Determinants of soil organic matter chemistry in maritime temperate forest ecosystems

While the influence of climate, vegetation, management and abiotic site factors on total carbon budgets and turn-over is intensively assessed, the influences of these ecosystem properties on the chemical complexity of soil organic matter (SOM) remains poorly understood. This study addresses the chemical composition of NaOH-extracted SOM from maritime temperate forest sites in Flanders (Belgium) by pyrolysis-GC/MS. The studied forests were chosen based on dominant tree species (Pinus sylvestris, Fagus sylvatica, Quercus robur and Populus spp.), soil texture and soil-moisture conditions. Differences in extractable-SOM pyrolysis products were correlated to site variables including dominant tree species, management of the woody biomass, site history, soil properties, total carbon stocks and indicators for microbial activity. Despite of a typical high intercorrelation between these site variables, the influence of the dominant tree species is prominent. The extractable-SOM composition is strongly correlated to litter quality and available nutrients. In nutrient-poor forests with low litter quality, the decomposition of relatively recalcitrant compounds (i.e. short and mid-chain alkanes/alkenes and aromatic compounds) appears hampered, causing a relative accumulation of these compounds in the soil. However, if substrate quality is favorable, no accumulations of recalcitrant compounds were observed, not even under high soil-moisture conditions. Former heathland vegetation still had a profound influence on extractable-SOM chemistry of young pine forests after a minimum of 60 years.     

Use of organic carbon and loss-on-ignition to estimate soil organic matter in different soil types and horizons

Summary Loss-on-ignition (LOI) and the organic C content have been used to estimate soil organic matter. Organic matter is often estimated from organic C by applying a factor of 1.724. Several authors have examined the relationship between LOI, used as an estimate of organic matter, and C by simple linear regressions. In the present study, this approach was examined in relation to two sets of data. LOI overestimates organic matter in soils with significant proportions of clay minerals because of bound water, and correcting for bound water gives some LOI: C ratios of less than 1. It is concluded that differences in the nature of the organic matter in different soils and horizons make the simple regression approach unsuitable. More attention needs to be paid to studies of the nature of the organic matter.     

Differences in the chemical composition of dissolved organic matter from waste material of different sources

The chemical composition of waste-material-derived dissolved organic matter (DOM) was characterized by chemolytic analyses and ¹H, ¹³C and ³¹P nuclear magnetic resonance (NMR) spectroscopy. Dissolved organic matter was extracted by water from an aerobic fermented urban waste compost, a sewage sludge and a pig slurry and then fractionated using the XAD-8 method. The amount of water-extractable dissolved organic carbon (DOC) ranged from 3% in the sewage sludge to 22% in the pig slurry. Dissolved organic matter isolated from pig slurry was equally distributed between hydrophilic and hydrophobic DOC, whereas in the sewage-sludge-derived material the hydrophobic fraction was predominant. Dissolved organic C from the urban waste compost was mainly within the hydrophilic fraction. Wet-chemical analysis and ¹H- and ¹³C-NMR spectra showed that both DOM fractions from the urban waste compost were low in neutral, acidic and amino sugars as well as in lignin-derived compounds. In turn, the materials were rich in low-molecular-weight aliphatic compounds. The chemical structure of both fractions is probably the result of the intensive transformation of urban waste compost during its fermentation. The hydrophilic fractions of DOM from sewage sludge and pig slurry contained considerable amounts of carbohydrates but were also rich in low-molecular-weight aliphatics. The respective hydrophobic fractions had the largest contents of CuO-extractable phenols which may in part derive from sources other than lignin. By contrast with the other materials, the hydrophobic fraction from the pig slurry seemed to contain polymeric rather than low-molecular-weight material. The ³¹P-NMR spectrum of the hydrophilic DOM fraction from urban waste compost did not show signals of inorganic or organic P compounds while the spectrum of the hydrophobic fraction revealed traces of monoester P, diester P, and orthophosphate. ³¹P-NMR spectroscopy suggested that both the hydrophobic and hydrophilic fractions from pig slurry did not contain organic P. The hydrophilic DOM fraction from sewage sludge contained orthophosphate, organic monoester P and a little pyrophosphate. The hydrophobic fraction contained mainly organic diester P and smaller amounts of teichoic acids and organic monoester P. Considering that water-soluble fractions of urban waste compost contained no easily plant-available P and a low content of labile organics, we conclude that this material contains less labile nutrients and is more refractory than the soluble constituents of pig slurry and sewage sludge.     

