Small Macroaggregates Research Articles

Effect of grassland harvesting frequency and N-fertilization on stocks and dynamics of soil organic matter in the temperate climate

ABSTRACTManagement of grassland may affect the dynamics of soil organic carbon (SOC). Objectives were to analyze the effect of different harvesting frequencies and nitrogen fertilization regimes on SOC and total N stocks in a field trial on a sandy loam to loamy sand soil of a grassland site near Kiel (Germany). Additionally, effects on microbial biomass C (Cmic) and ergosterol (as proxy for fungi) contents, water-stable aggregate size-classes and density fractions were studied. In the surface soil (0–10 cm), SOC and total N stocks, amounts of large water-stable macroaggregates (> 2000 µm) and contents of Cmic and ergosterol were significantly higher under a five cut regime. Cmic (rSpearman = 0.61) and ergosterol contents (rSpearman = 0.67) were correlated with amounts of large water-stable macroaggregates suggesting that fungi and microbial biomass play an important role in binding of small macroaggregates into large macroaggregates. The free light fraction of SOM showed significantly higher C concentrations under three cut compared to five cut at 30–60 cm, presumably related to the C/N ratio and the decomposability of root litter. This study indicates the importance of cutting frequency on SOC and total N stocks, amounts of large macroaggregates and contents of Cmic and ergosterol.

Structure and biodegradability of dissolved organic matter from Ultisol treated with long-term fertilizations

Dissolved organic matter (DOM) is a labile and reactive fraction of soil organic matter. Ultisol usually has low content of organic matter. Fertilization is a normal way in regulating the state of soil organic matter. This study aimed to investigate the effects of different fertilizations on the content, distribution, structure, and biodegradability of DOM from Ultisol. Ultisol treated by no fertilizer (Ck), chemical fertilizer (NPK), the mixture of chemical fertilizer and straw (NPKS), and animal manure (AM) were collected from a long-term fertilization experimental site. Soil DOM was extracted by deionized water with solid to water ratio of 1:5. The concentration of dissolved organic carbon and total soluble nitrogen in DOM was determined by TOC analyzer. The structure of DOM was investigated by fluorescence spectroscopy through the emission spectra, excitation spectra, synchronous-scan spectra, and excitation-emission matrix. Soil DOM was incubated for 21 days after the addition of soil microbial inoculum and nutrient solution, the biodegradability of DOM was estimated by calculating the percentage of dissolved organic carbon lost after 21 days of incubation. The content of dissolved organic carbon in the soil of Ck, NPK, NPKS, and AM was 22.14, 27.05, 36.08, and 56.04 mg/kg, respectively. The distribution of dissolved organic carbon on large macroaggregate increased from 47.0% in Ck to 65.0–71.5% in fertilized treatments but that on small macroaggregate decreased from 35.5% in Ck to 18.3–19.9% in fertilized treatments. Similar peak wavelength and a fluorophore at wavelength pair of 330–340ex/406–422em were found in all examined DOM. However, a sequence of AM > NPKS > NPK > Ck was found in the fluorescence intensity of examined DOM, indicating the simplification of DOM structure induced by fertilization. The biodegradability of DOM from the Ck, NPK, NPKS, and AM treatment after 21 days of incubation was 23.9, 28.7, 34.2, and 42.7%, respectively. Organic and inorganic fertilization increased the content of DOM from Ultisol. Compared to the Ck treatment, the fertilization treatments increased the distribution of dissolved organic carbon on large macroaggregate but reduced its distribution on small macroaggregate. Humic acid-like compounds were the dominant fractions in DOM from all treatments. DOM from Ck and NPK treatments had more complex molecular structure and lower biodegradability than that from NPKS and AM treatments.

