Humification Coefficient Research Articles

Low stabilization of aboveground crop residue carbon in sandy soils of Swedish long-term experiments

Soil organic carbon (SOC) storage can be increased by incorporating crop residues such as straw. However, the use of straw as a bioenergy source is an alternative option. There is currently great uncertainty concerning the effects of residue incorporation/removal, but estimates can be improved by using well-documented, frequently sampled long-term experiments (LTEs). This study examined the effect of straw incorporation on SOC stocks in six Swedish LTEs (duration of 27–56years), using data from 5 to 28 sampling occasions. A total of 16 pairs of straw incorporation (SI) and straw removal (SR) treatments were compared and modelled with the ICBM/2 model (two young pools with distinct humification coefficients, h), which enabled us to clearly isolate the effect of straw carbon input. The model results were compared to the Ultuna frame trial, as the first and only parameterization site of ICBM. At five out of six sites, the humification coefficient for straw (hlitter) was much smaller (0–0.09) than the ICBM default h value for plant material (0.125). The derived hlitter values and thus the stabilization of straw derived carbon increased significantly with clay content. An Italian site with five pairs of SI and SR treatments was used to test the performance of ICBM/2 under contrasting pedoclimatic conditions. Similar to the Swedish sites, the best model fits were found with hlitter values ranging from 0 to 0.05 increasing with nitrogen fertilization (range of 0–240kgNha−1yr−1), which was attributed to changes in substrate use efficiency of microbes. However, this trend was not consistent for all sites. For future applications of ICBM/2, we suggest using the validated clay function to derive hlitter for common levels of nitrogen fertilization. The efficiency of incorporating straw to increase SOC stocks depends on soil texture and using it for bioenergy production could be a more sustainable and climate-smart option.

Analysis of spatial distribution characteristics of dissolved organic matter in typical greenhouse soil of northern China using three dimensional fluorescence spectra technique and parallel factor analysis model.

The aim of the present work is to study the soil DOM characteristics in the vegetable greenhouse with a long-term of cultivation. Results showed that the soil DOM mainly consisted of three components, fulvic acid-like (C1), humic acid-like (C2) and protein-like (C3), with C1 as the majority one. The distribution of DOM in space was also studied. In vertical direction, C1 and C2 decreased significantly with the increase in soil depth, while C3 component decreased after increased. The humification coefficient decreased fast from 0-20 to 30-40 cm, and then increased from 30-40 to 40-50 cm. In the horizontal direction, the level of C2 component varied greatly in space, while that of C1 component changed little, and that of C3 component fell in between the above two. The change in the humification degree of each soil layer significantly varied spatially. Humification process of soil organic matter mainly occurred in the surface soil layer. In addition, the humification degree in space also changed significantly. The new ideas of this study are: (1) Analyze the composition and spatial heterogeneity of soil DOM in the vegetable greenhouse; (2) Use three dimensional fluorescence spectra technology and parallel factor analysis model successfully to quantify the components of soil DOM, which provides a new method for the soil DOM analysis.

Changes in organic carbon in topsoil and subsoil and microbial community composition caused by repeated additions of organic amendments and N fertilisation in a long-term field experiment in Sweden

The effects of 13 years of biennial application of four organic amendments (compost, green manure, farmyard manure and sewage sludge) on soil organic carbon (SOC) stocks, microbial biomass and community structure were compared with those of bare fallow, an unfertilised control and N-fertilised treatments. The experiment was conducted on a clay soil in western Sweden, in a randomised block design with four replicates. The crops grown were spring barley and oats. Changes in SOC mass were estimated by the ‘equivalent soil mass’ concept. Carbon inputs from crops were calculated from grain yields using linear allometric functions. The decomposition rate of soil organic matter was derived from a bare fallow treatment. Humification coefficients, defined here as the fraction of C input entering the SOC pool, for the organic amendments and crop residues were estimated by fitting a single-pool first order model to the final SOC stocks. The humification coefficient was highest for domestic waste compost (0.9) and lowest for above-ground crop residues (0.19). Crop yields were highest in sewage sludge-amended soil, probably due to favourable physical soil conditions, as indicated by lower bulk density (1.27–1.30gcm−3) than in the other treatments (1.32–1.38gcm−3). Despite addition of relatively high amounts of organic N, yields in other treatments receiving organic amendments were significantly lower than in those receiving mineral N fertiliser. In particular, the compost material was found to be highly recalcitrant, as indicated by its low C/N ratio (10) and low crop yield. Comparison of the correlation between phospholipid fatty acid (PLFA) concentration and SOC showed that compost supported relatively less microbial biomass than the other substrates tested. An important finding was that differences in SOC between treatments were significant to 40cm depth and that up to 27% of the SOC changes observed to 40cm depth occurred in the upper subsoil (25–40cm). Thus, SOC changes below ploughing depth should be considered in SOC balance studies.

