Aggregate-associated Soil Organic Carbon Research Articles

Assessment of Soil Aggradation through Soil Aggregation and Particulate Organic Matter by Riparian Switchgrass Buffers

The restoration of riparian zones has been an important issue in many regions for the recovery of ecosystem functions. The objective of this study was to assess soil aggradation in a 7-year established riparian switchgrass buffer (SGB) and in a non-buffered riparian zone with an annual row crop (ARC). We measured the aggregate size distribution and stability of macroaggregates, aggregate-associated soil organic carbon, soil organic matter fractions and the chemical composition of light particulate organic matter to monitor soil aggregation in a riparian soil following the conversion of agricultural row crops to switchgrass filters. Aggregate size fractions were separated by wet sieving using the aggregate size-stability protocol. The proportion of soil and total organic C was quantified for each aggregate size class. Soil organic matter fractions were isolated by size and density into light particulate organic matter and heavy particulate organic matter and mineral fraction organic matter. The categorization of aggregates by size and water stability (slaking resistance) showed a significantly larger (p < 0.001) proportion of water-unstable large macroaggregates (>2000 µm) under SGB (34%) compared to that under ARC (29%), while the proportion of water-unstable small macroaggregates (250–2000 µm) was significantly higher under ARC (14%) than under SGB (10%). Our results showed that the amounts of light and heavy particulate organic matter did not change in the short-term (7 years) after SGB establishment. It appears that the lower soil stabilization and soil organic C storage under SGB is related to (i) the large number of coarse roots; (ii) lower inputs of light and heavy particulate organic matter; (iii) no changes in the alkyl-C/O-alkyl-C ratio over time; and (iv) light particulate organic matter with a high C/N ratio.

Soil aggregation and aggregate-associated organic carbon under typical natural halophyte communities in arid saline areas of Northwest China

Information on the effects of halophyte communities on soil organic carbon (SOC) is useful for sequestrating C in arid regions. In this study, we identified four typical natural halophyte communities in the Manasi River Basin in Xinjiang Province, Northeast China, namely, Karelinia caspia (Pall.) Less., Bassia dasyphylla (Fisch. et C. A. Mey.) Kuntze, Haloxylon ammodendron (C. A. Mey.) Bunge, and Tamarix ramosissima Lour. We compared soil aggregation and aggregated-associated SOC under these communities. The aggregate fraction of 0.053–0.25 mm accounted for 47%–75% of the total soil mass, significantly more than the > 0.25 and < 0.053 mm fractions, under all the halophyte communities. Significant differences in soil aggregate size distribution were observed among the plant communities, with the H. ammodendron and B. dasyphylla communities showing the highest proportions of > 0.25 mm aggregates (13.3%–43.8%) and T. ramosissima community having more < 0.053 mm aggregates (14.1%–27.2%). Aggregate-associated SOC concentrations were generally the highest in the > 0.25 mm fraction, followed by the < 0.053 mm fraction, and were the lowest in the 0.053–0.25 mm fraction; however, because of their large mass, 0.25–0.053 mm aggregates contributed significantly more to the total SOC. Total SOC concentrations (0–60 cm depth) decreased in the order of H. ammodendron (5.7 g kg−1) > T. ramosissima (4.9 g kg−1) > K. caspia (4.2 g kg−1) > B. dasyphylla (3.4 g kg−1). The H. ammodendron community had the highest total SOC and aggregate-associated SOC, which was primarily because aggregate-associated SOC content at the 0–10 and 10–20 cm depths under this community were higher than those under other plant communities. The H. ammodendron community could be beneficial for increasing SOC in saline soils in the arid region.

