Soil Organic Matter Mineralization Research Articles

Biochar produced from the straw of common crops simultaneously stabilizes soil organic matter and heavy metals

The simultaneous stabilization of heavy metals and organic matter in polluted soil has received little research attention. In this study, we studied the immobilization of Cu and Cd and the mineralization of organic matter in the acidic soil amended with biochar produced from rice, wheat, corn, and rape straws through incubation experiments. Compared with that in the control treatment, the availability of Cu and Cd in the biochar amended soils decreased by 17–31% and 3–17%, respectively. The cumulative amount of CO2 released from each treatment in 60 days of incubation followed the order: control treatment (399 mg CO2-C kg−1) > rape straw biochar treatment (388 mg CO2-C kg−1) > rice straw biochar treatment (374 mg CO2-C kg−1) > corn straw biochar treatment (355 mg CO2-C kg−1) > wheat straw biochar treatment (288 mg CO2-C kg−1). The information implied that biochar produced from the straw of common crops can simultaneously stabilize both heavy metals and organic matter in the acidic soil. The transformation of Cu and Cd from acid soluble fraction to residual fraction was the potential mechanism of biochar in facilitating soil heavy metal immobilization. The significant decrease in soil β-glucosidase activity, which controlled the degradation of soil organic matter, was an important potential pathway of biochar in decreasing soil organic matter mineralization. A significant decrease in the content and a substantial increase in the structural complexity of soil dissolved organic matter could further the decrease of wheat straw biochar in soil organic matter mineralization. Thus, biochar produced from the straw of common crops is a promising amendment for simultaneously stabilizing both heavy metals and organic matter in the acidic soil.

Microbial carbon use efficiency and priming of soil organic matter mineralization by glucose additions in boreal forest soils with different C:N ratios

During the last decade it has been increasingly acknowledged that carbon (C) contained in root exudates can accelerate decomposition of soil organic matter (SOM), a phenomenon known as rhizosphere priming effect (RPE). However, the controlling factors and the role of different soil microorganisms in RPE are not yet well understood. There are some indications that the response of the soil microbial decomposers to labile C input in the rhizosphere depends on microbial demand of nutrients for growth and maintenance, especially that of C and nitrogen (N). To test this hypothesis, we assessed SOM decomposition induced by 13C-glucose additions during one week in forest soils with different C:N ratios (11.5–22.2). We estimated SOM respiration, the potential activity (concentration) of a range of extracellular enzymes, and incorporation of 13C and deuterium (D) in microbial phospholipid fatty acids (PLFAs).Glucose additions induced positive priming (a 12–52% increase in SOM respiration) in all soil types, but there was no linear relationship between priming and the soil C:N ratio. Instead, priming of SOM respiration was positively linked to the C:N imbalance, where a higher C:N imbalance implies stronger microbial N limitation. The total oxidative enzyme activity and the ratio between the activities of C and N acquiring enzymes were lower in soil with higher C:N ratios, but these findings could not be quantitatively linked to the observed priming rates. It appears as if glucose addition resulted in priming by stimulating the activity rather than the concentration of oxidative enzymes. Microbial incorporation of D and 13C into in PLFAs demonstrated that glucose additions stimulated both fungal and bacterial growth. The increased growth was mainly supported by glucose assimilation in fungi, while the increase in bacterial growth partly was a result of increased availability of C or N released from SOM. Taken together, the findings suggest that the soil C:N ratio is a poor predictor of priming and that priming is more dependent on the C:N imbalance, which reflects both microbial nutrient demand and nutrient provision.

Network analysis reveals bacterial and fungal keystone taxa involved in straw and soil organic matter mineralization

Understanding the effects of straw return and N fertilization on soil organic matter (SOM) transformations will help maintain crop production and soil function, which can ultimately contribute to mitigating climate change. In this study, we conducted a 100-day soil incubation experiment with the addition of 13C-labeled maize straw and/or N fertilization. Soils that were used in the study included soil without fertilizers (Control), with mineral fertilizer alone (NPK), and with mineral fertilizer and straw (NPK+Straw). Compared with the control, the NPK- and NPK+Straw-treated soils showed higher straw decomposition by 59% and 55%, and SOM mineralization by 27% and 37%, respectively, although the priming effect was decreased by 59% and 39%, respectively. The priming effect (PE) was higher with increased N content and lower with decreased N additions because of an improved C/N ratio for microorganisms. Straw additions compared to without straw increased the bacterial and fungal abundance by 1.4- and 4.9-fold, respectively. N fertilization lowered C/N ratios resulting in decreased fungal diversity. Although the bacterial abundance decreased, the diversity increased with the duration of incubation as the bacteria preferred to utilize the labile organic compounds that were abundant in the initial stages of incubation. In addition to the depletion of labile organic compounds, the fungal abundance increased. Bacteria (Firmicutes, Actinobacteria, and Proteobacteria phyla) and fungi (Ascomycota, Basidiomycota, and Mucoromycota phyla) dominated straw and SOM decomposition. Firmicutes were mostly involved in straw and SOM mineralization during the first day after straw addition. The edge number and ratio for pairwise correlations between environmental factors and fungal taxa (22.1–24.6%) were greater than those with bacterial taxa (1.0–2.9%) in the microbial correlation network. Overall, straw combined with a low level of added N benefited soil C sequestration by decreasing the PE. Compared to bacteria, the functional role of fungi in SOM mineralization is more prominent and should be considered during agricultural management.