Carbon stocks in the Guinea savanna of Ghana: estimates from three protected areas

AbstractSavannas are widespread in sub‐Saharan Africa (SSA) and play a major role in the global carbon balance. Extensive quantification of savanna carbon stocks inSSAwill therefore contribute to better accounting of the global carbon budget in the era of climate change. In this study, we investigated the spatial distribution of carbon stocks of different soil fractions and aboveground biomass within three forest reserves in the Guinea savanna zone of Ghana. Soil carbon stocks (SCSs) ranged from 4.80 to 12.61 Mg C/ha in surface soils (0–10 cm depth). HigherSCSs were associated with the silt +clay fraction than microaggregates and small macroaggregates in all three reserves. Relative to the dominant tree species (Vitellaria paradoxa), the highestSCSs were recorded under the sub‐canopy (SC), drip line (DL), and interspace (2 *SC+DL) zones for the Klupene, Sinsablegbinni, and Kenikeni forest reserves, respectively. The highest tree carbon stock was 60.01 Mg C/ha in Kenikeni. Sinsablegbinni had an average stock of 26.74 Mg C/ha and had the highest tree density. Average carbon capture by a single tree ranged from 0.04 to 0.34 Mg C. Aboveground grass carbon stock ranged from 0.08 to 0.47 Mg C/ha, while the belowground carbon stock ranged from 0.03 to 0.44 Mg C/ha. Accumulation of carbon in the aboveground grass biomass was greater at Klupene with low forest cover.

Nematodes and microbial community affect the sizes and turnover rates of organic carbon pools in soil aggregates

Soil aggregates provide microhabitats for nematodes and microorganisms, but how nematodes and microbial community interactively drive the dynamics of soil organic carbon (SOC) pool remains unclear. Here, we examined the relationships between bacterivorous nematodes, microbial community, and the sizes and turnover rates of three SOC pools in a red soil under four fertilization regimes. The abundance and community composition of nematode and bacterial communities were examined within aggregate fractions, including large macroaggregates (LMA), small macroaggregates (SMA), and microaggregates (MA). The sizes of SOC pools in soil aggregates increased with decreasing aggregate size, while the turnover rates of SOC pools followed the opposite trend. The ratios of bacteria to fungi (B/F) and Gram-positive to Gram-negative bacteria (GP/GN) were higher in the MA fraction than in the SMA and LMA fractions. The assemblages of bacterivorous nematodes in the LMA fraction were significantly different from those in the MA and SMA fractions, primarily because of the higher abundance of the dominant genus Protorhabditis. Results of structural equation modelling indicated that the ratios of B/F and GP/GN showed stronger positive correlations with the sizes and turnover rates of SOC pools in the MA fraction compared with the LMA fraction. Conversely, bacterivores exhibited indirect relationships with the sizes and turnover rates of SOC pools through the B/F ratio in the SMA and LMA fractions. Taken together, these results highlight the functional role of nematodes and microbial community in controlling SOC pool dynamics at the aggregate scale.

Soil organic carbon and nitrogen sequestration and turnover in aggregates under subtropical leucaena–grass pastures

Stabilisation and protection of soil organic carbon (SOC) in macroaggregates and microaggregates represents an important mechanism for the sequestration of SOC. Legume-based grass pastures have the potential to contribute to aggregate formation and stabilisation, thereby leading to SOC sequestration. However, there is limited research on the C and N dynamics of soil organic matter (SOM) fractions in deep-rooted legume leucaena (Leucaena leucocephala)–grass pastures. We assessed the potential of leucaena to sequester carbon (C) and nitrogen (N) in soil aggregates by estimating the origin, quantity and distribution in the soil profile. We utilised a chronosequence (0–40 years) of seasonally grazed leucaena stands (3–6 m rows), which were sampled to a depth of 0.3 m at 0.1-m intervals. The soil was wet-sieved for different aggregate sizes (large macroaggregates, >2000 µm; small macroaggregates, 250–2000 µm; microaggregates, 53–250 µm; and <53 µm), including occluded particulate organic matter (oPOM) within macroaggregates (>250 µm), and then analysed for organic C, N and δ13C and δ15N. Leucaena promoted aggregation, which increased with the age of the leucaena stands, and in particular the formation of large macroaggregates compared with grass in the upper 0.2 m. Macroaggregates contained a greater SOC stock than microaggregates, principally as a function of the soil mass distribution. The oPOM-C and -N concentrations were highest in macroaggregates at all depths. The acid nonhydrolysable C and N distribution (recalcitrant SOM) provided no clear distinction in stabilisation of SOM between pastures. Leucaena- and possibly other legume-based grass pastures have potential to sequester SOC through stabilisation and protection of oPOM within macroaggregates in soil.