Possible effect of soil organic carbon on its own turnover: A negative feedback

Soil carbon (C) sequestration plays an important role in mitigating climate change; therefore, it is essential to understand the mechanisms underlying soil organic carbon (SOC) turnover and dynamics. The humification coefficient of input organic materials (h) and the SOC decomposition rate (k) together control the SOC turnover, and they have been well known to be affected by factors including climatic factors, organic material qualities, soil characteristics, and anthropogenic activities. However, the SOC content changes over time, and thus far, whether the SOC content is an inherent factor has not been understood clearly. By using a new mathematical method, we found that SOC is possibly able to regulate its own turnover through a negative feedback: h decreases and k increases simultaneously with an increase in the SOC content. This feedback is caused by a series of chemical, biological, and physical mechanisms. We suggest that the response of the SOC pool to climate change is a function of the C input level to soil, temperature, and negative feedback of the SOC turnover; in addition, organic materials should be applied in priority to soils with low SOC content to enhance the soil C sequestration.

Biological activity of modern and buried dry-steppe chestnut soils

A comparative assessment of the biological activity of modern chestnut soils and chestnut soils buried beneath the Caucasian line set by Anna Ioannovna (1718–1720) on the dry steppes of the Volga Upland was carried out based on determination of enzymatic activity and the state of microbial communities. It was found that, in the modern soil, maximal values of microbial parameters and phosphatase activity, but a minimal value of the humification coefficient were observed in the A1 horizon, whereas in paleosoil, these values were found in the B1 horizon. This fact is consistent with the data on more humid and cold climate during the construction of the historical monument.

Influence of soil aggregation on SOC sequestration: A preliminary model of SOC protection by aggregate dynamics

Aggregation dynamics and soil organic carbon (SOC) fractions collected from long-term tillage trials at two sites in Illinois were used to develop a model to simulate the aggregate dynamics and physical protection of SOC. We used two litter pools which are surface litter and root litter and three SOC pools which are directly measurable from the fractionation: loose particulate organic matter (LPOM), aggregate-occluded particulate organic matter (OPOM), and humified fractions (HF). Decay rates of all of five pools were modified by soil temperature and moisture. In the model, the decay rate of LPOM was not influenced by any type of physical protection and the OPOM decay rate was influenced by dry aggregate mean weight diameter (DMWD) size. The effect of DMWD on OPOM decay rate was expressed as logistic equation based on the threshold value beyond which OPOM decay rate was influenced by the reactive mass concept which is that it is primarily outer layer of aggregates that participates in chemical and biological reactions. The decay of HF was influenced by clay contents. The relative aggregate turnover modified the humification coefficients. The faster aggregate turnover speeded the carbon transfer from LPOM to OPOM by providing more chances for organic matter to be incorporated with macroaggregates and retarded carbon transfer from OPOM to HF due to the fact that there is not enough time for organic mater to be associated with microaggregates and clay particles. Simulated results were compared against actual SOC fraction contents obtained from two long-term tillage trials located in Illinois, DeKalb (silty clay loam) and Monmouth (silt loam). Both actual and simulated data showed that after 10 and 17 years of no tillage (NT) practice adoption, OPOM content was increased at the surface in Monmouth and HF content was increased at the surface in DeKalb. Agreement between the output of aggregate dynamics-based model and actual data suggested that DMWD size, relative aggregate turnover, and their interaction with soil moisture and clay contents can be used to predict the inconsistent effects of tillage practices on SOC sequestration.

