Large Macroaggregates Research Articles

Tillage induces rapid loss of organic carbon in large macroaggregates of calcareous soils

Soil organic carbon (SOC) sequestration and protection not only mitigate CO2 emission but also promote soil fertility. Naturally, calcareous soils in the karst area contain significantly higher SOC concentration than the red soils in non-karst area of the same region. After cultivation, calcareous soils are susceptible to SOC loss, with substantial SOC loss occurring within two years. However, the underlying mechanisms are not well understood. To expand our understanding of the process of rapid SOC loss and the role played by soil aggregate disruption, we conducted a 1-year in situ study in a karst area in Southwest China. We tilled the soil at different intervals—every 6 (T1), 4 (T2), 2 (T3), and 1 (T4) months—causing aggregate disruption of different intensities. No-tillage soil (T0) was treated as a control. After 1 year, the SOC concentration in the soil under the tillage treatments (T1, T2, T3, and T4) was significantly reduced; however, in the no-tillage soil, no significant change was observed. The SOC loss within the first 3 months accounted for approximately 70 %–84 % of the total SOC loss, indicating that SOC loss was most pronounced during the initial tillage period. SOC was mainly stored in the 2–8 mm soil aggregates. Tillage-induced disruption of the 5–8 mm aggregates resulted in a decline of the physical protection of SOC, leading to substantial SOC loss from these aggregates. Besides, the SOC loss was positively correlated to the amount of Ca2+ leaching, indicating that enhanced carbonate dissolution might contribute to the decline in aggregate stability and associated SOC loss. Our results emphasize the importance of zero-tillage management and maintenance of large soil macroaggregates for sustainable land use of calcareous soils in karst regions.

Land use affects temperature sensitivity of soil organic carbon decomposition in macroaggregates but not in bulk soils in subtropical Oxisols of Queensland, Australia

The turnover of soil organic carbon (SOC) under different land uses plays an important role in terrestrial carbon (C) cycling. However, the effects of warming on C decomposition dynamics have not been resolved due to seemingly inconsistent results among different experiments. Hence, the objective of this work was to assess soil aggregate-associated C and temperature sensitivity of SOC decomposition within bulk soils and aggregates under two contrasting land management systems in subtropical southern Queensland.We examined the temperature sensitivity of SOC decomposition (Q10) and activation energy required for soil C mineralization (Ea) from bulk soils (<8 mm), large macroaggregates (2−8 mm) and small macroaggregates (0.25−2 mm), as well as key C and N cycling enzyme activities in the surface (0−10 cm) soils of an Oxisol. The two land uses studied were: undisturbed rainforest and 115-year-old kikuyu grass (Pennisetum clandestinum) pasture. Bulk soils as well as their large and small macroaggregates were separately incubated at two temperatures (17 °C and 27 °C) for 124 d. We observed that the pasture soils had more small macroaggregates but less microaggregates than the forest soils. However, all fractions in the forest contained significantly higher SOC than those in the pasture. Potential soil enzyme activities per unit SOC within bulk soils and macroaggregates for N-acetyl β-glucosaminidase, α-glucosidase, and phenol oxidase were higher in the pasture than in the forest. The cumulative C mineralization values of bulk soils under the forest vegetation were 16 % higher than for pasture at 17 °C and 97 % higher than pasture at 27 °C. Cumulative C mineralization from large macroaggregates of both land uses were more than that from small macroaggregates. However, the Ea and Q10 values of bulk soils were similar for both the forest and pasture soils. For the large macroaggregates, the Ea value was ∼78 % higher for the pasture soil than for the forest soil, with the Q10 value also being 73 % higher in the pasture soil. However, the opposite was observed for the small macroaggregates, with Ea and Q10 values being higher in the forest soil. The contrasting nature of Q10 from large and small macroaggregates in Oxisols suggests that the feedback from SOC decomposition in response to changing temperature, that is, global warming is closely associated with soil aggregation.

Variation in soil lignin protection mechanisms in five successional gradients of mixed broadleaf–pine forests

AbstractOver 200 yr of ecosystem succession in northeastern China, conifers have replaced broadleaf trees. Vegetation changes during forest succession may affect soil organic C (SOC) stability, which is associated with altered protection mechanisms. Lignin phenol composition, a predictor of soil organic matter variation, was assessed for macroaggregates, microaggregates, and silt–clay (SC) fractions from successional gradients in five forests aged 19 to 239 yr in the Changbai Mountains nature reserve. The large macroaggregates (4.00–8.00 and 2.00–4.00 mm) comprised 45.17 to 59.87% of bulk dry weight and 40.22 to 60.89% of SOC in the 19‐, 32‐, and 48‐yr‐old pioneer forests. However, we detected increased mass proportions and SOC contents in small macroaggregates (1.00–2.00 and 0.25–1.00 mm) in the mixed broadleaf–Korean pine (Pinus koraiensis Siebold &amp; Zucc.) forest after 122 yr. Lignin in bulk soil and aggregates in the 122‐yr‐old stand had lower acid/aldehyde ratios for the vanillyl and syringyl types than other stands, indicating less lignin decomposition. The highest C (198.20 g kg−1 soil) and lignin concentrations (6.14 mg 100 mg−1 C) were detected in the bulk soil from the 239‐yr‐old stand, where SC fraction occupied 56.18% of the C and 84.17% of the lignin content in bulk soil. Forest succession from broadleaf to a broadleaf–pine mixture shifted SOC sequestration and lignin protection from aggregates to SC fractions, along with the development of plant litter composition, fine‐root biomass and turnover, and microorganism biomass, which prompted effective long‐term SOC accumulation in successional forest communities.