Testing the environmental controls of microbial nitrogen-mining induced by semi-continuous labile carbon additions in the subarctic

Climate warming and shrubification will affect soil carbon (C) cycling in arctic ecosystems. Rhizosphere inputs from increased plant productivity by shrubification may stimulate the mineralization of old soil organic matter (SOM), termed the “priming effect”. However, increased soil nitrogen (N) availability due to warming-accelerated mineralization, and litter inputs associated with shrubification could modulate this response. In this study, we investigated how N-availability affects the priming of SOM mineralization in subarctic soils by adding labile organic matter (OM) including 13C-glucose with and without mineral N, or 13C-alanine, into soils with different N availabilities resulting from inorganic N and/or litter addition field-treatments. Rather than as a single pulse addition, labile OM additions were administered semi-continuously every other day to simulate rhizosphere conditions. We found that semi-continuous additions of labile OM induced a sustained priming of SOM mineralization, and this was linked to a sustained stimulation of bacterial and fungal growth over time, despite a reduced microbial growth efficiency. The priming of soil N mineralization was higher than the priming of soil C mineralization, indicating a selective microbial N-mining that was particularly pronounced in more N-poor soils. However, microbial N-mining showed a declining trend over time, suggesting a shift from the most N-rich compounds to less N-rich compounds, presumably as reservoirs were exhausted. The priming effect controlled by N-mining was associated with a stimulation of bacterial and fungal growth depending on the form of labile OM: Alanine induced higher priming of soil C mineralization by stimulating bacterial growth, while glucose induced lower priming of soil C mineralization by stimulating fungal growth. These results indicate that bacteria and fungi can both drive the priming of SOM mineralization in subarctic soils. Based on microbial biomass and growth rates, it could be estimated that over 90% of the observed priming of both soil C and N mineralization were due to changes in turnover of “old” SOM rather than of the microbial biomass pool (i.e. “real” rather than “apparent” priming). Overall, our findings suggest that increased rhizosphere inputs could increase soil N availability by enhanced microbial N-mining, generating a positive feedback to plant productivity in the subarctic. In contrast, increased soil N availability could reduce soil C release through alleviated microbial demand for N.

Wheat nitrogen utilization efficiency and yield as affected by nitrogen management and environmental conditions

Nitrogen (N) is the main nutrient for plant nutrition, however, its fertilization management is still very complex. To evaluate wheat N utilization efficiency (NUtE) and yield in response to N fertilization management considering the influence of environmental conditions, an experiment was carried out in three field conditions in Southern Brazil: Londrina in rainfed and irrigation conditions, and Ponta Grossa in rainfed. A complete (2 × 2 × 2) + 1 factorial arrangement evaluated two N rates (40 and 80 kg ha-1), two N sources (ammonium nitrate and urea), two N timings of fertilization (at sowing or at the beginning of plant tillering), and additional treatment without N fertilization. Agronomic characteristics related to wheat productivity and N plant nutrition were evaluated. Irrigation increased the density of fertile spikes and the N accumulated in the shoot dry matter at anthesis, which partially explained the better grain yield in the irrigated condition. The higher N accumulation in the shoot dry matter was essential for grain yield increase in the environments with lower water deficit, based on their higher NUtE for grain yield. Nitrogen fertilization reduced NUtE for grain yield in Londrina (in rainfed and irrigation conditions), and increased NUtE for shoot dry matter production in all environments. In a colder condition (i.e. Ponta Grossa), the use of a higher N rate at sowing provided greater grain yield, thousand-grain weight, and density of fertile spikes. Nitrogen rate can be reduced in warmer and wetter environmental conditions that favor the mineralization of soil organic matter and the decomposition of soybean straw. The choice for urea or ammonium nitrate can be based on economic criteria in environments with low water deficit and low potential for NH3 volatilization. Nitrogen fertilization carried out exclusively at wheat sowing may be suitable to supply spring wheat N requirements.