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.

Effects of soil type and depth on carbon distribution within soil macroaggregates from temperate grassland systems

Grassland soils have been highlighted as a global soil carbon (C) sink, and have the potential to sequester additional C. Sequestration of C can occur through incorporation of soil organic carbon (SOC) within microaggregates and the silt and clay fractions. The distribution of SOC within macroaggregate fractions gives an insight into both SOC dynamics and its incorporation into the soil. Research to date on soil C has tended to focus on the topsoil (0–30cm). While many studies have assessed the changes in aggregation and SOC dynamics after land use or management change, this paper assesses aggregation and SOC dynamics in the topsoil and subsoil of twenty-one temperate grassland sites covering four soil types (Haplic Luvisol, Haplic Stagnosol, Haplic Cambisol, Stagnic Cambisol). Results show that there are no differences in SOC between soil types in the surface 0–30cm, except a decrease in the quantity of microaggregates within macroaggregates in Haplic Stagnosols. In the subsoil, the silt and clay fraction of clay-illuviated soils had a lower percentage of SOC. Soils with clay illuviation and reducing conditions had a decreased proportion of SOC in microaggregates and silt plus clay within small macroaggregates in the subsoil. This could be caused by a combination of (i) reduced incorporation of SOC into smaller fractions, because POM inputs could be limited due to soil saturation limiting root growth, and (ii) reduced mineralisation and subsequent incorporation of POM into microaggregates and silt plus clay within macroaggregates. These results enable elucidation of the mechanisms driving aggregate formation (and thus C sequestration in microaggregates and silt plus clay fractions) in topsoil and subsoil. This study shows that the dynamics of SOC in subsoil horizons is soil-type dependant and that differences between soil types cannot be elucidated when the sampling is limited to 30cm. This suggests that the IPCC guidelines for SOC measurements should also include the sampling of subsoil horizons in order to get valuable information that allows discerning between soil types.

Macroaggregation and soil organic carbon restoration in a highly weathered Brazilian Oxisol after two decades under no-till

Conclusions based on studies of the impacts of soil organic carbon (SOC) fractions and soil texture on macroaggregation and SOC stabilization in long-term (>20years) no-till (NT) fields remain debatable. This study was based on the hypothesis that the amount and frequency of biomass-C input associated with NT can be a pathway to formation of macroaggregates and to SOC buildup. The objectives were to: 1) assess the macroaggregate distribution (proportional mass, class mass) and the SOC and particulate organic carbon (POC) stocks of extra-large (8–19mm), large (2–8mm) and small (0.25–2mm) macroaggregate size classes managed for two decades by NT, and 2) assess the recovery of SOC stocks in extra-large macroaggregates compared to adjacent native vegetation (Andropogon sp., Aristida sp., Paspalum sp., and Panicum sp.). The crop rotation systems were: soybean (Glycine max L.), maize (Zea mays L.) and beans (Phaseolus vulgaris L.) in summer; and black oat (Avena strigosa Schreb), white oat (Avena sativa), vetch (Vicia sativa L.), black oat.+vetch (Avena strigosa Schreb+vetch) and wheat (Triticum aestivum L.) in winter. The experimental was laid out as 2×2 randomized block factorial with 12 replicates of a NT experiment established in 1997 on two highly weathered Oxisols. The factors comprised of: (a) two soil textural types: clay loam and sandy clay, and (b) two sampling depths: 0–5 and 5–20cm. The three classes of macroaggregates were obtained by wet sieving, and the SOC content was determined by the dry combustion method. The extra-large macroaggregate classes in 0–20cm depth for sandy clay (SdC) and clay loam (CL) Oxisol represented 75.2 and 72.4% of proportional mass, respectively. The SOC and POC stocks among macroaggregate classes in 0–5 and 5–20cm depths decreased in the order: 8–19mm>2–8mm ≈ 0.25–2mm. The SdC plots under soybean/maize at 3:1 ratio recovered 58.3%, while those at 1:1 ratio (high maize frequency) in CL recovered 73.1% of SOC stock in the extra-large macroaggregates compared with the same under native vegetation for 0–20cm depth. Thus, partial restoration of the SOC stock in original extra-large macroaggregate confirms the hypothesis that NT through higher maize cultivation frequency can be a pathway to fomation of macroaggregates and SOC buildup.