植茶年限对土壤团聚体线虫群落结构的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 植茶年限对土壤团聚体线虫群落结构的影响 DOI: 10.5846/stxb201904010627 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（41271307）；四川省科技厅支撑计划项目（2013NZ0044） Effects of tea plantation age on soil nematode community characteristics within soil aggregate Author: Affiliation: Fund Project: China Postdoctoral Science Foundation, 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:在野外调查的基础上，采用干筛法和改良的Baermann漏斗法，研究植茶年限（19年、26年、34年和56年）对土壤团聚体线虫群落结构的影响。本研究共捕获线虫6602条，隶属于46属，以Acrobeloides为优势属，食细菌线虫为优势营养类群。线虫总数和大多数营养类群数量均随植茶年限的延长先升高后降低，在植茶26年时数量达到最高，且随粒径的减小而减少。线虫类群数则随着植茶年限的延长，虽随着粒径的减小而减少，但对 > 2 mm粒径团聚体中响应不敏感。线虫结构指数（Structure index，SI）多数小于50，富集指数（Enrichment index，EI）均大于50。随着植茶年限的延长，茶园生态系统土壤食物网逐渐退化；与大粒径团聚体相比， < 0.25 mm粒径团聚体中食物网受扰动相对较小。以上结果表明，植茶26年土壤更适合线虫繁殖且大粒径团聚体更有利于线虫生存；随着植茶年限的延长，土壤食物网退化， < 0.25 mm粒径团聚体受到的干扰较少，这一结果有助于揭示茶园土壤团聚体中线虫群落变化和土壤食物网对植茶年限的响应特征。 Abstract:This study began in July 2018 at Zhongfeng long-term agricultural research site of Sichuan Agricultural University in Ya'an, Sichuan, China. The prevailing climate surrounding the study site is a subtropical monsoon climate. Mean annual temperature is 15.4℃, with the lowest and highest mean daily temperatures of 4.3℃ and 35.2℃, respectively. Mean annual precipitation is 1500 mm with 72.6% of the precipitation occurring during July and September. The layer being exposed belongs to sedimentary rock predominantly formed since the Mesozoic age, with the soil being Luvisols with a texture of clay loam. "Sichuan tea" has been cultivated since 1950s, forming a large-scale tea plantation with different ages. As a useful approach, the space-for-time substitution method is often used to monitor soil changes occurring along a tea plantation chronosequence that has developed with similar soils and similar climatic conditions. Establishment of tea plantations at different times creates an opportunity for elucidating the tea planting process because the geomorphologic processes can determine soil conditions before tea planting. In order to explore the effects of tea plantation age on the soil nematode community characteristics within soil aggregate and provide theoretical basis for formulating scientific and reasonable soil management measures in tea plantations, soil samples were obtained from tea plantations with different ages (19, 26, 34 and 56 a). The four tea plantations were collected on similar geomorphologic units, slope direction and gradient, and fertilization practices Soil aggregates were separated into four fractions:large (> 2 mm), medium (2-1 mm), small macro-aggregates (1-0.25 mm), and micro-aggregates (< 0.25 mm). The nematodes were extracted from 50 g fresh soil by a modified Baermann funnel methodology. In this study, a total of 6602 nematodes were captured, which belonged to 46 genera and the mean density was 421 ind·100g-1 dry soil. Acrobeloides was the dominant genus and bacterial nematodes were the dominant trophic group. Soil aggregate-associated total nematode abundance decreased significantly with decreasing of aggregate size regardless of the tea plantation age. Additionally, the abundance of different trophic groups (including bacterivores, plant-parasites and fungivores) showed a trend similar to the abundance of total nematodes within different aggregates. In the macro-aggregates, the abundance of total nematodes as well as different trophic groups, with the exception of omnivores-predators, was significantly higher in the 26a tea plantation compared to that in other plantations. In the micro-aggregates, on the contrary, no significant differences were observed among the different aged tea plantations. Nematode structure index (SI) was less than 50, and enrichment index (EI) was more than 50. In this tea plantation ecosystem, the soil food web gradually degraded alongside plantations. Compared with the large macro-aggregates, the structure of soil food web was relatively less disturbed in the micro-aggregates. Notably, the degradation of soil food web alongside plantations underscores the need for sustainable soil management practices that would maintain soil health in the hilly area of western Sichuan in China. 参考文献 相似文献 引证文献