Impacts of hydrological conditions on the activities of soil enzymes in temperate floodplain forest sites

Context The development and survival of floodplain forests are dependent on the occurrence of seasonal flooding that provides soil moisture and nutrient availability suitable for the development of soil microorganisms, which are the main sources of soil enzymes involved in the decomposition and mineralisation of soil organic matter. Aims We compared the soil enzyme activities of a floodplain forest in the Lower Vistula Valley, Poland, cut off for 80 years from the river by artificial embankment, and of two other floodplain forests located in close vicinity and subjected to seasonal flooding. We hypothesised that inhibiting the inflow of the river water to the floodplain forest will alter the soil water and mineral conditions, decreasing the activity of extracellular soil enzymes. Methods Enzyme activity, soil moisture content, and pH were monitored for 3 years in the upper soil level (0–30 cm). The activities of soil β-glucosidase, acid phosphatase, alkaline phosphatase, and nonspecific dehydrogenase were determined by spectrophotometry. Key results Inhibition of floods decreased soil moisture, the concentrations of carbon, nitrogen, and phosphorus, as well as the activity of soil enzymes. Conclusions Forest site and soil depth, but not season, had significant effects on soil chemical and biological features. Soil enzyme activities were significantly positively correlated with soil moisture and the levels of total and organic carbon, total nitrogen, and available phosphorus, as well as the carbon/nitrogen ratio. Implications This information is essential for the protection and restoration of riverine habitats destroyed by human activity.

How Physical Disturbance and Nitrogen Addition Affect the Soil Carbon Decomposition?

The decomposition of soil organic carbon (SOC) plays a critical role in regulating atmospheric CO2 concentrations and climate dynamics. However, the mechanisms and factors controlling SOC decomposition are still not fully understood. Here, we conducted a 60 days incubation experiment to test the effects of physical disturbance and nitrogen (N) addition on SOC decomposition. N addition increased the concentration of NO3- by 51% in the soil, but had little effect on the concentration of NH4+. N addition inhibited SOC decomposition, but such an effect differed between disturbed and undisturbed soils. In disturbed and undisturbed soils, application of N decreased SOC decomposition by 37% and 15%, respectively. One possible explanation is that extra N input suppressed microbial N mining and/or increased the stability of soil organic matter by promoting the formation of soil aggregates and incorporating part of the inorganic N into organic matter, and consequently decreased microbial mineralization of soil organic matter. Physical disturbance intensified the inhibition of N on SOC decomposition, likely because physical disturbance allowed the added N to be better exposed to soil microbes and consequently increased the availability of added N. We conclude that physical disturbance and N play important roles in modulating the stability of SOC.