The effects on soil aggregation and carbon fixation of different organic amendments for restoring degraded soil in semiarid areas

SummaryThe addition of organic wastes as a strategy to restore degraded soil in semiarid regions can increase soil aggregation and carbon fixation. These processes can be affected by both the characteristics of the organic amendments and the soil microbial community structure. This study examines the effect of single additions (150 t ha−1) of different organic residues for soil restoration (fresh and composted sewage sludge, vegetal residue and farmyard manure) on aggregation and carbon (C) retention in a semiarid degraded soil. Changes in the distribution of microbial biomass in soil aggregates were also evaluated. Sewage sludge compost was added at rates of 150 and 300 t ha−1 to determine the effect of the amount applied on the distribution processes. Large macroaggregates (2000–8000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the < 53‐μm soil fraction were obtained, and organic carbon (Corg) concentration was determined in these fractions in addition to the total phospholipid‐derived fatty acids (PLFAs). Both the characteristics of the amendments and the amount applied affected soil aggregation and Corg concentration in the aggregate size classes. The manure and the compost added at 300 t ha−1 contributed to the fixation of stable Corg in soil aggregates to a greater extent than the fresh sludge, vegetal residue or compost added at 150 t ha−1. In all soil fractions, the total concentration of PLFAs was larger in restored soil than in the control soil, and increases depended on the aggregate size class and characteristics of the amendment. The largest microbial abundance occurred in both large and small macroaggregates.Highlights Effects of type and amount of amendment on aggregation and C fixation in semiarid soil are elucidated. We explored distribution of the microbial community in soil aggregates in response to new soil C pools. Carbon fixation and microbial abundance of aggregates are affected by type and amount of amendment. Restored and degraded soils differ in terms of the breakdown of new organic matter inputs.

Effect of straw return mode on soil aggregation and aggregate carbon content in an annual maize-wheat double cropping system

Crop residue is a commonly used organic soil amendment in summer maize (June–October)–winter wheat (October–June of next year) rotation systems. However, the effects of different straw return modes on soil aggregation and soil organic carbon (SOC) stocks in different water-stable aggregates have not been extensively investigated in these cropping systems. The objective of this study was to quantify the long-term (7 yr) impact of C input on the SOC content of four soil aggregate size classes (large macroaggregates; small macroaggregates; microaggregates, silt plus clay fraction) and in explicit SOC fractions (free light fraction, free LF; intra-aggregate particulate organic matter, iPOM; mineral-associated matter, mSOM) within the top 40cm of soil in a wheat–maize double cropping system in Northwest China. Four treatments were examined: (i) no return (control); (ii) return of wheat straw only (WR); (iii) return of maize straw only (MR); and (iV) return of both maize and wheat straw (MR-WR). Over the experimental period, the change in SOC under the four treatments ranged from −0.96 to 5.83Mgha−1 and a significant linear relationship between SOC change and cumulative C input (R2=0.9882, P<0.05) was observed. Relative to the control, the proportion of large and small macroaggregates in the 0–20cm soil layer increased the most in MR-WR (32% and 24%), followed by MR (22% and 13%), and WR (11% and 10%). Straw return significantly increased the SOC content in each soil aggregate size class relative to no straw return. The order of SOC fractions with respect to SOC content was mSOM>fine iPOM>coarse iPOM>free LF. Straw return significantly increased the C stock in iPOM and mSOM relative to the control. Coarse iPOM was the most sensitive indicator of C change and mSOM was the main form of SOC under long-term straw return. A significant linear relationship existed between cumulative C input and the mass proportion of macroaggregates as well as the C content of SOC fractions (or aggregate fractions). Soil depth had a significant influence on almost all measurements, with greater values observed in the 0–20cm layer than in the 20–40cm layer. Overall, return of both maize and wheat straw was the best strategy for improving soil structure, soil organic carbon, and crop yield. However, straw return from one crop was sufficient to maintain initial SOC levels, and conserved straw could be used for cellulosic feedstocks.