The impact of agroforestry combined with water harvesting on soil carbon and nitrogen stocks in central Chile evaluated using the ICBM/N model

Soil organic carbon (SOC) and total nitrogen (N) stocks in an agroforestry system with water harvesting were analysed in a field experiment and the results compared with those of other crop management systems in the Mediterranean zone of central Chile. Agroforestry with water harvesting showed higher positive effects on N stocks, mainly in the upper soil layer, than the other crop management systems. However, soil analysis revealed a lack of differences between treatments, a fact that might be related mainly to the short study time (12 years) and the high spatial variability in these soil properties at the experimental site. In addition, the Introductory Carbon Balance Model that simulates N processes (ICBM/N) was evaluated for simulating trends in SOC and N stocks in the field experiment. Soil data collected between 1996 and 2008 in the field experiment and primarily literature data sets were used to test ICBM/N and its performance was evaluated by considering uncertainty in model inputs using Generalised Likelihood Uncertainty Estimation (GLUE) methodology. The GLUE estimates (5% and 95%) and measured SOC and N stocks were in satisfactory agreement. The observed SOC and N stocks were bracketed by the uncertainty bands in 70% and 80% of the simulations, respectively. Sensitivity analysis showed the model to be most sensitive to C parameters, such as the humification coefficient (h). The results of this study show that ICBM/N can be an effective tool for estimating SOC and N stocks from agroforestry combined with water harvesting systems in the Mediterranean zone of central Chile over the medium term. However, they also indicate that additional data sets are needed to redefine the parameter distributions in the model and thus to predict trends in SOC and N stocks in the future.

Building Up Organic Matter in a Subtropical Paleudult under Legume Cover‐Crop‐Based Rotations

The potential of conservation management systems to ameliorate degraded agricultural soils and mitigate global warming is related to their potential for long‐term stabilization of soil organic matter (SOM). This study was performed in a 19‐yr‐old experiment that was set up on a degraded Paleudult (220 g kg−1 clay) in southern Brazil to (i) evaluate the effect of seven no‐till crop rotations (grass‐ and legume‐based cover crop systems) and mineral N fertilization (0 and 145 kg ha−1 yr−1) on soil organic C (SOC) stocks (0–17.5‐cm depth) and (ii) estimate rates of SOM dynamics in these systems under subtropical climate conditions. Annual C input (shoot + root) ranged from 2.61 to 7.84 Mg ha−1, with the highest values in legume‐based and N‐fertilized cropping systems. The SOC stocks were closely related to C input levels, and a minimum C input of 4.05 Mg ha−1 yr−1 was estimated to maintain the original SOC stock of 31.38 Mg ha−1 Based on the one‐compartment model of SOM dynamics, the SOM decomposition rate was estimated to be 1.2% and the humification coefficient was estimated to be 9.6%. After 19 yr, the stock of the original SOC decreased to about 24.78 Mg ha−1, while accumulation of SOC derived from the crops ranged from 4.26 to 12.79 Mg ha−1 Our results highlighted the benefits of legume cover crop species in no‐till systems for the stabilization of SOC in degraded agricultural soils.

Residual Nitrogen Effects in Organically Cultivated Beetroot Following a Harvested/Green Manured Grass-Clover Ley

ABSTRACT The residual nitrogen (N) effect on beetroot following harvested grass-clover (H-ley) or corresponding twice-cut green manure (GM-ley) was compared with a barley control. The residual N at beetroot harvest, calculated as N in beetroot crop and mineral N in the 0–90 cm soil layer, was 83 kg ha−1 for the GM-ley when barley was set to zero. The effect of GM-ley exceeded that of 3H-ley (3 harvests) by 74 kg N ha−1 and 2H-ley (2 harvests) by 42 kg N ha−1. Prediction of residual effects was studied using a model with a humification coefficient (hc) of 0.35 and another with different hc values for different plant materials. The former gave the best fit to observed data in regression analysis. The latter gave the best fit to observed data in absolute values. The residual N effect of GM biomass from summer cuttings corresponded to 40–70% of the potential effect.