Changes in soil organic matter composition after introduction of riparian vegetation on shores of hydroelectric reservoirs (Southeast of Brazil)

This work is part of a research program with the general objective of evaluating soil sustainability in areas surrounding hydroelectric reservoirs, which have been planted with riparian forest. The specific aims were: (i) to assess if and how the soil organic matter (SOM) chemical composition has changed in such areas, and (ii) to contribute to the knowledge of SOM chemistry in Brazil. To this end, we sampled litter and soil (Anionic Acrustox) in two adjacent areas: one under native vegetation and another forested with riparian species in 1992. The native vegetation was Brazilian savannah orcerrado. In this case, it was a ‘grassy cerrado’, dominated by grasses with few shrubs. Litter was collected and humic substances were extracted from soil by an alkaline solution. Both were characterised by a combination of cross-polarisation-magic angle spinning (CPMAS) solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy and pyrolysis-gas-chromatography/mass-spectrometry (Py-GC/MS). Eight years after forestation, the addition of the forest litter had changed SOM chemical composition. The C input pattern exerted a key role on the observed alterations. In the grassy cerrado, litter addition is predominantly below-ground and the litter is richer in carbohydrate-derived compounds and poorer in lignin moieties. In the forested area, C input is largely above-ground and grass litter has been partially replaced by a relatively more recalcitrant material. As a result, topsoil under forest was chemically strongly different from that under cerrado. Factor analysis indicated that the largest differences were between topsoil under forest and deepest subsoil under cerrado, where there is influence of remaining cerrado-derived C. Both semi-quantification and factor analysis of pyrolysis data gave further insight on the extent of alterations, but more research on such a quantitative approach should be developed to detail its application in SOM studies.     

Effects of different sources of organic matter on soil aggregate formation and stability: A laboratory study on a Lithic Rhodoxeralf from Turkey

In this study the influences of three different organic sources on aggregate formation and stability in soil were investigated.Our approach involves establishing plot experiment in the greenhouse with a Lithic Rhodoxeralf and a variety of carbon sources having specific chemical properties applied to the soil of individual plots. Treatments include K-Humate (KH, 25, 50 and 100 kg/ha), Concentrated Plant Extract (CPE, 50, 100, 200 kg/ha) and Molasses (M, 50, 100, 200 kg/ha). After a seven-month incubation period, aggregate formation and their stability were determined and their correlation to different C sources was developed.The effect of KH on aggregate formation was significant for 2–1 mm aggregates (P < 0.05) and 0.5–0.25 mm aggregates (P < 0.01). The effect of CPE on aggregate formation was significant for 0.5–0.25 aggregates (P < 0.05), while the M had no significant effect on aggregate formation in any aggregate size. The effect of KH on aggregate stability was significant for 8–4 mm and 1–0.5 mm aggregates (P < 0.01). The effect of CPE on aggregate stability was significant for 0.5–0.25 mm aggregates (P < 0.05), while the M was significant for 8–4 mm aggregates (P < 0.05).     

作物连作与自然恢复下黑土密度组分中碳、氮分布特征

本文以海伦农田生态系统国家野外科学观测站长期定位试验的黑土为研究对象, 通过对不同作物连作(玉米、大豆、小麦)和自然恢复(草地、裸地)下土壤及其密度分组中有机碳、氮含量的测定, 比较分析了土壤总有机碳、全氮以及密度组分碳、氮分布的变化特征。结果表明不同作物连作下土壤总有机碳的含量差异不显著。0~10 cm和10~20 cm土层农田土壤游离态轻组有机碳含量具有显著差异(P<0.05)。大豆连作的农田土壤游离态轻组有机碳主要分布在0~10 cm土层, 而小麦连作的土壤游离态轻组有机碳主要分布在10~20 cm土层。不同作物连作下土壤闭蓄态轻组有机碳含量差异不显著。不同组分中氮素具有与碳相似的分布特征。游离态轻组与闭蓄态轻组C/N比值之间呈显著负相关(P<0.05)。草地与农田、裸地相比显著提高了土壤总有机碳和全氮的含量。草地土壤游离态轻组、闭蓄态轻组和重组有机碳含量显著高于农田和裸地。作物连作和自然恢复导致土壤碳、氮的重新分配, 改变了土壤碳、氮的赋存特征。     