Effects of straw and biochar amendments on aggregate stability, soil organic carbon, and enzyme activities in the Loess Plateau, China

Soil from the Loess Plateau of China is typically low in organic carbon and generally has poor aggregate stability. Application of organic amendments to these soils could help to increase and sustain soil organic matter levels and thus to enhance soil aggregate stability. A field experiment was carried out to evaluate the effect of the application of wheat straw and wheat straw-derived biochar (pyrolyzed at 350-550°C) amendments on soil aggregate stability, soil organic carbon (SOC), and enzyme activities in a representative Chinese Loess soil during summer maize and winter wheat growing season from 2013 to 2015. Five treatments were set up as follows: no fertilization (CK), application of inorganic fertilizer (N), wheat straw applied at 8tha-1 with inorganic fertilizer (S8), and wheat straw-derived biochar applied at 8tha-1 (B8) and 16tha-1 (B16) with inorganic fertilizer, respectively. Compared to the N treatment, straw and straw-derived biochar amendments significantly increased SOC (by 33.7-79.6%), microbial biomass carbon (by 18.9-46.5%), and microbial biomass nitrogen (by 8.3-38.2%), while total nitrogen (TN) only increased significantly in the B16 plot (by 24.1%). The 8tha-1 straw and biochar applications had no significant effects on soil aggregation, but a significant increase in soil macro-aggregates (>2mm) (by 105.8%) was observed in the B16 treatment. The concentrations of aggregate-associated SOC increased by 40.4-105.8% in macro-aggregates (>2mm) under straw and biochar amendments relative to the N treatment. No significant differences in invertase and alkaline phosphatase activity were detected among different treatments. However, urease activity was greater in the biochar treatment than the straw treatment, indicating that biochar amendment improved the transformation of nitrogen in the soil. The carbon pool index and carbon management index were increased with straw and biochar amendments, especially in the B16 treatment. In conclusion, application of carbonized crop residue as biochar, especially at a rate of 16tha-1, could be a potential solution to recover the depleted SOC and enhance the formation of macro-aggregates in Loess Plateau soils of China.

Organic carbon associated with tillage-induced aggregates of some quartz-dominated loamy soils in a semi-arid region of South Africa

Tillage breaks down soil aggregates exposing the associated organic matter to increased degradation. The extent of degradation depends on the soil properties, mainly texture and mineralogy. The objective of this study was to determine the amount of soil organic carbon (SOC) in various aggregates under conventionally tilled and no-till systems in some quartz-rich loamy soils in South Africa. The aggregate-associated SOC was significantly (p < 0.001) influenced by tillage systems. The amount of aggregate-associated SOC was 1.67 times higher in tilled than no-till soils because of the crop residue that was incorporated annually during tillage. In the untilled soil, vegetation was grazed by cattle, depriving the soil of fresh organic matter input. Quartz mineralogy and the semi-arid climate were generally responsible for the low SOC content in both tillage systems. It was concluded that aggregate-associated SOC content increased in the loamy tilled soils relative to no-till soils because of the incorporated crop residue.

Increased cropping intensity improves crop residue inputs to the soil and aggregate-associated soil organic carbon stocks

Many South American agroecosystems are based mainly on soybean [Glycine max (L.) Merr.] as a sole crop in the year, which has increased concerns regarding soil conservation and ecosystems sustainability. The increase in cropping intensity (CI) has been suggested as a strategy to improve crop residue inputs, which in turn, may increase soil aggregation and soil organic C (SOC) storage, while maintaining or even increasing total sequence yields. Our objective was to evaluate the relationships between CI and crop residue input with SOC storage and soil aggregation in two contrasting northeastern Argentinean Pampas soils under no-till. Two parallel experiments were established in a Mollisol and a Vertisol evaluating six cropping sequences, starting from soybean monoculture and increasing the number of crops per year and crop diversity. Crop residue inputs to the soil (aboveground biomass, belowground biomass and total biomass), grain yield, the amount of macroaggregates (MA), SOC stored inside macroagregates (SOCMA) and total SOC stocks were measured in both soils two years after the beginning of cropping sequences, at three soil depths. Soil organic C stocks, MA and SOCMA were all positively related with CI in both soils at 0–5cm depth. All soil variables were lowest in simple rotations (soybean monoculture) and increased in more complex rotations (double cropping with cereals and legumes), although differences were significant (P<0.05) only in the top soil (0–5cm depth). Grain yields and crop residues followed a similar pattern being higher in rotations that included maize (with yields expressed as grain mass or as glucose equivalent mass) and lower in soybean monocultures. The highest protein yields were obtained in sequences with wheat and soybean double cropping. Increases in CI under no-till seem to be a useful strategy to improved residue inputs, soil aggregates and SOC stocks. Our results provide valuable evidence for stakeholders and policy-makers to improve SOC sequestration and soil health in agroecosystems of humid temperate croplands.