Carbon sequestration and mineralization in soil aggregates under long-term conservation tillage in the North China Plain

Understanding the process of soil organic carbon (SOC) sequestration and mineralization in aggregates is pertinent to mitigate climate change and minimize risks of soil degradation. Thus, soil samples were obtained after a 10-year field experiment to identify the influences of tillage on aggregate-associated SOC sequestration and mineralization in the North China Plain (NCP). Four tillage practices investigated were as follows: no-till with straw retention (NTS, conservation tillage), rotary tillage with straw incorporation (RTS), conventional tillage with straw incorporation (CTS), and conventional tillage with straw removal (CT). Significantly negative correlations were observed between SOC concentration and potentially mineralized carbon in aggregates under different treatments for the 0–10 cm soil layer. The large macro-aggregates (>2 mm) with the highest proportion of size distribution represented the major pool of SOC stock (47.3–51.2%) and mineralization amount (38.2–43.6%) in the 0–30 cm layer, followed by that in the small macro-aggregates (0.25–2 mm), regardless of tillage practices. However, the mineralization quotient (mineralization per unit SOC concentration) of macro-aggregates (>0.25 mm) was lower than that for the other size classes. The NTS enhanced the macro-aggregate formation in the 0–20 cm layer and associated SOC concentration in the 0–10 cm layer. Furthermore, NTS decreased total potential mineralization in the 0–30 cm layer compared with the other tillage practices, attributed to decrease in the large macro-aggregates (30.0–51.4%) with low particulate organic carbon (POC) concentration. The NTS with low straw inputs had higher incremental efficiency with straw incorporation than that in the RTS and CTS by 45.0% and 13.5%, respectively (P < 0.05). Overall, the higher proportion of macro-aggregates recorded under NTS decreased carbon mineralization, and consequently, increased incremental efficiency with straw incorporation, and improved SOC sequestration in the surface soil layer in the NCP.

Impact of land use change and soil type on total phosphorus and its fractions in soil aggregates

AbstractConversion of land from native vegetation to long‐term agricultural cropping adversely affects soil phosphorus (P) reserves. We examined surface soils (0–0.1 m) from three paired sites from four land uses (native vegetation, cropping, pasture, and plantation, up to 115 years) from subtropical Australia (a Vertisol from Warra, an Oxisol from Lamington, and an Oxisol from Kingaroy) for the changes in the total P concentrations as well as the fractionation of P in bulk soil and in aggregate fractions. Among different land uses and soil types, the conversion of the Kingaroy Oxisol from native vegetation to cropping decreased soil aggregation markedly. Across all three soils, land use change from native vegetation to cropping decreased total organic C (TOC) concentrations by 68–85% and organic P (Po) by 55–63%. The corresponding decreases in Po upon conversion to pasture and plantation were lower, being 12–23%. These decreases in Po were primarily associated with a decrease in the NaOH‐Po fraction (relatively available P). Although land use change to cropping reduced the proportion of large macroaggregates, the P concentration, including NaOH‐Po fraction was similar across all aggregate sizes. Finally, although the overall contribution of the silt+clay fraction was small, the C and Po reserves in this fraction tended to be more resilient and the C/Po ratio remained relatively constant. This study has demonstrated the impacts of cropping on soil P reserves, but that conversion of cropping to pasture or plantation can largely reverse these adverse effects.

Coastal saline soil aggregate formation and salt distribution are affected by straw and nitrogen application: A 4-year field study

Little is known about the effects of straw incorporation on saline soil aggregate formation or salt distribution in coastal zones. In this study, a 4-year coastal wheat/maize rotation field experiment was employed. In each growing season, maize/wheat straw was applied at the rates of 5.0 × 103 kg ha−1 (S) and 1.0 × 104 kg ha−1 (2S), and inorganic N was applied at the rates of 75 kg ha−1 (N1/2), 150 kg ha−1 (N), and 300 kg ha−1 (N2). Treatment without straw addition and applied with 150 kg ha−1 inorganic N was used as the control (CK). Dry- and wet-sieving techniques were used to fractionate the soils into large macroaggregates (> 2 mm, LM), small macroaggregates (0.25–2.0 mm, SM), microaggregates (0.053–0.25 mm, MI), and silt-plus-clay particles (< 0.053 mm, CS). Results revealed that the proportion of dry and water-stable macroaggregate fractions (> 0.25 mm, LA + SA) ranged from 74.2 %–88.3 % and 28.4 %–37.6 % in straw applied treatments, which significantly increased by 14.7 %–19.0 % and 21.1 %–32.4 % compared to CK, respectively (p < 0.05). The mean weight diameter (MWD) and aggregate stability rate (AR) enhanced as straw and N application rate increased. After straw application, soil organic carbon (SOC) preferentially accumulated in MI fractions, which was significantly higher than other aggregate fractions, except SN1/2 and CK (p < 0.05). MWD and AR were significantly negatively correlated with soil salinity (p < 0.05), and soil salinity decreased by 20.5 %–26.9 % in straw treatments compared to the initial soil salinity. In the CK and SN1/2 treatments, soil salinity of LA + SA fractions was significantly higher than < 0.25 mm fractions (MI + CS) (p < 0.05). The same difference in soluble Ca2+ was also observed in CK, SN1/2, or SN. In conclusion, saline soil aggregate formation and stability improved after straw and N application, which caused the salinity to decrease and affected salt distribution in aggregates. The findings of this study suggest that adequate N should be applied in order to maximize saline soil reclamation efficiency with straw.