Total, particulate organic matter and structural stability of a Calcixeroll soil under different wheat rotations and tillage systems in a semiarid area of Morocco

Wheat production in Morocco is constrained by both scarce climate and degraded soil quality. There is an urgent need to revert production decline while restoring country’s soils. Among conservation tillage systems known for their improvement in yield, no-till technology was found to influence soil quality as well. Soil quality indices are also affected by wheat rotations at medium and long-terms. This paper discusses changes in selected properties of a Calcixeroll soil, including total and particulate soil organic matter (SOM), pH, total N and aggregation, subjected, for 11 consecutive years, to various conservation and conventional agricultural systems. Tillage systems included no-tillage (NT) and conventional tillage (CT). Crop rotations were continuous wheat, fallow–wheat, fallow–wheat–corn, fallow–wheat–forage and fallow–wheat–lentils. Higher aggregation, carbon sequestration, pH decline and particulate organic matter (POM) buildup are major changes associated with shift from conventional- to NT system. Better stability of aggregates was demonstrated by a significantly greater mean weight diameter under NT (3.8 mm) than CT system (3.2 mm) at the soil surface. There was 13.6% SOC increase in (0–200 mm) over the 11-year period under NT, while CT did not affect much this soil quality indicator. Another valuable funding is the stratification of SOC and total nitrogen in NT surface horizon (0–25 mm) without their depletion at deeper horizon compared to tillage treatments. Fallow–wheat system resulted in reduction of SOC compared to WW, but 3-year wheat rotation tended to improve overall soil quality. Benefits from crop rotation in terms of organic carbon varied between 2.6 and 11.7%, with fallow–wheat–forage exhibiting the maximum. Combined use of NT and 3-year fallow rotation helped to improve soil quality in this experiment.     

利用地统计学研究土壤有机质空间分布：评估意大利中部地区土地退化的重要指标

Soil organic matter (SOM) content is one of the main factors to be considered in the evaluation of soil health and fertility. As timing, human and monetary resources often limit the amount of available data, geostatistical techniques provide a valid scientific approach to cope with spatial variability, to interpolate existing data and to predict values at unsampled locations for accurate SOM status survey. Using geostatistical and geographic information system (GIS) approaches, the spatial variability of some physical and chemical soil parameters was investigated under Mediterranean climatic condition in the Abruzzo region of central Italy, where soil erosion processes accelerated by human induced factors are the main causes of soil degradation associated with low SOM content. Experimental semivariograms were established to determine the spatial dependence of the soil variables under investigation. The results of 250 soil sampling point data were interpolated by means of ordinary kriging coupled with a GIS to produce contour maps distribution of soil texture, SOM content related to texture, and C/N ratio. The resulting spatial interpolation of the dataset highlighted a low content of SOM in relation with soil texture in most of the surveyed area (87%) and an optimal C/N ratio for only half of the investigated surface area. Spatial location of degraded area and the assessment of its magnitude can provide decision makers with an accurate support to design appropriate soil conservation strategies and then facilitate a regional planning of agri-environmental measures in the framework of the European Common Agricultural Policy.     

A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO2 world

Understanding how elevated atmospheric CO₂ alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO₂ induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphere's concentration of CO₂. However, the long turnover times of many SOC pools - decades to centuries - make the detection of changes in the soil's pool size difficult. Long-term CO₂ enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO₂ has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO₂; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO₂ influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO₂. Specifically, we need to understand how elevated CO₂ influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.     

