and ranged from 2.67 to 3.15 in the different regions. While the magnitude of N mineralized varied greatly, the temperature response did not differ significantly across regions and was not affected by SOM content and quality, texture, electrical conductivity, or pH. Coupling the temperature response curve with daily soil temperature data measured in the field highlighted the differences in regional and seasonal mineralization rates and underscored the importance of taking soil temperature into account when determining N mineralization rates for N budgets. This is especially important in regions where crops are grown year-round.     

Factors affecting the molecular structure and mean residence time of occluded organics in a lithosequence of soils under ponderosa pine

Occluded, or intra-aggregate, soil organic matter (SOM) comprises a significant portion of the total C pool in forest soils and often has very long mean residence times (MRTs). However, occluded C characteristics vary widely among soils and the genesis and composition of the occluded organic matter pool are not well understood. This work sought to define the major controls on the composition and MRT of occluded SOM in western U.S. conifer forest soils with specific focus on the influence of soil mineral assemblage and aggregate stability. We sampled soils from a lithosequence of four parent materials (rhyolite, granite, basalt, and dolostone) under Pinus ponderosa. Three pedons were excavated to the depth of refusal at each site and sampled by genetic horizon. After density separation at 1.8 g cm⁻³ into free/light, occluded and mineral fractions, the chemical nature and mean residence time of organics in each fraction were compared. SOM chemistry was explored through the use of stable isotope analyses, ¹³C NMR, and pyrolysis GC/MS. Soil charcoal content estimates were based on ¹³C NMR analyses. Estimates of SOM MRT were based on steady-state modeling of SOM radiocarbon abundance measurements. Across all soils, the occluded fraction was 0.5–5 times enriched in charcoal in comparison to the bulk soil and had a substantially longer MRT than either the mineral fraction or the free/light fraction. These results suggest that charcoal from periodic burning is the primary source of occluded organics in these soils, and that the structural properties of charcoal promote its aggregation and long-term preservation. Surprisingly, aggregate stability, as measured through ultrasonic dispersion, was not correlated with occluded SOM abundance or MRT, perhaps raising questions of how well laboratory measurements of aggregate stability capture the dynamics of aggregate turnover under field conditions. Examination of the molecular characteristics of the occluded fraction was more conclusive. Occluded fraction composition did not change substantially with soil mineral assemblage, but was increasingly enriched in charcoal with depth relative to bulk SOM. Enrichment levels of ¹³C and ¹⁵N suggested a similar degree of microbial processing for the free/light and occluded fractions, and molecular structure of occluded and free/light fractions were also similar aside from charcoal enrichment in the occluded fraction. Results highlight the importance of both fire and aggregate formation to the long-term preservation of organics in western U.S. conifer forests which experience periodic burning, and suggest that the composition of occluded SOM in these soils is dependent on fire and the selective occlusion of charcoal.     

Soil and nutrient losses due to root crops harvesting: a case study from southwestern Iran

Although root crops are widely cultivated in Iran, little is known about soil loss due to crop harvesting (SLCH). We assessed the annual exported soil, soil organic matter (SOM) and nutrients from 47 farms under root crops in southwestern Iran. Soil losses for garlic, potato, sugar beet, radish and beetroot were estimated as 6.27, 2.52, 2.26, 4.10 and 6.95, Mg ha^?1, respectively, which on average was of the order of soil losses by water erosion in the watershed basins of Iran. Total N, P₂O₅ and K₂O losses were estimated as 36.61, 1.10 and 31.50 kg ha^?1 and their costs as 18.24, 0.74 and 19.93 USAlthough root crops are widely cultivated in Iran, little is known about soil loss due to crop harvesting (SLCH). We assessed the annual exported soil, soil organic matter (SOM) and nutrients from 47 farms under root crops in southwestern Iran. Soil losses for garlic, potato, sugar beet, radish and beetroot were estimated as 6.27, 2.52, 2.26, 4.10 and 6.95, Mg ha^?1, respectively, which on average was of the order of soil losses by water erosion in the watershed basins of Iran. Total N, P₂O₅ and K₂O losses were estimated as 36.61, 1.10 and 31.50 kg ha^?1 and their costs as 18.24, 0.74 and 19.93 US$ ha^?1, respectively. For the whole country, total soil, N, P₂O₅, K₂O and SOM losses for garlic, potato and sugar beet were estimated as 731.7 × 10³, 836, 27, 476 and 14.5 × 10³ Mg, respectively (radish and beetroot were excluded due to no reliable data on their planted areas). Correlation analysis showed no significant relationship between soil properties and SLCH (except soil moisture content for radish and clay content for beetroot). The findings indicated that the exported soil, SOM and nutrients were at such levels that SLCH should be considered in soil erosion studies.     

Density separation of soil organic matter across three land uses in calcareous soils of Iran

ABSTRACT

The aim of this study was to examine the usefulness of physical and chemical fractionation in quantifying soil organic matter (SOM) in different stabilized fraction pools. Soil samples from three land use types in Lorestan province, Southwest Iran were examined to account for the amount of organic carbon and nitrogen in different SOM fractions. Size/density separation and chemical oxidation methods were applied to separate the SOM fractions including particulate organic matter (POM), Si + C (silt and clay), DOC (dissolved organic C), rSOM (oxidation-resistant organic carbon and nitrogen) and S + SA (sand and stable aggregates). The values obtained for TOC, TN, and HWC were highest in forest lands followed by the range and agricultural lands. Among the SOM fractions, S + SA showed the highest values (5.75, 5.77 and 20.6 g kg^?1 for agriculture, range and forest lands respectively) followed by POM, Si + C, rSOM, and DOC. The concentrations of C and N in the labile fractions obtained the higher values than in the stabilized fractions. Forest lands had the highest amounts of organic C and N among all fractions whereas agricultural lands showed highest values for inorganic C content of soils in different fractions.