Variations in aggregate-associated organic carbon and polyester microfibers resulting from polyester microfibers addition in a clayey soil

Organic carbon is an essential element for sustainable soil management. While the effects of microplastics on soil physical and biological properties are presenting, it remains unclear whether the organic carbon dynamics of soil are altered by increased microplastic accumulation. The objectives of this study were to evaluate the influences of different polyester microfiber (PMF 0, 0.1% and 0.3% of soil dry weight) and organic material (OM 0, 1%, 2% and 3% of soil dry weight) addition levels on soil organic carbon and to determine the PMF distribution in aggregates from a pot experiment. After 75 days of incubation under 6 wet-dry cycles, the concentrations of soil total organic carbon did not differ significantly between the PMF (9.7 ± 6.6 g kg−1) and control (9.7 ± 6.9 g kg−1) treatments. However, PMF addition significantly reduced the organic carbon concentration in the large (>2 mm) macro-aggregates compared to the control treatment (10.6 ± 4.8 g kg−1 vs. 11.7 ± 4.4 g kg−1), but the results were opposite in the small (2–0.25 mm)macro-aggregates (10.2 ± 4.9 g kg−1 vs. 8.4 ± 3.8 g kg−1). In this study, less than 30% of added PMFs were incorporated into soil aggregates. In addition, the abundance and average length of aggregate-associated PMF in the large (2210 ± 180 particles per g aggregate and 2.08 ± 0.17 mm) and small (1820 ± 150 particles per g aggregate and 1.68 ± 0.11 mm) macro-aggregates were significantly greater than those in the micro-aggregates (1010 ± 70 particles per g aggregate and 0.72 ± 0.05 mm). Our results demonstrate that the distribution of organic carbon in soil macro-aggregates is affected by PMFs addition. Thus, we propose that the behavior of microplastics inside soil aggregates should be further explored to clarify their effects on the physical protection of soil organic carbon.

Soil aggregate stability and aggregate‐associated organic carbon under different land use or land cover types

AbstractSoil aggregate stability (SAS) is an indicator for soil condition and is greatly influenced by land use or land cover (LULC) type and other soil and environmental attributes. This study investigated the soil aggregate‐size distribution, SAS, aggregate‐associated organic carbon (AAOC) and the relative importance of factors affecting SAS and AAOC. Based on conditioned Latin hypercube sampling, soil aggregate samples were collected from the “A” horizon and wet sieved into large macroaggregates (&gt;2.0 mm), small macroaggregates (0.25–2.0 mm), microaggregates (0.053–0.25 mm) and mineral fraction (&lt;0.053 mm). The large macroaggregates accounted for 86% to 93% of the total aggregates under all LULC types except under dry land (64%) and paddy land (35%). The SAS under different LULC decreased in the order fir &gt; shrubland &gt; natural grassland &gt; orchard &gt; blue pine &gt; broadleaf &gt; mixed conifer &gt; dry land &gt; paddy land. The AAOC of the large macroaggregates constituted for 76%–90% of the total AAOC under all LULC types except under dry land (65%) and paddy land (38%). While SAS was largely influenced by the AAOC of small macroaggregates, microaggregates and large macroaggregates and LULC type, the AAOC of different aggregate fractions was mostly affected by LULC type, altitude and slope. SAS did not exhibit any significant relationship with the AAOC of different aggregate fractions under the natural LULC types but showed a strong relationship under the agricultural land indicating that AAOC is more critical for SAS under the agricultural land than under the natural LULC.

Soil aggregation and aggregate-associated carbon, nitrogen, and phosphorus under different aged tea (Camellia sinensis L.) plantations in hilly region of southern Guangxi, China