Near infrared reflectance spectroscopy: A tool to characterize the composition of different types of exogenous organic matter and their behaviour in soil

In addition to total organic carbon and nitrogen, potential organic carbon mineralization under controlled laboratory conditions and indicators such as the indicator of remaining organic carbon in soil (I_ROC), based on Van Soest biochemical fractionation and short-term carbon mineralization in soil, are used to predict the evolution of exogenous organic matter (EOM) after its application to soils. The purpose of this study was to develop near infrared reflectance spectroscopy (NIRS) calibration models that could predict these characteristics in a large dataset including 300 EOMs representative of the broad range of such materials applied to cultivated soils (plant materials, animal manures, composts, sludges, etc.). The NIRS predictions of total organic matter and total organic carbon were satisfactory (R²_P = 0.80 and 0.85, ratio of performance to deviation, RPD_P = 2.2 and 2.6, respectively), and prediction of the Van Soest soluble, cellulose and holocellulose fractions were acceptable (R²_P = 0.82, 0.73 and 0.70, RPD_P = 2.3, 1.9 and 1.8, respectively) with coefficients of variation close to those of the reference methods. The NIRS prediction of carbon mineralization during incubation was satisfactory and indeed better regarding the short-term results of mineralization (R²_P = 0.78 and 0.78, and RPD_P = 2.1 and 2.0 for 3 and 7 days of incubation, respectively). The I_ROC indicator was predicted with fairly good accuracy (R²_P = 0.79, RPD_P = 2.2). Variables related to the long-term C mineralization of EOM in soil were not predicted accurately, except for I_ROC which was based on analytical and well-identified characteristics, probably because of the increasing interactions and complexity of the factors governing EOM mineralization in soil as a function of incubation time. This study demonstrated the possibility of developing NIRS predictive models for EOM characteristics in heterogeneous datasets of EOMs. However, specific NIRS predictive models still remain necessary for sludges, organo-mineral fertilizers and liquid manures.     

环境与人为因子对水稻土有机质变化的影响——以长江中下游平原为例

Changes in soil organic matter (SOM) can affect food security,soil and water conservation,and climate change.However,the drivers of changes in SOM in paddy soils of China are not fully understood because the effects of agricultural management and environmental factors are studied separately.Soil,climate,terrain,and agricultural management data from 6 counties selected based on representative soil types and cropping systems in China were used in correlation analysis,analysis of variance,and cforest modeling to analyze the drivers of changes in SOM in paddy soils in the Middle and Lower Yangtze River Plain from 1980 to 2011.The aims of this study were to identify the main factors driving the changes in SOM and to quantitatively evaluate their individual impacts.Results showed that the paddy SOM stock in the study area increased by 12.5％ at an average rate of 0.023 kg m-2 year-1 over the 31-year study period.As a result of long-term rice planting,agricultural management practices had a greater influence than soil properties,climate,and terrain.Among the major drivers,straw incorporation,the most influential driver,together with fertilization and tillage practices,significantly increased the accumulation of SOM,while an increase in temperature significantly influenced SOM decomposition.Therefore,to confront the challenge of rising temperatures,it is important to strengthen the positive effects of agricultural management.Rational fertilizer use for stabilizing grain production and crop straw incorporation are promising measures for potential carbon sequestration in this region.     

Effect of the long-term application of organic matter on soil carbon accumulation and GHG emissions from a rice paddy field in a cool-temperate region,Japan-II. Effect of different compost applications-

ABSTRACT

The influence of long-term application of different types of compost on rice grain yield, CH₄ and N₂O emissions, and soil carbon storage (0 ? 30 cm) in rice paddy fields was clarified. Two sets of paddy fields applied with rice straw compost or livestock manure compost mainly derived from cattle were used in this study. Each set comprised long-term application (LT) and corresponding control (CT) plots. The application rates for rice straw compost (42 years) and livestock manure compost (41 years in total with different application rates) were 20 Mg fresh weight ha^–1. Soil carbon storage increased by 33% and 37% with long-term application of rice straw compost and livestock manure compost, respectively. The soil carbon sequestration rate by the organic matter application was 23% higher with the livestock manure compost than with the rice straw compost. The rice grain yield in the LT plot was significantly higher than that in the corresponding CT plot with both types of compost. Although the difference was not significant in the rice straw compost, cumulative CH₄ emissions increased with long-term application of both composts. Increase rate of CH₄ emission with long-term application was higher in the livestock manure compost (99%) than that in the rice straw compost (26%). In both composts, the long-term application did not increase N₂O emission significantly. As with the rice straw compost, the increase in CH₄ emission with the long-term application of livestock manure compost exceeded the soil carbon sequestration rate, and the change in the net greenhouse gas (GHG) balance calculated by the difference between them was positive, indicating a net increase in the GHG emissions. The increase in CH₄ and net GHG emissions owing to the long-term application of the livestock manure compost could be higher than that of the rice straw compost owing to the amount of applied carbon, the quality of compost and the soil carbon accumulation. The possibility that carbon sequestration in the subsoil differs depending on the type of composts suggests the importance of including subsoil in the evaluation of soil carbon sequestration by long-term application of organic matter.     