Temporal changes in soil organic carbon and aggregate-associated organic carbon after reclamation of abandoned, salinized farmland

SUMMARYA field experiment was conducted to quantify changes in soil aggregation and aggregate-associated soil organic carbon (SOC) concentration 1, 3, 5 and 10 years after abandoned, salinized land in the Manasi River Basin was reclaimed for cotton (Gossypium hirsutum L.). Results showed that reclamation significantly increased SOC concentrations and SOC stocks. Specifically, 10 years of cotton production increased SOC concentrations by 45% in the 0–60 cm depth and SOC stocks by 35%. The SOC concentrations and stocks decreased as soil depth increased. Reclamation time, season and soil depth had significant interaction effects on SOC. The SOC concentrations were significantly and positively correlated with available soil nitrogen and available soil phosphorus. Compared with abandoned farmland, macro-aggregate-associated (&gt;250 µm) SOC concentrations in the 0–60 cm depth increased by 47% after 5 years of cotton production and by 53% after 10 years of cotton production. The contribution of macro-aggregate-associated SOC to total SOC in the 0–60 cm depth increased by 87% after 5 years of cotton production and by 69% after 10 years of cotton production. The findings indicate that soil aggregates were more stable after abandoned, salinized farmland was reclaimed for cotton production. Furthermore, cotton production can increase SOC concentrations and sequester C in this arid area.

The role of poorly crystalline iron oxides in the stability of soil aggregate-associated organic carbon in a rice–wheat cropping system

The stability of the soil organic carbon (SOC) in a rice–wheat rotation paddy soil has been of interest because of the global emission of greenhouse gases from the soil and the sequestration of atmospheric carbon dioxide in the soil, as well as in terms of soil fertility in subtropical Asia. The purpose of this study was to explore the relationship between poorly crystalline iron oxides (Feo) and SOC in bulk soil and soil aggregates. Soil samples were collected after a wheat harvest in June 2014 and after a rice harvest in October 2014 and were separated into large macroaggregates, small macroaggregates, microaggregates, and silt and clay by wet-sieving. The long-term (5-year) application of pig manure compost plus chemical fertilizer (NPKM) increased the SOC content more efficiently than other treatments. The specific carbon mineralization rate (SCMR, rate per unit SOC) increased in the following order: microaggregate<macroaggregate<silt and clay, suggesting that SOC in the microaggregates is more stable than in the silt and clay fraction. The Feo concentration was significantly positively correlated with the SOC content in the bulk soil (P<0.001) and the soil aggregates (P<0.001), but negatively correlated with the SCMR (P<0.001). Compared with chemical fertilization alone (NPK), NPKM not only significantly improved soil aggregation but also efficiently activated the iron oxides as indicated by an increase in the aggregate mean-weight diameter (MWD) and the Feo content. Therefore, we suggested that the seasonal variation of wetting and drying drives the redox transformation of iron oxides and the mobility of Feo, and then affects the distribution of Feo in soil aggregates, which may endow paddy soil with a physico-chemical protection of SOC while application of organic fertilizers enhances this protection.