西南喀斯特区坡耕地秸秆覆盖对土壤生态化学计量特征及产量的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 西南喀斯特区坡耕地秸秆覆盖对土壤生态化学计量特征及产量的影响 DOI: 10.5846/stxb202104030867 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（31760243，32060372）；贵州省科技计划项目（黔科合支撑[2021]一般462号） Effects of straw mulching on soil ecological stoichiometry characteristics and yield on sloping farmland in karst area, Southwestern China Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤养分亏缺是限制作物生长的关键因素，同时也是制约作物产量的重要影响因子。为提高西南喀斯特区坡耕地土壤肥力和作物产量，于2018-2019年连续两年在贵州省黔西县开展了野外原位径流小区观测试验。通过该试验探讨不同秸秆覆盖率下土壤碳（C）、氮（N）、磷（P）、钾（K）含量及其生态化学计量特征，揭示不同秸秆覆盖率下土壤养分状况及土壤改良效果。共设6个秸秆覆盖梯度，玉米单作＋秸秆覆盖（SM0-SM5，0，1111、2222、3889、5556，6944 kg/hm2），其中SM0为对照组（CK）。结果表明：（1）秸秆覆盖不同程度增加了土壤有机碳（SOC）、全氮（TN）及全磷（TP）含量，总体上随覆盖量的增加而增加，尤其是覆盖量较多的情况下（SM4，SM5），均显著高于对照（P<0.05），但两者之间差异不显著（P > 0.05），而全钾（TK）则随着覆盖量的增加而减少。（2）在高秸秆覆盖条件下（SM4，SM5），除了N ： P外，两年各土壤养分元素的化学计量比均显著高于对照（P<0.05），但SM4和SM5之间差异不显著（P > 0.05）。（3）秸秆覆盖条件下土壤C ： N、C ： P较低，加速了土壤有机质矿化分解，同时也说明P的有效性较高。（4）高秸秆覆盖率条件下对作物增产较为显著，其中SM4处理作物产量最高。研究表明覆盖于地表的秸秆作为土壤营养元素的重要来源，可将其作为一种补充手段以补给土壤中被作物吸收和随水土流失而流失的土壤养分。综合主成分、灰色关联度及差异显著性分析，同时结合作物产量，建议在西南喀斯特坡耕地覆盖6000 kg/hm2左右的秸秆以促进农田生态系统的恢复，从而达到较好的土壤增肥、作物增产效果。 Abstract:Soil nutrient deficiency is a key factor limiting crop growth and yield, but also threatens the survival of crops. In order to improve soil fertility and crop yield of sloping farmland in karst area of Southwestern China, the experiments were carried out in Qianxi county, Guizhou province for two consecutive years from 2018 to 2019. We explored the contents and the ecological stoichiometry characteristics of soil carbon, nitrogen, phosphorus, potassium and revealed the soil nutrient status and soil improvement effect under different straw mulching application rates through this experiment. There were six straw mulching gradients of maize monoculture+straw mulching (SM0-SM5, 0, 1111, 2222, 3889, 5556, 6944 kg/hm2), in which SM0 was the control cheak (CK). The results showed that:(1) straw mulching increased the contents of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) in different degrees, and generally increased with the increase of mulching rate. Especially in SM4 and SM5, which were significantly higher than those of the CK (P<0.05), but there was no significant difference between them (P > 0.05), while total potassium (TK) decreased with the increase of mulching rate. (2) Under the condition of high straw mulching (SM4, SM5), the stoichiometry ratio of soil nutrient elements in two years was significantly higher than that of the CK (P<0.05), except for N:P, but there was no significant difference between SM4 and SM5 (P>0.05). (3) Under the condition of straw mulching, the C:N and C:P in soil were lower, which accelerated the mineralization and decomposition of soil organic matter, and also showed that the availability of P was higher. (4) The crop yield of each mulching treatment was significantly higher than that of the CK (P<0.05), especially under the condition of high straw mulching, the yield of crop increased significantly, and the yield of SM4 was the highest. The research showed that straw mulch on the ground is an important source of soil nutrients, which can be used as a supplementary means to supplement the soil nutrients absorbed by crops and lost with soil erosion. Based on the crop yield, the principal component analysis, grey correlation analysis, difference significance, it is suggested that straw mulching in about 6000 kg/hm2 should be adopted for sloping farmland in the karst area of Southwestern China to promote the restoration of farmland ecosystem and achieve better effect of soil fertility and crop yield. 参考文献 相似文献 引证文献

Plant-microbial linkages underpin carbon sequestration in contrasting mountain tundra vegetation types

Tundra ecosystems hold large stocks of soil organic matter (SOM), likely due to low temperatures limiting rates of microbial SOM decomposition more than those of SOM accumulation from plant primary productivity and microbial necromass inputs. Here we test the hypotheses that distinct tundra vegetation types and their carbon supply to characteristic rhizosphere microbes determine SOM cycling independent of temperature. In the subarctic Scandes, we used a three-way factorial design with paired heath and meadow vegetation at each of two elevations, and with each combination of vegetation type and elevation subjected during one growing season to either ambient light (i.e., ambient plant productivity), or 95% shading (i.e., reduced plant productivity). We assessed potential above- and belowground ecosystem linkages by uni- and multivariate analyses of variance, and structural equation modelling. We observed direct coupling between tundra vegetation type and microbial community composition and function, which underpinned the ecosystem's potential for SOM storage. Greater primary productivity at low elevation and ambient light supported higher microbial biomass and nitrogen immobilisation, with lower microbial mass-specific enzymatic activity and SOM humification. Congruently, larger SOM at lower elevation and in heath sustained fungal-dominated microbial communities, which were less substrate-limited, and invested less into enzymatic SOM mineralisation, owing to a greater carbon-use efficiency (CUE). Our results highlight the importance of tundra plant community characteristics (i.e., productivity and vegetation type), via their effects on soil microbial community size, structure and physiology, as essential drivers of SOM turnover. The here documented concerted patterns in above- and belowground ecosystem functioning is strongly supportive of using plant community characteristics as surrogates for assessing tundra carbon storage potential and its evolution under climate and vegetation changes.