Effects of long-term grassland management on the carbon and nitrogen pools of different soil aggregate fractions

Common grassland management practices include animal grazing and the repeated addition of lime and nutrient fertilizers to soils. These practices can greatly influence the size and distribution of different soil aggregate fractions, thus altering the cycling and storage of carbon (C) and nitrogen (N) in grassland soils. So far, very few studies have simultaneously addressed the potential long-term effect that multiple management practices might have on soil physical aggregation. Here we specifically ask whether and how grazing, liming and nutrient fertilization might influence C and N content (%) as well as C and N pools of different soil aggregate fractions in a long-term grassland experiment established in 1991 at Silwood Park, Berkshire, UK.We found that repeated liming applications over 23years significantly decreased the C pool (i.e. gCKg−1 soil) of Large Macro Aggregate (LMA>2mm) fractions and increased C pools within three smaller soil aggregate fractions: Small Macro Aggregate (SMA, 250μm–2mm), Micro Aggregate (MiA, 53–250μm), and Silt Clay Aggregate (SCA<53μm). Soil C (and N) accrual in smaller fractions was mainly caused by positive liming effects on aggregate fraction mass rather than on changes in soil C (and N) content (%). Liming effects could be explained by increases in soil pH, as this factor was significantly positively related to greater soil C and N pools of smaller aggregate fractions. Long-term grazing and inorganic nutrient fertilization had much weaker effects on both soil aggregate-fraction mass and on soil C and N concentrations, however, our evidence is that these practices could also contribute to greater C and N pools of smaller soil fractions.Overall our study demonstrates how agricultural liming can contribute to increase C pools of small (more stable) soil fractions with potential significant benefits for the long-term C balance of human-managed grassland soils.

Relationships between soil macroaggregation and humic carbon in a sandy loam soil following conservation tillage

Humic substances are recalcitrant and might act as persistent binding agents to form macroaggregates. The focus of this study is in investigating the contribution of humic carbon (HC) to soil aggregation in response to various tillage and residue managements. Arable soils following 8-year contrasting managements were collected to determine aggregate size distribution and stability and HC fractions including humic acid (HA) and fulvic acid (FA). The contribution of HC to aggregation was divided into three special effects including positive effect (PE), negative effect (NE), and combined effect (CE), and these effects were measured using aggregate fractionation techniques. As well as to promote structural stability, HC bounds predominantly with the silt + clay fraction and secondarily with microaggregates to form larger aggregates. The PE increased with increasing aggregate size, whereas the NE followed the opposite pattern. A positive CE was observed for large and small macroaggregates, whereas the CE for microaggregates and the silt + clay fraction was negative. Compared to continuous tillage, reduced- and no-tillage decreased the PE for large and small macroaggregates by 1.58–30.98% at the 0–20 cm depth, and straw returning also slightly decreased the corresponding PE relative to straw removing. By contrast, a significantly higher NE for small macroaggregates at the 0–10 cm depth while 6.33–81.11% decreases in CE for large and small macroaggregates at the 0–10 cm depth as well as for large macroaggregates at the 10–20 cm depth, were observed under reduced- and no-tillage. The extraction of HC significantly reduced the aggregate stability and reduced- and no-tillage effectively limited its decrease magnitude. Small macroaggregates and microaggregates made larger contributions to soil HC accumulation than did other fractions. An averagely increased contribution from large or small macroaggregates was observed under both reduced-/no-tillage and straw returning at the 0–20 cm depth. A significant and positive relationship was found between the mass proportion of macroaggregates and the HC accumulation in 0–20 cm soil. Large macroaggregates had significantly higher HA/FA ratios than small macroaggregates, and reduced- and no-tillage significantly increased these ratios both in large and in small macroaggregates. The CE for large or small macroaggregates was also significantly negatively correlated with their HA/FA ratios. Overall, the HC accumulation in soil is likely to play a key role in macroaggregation, but conservation tillage might decrease the contribution magnitude of HC to large or small macroaggregation through increasing the corresponding HA/FA ratios.