干旱区典型盐生植物群落土壤团聚体组成及有机碳分布

PDF HTML阅读 XML下载 导出引用 引用提醒 干旱区典型盐生植物群落土壤团聚体组成及有机碳分布 DOI: 10.5846/stxb201405221057 作者: 作者单位: 石河子大学 石河子,石河子大学 石河子 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(U1203282, 31360320); 教育部科学技术研究项目(213039A) Distribution of soil aggregates and aggregate-associated organic carbon from typical halophyte community in arid region Author: Affiliation: Shihezi University,Shihezi University;China 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:以干旱区玛纳斯河流域扇缘带为研究区,分析了花花柴(Karelinia caspia)、雾冰藜(Bassia dasyphylla)、梭梭(Haloxylon ammodendron)和柽柳(Tamarix ramosissima)4个典型盐生植物群落土壤团聚体的组成及有机碳分布。研究表明:不同植物群落土壤团聚体均以0.25-0.053 mm粒径为主,占了46.7%-74.6%,且与其他粒径差异显著(P < 0.05), > 0.25 mm和 < 0.053 mm粒径土壤团聚体含量较少,仅占7.8%-43%。梭梭群落 > 0.25 mm团聚体平均含量较高,达32%。不同植被群落土壤总有机碳介于2.01-8.73 g/kg之间,不同粒径团聚体有机碳介于1.70-13.68 g/kg之间。不同群落之间,梭梭和柽柳群落总有机碳和团聚体有机碳含量均相对较高,且随着土层深度下降而下降。不同粒径之间,有机碳含量在0.25-0.053 mm粒径最低,在 > 0.25 mm和 < 0.053 mm粒径中最高,呈现"V"型分布且差异显著(P < 0.05)。0.25-0.053 mm团聚体中有机碳含量的贡献率最高,达43.43%,而 < 0.053 mm粒级贡献率较低,但有机碳含量较高,说明了小粒径团聚体对有机碳保护能力较强。土壤有机碳含量与0.25-0.053 mm团聚体含量呈显著负相关(P < 0.05)。而从整体来看,梭梭群落 > 0.25 mm团聚体比例较高,且土壤有机碳和团聚体有机碳含量也较高,说明了在该研究区,梭梭群落聚集土壤养分能力较强,相对其他群落更有利于土壤有机碳的积累。 Abstract:Soil aggregates are organic-inorganic complexes and the basic unit of soil structure. Their formation and stability are closely related to soil organic carbon content. The distribution of aggregate-associated organic carbon determines the capacity for soil organic carbon storage and retention. Currently, research on soil aggregate-associated organic carbon mainly focuses on agricultural and artificial ecosystems; relatively few studies have focused on natural ecosystems. In particular, little is known about distribution of aggregate-associated organic carbon in halophyte communities in Xinjiang. In light of this, a field experiment was conducted on an alluvial fan in the Manasi River Basin to quantify changes in soil aggregation and aggregated-associated soil organic carbon associated with four halophytes (Karelinia caspia, Bassia dasyphylla, Haloxylon ammodendron, and Tamarix ramosissima). Our objective was to determine 1) the particle size that is most conducive to carbon retention and 2) which of the four halophytes is associated with the accumulation of the most soil organic carbon. The results showed: 1) 46.7% to 74.6% of the total soil mass comprised particles of intermediate diameter (0.25-0.053 mm). This was significantly greater (P < 0.05) than the other two classes of particle size defined in this study ( > 0.25 mm and < 0.053 mm). The > 0.25 mm or < 0.053 mm classes only comprised 7.8% to 43% of soil mass. The > 0.25 mm class was most abundant in the Haloxylon ammodendron community, accounting for 32% of soil mass. 2) Soil total organic carbon content was 2.01-8.73 g/kg, and aggregate-associated soil organic carbon content varied from 1.70 to 13.68 g/kg among communities. Total organic carbon and aggregate-associated organic carbon were greater in the Haloxylon ammodendron and Tamarix ramosissima communities than in the other communities studied. These measures decreased in all communities with depth. 3) The aggregate-associated organic carbon content was lowest in the 0.25-0.053 mm class, and significantly higher (P < 0.05) in the >0.25 mm and < 0.053 mm classes. 4) Because the 0.25-0.053 mm class was the most abundant, aggregate-associated organic carbon in this fraction contributed more to soil total organic carbon contained 43.43%. There was more aggregate-associated organic carbon in the < 0.053 mm class, but this contributed less to total soil organic carbon, indicating that these micro-aggregates have the greatest potential to retain soil organic carbon. There was a significant negative relationship between the abundance of 0.25-0.053 mm soil aggregate particles and soil organic carbon content.Total soil organic carbon, aggregate-associated soil organic carbon and abundance of >0.25 mm aggregate particles were highest in the Haloxylon ammodendron community, which implies that this community has the greatest potential to gather soil nutrients of the four communities studies, could be useful for accumulating soil organic carbon in arid areas. 参考文献 相似文献 引证文献