To have a comprehensive understanding towards the dynamics of soil carbon (C), nitrogen (N), and phosphorus (P) during tea planting, it requires to evaluate the distribution of these elements within soil aggregates at various sizes. In this study, soil organic C (OC), labile OC (LOC), total nitrogen (TN), alkali-hydrolyzable N (AN), total P (TP), and available P (AP) had been detected within soil aggregates obtained at the 0–20 cm depth from tea plantations of various ages (8-, 17-, 25-, and 43-yr.) in hilly region of southern Guangxi, China. Soil aggregates were divided into micro-aggregates (<0.25 mm), small (1-0.25 mm), medium (2-1 mm), and large (>2 mm) macro-aggregates through a dry-sieving procedure. As main carriers of soil C, N, and P, both large macro-aggregates and micro-aggregates had relatively high stores of these elements in all of tea plantations. Tea planting age significantly influences the dynamics of C, N, and P stores in whole-soil, except for total P. During the 35-yr. of tea planting (from 8- to 43-yr.), the accumulation rates of soil OC, TN, AN, and AP were 27.91 g OC m−2 yr.-1, 0.96 g TN m−2 yr.-1, 0.06 g AN m−2 yr.-1, and 0.04 g AP m−2 yr.-1, respectively. Notably, the accumulation rates of these elements in whole-soil differed with the stage of tea planting. For example, soil OC store was increased by 54.94 g OC m-2 yr.-1 at early (from 8- to 17-yr.) stage of tea planting, while by 25.82 and 15.31 g OC m-2 yr.-1 at middle (from 17- to 25-yr.) and late (from 25- to 43-yr.) stages, respectively. Also, similar trends were observed in the TN, AN, and AP in whole-soil. Differently, soil LOC store was first increased and then decreased, and the store was significantly higher in the 17-yr. tea plantations relative to other tea plantations. These results indicated that long-term tea planting brought various benefits, such as the accumulation of soil OC, TN, AN, and AP. However, their accumulation rates in whole-soil decreased with the increase of tea planting age. In addition, it is of critical importance to develop and implement an appropriate management scheme to maintain the soil as a LOC sink after 17-yr. of tea planting in hilly region of southern Guangxi, China.

Increased soil aggregate stability is strongly correlated with root and soil properties along a gradient of secondary succession on the Loess Plateau

The stability of soil aggregates is an indicator of restoration of soil in degraded ecosystems. A multitude of factors such as properties of plant roots and soil have been suggested to contribute to aggregate stability, but little information is available on the relative importance of these factors in temperate grass zones. We examined how root and soil properties modified aggregate stability along a gradient of secondary succession grassland on the Loess Plateau in China. We selected three cropland abandoned for 3, 10, and 16-year and measured the distribution of aggregates, mean weight diameter (MWD), bulk and aggregate-associated soil organic carbon (SOC) and glomalin-related soil protein (GRSP) contents, root biomass density, root length density, and specific root length (SRL). Compared with 3-year site, the amount of large macroaggregates (>2 mm) and aggregate stability (indicated by MWD) at 16-year site increased by 25.6% and 8.5%. The higher MWD contributed the most to the accumulation of SOC in large and small macroaggregates and to the accumulation of GRSPs in microaggregates (<0.25 mm). SRL was significantly positively correlated with MWD. Redundancy analysis (RDA) showed that soil and plant variables together explained 89.1% of the aggregate distribution variation. Partial RDA further revealed that soil variables solely explained 6.4% of the variation, plant root variables explained 47.9% of the variation, and interaction of soil and plant variables accounted for 34.8% of the variation. Our study indicated that increased soil aggregate stability during plant secondary succession depended on both plant roots and aggregate-associated SOC and GRSPs, and plant root exerted a stronger influence on soil aggregate stability than soil. Allowing secondary succession may be a promising strategy for restoring degraded ecosystems on the plateau.

Physicochemical determinants in stabilizing soil aggregates along a hydrological stress gradient on reservoir riparian habitats: Implications to soil restoration

Stability of soil aggregates, playing a significant role in soil quality, has been greatly impacted by wetting-drying cycles. Yet, determinants in stabilizing soil aggregate structure have been rarely clarified under the hydrological stress. Therefore, under the hydrological stress, this study evaluated the relative importance of soil physicochemical properties in stabilizing soil aggregates on the water level fluctuating zones of the Three Gorges Reservoir riparian at China's Yangtze River. Water stable aggregates of soils (large macroaggregates: > 2 mm, small macroaggregates: 0.25–2 mm, microaggregates: 0.053–0.25 mm, silt- and clay-sized aggregates: 0–0.053 mm) were measured by wet sieving, and soil aggregate stability was evaluated as mean weight diameter (MWD), geometric mean diameter (GMD) and fractal dimension (D) of soil aggregate and soil erodibility parameter (K). Physicochemical properties of the riparian soils were analyzed as total carbon, nitrogen, organic matter, phosphorus and potassium content, C/N ratio, soil mechanical composition, pH, bulk density and moisture content. The results indicated that soil total carbon, nitrogen, organic matter, phosphorus and potassium were highly concentrated in the areas under strong hydrological stress; the pH was mainly alkalinity, pH value and bulk density both decreased along the incremental stress gradient. The findings revealed that the proportion and stability of water stable aggregates were affected by hydrological stress, and stability indices were interpreted by pH, C/N ratio, potassium and silt contents. Aggregates showed the highest and lowest stability under intermediate and strong stress, respectively, however, no significant difference was found between weak and none stress. Finally, our results suggested that the riparian ecosystem may have the ability of autogenic restoration for soil stability. Under intermediate hydrological stress level, autogenic restoration with less human intervention would be recommended for stabilizing the riparian soil. However, under strong hydrological stress, anthropogenic restoration is still needed to stabilize soil structure and reduce soil erosion in the reservoir riparian habitats.