Changes in soil organic matter under different land management in misiones province (Argentina)

Highly weathered tropical soils rapidly loose soil organic matter (SOM) and may be affected by water erosion and soil compaction after deforestation and intensive cultivation. With the main objective to estimate the SOM balances in a subtropical soil we determined the dynamics of SOM in a degraded yerba mate (Ilex paraguaiensis Saint Hil.) plantation introduced after deforestation and with elephant grass (Pennisetum purpureum L.) as a cover crop. The study site was in Misiones, Argentina, and we use the natural 13C abundance methodology and a descriptive model. The study was conducted on three contiguous 50 x 100 m plots of a typic Kandihumult soil with: (i) native forest, (ii) 50 years of continuous yerba mate monoculture with intensive tillage, and (iii) yerba mate associated with elephant grass as a cover crop and no tillage. We determined bulk density, carbon (C), nitrogen (N) and 13C content of the soil (0 - 0.05, 0.05 - 0.15 m layers) and the grass biomass. Yerba mate monoculture reduced soil C and N content as well as porosity at 0 - 0.15 m depth by 43 and 23%, respectively, as compared to the native forest. After ten years of yerba mate - elephant grass association soil C and N contents at the same depth increased by 19 and 12%, respectively, compared to the yerba mate monoculture, while soil porosity remained similar. Total C input,13C, and soil organic C were incorporated into a three compartment model to evaluate elephant grass C dynamics. Through the natural 13C abundance methodology we tracked the elephant grass C incorporation and the "old" soil C loss, and determined the model parameters - humification (k1) and mineralization (k) coefficients and stable C (Cs)- unambiguously. The high k1 and k predicted by the model are probably explained by elephant grass root system incorporation under no tillage and humid subtropical climate, respectively. In soil under yerba mate monoculture, Cs was counted as 91% of the total soil organic C.

Modeling consequences of straw residues export on soil organic carbon

Cereal straw, which is most often returned to the soil in arable cropping systems, is of renewed interest as a potential source of bioenergy. However, the sustainability of this practice which implies systematic removal of aerial biomass of cereal crops is a controversial issue, particularly in soils having a low soil organic carbon (SOC) content. This study aims at evaluating a simple model (AMG) to predict the consequences of straw export on SOC evolution in various cropping and pedoclimatic conditions. The model was tested on nine long-term field experiments (18–35 yr) dominated by cereal crops and differing in climate, soil type and carbon inputs. The model was able to provide satisfactory simulations of the evolution of SOC in most experiments with a unique set of parameters. The sensitivity analysis indicated that the quality of fit was very sensitive to humification coefficient, moderately sensitive to the size of the stable SOC pool and weakly affected by the ratio of belowground: aerial C input. The dependence of model parameters (humification and mineralization rates) on pedoclimatic conditions (soil clay content and temperature) was analyzed and compared to those proposed in other models (DAISY, CENTURY, ROTHC, CN-SIM) since they vary widely between models. AMG functions provided the best fit in seven out of nine experiments. More generally, the best fit was obtained by assuming that clay content had a small or no effect on humification coefficient and a marked effect on mineralization rate, in accordance with incubation studies in literature. The AMG model was used to simulate the impact of a straw export scenario in nine experiments considering a systematic straw removal one year out of two. With this scenario, straw removal vs. incorporation would reduce carbon stocks by 2.5–10.9% of the initial SOC after 50 yr, depending primarily on the experiment (soil, climate, productivity) and secondarily on the size of the stable C pool (varying from 10% to 65%).