Conservation Practices in Tobacco Production Increase Large Aggregates and Associated Carbon and Nitrogen

Burley tobacco (Nicotiana tobacum L.) production in Kentucky is typically tillage intensive and is often mono-cropped. Intensive tillage has been reported to degrade soil structure and deplete soil organic carbon (SOC) and total soil nitrogen (TSN) stocks. This study assessed the effect of tillage, crop rotation, and N fertilizer application on soil structure, water-stable aggregation, and aggregate-associated SOC and TSN concentrations and stocks for rotations including burley tobacco. Four burley tobacco production systems (main plots) were established in 2007 on a Bluegrass-Maury (fine, mixed, active, mesic Typic Paleudalf) silt loam soil including: (i) no-tillage continuous tobacco (NT-Tobacco); (ii) conventional tillage continuous tobacco (CT-Tobacco); (iii) 2-yr sod (Festuca arundinacea Schreb.) 1-yr no-tillage tobacco (NT-Rotation); (iv) 2-yr sod and 1-yr conventional tillage tobacco (CT-Rotation). In 2012, two N rates (0 and 280 kg N ha-1) were applied to split plots. Soil samples were taken in Spring 2013 to determine water-stable aggregate-size distributions and associated SOC and TSN concentrations and stocks. Compared with conventional tillage and tobacco monoculture, no-tillage and rotation with sod significantly increased macroaggregate fractions (>250 μm), and associated SOC and TSN stocks at the expense of the silt-clay size class (<53 μm) and associated SOC and TSN stocks. Although previous fertilizer N application had no significant effect on whole-soil SOC and TSN stocks, a significant interaction with crop rotation on macroaggregate distribution and macroaggregate-associated SOC and TSN stocks was observed. Using NT and/or rotation practices in burley tobacco production maintains desirable soil physical and chemical properties via macroaggregate stabilization, which leads to the conservation of SOC and TSN stocks.

Dynamics of soil aggregate-associated organic carbon along an afforestation chronosequence

The objectives of this study were to determine the dynamics of aggregate-associated organic carbon (OC) along an afforestation chronosequence on abandoned farmland of China, and to examine the contributions of changes in aggregate-associated OC to changes in total soil OC. We investigated the dynamics of OC associated with aggregates along an afforestation chronosequence. Water-stable aggregates were isolated, and the OC concentrations in total soil and the aggregates were measured. Averaged across the entire chronosequence, afforestation led to 116, 128 and 108 % average increases in OC concentrations in macroaggregates, microaggregates and the <0.053 mm size class, respectively, in the top 20 cm of soil. The OC stocks in macroaggregates increased by averages of 651 and 473 % at 0–10 and 10–20 cm depths, respectively, mostly within the first 24 years. The OC stocks in microaggregates decreased during the first 35 years and then increased during 48–200 years of afforestation. Averaged across the entire chronosequence, the increases in OC stocks in macroaggregates accounted for 83 and 100 % of the total increase in OC stocks in soils at 0–10 and 10–20 cm depths, respectively. Our results indicated that the accumulation of OC in soils after afforestation on abandoned farmland was mainly due to the accumulation of OC in macroaggregates.

Effects of no-tillage systems on soil physical properties and carbon sequestration under long-term wheat–maize double cropping system