Effects of Organic Amendments on Soil Physical Attributes and Aggregate-Associated Phosphorus Under Long-Term Rice-Wheat Cropping

Abstract The quantification of phosphorus (P) in bulk soil and P distribution in different size fractions of water-stable aggregates (WSAs) are important for assessing potential P loss through runoff. We evaluated available and total P distribution within WSAs of a sitty clay to clay soil in a long-term fertility experiment of a rice-wheat cropping system in India. Surface soil samples were collected from seven plots amended with NPK fertilizers in combination with or without organic amendments, farmyard manure (FYM), green manure (GM), and paddy straw (PS). The plot with no NPK fertilizers or organic amendments was set as a control. The soil samples were separated by wet sieving into four soil aggregate size fractions: large macroaggregates (> 2.0 mm), small macroaggregates (0.25–2.0 mm), fine microaggregates (0.05–0.25 mm), and a silt + clay-sized fraction ( small macroaggregates > fine microaggregates > large macroaggregates. Within a size class, aggregate-associated available and total P contents in the organically amended soil were in the following order: FYM > PS ≥ GM. The available P content of the microaggregates ( 0.25 mm), and the total P content of the microaggregates was 4- to 5-times higher than that of the macroaggregates. Cultivation without organic amendments resulted in more microaggregates that could be checked by the application of organic amendments such as FYM and GM, which increased the proportion of water-stable macroaggregates by consolidating microaggregates into macroaggregates.

Soil organic carbon stocks and fractionation under different land uses in the Peruvian high-Andean Puna

Soil organic C (SOC), the largest terrestrial C pool, has been shown to be sensitive to agricultural disturbance. In the Andean Puna, agricultural expansion is thought to be jeopardizing the large SOC stock sequestered in its soils. In this paper, we assessed the impact changes in land use have on C sequestration, fractionation and δ13C natural abundance featuring two case studies in the Peruvian Central Puna. We found SOC stocks greater than average temperate grassland soils (between 123±4 and 136±4MgCha−1 in the 0–30cm soil profile); however, they did not differ between land uses. In the first case study, depletion of δ13C in mineral associated C in long fallow (>7yr) and maca (Lepidium meyenii Walpers) croplands was hypothesized to be caused by disruption of soil aggregates due to tillage and initial greater SOC concentration in maca plots. In addition, SOC loss was found in long fallow plots, and was correlated to steep and low plant cover plots; evidencing that soil degradation does not occur during cropping activity but due to erosion after land abandonment. In the second case study, a long-established perennial ryegrass (Lolium perenne L.) - white clover (Trifolium repens L.) association (>40yr) was studied. We did not find an increase in SOC stock under the cultivated pasture. However, we found a better soil aggregation and an increase of 17.3 and 12.2gCkg−1WSA in particulate organic C, and SOC within small macroaggregates in cultivated pasture soils. Soil δ13C natural abundance was about 1‰ depleted in all measured C fractions of cultivated pastures when compared to native grasslands, suggesting that pasture-fixed C forms labile and recalcitrant SOC fractions.

Effects of maize roots on aggregate stability and enzyme activities in soil

Soil aggregation and microbial activities within the aggregates are important factors regulating soil carbon (C) turnover. A reliable and sensitive proxy for microbial activity is activity of extracellular enzymes (EEA). In the present study, effects of soil aggregates on EEA were investigated under three maize plant densities (Low, Normal, and High). Bulk soil was fractionated into three aggregate size classes (>2000μm large macroaggregates; 2000–250μm small macroaggregates; <250μm microaggregates) by optimal-moisture sieving. Microbial biomass and EEA (β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG), l-leucine aminopeptidase (LAP) and acid phosphatase (acP)) catalyzing soil organic matter (SOM) decomposition were measured in rooted soil of maize and soil from bare fallow. Microbial biomass C (Cmic) decreased with decreasing aggregate size classes. Potential and specific EEA (per unit of Cmic) increased from macro- to microaggregates. In comparison with bare fallow soil, specific EEA of microaggregates in rooted soil was higher by up to 73%, 31%, 26%, and 92% for BG, NAG, acP and LAP, respectively. Moreover, high plant density decreased macroaggregates by 9% compared to bare fallow. Enhanced EEA in three aggregate size classes demonstrated activation of microorganisms by roots. Strong EEA in microaggregates can be explained by microaggregates' localization within the soil. Originally adhering to surfaces of macroaggregates, microaggregates were preferentially exposed to C substrates and nutrients, thereby promoting microbial activity.