Projections of 30-year soil carbon balances for a semi-natural grassland under elevated CO2 based on measured root decomposability

Abstract Long-term soil C projections were made for a semi-natural grassland under CO 2 elevation in Central Sweden based on treatment-induced differences in input quantity, input quality and measured microclimate. Three treatments were applied to the grassland during 6 years: Ambient and Elevated (+350 μmol mol −1 ) CO 2 levels in open-top chambers and Control. Roots grown during the fifth experimental year were incubated under controlled conditions for 160 days and showed significant treatment differences in decomposition rates. The fraction of C lost during incubation of Elevated was 57% while only 53% for Ambient and 52% for Control. The incubation results were fit to a decomposition model and a humification coefficient was used to account for treatment-induced differences in root quality. Annual soil C input was calculated considering both root and shoot input estimates and was 103, 74 and 92 g C m −2 year −1 for Elevated, Ambient and Control, respectively. A climate factor represented measured microclimate differences. Due to drier conditions this factor was somewhat lower for Ambient than for Control and Elevated. The ICBM model was used for 30-year soil C projections. The input, quality and climate parameters for Control resulted in projections indicating that the present measured soil C store, 5.5 kg C m −2 at 0–15 cm, is near steady-state. The soil C pool in Ambient was projected to lose 90 g C m −2 in 30 years primarily due to the decreased input. Although Elevated had the greatest input, this did not compensate for the increased root decomposability and Elevated was projected to lose 70 g C m −2 . We discuss the validity of the projections and test other possible scenarios. A tentative conclusion is that the expected CO 2 and temperature increase during the next 30 years will have only minor effects on the soil carbon content of this system, unless plant production is severely reduced by weather irregularities or even disasters.

Adição de carbono e nitrogênio e sua relação com os estoques no solo e com o rendimento do milho em sistemas de manejo

Experimentos de longa duração constituem ferramenta valiosa para o estudo da dinâmica da matéria orgânica no solo. Com esse objetivo, realizou-se a presente pesquisa a qual foi baseada em experimento instalado, em 1985, em um Argissolo Vermelho degradado pelo cultivo inadequado por 16 anos. O experimento está localizado na Estação Experimental Agronômica da UFRGS, em Eldorado do Sul (RS), e consiste em três sistemas de preparo de solo (preparo convencional-PC, preparo reduzido-PR, e plantio direto-PD) em parcelas principais, três sistemas de culturas (aveia/milho-A/M, ervilhaca/milho-E/M, e aveia + ervilhaca/ milho + caupi-A + E/M + C), em subparcelas, e duas doses de N mineral na cultura do milho (0 e 139 kg ha-1), em subblocos, distribuídos segundo um delineamento de blocos ao acaso, com três repetições. As adições de C e N pelas culturas foram estimadas para o período experimental de 13 anos; o solo foi amostrado, em setembro de 1998, em seis camadas, até à profundidade de 0,30 m, analisado em relação aos teores de carbono orgânico total (COT) e nitrogênio total (NT), tendo sido calculados os estoques desses elementos nas camadas de 0-0,175 e 0,175-0,30 m. As leguminosas contribuíram para a maior adição anual de C e N ao solo pelos sistemas de culturas, que variaram de 4,17 a 8,39 Mg ha-1 e de -21 a 178 kg ha-1, respectivamente. As maiores adições de C e N refletiram-se num maior acúmulo de COT e NT na camada de 0-0,175 m do solo em PD e em gradativa elevação do rendimento do milho nos três sistemas de preparo. A adição anual de C (A) necessária para manter o estoque inicial de COT (dC/dt = zero) foi estimada em 4,2 Mg ha-1 em PD, 7,3 Mg ha-1 em PR e 8,9 Mg ha-1 em PC. De forma similar, a adição anual de N necessária para manter os estoques iniciais (dN/dt = zero) foi estimada em 5 kg ha-1 para o PD, 31 kg ha-1 para o PR e 94 kg ha-1 para o PC. Estimou-se, pelo coeficiente angular da regressão linear que relaciona as taxas anuais de adição de C (A) e a variação anual nos estoques de COT na camada de 0-0,175 m do solo (dC/dt), que 12,9, 8,1 e 11,5 % do C adicionado ao solo foi retido na matéria orgânica do solo em PD, PR e PC, respectivamente, o que corresponde a uma estimativa aproximada do coeficiente de humificação (k1). Analogamente, estimou-se que 49,7, 21,0 e 33,1 % da quantidade líquida de N adicionada foi retida como NT no solo em PD, PR e PC, respectivamente. A taxa de perda de COT do solo (k2), calculada para a condição dC/dt = zero, foi de 0,0166 ano-1 no solo em PD, 0,0182 ano-1 no solo em PR, e 0,0314 ano-1 no solo em PC. O plantio direto, pela diminuição da taxa de perda de matéria orgânica (k2), e os sistemas de culturas com leguminosas, pela alta adição de C fotossintetizado e de N fixado simbioticamente, são boas alternativas para recuperar os estoques de COT e NT do solo e aumentar o rendimento do milho na região subtropical do Sul do Brasil.