Soil Carbon Storage and Its Determinants in Forest Fragments of Differentiated Patch Size

Research Highlights: Soil carbon storage (SOC) decreased due to forest fragmentation through lower proportion of macroaggregate distribution, higher storage of fine roots and litter falls, and lower fine root production rate. Background and Objectives: Globally, forest fragmentation processes lead to enormous losses of SOC in forests. We investigated SOC and its determinants in forest fragments experiencing edge disturbances in south China. Materials and Methods: Soil aggregate characteristics, dynamics of fine roots, and litter fall were studied from forest edges to interiors. Generalized linear mixed models were used to model the contributions of fine root and litter fall dynamics to carbon concentration in aggregates. Results: Large and small macroaggregates had higher proportion of aggregate distribution and contributed more carbon to SOC in all types of plots in the present study. SOC significantly increased from forest edges to interiors due to carbon concentration of these two aggregate types increasing from edges to interiors, while the proportion of different aggregate distributions was similar within each plot. The same trend was found with increasing forest patch size. Fine root biomass storage had the strongest impact on carbon concentration in large macroaggregates and microaggregates, with higher fine root biomass storage associated with lower carbon concentration. In addition, biomass storage and production rates of both fine roots and litter falls decreased from forest interiors to edges. Our results showed that SOC was significantly decreased due to the lower proportion of large and small macroaggregate distribution, and lower fine root production rate in forest fragments. Conclusions: SOC loss due to effects of forest fragmentation and forest edges occurred through decreased concentrations of soil aggregates and fine root production rates. Results from this study will enhance our ability to evaluate soil aggregate, fine root, and leaf litter fall contributions to SOC within forest fragments, and to suggest basic recommendations for the management and conservation of these forest fragments.

Plant roots and deep-banded nutrient-rich amendments influence aggregation and dispersion in a dispersive clay subsoil

The ameliorating effect of deep banding of nutrient-rich organic amendments, termed subsoil manuring, on improving physical structure of sodic high-clay subsoils, has been often attributed to organic amendments per se. However, this cannot explain the transformation of soil physical properties between the rip-lines, away from the amendments. This study assessed the effect of deep-banding nutrient-rich amendments on aggregation and dispersion of a clay subsoil in the presence and absence of wheat (Triticum aestivum) roots under controlled environment conditions. A specially-designed dual-column was set up to simulate a soil profile where a well-structured topsoil overlaid a sodic clay subsoil with an exchangeable sodium percentage (ESP) of 21%. The five amendments include a control (zero amendments), fertilizer nutrients (NPKS), wheat straw + fertilizer nutrients (straw/NPKS), poultry litter (PL) and poultry litter + controlled-release fertilizer (PL/mac). All amendments were added to the centre of the subsoil, 6 cm below the base of the topsoil. Our results showed that the presence of deep-placed nutrient-rich amendments, such as straw/NPKS and PL/mac, greatly enhanced deep root proliferation in this sodic clay subsoil, and resulted in the rapid build-up of large (>2000 μm) water-stable macroaggregates. There was a significant (P < 0.05) positive linear relationship between the root length density and the formation of large macroaggregates in the subsoil adjacent to and below the amendment. The stimulation of microbial growth by root exudates or by mucilage, as indicated by a significantly higher bacterial and fungal abundance (P < 0.05) in the planted than unplanted soils, is likely to have contributed to the formation of these macroaggregates. The effectiveness of wheat straw/NPKS in promoting the formation of macroaggregates in the unplanted soil could be attributed to the ‘straw effect’ which induced a marked increase in fungal growth (P < 0.05). Soil electrical conductivity (EC) and aggregate size were the key determinants of clay dispersion in the aggregated subsoil. Plant roots showed a contrasting effect on clay dispersion: increasing clay dispersion by reducing soil EC while suppressing clay dispersion via root-induced increases in large macroaggregates. We argue that the degree of slaking or disaggregation is likely to determine the net effect of roots on clay dispersion, and that root effect on increasing dispersion of macroaggregates in wet subsoil is limited. The major finding of the study is that increased aggregation in a dispersive clay subsoil can occur when wheat roots grow actively in these layers, in response to deep-placed nutrient-rich amendments.

Rotation and Organic Fertilizers Stabilize Soil Water-Stable Aggregates and Their Associated Carbon and Nitrogen in Flue-Cured Tobacco Production

Long-term mono-cropping and excessive chemical fertilizer application reduce water-stable soil aggregate stability and soil N and C stocks in tobacco production. This study tested the hypothesis that rotation and fertilizer type affected the proportion of water-stable aggregates and aggregate-associated soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations. Two planting systems (tobacco mono-cropping and tobacco-maize rotation) with five fertilizer treatments (0 and 75 kg N ha−1, 450 kg oil cake + 75 kg N ha−1, 15,000 kg farmyard manure + 60 kg N ha−1, and 3000 kg straw + 75 kg N ha−1) were established in 2007. After 10 years, rotation and fertilizer type significantly affected soil aggregates and associated SOC and TSN at different soil depths. Rotation significantly increased mean weight diameter (MWD) and geometric mean diameter (GMD); GMD increased with increasing soil depth. Rotation and organic fertilizer effectively increased the proportion of large macro-aggregates and MWD and GMD, while significantly reducing the proportion of micro-aggregates. The effect of fertilization type and rotation on the SOC content of large macro-aggregates was more obvious than that on TSN content, but there was no significant effect on SOC and TSN contents of aggregates 250 μm). However, this trend weakened with increasing soil depth and decreasing aggregate size. Rotation and organic fertilizers used in tobacco production stabilized the proportion of macro-aggregates in surface soil and maintained SOC and TSN better.