Addition of iron to agricultural topsoil and subsoil is not an effective C sequestration strategy

The interaction of soil organic matter (SOM) with Fe-containing minerals represents a key mechanism that promotes carbon (C) stabilisation in soil. The addition of Fe-rich industrial by-products to soil may therefore help accelerate C storage. Our understanding of the effects of exogenous Fe addition (Fe (oxy)hydroxide vs. Fe chloride) on SOM dynamics and C dynamics in agricultural soils, especially in subsoils, however, remains poor. Here, we simulate the addition of Fe in an arable soil context and assess its effectiveness based on CO2 emissions and soil chemistry. We hypothesised that insoluble and soluble Fe would reduce the mineralization of newly added unprotected organic materials more than native SOM and that soluble Fe would cause mineralisation of native SOM. To investigate this, insoluble Fe(OH)3 or soluble FeCl2 (0–5 g kg−1) were added to arable top- (0–10 cm) or subsoils (50–60 cm) and CO2 emissions, pH and nutrient dynamics (e.g. P, N) measured in a laboratory incubation over a 45 d period. We also compared the effect of Fe on the turnover of native SOM and newly added C (i.e. 14C-labelled glucose, citrate and crop residues) which was pre-mixed with exogenous Fe. We found that: (1) despite a reduction in P and DOC, Fe(OH)3 did not suppress total CO2 efflux; (2) high FeCl2 rates induced a rapid and significant release of CO2, which we attribute almost entirely to FeCl2-induced soil acidification increasing DOC availability and carbonate dissolution; (3) 14C-substrate mineralisation was weakly suppressed by Fe(OH)3 but strongly by FeCl2 following the series: citrate < glucose < crop residues; and (4) Fe addition to subsoils induced a stronger C mineralisation response but weaker effect on soil solution chemistry compared to topsoil, possibly due to subsoils having a lower buffering ability and less microbial biomass. We conclude that addition of extra Fe was not effective in promoting greater C sequestration in the arable soil we tested.

Evolution CO2-C induced by plant-derived carbon soil input: Evaluation of the priming effect promoted by Meliaceae by-products

The application of organic fertilizer to soils may affect the rate of native soil organic matter mineralisation, inducing a positive priming effect (PE), with an increase of evolution of CO2 and a release of nutrient that may be in excess of crop demand. The objective of this study was to evaluate the effect of different Meliaceae derivatives, characterised by different C:N ratios, on the total soil CO2-C evolution, CO2-C fractions from the derivatives, PE magnitude, and to understand the mechanism that regulates PE. In a laboratory incubation experiment, the effects of three Meliaceae derivatives, named neem 5, neem 6, and melia leaves, were evaluated using the natural 13C abundance method, employing a naturally C4-plant labelled soil. Total CO2-C losses were proportional to the plant-derived C inputs. Soil CO2-C produced by the addition of the derivatives ranged between 22% (neem 6) and 40% (neem 5 and melia leaves) of the total C incorporated into the soil. All the derivatives studied induced a positive PE that increased with the amount of C added, but was not related to their chemical structure. Priming effect was also unaffected by soil mineral nitrogen, showing a response with little or no correlation with the ability of microbial biomass to locate nutrients in the soil (N mining). At the end of the experiment, the proportion of C primed corresponded to approximately 24%–28% of the added C, with no difference among the derivatives, and contributed to total CO2-C loss by approximately 31%, 30%, and 27% for neem 5, melia leaves, and neem 6, respectively.

Comparing GHG Emissions from Drained Oil Palm and Recovering Tropical Peatland Forests in Malaysia

For agricultural purposes, the drainage and deforestation of Southeast Asian peatland resulted in high greenhouse gases’ (GHGs, e.g., CO2, N2O and CH4) emission. A peatland regenerating initiative, by rewetting and vegetation restoration, reflects evidence of subsequent forest recovery. In this study, we compared GHG emissions from three Malaysian tropical peatland systems under the following different land-use conditions: (i) drained oil palm plantation (OP), (ii) rewetting-restored forest (RF) and (iii) undrained natural forest (NF). Biweekly temporal measurements of CO2, CH4 and N2O fluxes were conducted using a closed-chamber method from July 2017 to December 2018, along with the continuous measurement of environmental variables and a one-time measurement of the soil physicochemical properties. The biweekly emission data were integrated to provide cumulative fluxes using the trapezoidal rule. Our results indicated that the changes in environmental conditions resulting from draining (OP) or rewetting historically drained peatland (RF) affected CH4 and N2O emissions more than CO2 emissions. The cumulative CH4 emission was significantly higher in the forested sites (RF and NF), which was linked to their significantly higher water table (WT) level (p &lt; 0.05). Similarly, the high cumulative CO2 emission trends at the RF and OP sites indicated that the RF rewetting-restored peatland system continued to have high decomposition rates despite having a significantly higher WT than the OP (p &lt; 0.05). The highest cumulative N2O emission at the drained-fertilized OP and rewetting-restored RF sites was linked to the available substrates for high decomposition (low C/N ratio) together with soil organic matter mineralization that provided inorganic nitrogen (N), enabling ideal conditions for microbial mediated N2O emissions. Overall, the measured peat properties did not vary significantly among the different land uses. However, the lower C/N ratio at the OP and the RF sites indicated higher decomposition rates in the drained and historically drained peat than the undrained natural peat (NF), which was associated with high cumulative CO2 and N2O emissions in our study.