Studies of the preceding crop effect of ley in ecological agriculture

AbstractTwo field experiments were conducted from 1991 to 1996 on clay soils in central Sweden to provide information for improving soil fertility, minimizing N leaching, and increasing the benefits of ley to subsequent crops in organic farming. The results show that it is possible to calculate the amount of N mineralized during the 2-year period following incorporation of ley-crop residues, based on the proportion of incorporated organic matter stabilized in the more resistant humus fractions (i.e., the humification coefficient, calculated to be 35–40%), C:N ratios of the ley biomass, and ley age (humification appears to be higher in older crop residues). The fractions of potentially mineralizable N that are actually mineralized in the first and second years after ley incorporation vary depending on ley age and botanical composition and climatic conditions.

Long-term agricultural field experiments in Northern Europe: analysis of the influence of management on soil carbon stocks using the ICBM model

Land use in general and particularly agricultural practices can significantly influence soil carbon (C) storage. Changes in topsoil C mass measured in long-term agricultural field trials in Sweden and other Northern European countries were compiled and aggregated into seven treatment classes, including manured, fertilized and straw exported. The impact of crop rotations and management practices on C stocks in the topsoil was analyzed using both a static model and a dynamic soil carbon model (introductory C Balance Model; ICBM). ICBM consists of two state variables and four fluxes (governed by four rate-determining parameters), and one parameter, r e, combining most external factors affecting C mineralization (temperature, precipitation, drainage, etc.). Simple `front-end models' were used to estimate values for two of the parameters, i (annual C input) and h (humification coefficient) initially based on incomplete records from the field trials, official agricultural statistics and other literature. The r e parameter was then optimized for each class of treatments, using an algorithm for non-linear least squares. Initial soil C mass, present C inputs and abiotic conditions, such as soil temperature and moisture, were the deciding factors in whether C stocks declined or increased. Steady-state values calculated using the static and dynamic model were similar for both models, but differed greatly between treatment classes. For cereal-dominated cropping systems where the straw was removed, manure application increased steady-state values about three times (from 3 to 9 g C m −2), compared with corresponding treatments. Incorporation of straw resulted in intermediate steady-state values (5–6 g C m −2). C mineralization rates were highest in bare fallow treatments. For one class of soils, C retention in the soil was found to increase significantly with increasing clay content. The hypothesis that the climatic gradients in Northern Europe affect decomposition and primary production rates similarly, leading to the same soil C stocks, could not be rejected.

Nitrogen mineralization in relation to C:N ratio and decomposability of organic materials

A desk study was made on N mineralization of various organic materials. Data were obtained from the literature containing information on mineralized organic N, C/N of the substrate and some index for decomposability. Per class of substrate decomposability, linear regression lines between N mineralization and substrate C/N were established, from which ‘apparent initial age’ and humification coefficient were calculated to indicate decomposability. For quantitative grading, a ‘resistance index’ was formulated comprising the concentrations of lignin and polyphenols. The results confirmed that the fraction of mineralized organic N is linearly related to substrate C/N for equally decomposable materials. The nature of the organic materials and the processes to which they had been subjected were reflected in the ranking by decomposability.

Effects of changing land use in the Netherlands on net carbon fixation.

The changed crop rotation on arable land, the decreasing grassland area and the increase in forest area in the Netherlands resulted in a decrease in C pool size. For the calculation of this C pool a method requiring only three input data (average amount of crop or tree residue rate, soil organic matter decomposition and the humification coefficient) has been applied. However the method can only be applied to situations in equilibrium where all three input data are equal. For a changing land use a new state of equilibrium and rate of change in C pool size can be calculated. (Abstract retrieved from CAB Abstracts by CABI’s permission)