A thorough assessment of changes in soil quality associated with soil management practices is vital for the selection and establishment of sustainable agricultural management. The winter wheat (Triticum aestivum L.)–maize (Zea mays L.) double cropping system was used to study the integrated effects of a 9-year-old tillage coupled with fertilization on soil carbon sequestration and other physical properties in the Yellow River Delta (YRD). Three tillage systems were selected: no-tillage with straw cover plus recommended urea nitrogen rate (NTS), no-tillage with straw removed and manure applied plus recommended urea nitrogen rate (NTM), and conventional tillage with straw removed plus conventional urea nitrogen application rate (CT). There were three replicates of each treatment organized in a randomized block design. NTS and NTM treatments were found to result in a slightly decrease in the soil bulk density (BD), and significantly increased the proportion of water stable aggregates (WSA) (>2mm), as well as the water infiltration capacity. The proportion of water stable macroaggregates (>0.25mm), the mean weight diameter (MWD) and the geometric mean diameter (GMD) of aggregates in the 0–20cm layer were unchanged by NTS and NTM. The total soil organic carbon (SOC) stock at a depth of 0–60cm was not significantly different among the treatments. Both aggregate-associated SOC concentration and stocks (0–5cm) were significantly greater for NTS and NTM compared with CT, while CT led to a greater OC accumulation in the 20–60cm soil layer compared with NTS and in the 10–20cm compared with NTM. The mesoaggregate fraction (2–0.25mm) and its associated OC pool accounted for the highest percentages in the whole soil profile under the CT treatment. The NT system was found to have a positive effect on the investigated soil physical properties and increase soil carbon content in the soil surface layer. The CT system in conjunction with the wheat–maize double cropping system, however, improved soil aggregation in the soil profile (0–60cm) and also maintained a higher fraction of SOC in the subsoil compared with the NT systems.

Changes in Soil Aggregate-Associated Organic Carbon and Nitrogen after Ten Years under Different Land-Use and Soil-Management Systems in Indo-Gangetic Sodic Soil

A study was conducted to evaluate the effects of different land uses and soil-management systems (LU and SMS) on key soil physicochemical indicators [aggregate stability, distribution of soil organic carbon (SOC), and nitrogen (N) in aggregate fractions] and to interpret significance of long-term cultivation of agroforestry plantations [Prosopis juliflora L. (AFP) and Casuarina equisetifolia L. (AFC)], horticultural plantations [(Tamarindus indica (HI) and Syzygium cumini (HJ)], and rice–wheat system (RW) in sodic soil of the Indo-Gangetic plain. Soil samples collected from the different LU and SMS plots were analyzed. The barren sodic soil (BSS) exhibited the least mean weight aggregate diameter (0.21), whereas AFP recorded the greatest (0.59). Total N content in surface soil under RW system was about the same as AFP, AFC, HI, and HJ and significantly greater than BSS. Across the LU and SMS except BSS, microaggregates recorded a narrower C/N ratio than macro- and mesoaggregates.

Stability and storage of soil organic carbon in a heavy-textured Karst soil from south-eastern Australia

Both aggregation and mineral association have been previously found to enhance soil organic carbon (SOC) storage (the amount of organic C retained in a soil), and stability (the length of time organic C is retained in a soil). These mechanisms are therefore attractive targets for soil C sequestration. In this study, we investigate and compare SOC storage and stability of SOC associated with fine minerals and stored within aggregates using a combination of particle-size fractionation, elemental analysis and radiocarbon dating. In this heavy-textured, highly aggregated soil, SOC was found to be preferentially associated with fine minerals throughout the soil profile. By contrast, the oldest SOC was located in the coarsest, most highly aggregated fraction. In the topsoil, radiocarbon ages of the aggregate-associated SOC indicate retention times in the order of centuries. Below the topsoil, retention times of aggregate-SOC are in the order of millennia. Throughout the soil profile, radiocarbon dates indicate an enhanced stability in the order of centuries compared with the fine mineral fraction. Despite this, the radiocarbon ages of the mineral-associated SOC were in the order of centuries to millennia in the subsoil (30–100 cm), indicating that mineral-association is also an effective stabilisation mechanism in this subsoil. Our results indicate that enhanced SOC storage does not equate to enhanced SOC stability, which is an important consideration for sequestration schemes targeting both the amount and longevity of soil carbon.