Wheat root system architecture and soil moisture distribution in an aggregated soil using neutron computed tomography

Non-invasive techniques are essential to deepen our understanding of root-soil interactions in situ. Neutron computed tomography (NCT) is an example of such techniques that have been successfully used to study these interactions in high resolution. Many of the studies using NCT however, have invariably focused on lupine plants and thus there is limited information available on other more commercially important staple crop plants such as wheat and rice. Also considering the high neutron sensitivity to hydrogen (e.g. water in roots or soil organic matter), nearly all previous in-situ NCT studies have used a relatively homogeneous porous media such as sand, low in soil organic matter and free from soil aggregates, to obtain high-quality images. However to expand the scope of the use of NCT to other more commercially important crops and in less homogenous soils, in this study we focused on wheat root growth in a soil that contained a considerable amount of soil organic matter (SOM) and different sized aggregates. As such, the main aims of this research were (1) to unravel wheat (Triticum aestivum cv. Fielder) root system architecture (RSA) when grown in an aggregated sandy loam soil (<4 mm) with 4% SOM content, (2) Map in 3D, soil water distribution after a brief drying period and (3) to understand how the root system interacts with soil moisture distribution brought about by soil structural heterogeneity. To achieve these, wheat seedlings were grown for 13-days in aluminium tubes (100 mm height and 18 mm diameter) packed with soil and imaged for the first time at the IMAT neutron beamline (in the Rutherford Appleton Laboratory, UK). To the best of our knowledge, this is also the first study to use NCT to study wheat root architectural development. Our study proved that NCT can successfully be used to reveal wheat RSA in a heterogeneous aggregated soils with moderate amounts of SOM. Lateral root growth within the soil column was increased in regions with increased finer soil separates. NCT was also able to successfully map water distribution in a 3D and we show that large macro-aggregates preferentially retained relatively higher soil moisture in comparison to the smaller soil separates within our samples (Fig. 1). This highlights the importance large macro-aggregates in sustainable soil management as they may be able to provide plants water during periodic dry spells. More in situ investigations are required to further understand the impact of different aggregate sizes on RSA and water uptake.

Dominant tree species and earthworms affect soil aggregation and carbon content along a soil degradation gradient in an agricultural landscape

Soil organic matter (SOM) is considered an important determinant of soil fertility in tropical agroecosystems. While numerous studies have shown the value of agroforestry in increasing soil nutrients and improving crop yield, few have addressed the systematic impacts of duration of cultivation on soil aggregation and C storage in such systems. A study was conducted in South Nandi (Kenya) to assess spatial influence of three dominant trees (Croton megalocarpus, Eucalyptus grandis and Zanthoxylum gilletii) on soil aggregation and C content in agroforestry systems. The study was conducted in a chronosequence experimental set-up where farms were continuously cultivated for 10, 16 and 62 years since conversion from primary forest. It was hypothesized that soil aggregates and whole soil and aggregate-associated C would decrease with duration of cultivation, with the magnitude of influence being reduced by the presence of trees and abundance of earthworms and termites. Greater abundance of small macroaggregates and microaggregates were recorded in soils under the canopy of Z. gilletii with an average weight of 62.8 g and 9.4 g 100 g−1 of soil compared to 53.9 g and 3.1 g 100 g−1 in soils under C. megalocarpus and 48.7 g and 3.9 g 100 g−1 in soils under E. grandis, respectively. These differences could be attributed to the high number of endogeic earthworm species, Nematogenia lacuum (Ocnerodrilidae) in soils under the canopy of Z. gilletii trees. Since N. lacuum is a small-sized species (40–55 mm long), it produces small faecal pellets and thus, we could infer that this species may have contributed to the fragmentation of large macroaggregates into small macroaggregates and microaggregates. The C content decreased by almost 40% in soils under longer duration of cultivation, with higher magnitude of differences associated with Z. gilletii trees. Increased microbial population in earthworms’ casts can increase mineralization rates, which may explain the low aggregate-associated C content under Z. gilletii trees where high number of N. lacuum were recorded. This study shows the significance of specific trees in shaping soil aggregation process and soil C content which could have far-reaching implications for the long-term C storage in the soil and hence net contributions to climate change mitigation.