Distribution of soil organic matter fractions are altered with soil priming

Soil organic matter (SOM) plays a central role in mediating soil productivity through its impacts on nutrient cycling and retention, aggregate stability and water retention. Thus, management techniques or technologies including novel soil amendments could benefit farmers through the accumulation of carbon (C) and other nutrients in SOM. However, these same inputs can also lead to accelerated mineralization of native SOM through the process known as priming. This unresolved paradox may be due to the limited understanding of how different SOM fractions respond to priming and in which direction. In this study, we examine the response of functionally distinct SOM fractions to priming when soils are amended with lactobionate, a low molecular weight sugar acid byproduct of cheese manufacturing. Liquid-based 13C lactobionate was added to an agricultural silty loam soil to study its persistence, priming effects, and response of different SOM fractions to lactobionate over 84 days. Cumulative soil carbon dioxide (CO2) was greater in lactobionate-amended soils versus control and by the end of the experiment, 53% of added lactobionate was mineralized. In total, positive priming of 40% of extant SOM was observed from 14 to 84 days. Lactobionate-induced changes to SOM fractions were determined at days 14, 28, 56 and 84 of the incubation to examine if and how priming altered the distribution of C between fast and slow-cycling SOC fractions. In response to lactobionate, the total C content of the water extractable organic matter (WEOM) fraction initially increased by 100% from the dissolved lactobionate we added, but then declined and at a faster rate than other SOM fractions. In addition, the total C of the light-fraction particulate organic matter (LF-POM) fraction also declined. At the same time, we observed total C increases in the slower-cycling sand-sized POM (H-POM) and mineral-associated organic (MAOM) C fractions, in response to lactobionate additions. We also saw a marginal increase in total soil C in the lactobionate-amended soils. Our findings therefore suggest that the application of lactobionate to soils may induce positive priming of the faster cycling LF-POM and WEOM fractions, but also concurrent gains in the H-POM and MAOM C fractions associated with long-term persistence and relative resiliency to disturbance with no net loss of total soil carbon. Thus, the application of low-molecular weight C-based materials such as lactobionate presents an avenue to building more persistent SOM through its impacts on the internal cycling and transformation of SOM fractions.

Distribution of soil organic carbon and nitrogen under reduced nitrogen application in the Guanzhong plain of China#

AbstractBackgroundThe problems of soil quality degradation and environmental pollution caused by excessive use of nitrogen fertilizer seriously affect sustainable development of agriculture.AimsWe investigated the effects of reduced nitrogen application with simultaneous full straw return on carbon input, aggregate size distribution, aggregate‐associated organic carbon and total nitrogen (TN), soil organic carbon (SOC) stock, SOC fractions, and crop yield.MethodsA long‐term field experiment was performed in the dryland region of Guanzhong. The study involved three nitrogen fertilizer (NF) amounts: 337.5 (conventional NF, CNF), 286.4 (15%NF reduction, 15%RNF), and 236.3 (30%NF reduction, 30%RNF) kg N ha–1 y–1 with full straw return.ResultsCompared with CNF, 30%RNF increased the content of soil organic carbon by 5.7% and increased maize yield by 13.1%, under straw return conditions. The 30%RNF + straw application treatment promoted the formation of large macro‐aggregates more effectively than CNF application. The 30%RNF + straw treatment significantly increased SOC by 10.6% and 4.7%, respectively, in large and small macroaggregates relative to conventional NF. Moreover, the 30%RNF + straw treatment resulted in a significant increase in the SOC content in the density fractions of mineral soil organic matter (mSOM) for both small and large macro‐aggregates. This treatment also resulted in a significant increase in the content of TN in the mSOM and density fractions of particulate matter that had coarse and fine intra‐aggregates of the large macro‐aggregates.ConclusionFull straw return with reduced nitrogen (N) application could be a promising option to improve the structure of soil aggregates, soil organic carbon, and nitrogen sequestration.