Effect of Suaeda salsa on Soil Aggregate-Associated Organic Carbon and Nitrogen in Tidal Salt Marshes in the Liaohe Delta, China

In coastal wetlands, the effect of vegetation on soil organic carbon (C) quality is not clear. In this study, we investigated soil organic C and total nitrogen (N) in the whole soils, particulate organic matter (POM) and different aggregate fractions in the 0–15 and 15–30 cm layers in Suaeda salsa clusters and bare grounds in the salt marshes of the Liaohe Delta, China. S. salsa increased soil organic C, total N, POMC and POMN concentrations in both soil layers. Interestingly, the presence of S. salsa caused an increase in the amount of macroaggregates (>0.25 mm) in both 0–15 and 15–30 cm depths, whereas the vegetation effect on microaggregates (0.053–0.25 mm) and silt- and clay-sized fractions (<0.053 mm) differed with soil depth. Compared with bare grounds, S. salsa clusters had higher soil organic C and N contents at each aggregate size class. Among the three aggregate size classes, the magnitude of increases in soil organic C and N contents in the macroaggregates was the largest. Our results suggest that vegetation substantially increases the proportion of relatively labile soil organic C and contributes to soil organic C sequestration in tidal salt marshes in the Liaohe Delta, China.

Aggregate-associated soil organic carbon and total nitrogen following amendment of puddled and sawah-managed rice soils in southeastern Nigeria

Puddling during sawah rice cultivation destabilizes the soil structure. The re-formation of soil water-stable aggregates (WSA) following puddling and amendments, and their associated organic carbon (SOC) and total N were studied at Akaeze and Ikwo in south-eastern Nigeria. The amendments, which were randomized in triplicate, include control, NPK fertilizer, poultry dropping, rice husk powder and rice husk ashes (RHA). Soil samples from 0 to 15 cm depth were taken from the field after 2 years of cultivation. Most of the SOC were found in the very fine aggregates. There was no consistent trend in the treatment effects. However, the NPK-amended soils showed the lowest values of WSA > 2 mm in both locations, whereas the poultry dropping-amended soils showed the least and the highest mean-weight diameter (MWD) values at Akaeze and Ikwo, respectively. The SOC of the whole soil in Akaeze correlated positively with MWD (r = 0.92*). Irrespective of location, SOC in soils and WSA > 2.00 mm correlated positively with MWD (r = 0.56*; 0.65*, respectively) while SOC in WSA 0.50–0.25 mm accounted for low MWD values. More carbon was sequestered at Akaeze than at Ikwo, with the RHA-amended soils being the highest at both locations.

Stand age, fire and clearcutting affect soil organic carbon and aggregation of mineral soils in boreal forests

Soil organic carbon (SOC) in mineral soil accounts for a large portion of total ecosystem carbon (C) in boreal forests. We evaluated the effects of stand age and disturbance origin on SOC, soil aggregate stability, and aggregate-associated SOC in the boreal forests of Ontario, Canada. Mineral soils at 0–15 cm depth were sampled in 27 stands of six post-fire age classes (2- to 203-year-old) and three post-clearcut age classes (2- to 29-year-old), each with three replications. In post-fire stands, the SOC pool increased from 2- and 10-year-old to 29-, 85- and 140-year-old, and then decreased in 203-year-old stands. Aggregate-associated SOC showed a similar trend. Abundance of water stable aggregates (>0.25 mm in diameter) was the highest in 2-year-old stands. Compared with the same-aged post-fire stands, the SOC pool and aggregate-associated SOC were higher, and aggregate stability was lower in 2- and 10-year-old post-clearcut stands. But the differences in SOC pool, aggregate-associated SOC, and aggregate stability between the two stand origins diminished or became less dramatic in 29-year-old stands. Our results indicate that aggregate stability is more dependent on thermal modification of SOC by fire than on aggregate-associated SOC. Our results also show higher SOC pool and aggregate-associated SOC but lower aggregate stability in post-clearcut than post-fire stands shortly after disturbance; however, differences between the two stand origins diminish when stands become older.