Changes in δC composition of soil carbonates driven by organic matter decomposition in a Mediterranean climate: A field incubation experiment

The current view on the relationship between the δ¹³C of pedogenic carbonates and soil organic matter is based on static studies, in which soil profiles are analysed at a given moment of their development. A dynamic approach to this question should also be possible by studying under field conditions how the δ¹³C of carbonates changes as organic matter decomposes. No such study has been undertaken owing to the slowness of the changes in the δ¹³C of carbonates, since it has been calculated that a detectable change will occur only after millenia. Nevertheless, this may not be true where soil CO₂ efflux is intense, as expected in soil zones with high microbial activity. In this paper we test the latter assumption by incubating mixtures of plant material and carbonate-rich red earth in the field at depths of 5, 20 and 40 cm. Four types of plant material were tested: Medicago sativa, Eucalyptus globulus, Quercus ilex and Pinus halepensis. Because the isotopic composition of these plant materials is known, we can determine the isotopic composition of the respired C and study how it relates to the (expected) changes in the δ¹³C. After two years of field incubation, the changes in δ¹³C of carbonates were high enough to be reliably detected and quantified, thus showing that the isotopic composition of soil carbonates can change quite rapidly in biologically active soil horizons. The observed changes are possible only if we assume that the increase in δ¹³C in the overall path respired C → pedogenic carbonate is much higher than the usually applied standard factors (about 15‰). These enrichments can be explained by assuming, as does the currently accepted paradigm, that the precipitation of new carbonates occurs in an open system in which the penetration of free-air CO₂ plays a major role. On the other hand, these enrichments can also be explained by an alternative interpretation, which assumes that the dissolution–precipitation carbonate cycles occur in systems that can be at least temporarily closed. Thus, we suggest that both possibilities (carbonate dissolution and precipitation in either an open or closed system) can coexist in a given soil, even though one or the other will dominate in any given time period.     

Effect of the long-term application of organic matter on soil carbon accumulation and GHG emissions from a rice paddy field in a cool-temperate region,Japan. -I. Comparison of rice straw and rice straw compost -

ABSTRACT

The influence of the long-term combination of rice straw removal and rice straw compost application on methane (CH₄) and nitrous oxide (N₂O) emissions and soil carbon accumulation in rice paddy fields was clarified. In each of the initial and continuous application fields (3 and 39?51 years, respectively), three plots with different applications of organic matter were established, namely, rice straw application (RS), rice straw compost application (SC) and no application (NA) plots, and soil carbon storage (0?15 cm), rice grain yield and CH₄ and N₂O fluxes were measured for three years. The soil carbon sequestration rate by the organic matter application was higher in the SC plot than in the RS plot for both the initial and continuous application fields, and it was lower in the continuous application field than in the initial application field. The rice grain yield in the SC plot was significantly higher than those in the other plots in both the initial and continuous application fields. Cumulative CH₄ emissions followed the order of the NA plot < the SC plot < the RS plot for both the initial and continuous application fields. The effect of the organic matter application on the N₂O emissions was not clear. In both the initial and continuous application fields, the increase in CH₄ emission by the rice straw application exceeded the soil carbon sequestration rate, and the change in the net greenhouse gas (GHG) balance calculated by the difference between them was a positive, indicating a net increase in the GHG emissions. However, the change in the GHG balance by the rice straw compost application showed negative (mitigating GHG emissions) for the initial application field, whereas it showed positive for the continuous application field. Although the mitigation effect on the GHG emissions by the combination of the rice straw removal and rice straw compost application was reduced by 21% after 39 years long-term application, it is suggested that the combination treatment is a sustainable management that can mitigate GHG emissions and improve crop productivity.