Soil organic matter and glomalin-related soil protein contents do not explain soil aggregate stability after freeze-thaw cycles at contrasting soil moisture contents

ABSTRACT Soil organic carbon (SOC) and glomalin-related soil protein (GRSP) contributes to the formation and stability of soil aggregates, but it remains unknown whether the frequency of freeze-thaw cycles (FTCs) and soil moisture content (SMC) at freezing changes aggregate stability and whether SOC and GRSP regulate these changes. Effects of FTCs (0, 1, 3 and 9 times) and initial SMC (40% and 80% field capacity) on aggregate distribution, SOC, and easily extractable, difficultly extractable and total GRSP content in soil aggregates (> 2000 μm, 250–2000 μm, and <250 μm) were evaluated in Chinese pine forest soil. FTCs significantly reduced MWD by disrupting large macro-aggregates. On the other hand, higher SMC increased MWD by binding soil particles, the effect of which was much greater than the disruptive effects of FTCs. SOC in bulk soil and aggregate fractions slightly decreased with the frequency of FTCs. Bulk and aggregate-associated GRSP fractions first decreased, and then increased with the frequency of FTCs. SOC and GRSP fractions correlated insignificantly with MWD. Our results indicated that increasing SMC could augment soil aggregate stability, and that SOC and GRSP might not be the primary control on aggregate stabilization during freeze-thaw conditions in Chinese pine forest soil.

Response of organic carbon fractions and microbial community composition of soil aggregates to long-term fertilizations in an intensive greenhouse system

PURPOSE: Soil organic carbon (SOC) content and stability, which are regulated by microbial communities, vary depending on aggregate size. The objectives of this study were to investigate the distribution of the SOC fraction and microbial community composition within soil aggregates when subjected to different fertilization treatments. MATERIALS AND METHODS: This study used high-throughput sequencing technology to analyze the soil microbial community distribution in different-sized aggregate fractions [> 2 mm (large macro-aggregates), 2–0.25 mm (small macro-aggregates), and < 0.25 mm (microaggregates)] when they were subjected to different fertilization treatments in a 16-year greenhouse experiment. The three treatments were chemical fertilizer (CF), 50% organic fertilizer + 50% chemical fertilizer (MF), and organic fertilizer (OF). RESULTS AND DISCUSSION: The results showed that the application of organic fertilizer significantly increased the soil organic carbon (SOC), dissolvable organic carbon (DOC), and microbial biomass carbon (MBC) contents and changed microbial community composition in all the different-sized soil aggregates. We observed that OF significantly reduced the relative abundance of bacterial communities in all aggregates and significantly increased the relative abundances of fungal communities in small macro-aggregates. The larger fungal communities in small macro-aggregates could promote soil aggregation and C sequestration. CONCLUSIONS: Organic fertilization increased the substrate contents in soil aggregates and enhanced soil aggregation and microbial activity in macro-aggregates, which are critical factors that facilitate C transformation and sequestration. These improvements resulted in significantly improved soil fertility and increased vegetable yield in the greenhouses.

Effects of different tillage and straw retention practices on soil aggregates and carbon and nitrogen sequestration in soils of the northwestern China

Soil tillage and straw retention in dryland areas may affect the soil aggregates and the distribution of total organic carbon. The aims of this study were to establish how different tillage and straw retention practices affect the soil aggregates and soil organic carbon (SOC) and total nitrogen (TN) contents in the aggregate fractions based on a long-term (approximately 15 years) field experiment in the semi-arid western Loess Plateau, northwestern China. The experiment included four soil treatments, i.e., conventional tillage with straw removed (T), conventional tillage with straw incorporated (TS), no tillage with straw removed (NT) and no tillage with straw retention (NTS), which were arranged in a complete randomized block design. The wet-sieving method was used to separate four size fractions of aggregates, namely, large macroaggregates (LA, >2000 μm), small macroaggregates (SA, 250–2000 μm), microaggregates (MA, 53–250 μm), and silt and clay (SC, <53 μm). Compared to the conventional tillage practices (including T and TS treatments), the percentages of the macroaggregate fractions (LA and SA) under the conservation tillage practices (including NT and NTS treatments) were increased by 41.2%–56.6%, with the NTS treatment having the greatest effect. For soil layers of 0–5, 5–10 and 10–30 cm, values of the mean weight diameter (MWD) under the TS and NTS treatments were 10.68%, 13.83% and 17.65%, respectively. They were 18.45%, 19.15% and 14.12% higher than those under the T treatment, respectively. The maximum contents of the aggregate-associated SOC and TN were detected in the SA fraction, with the greatest effect being observed for the NTS treatment. The SOC and TN contents were significantly higher under the NTS and TS treatments than under the T treatment. Also, the increases in SOC and TN levels were much higher in the straw-retention plots than in the straw-removed plots. The macroaggregates (including LA and SA fractions) were the major pools for SOC and TN, regardless of tillage practices, storing 3.25–6.81 g C/kg soil and 0.34–0.62 g N/kg soil. Based on the above results, we recommend the NTS treatment as the best option to boost soil aggregates and to reinforce carbon and nitrogen sequestration in soils in the semi-arid western Loess Plateau of northwestern China.