Short-term effects of tillage practices on soil aggregate fractions in a Chinese Mollisol

Soil aggregate-size distribution and soil aggregate stability are used to characterize soil structure. Quantifying the changes of structural stability of soil is an important element in assessing soil and crop management practices. A 5-year tillage experiment consisting of no till (NT), moldboard plow (MP) and ridge tillage (RT), was used to study soil water-stable aggregate size distribution, aggregate stability and aggregate-associated soil organic carbon (SOC) at four soil depths (0–5, 5–10, 10–20 and 20–30 cm) of a clay loam soil in northeast China. Nonlinear fractal dimension (Dm) was used to characterize soil aggregate stability. No tillage led to a significantly greater aggregation for >1 mm aggregate and significant SOC changes in this fraction at 0–5 cm depth. There were significant positive relationships between SOC and >1 mm aggregate, SOC in each aggregate fraction, but there was no relationship between soil aggregate parameters (the proportion of soil aggregates, aggregate-associated SOC and soil stability) and soil bulk density. After 5 years, there was no difference in Dm of soil aggregate size distribution among tillage treatments, which suggested that Dm could not be used as an indicator to assess short-term effects of tillage practices on soil aggregation. In the short term, > 1 mm soil aggregate was a better indicator to characterize the impacts of tillage practices on quality of a Chinese Mollisol, particularly in the near-surface layer of the soil.

Short-Term Impacts of No Tillage on Aggregate-Associated C in Black Soil of Northeast China

In order to get a good indicator to evaluate the impacts of no tillage (NT) on soil structure and soil quality, we studied the dynamics of total soil organic carbon (SOC) and aggregate-associated SOC, and their relationships in the plow layer (30 cm) in black soil of Northeast China under NT practice. The tillage experiment was established in Dehui County, Jilin Province, China, in 2001. The total SOC and aggregate-associated SOC under 5-yr tillage treatments were measured. NT practices did not lead to the increase of average SOC content at 0-30 cm depth, but it did significantly increase SOC at the top soil (0-5 cm). In NT plots, the change of SOC in > 1 000 μm aggregate was the same with that of total SOC, but the effect of NT on SOC in > 1 000 μm aggregate was greater than the effect on total SOC, suggesting that > 1 000 μm aggregate had more sensitive response to the impact of tillage practices. Also, significant positive correlation occurred between total SOC and SOC in > 1 000 μm aggregate in black soil. Consequently, in the short term soil macroaggregate >1 000 μm could be used as an indicator to evaluate the impacts of tillage practices on soil structure in black soil of Northeast China.

CHANGES IN ORGANIC CARBON AND PHYSICAL PROPERTIES OF SOIL AGGREGATES UNDER FIBER FARMING

Interest in intensive forestry systems to produce woody or fiber farming (FF) is increasing because of high energy costs and the risks of global warming. The FF-induced changes in soil structural properties at the microscale or aggregate level important to soil organic carbon (SOC) sequestration and soil-water dynamics are, however, not well understood. Thus, we compared differences in soil physical properties and SOC concentrations in aggregates among tree species after 7 years of plantation establishment on an Onaway fine sandy loam (fine loamy, mixed, active, frigid Inceptic Hapludalfs) at the Upper Peninsula Tree Improvement Center near Escanaba, Mich. Soil samples from 16.8 x 16.8-m plots under six tree taxa and three rates of wood ash application (0, 9, and 18 Mg ha -1 ) were collected in May, 2005. The six tree taxa were European larch (Larix deciduas Mill.), hybrid aspen (Populus tremula x Populus tremuloides), and four poplar taxa: NE-222 (Populus deltoids x Populus nigra var. caudina), DN-5 (Populus euramericana, cv. Gelrica), DN-34 (P. euramericana, cv. Eugenei), and NM-6 (P. nigra x Populus maximowiczii). Tree species induced measurable changes in most soil aggregate properties. Larch and DN-34 increased aggregate mean weight diameter more than N-222 and NM-6. Macroaggregates contained 20% more SOC concentration than microaggregates. Ash application increased tensile strength, soil water retention, and SOC concentration, indicating the benefits of applying ash in FF systems. Within both macroaggregates and microaggregates, NM-6 had the highest SOC concentration, whereas aspen had the lowest of all tree species. The SOC concentration increased with increase in biomass input. Results show that differences in aggregate physical properties and aggregate-associated SOC concentration among tree species were relatively small and depended on the soil property.