Occurrence and stability of organic intercalation in clay minerals from permafrost-affected soils in the High Arctic – A case study from Spitsbergen (Svalbard)

Climate warming is responsible for many environmental changes in the Arctic, which lead to the decomposition of soil organic matter (SOM) and emissions of greenhouse gases from the soil into the atmosphere. Soil minerals play a crucial role in SOM stabilization. However, little is known about the occurrence and stability of organo-mineral associations in permafrost-affected soils in the Arctic. The main aims of this study were: 1) to determine the potential occurrence of SOM within interlayers of swelling clay minerals in the permafrost-affected soils in central part of Spitsbergen (Svalbard, High Arctic) and 2) to determine resistance of the intercalated SOM within the clay minerals against thermal and chemical oxidation. The obtained results indicate that 10% to 15% of organic carbon and 30% to 45% of total nitrogen occurring in the clay fraction of the Arctic permafrost-affected soils are intercalated within the swelling clay minerals. We also report that this part of SOM is highly resistant to both chemical and thermal oxidation. These findings should be taken into consideration in the refinement of climate models and in studies concerning the thawing of permafrost, SOM mineralization, and emissions of greenhouse gases from the soil into the atmosphere.

Potential of combined neutron and X‐ray imaging to quantify local carbon contents in soil

AbstractIn this study, we investigated the potential and limitations of using joint X‐ray and time‐of‐flight (TOF) neutron imaging for mapping the 3‐dimensional organic carbon distribution in soil. This approach is viable because neutron and X‐ray beams have complementary attenuation properties. Soil minerals consist to a large part of silicon and aluminium, and elements that are relatively translucent to neutrons but attenuate X‐rays. In contrast, attenuation of neutrons is strong for hydrogen, which is abundant in soil organic matter (SOM), while hydrogen barely attenuates X‐rays. In theory, TOF neutron imaging does further more allow the imaging of Bragg edges, which correspond to d‐spacings in minerals. This could help to distinguish between SOM and clay minerals, the mineral group in soil that is most strongly associated with hydrogen atoms. We collected TOF neutron image data at the IMAT beamline at the ISIS facility and synchrotron X‐ray image data at the I12 beamline at the Diamond Light source, both located within the Rutherford Appleton Laboratory, Harwell, UK. The white beam (the full energy spectrum) neutron image clearly showed variations in neutron attenuation within soil aggregates at approximately constant X‐ray attenuations. This indicates a constant bulk density with varying organic matter and/or clay content. Unfortunately, the combination of TOF neutron and X‐ray imaging was not suited to allow for a distinction between SOM and clay minerals at the voxel scale. While such a distinction is possible in theory, it is prevented by technical limitations. One of the main reasons is that the neutron frequencies available at modern neutron sources are too large to capture the main d‐spacings of clay minerals. As a result, inference to voxel scale SOM concentrations is presently not feasible. Future improved neutron sources and advanced detector designs will eventually overcome the technical problems encountered here. On the positive side, combined X‐ray and TOF neutron imaging demonstrated abilities to identify quartz grains and to distinguish between plastics and plant seeds.Highlights Full understanding of biogeochemical processes requires three‐dimensional (3‐D) maps of organic matter in soil (SOM). This study investigates a novel method to map voxel‐scale SOM contents with 3‐D resolution. The method is based a combination of X‐ray and time‐of‐flight neutron tomography. At present, technical limitations prevent distinguishing between SOM and clay mineral contents. More advanced neutron sources are required to overcome the encountered technical obstacles.

Key predictors of soil organic matter vulnerability to mineralization differ with depth at a continental scale

Soil organic matter (SOM) is the largest terrestrial pool of organic carbon, and potential carbon-climate feedbacks involving SOM decomposition could exacerbate anthropogenic climate change. However, our understanding of the controls on SOM mineralization is still incomplete, and as such, our ability to predict carbon-climate feedbacks is limited. To improve our understanding of controls on SOM decomposition, A and upper B horizon soil samples from 26 National Ecological Observatory Network (NEON) sites spanning the conterminous U.S. were incubated for 52 weeks under conditions representing site-specific mean summer temperature and sample-specific field capacity (−33 kPa) water potential. Cumulative carbon dioxide respired was periodically measured and normalized by soil organic C content to calculate cumulative specific respiration (CSR), a metric of SOM vulnerability to mineralization. The Boruta algorithm, a feature selection algorithm, was used to select important predictors of CSR from 159 variables. A diverse suite of predictors was selected (12 for A horizons, 7 for B horizons) with predictors falling into three categories corresponding to SOM chemistry, reactive Fe and Al phases, and site moisture availability. The relationship between SOM chemistry predictors and CSR was complex, while sites that had greater concentrations of reactive Fe and Al phases or were wetter had lower CSR. Only three predictors were selected for both horizon types, suggesting dominant controls on SOM decomposition differ by horizon. Our findings contribute to the emerging consensus that a broad array of controls regulates SOM decomposition at large scales and highlight the need to consider changing controls with depth.

Inorganic Fertilization at High N Rate Increased Olive Yield of a Rainfed Orchard but Reduced Soil Organic Matter in Comparison to Three Organic Amendments

Strategies for waste valorisation from domestic and agro-industrial activities must be pursued, and its use as a soil amendment is an interesting possibility. In this four-year study, the effect of applying municipal solid waste (MSW), farmyard manure (FYM), bottom wood ash supplemented with nitrogen (Ash + N), the inorganic fertilization common in the region (50 kg ha−1 N, P2O5 and K2O) (Control) and this inorganic fertilization supplemented with 70 kg N ha−1 (High N) was assessed in a rainfed olive grove planted in a shallow soil with low organic matter and managed with conventional tillage. The High N treatment significantly increased olive yield in comparison to the other treatments (165% more than MSW), and soil available N proved to be the main driver for tree productivity. MSW and FYM increased soil organic matter, as well as the levels of phosphorus and cation exchange capacity, leaving good indications for future production cycles, although during the four years of the study these treatments provided little N to the trees. The High N treatment significantly reduced soil organic matter (63% less than MSW). The result was attributed in part to the soil management system that did not allow the development of herbaceous vegetation, but also to an effect known as “added N interaction”, in which the excess of inorganic N in the soil might have contributed to accelerate the mineralization of native soil organic matter, an aspect that compromises the sustainability of this fertilization strategy. Although MSW and wood ash are sometimes associated with risks of environmental contamination with heavy metals, in this study the levels of heavy metals in soils and in plant tissues were not of concern.

On the Adsorption Mechanism of Humic Substances on Kaolinite and Their Microscopic Structure

Soil organic matter (SOM) and various inorganic minerals represent key components of soils. During pedogenesis and due to biological activity these species interact, having a crucial impact on the formation of an aggregated soil structure with a hierarchical arrangement from nano to macro scale. In this process, the formation of organo–mineral microaggregates represents a dominant factor affecting soil functions and properties. This study focuses on the interactions between humic substances (HSs) and the mineral kaolinite as typical representatives of SOM and soil minerals. By performing classical molecular dynamics (MD) simulations on models of HSs and kaolinite, we demonstrate how two dominant but chemically different kaolinite surfaces affect the stability of HSs microaggregates. By analyzing volumetric, structural, and energetic properties of SOM–kaolinite models, we explain possible mechanisms of the formation of stable SOM–clay aggregates and show how a polarized environment affects the electrostatic interactions, stabilizing the microscopic structure of SOM–mineral aggregates. Our results showed that when stable aggregates of HSs are confined in kaolinite nanopores, their interactions with kaolinite surfaces disintegrate them into smaller subaggregates. These subaggregates are adsorbed more strongly on the polar aluminol surface of kaolinite compared to less the active hydrophobic siloxane surface.

Contribution of above ground litterfall and roots to the soil CO2 efflux of two sub-tropical Cunninghamia lanceolata and Castanopsis carlesii forests

Soil respiration (RS) is the largest terrestrial carbon (C) flux to the atmosphere, and it can be influenced by changing input of plant C from above- and/or below ground. Especially in tropical and sub-tropical ecosystems, the contributions of litter respiration (RL), autotrophic respiration (RA) and mineral soil respiration (RM) are still poorly understood. In the present study, RS was measured under untreated control (CT), root exclusion (NR), litterfall exclusion (NL), and combined litterfall and root exclusion (NRNL) in a subtropical Cunninghamia lanceolata plantation and a secondary Castanopsis carlesii forest for three years. In addition, litter input, litter and soil chemistry, and microbial biomass and community structure (PLFAs) were assessed. RS was significantly higher in the C. carlesii forest than in the coniferous C. lanceolata forest. RL and RA were significantly higher in the C. carlesii forest than in the C. lanceolata forest, while there was no significant difference in RM. RM, RA, and RL contributed 55%, 29%, and 16% to RS under C. lanceolata, and 39%, 32%, and 29% under C. carlesii, respectively. Above ground litter input and microbial biomass were lower in the coniferous C. lanceolata forest. Soil microbial biomass was significantly lower in NL, NR and NRNL in both forests. NL had most pronounced effects on the microbial community composition in the C. carlesii soil, whereas NR and NRNL affected the community composition in C. lanceolata soil. Overall, the unexpectedly small and only insignificant additive effects of litter exclusion and root exclusion in the combined treatment (NRNL) suggest that yet unresolved interactions had accelerated the decomposition of mineral soil organic matter and RM under this lowest plant C-input scenario. Hence, in the case that above and below ground plant C inputs change simultaneously, effects on RS and its components might be more complex than suggested by single-C-source manipulation studies.