Soil organic carbon stock in relation to aggregate size and stability under tree-based cropping systems in Typic Ustochrepts

The growing of tree crops along with the field crops is a common practice in foothills of lower Himalayas with the twin objective of checking soil erosion by water and covering the risk of crop failure due to frequently occurring droughts. A study was conducted to evaluate dry and water stable aggregates for their soil organic carbon (SOC) stocks under different tree-based cropping systems. The treatments consisted of three cropping systems viz. maize-wheat (sole crop), agroforestry and agrohorticulture (tree-based) in the similar soil texture and in 6 year old plantations. The soil samples were obtained from different layers (0–15, 15–30, 30–60, 60–90 and 90–120 cm) and analyzed for SOC. The undisturbed aggregate samples (as big clods) were collected from 0–15 and 15–30 cm soil layer for dry and wet aggregate stability. The data so obtained was analyzed by using CRD factorial deigns at LSD (P ≤ 0.05). The SOC concentration decreased with soil depth, the decrease was higher (89.6%) in soils under maize-wheat than in soils under agrohorticulture (81.3%) and agroforestry (77.8%). The mean SOC concentration decreased with the size of the dry stable aggregates (DSA) and water stable aggregates (WSA). In DSA, the mean SOC concentration was 58.06 and 24.2% higher in large and small macroaggregates than in microaggregates respectively; in WSA it was 295.6 and 226.08% higher in large and small macroaggregates than in microaggregates respectively in surface soil layer. The mean SOC concentration in surface soil was higher in DSA (0.79%) and WSA (0.63%) as compared to bulk soil (0.52%). The SOC concentration and stock being highest in soils under agroforestry resulted in higher SOC concentration in dry as well as WSA.

Long-term plastic film mulching and fertilization treatments changed the annual distribution of residual maize straw C in soil aggregates under field conditions: characterization by 13C tracing

Plastic film mulching and fertilization strongly affect soil aggregation and dynamics of the total soil organic carbon (C) pool. However, there is limited information on how these agricultural management practices influence the fate and seasonal dynamics of crop residue-derived C in soil aggregates. Therefore, a better understanding of the fate of C derived from crop residues and their location in soil aggregates is crucial to improve our prediction of C sequestration and stabilization in soil. In this study, an in situ 13C-tracing technique was used to identify the dynamic distribution and accumulation patterns of crop residue-derived C in soil aggregates as affected by long-term plastic film mulching and four fertilization treatments (control, CK; nitrogen, N2; organic manure, M2; nitrogen and organic manure, M1N1). The fate of 13C-labeled maize straw in loam soil was studied over 360 days using in situ incubation of 0.2% equivalent dry straw incorporated into the soil and aggregate size fractionation. Soil samples were separated into four particle-size fractions [large (>2 mm) and small (1–2 mm) macroaggregates; large (0.25–1 mm) and small ( M1 N1 > N2 > CK. Both the content of straw-derived 13C in aggregate fractions and the proportion of 13C in total soil samples decreased considerably from the microaggregate fractions and increased moderately in the macroaggregate fractions in a manner enhanced by plastic film mulching and by seasonal transitions, specifically from spring (day 0) to summer (day 60). In addition, the decomposition of maize straw and soil aggregation exhibited a direct correlation in which the content of 13C-SOC and mean weight diameters decreased over the course of the 360-day experiment. Our results suggest that certain amounts of straw-derived 13C could accumulate in microaggregates, which play an important role in long-term C sequestration, while a large part of straw-derived C tends to transfer from microaggregates to macroaggregates over time as affected by long-term plastic film mulching coupled with fertilization. This study improves our understanding of the effect of plastic film mulching and different fertilization regimes on the retention and stabilization processes of straw-derived C in soil.

Biochar additions can enhance soil structure and the physical stabilization of C in aggregates

Biochar soil amendments are often considered as a soil carbon (C) sequestration strategy that can have beneficial impacts on a range of soil properties and plant production. We investigated the impact of two distinct types of biochar on soil chemical properties, microbial communities, soil aggregation and aggregate-associated C within two California agricultural soils in a laboratory incubation study (60weeks). Water stable aggregation and associated C were examined via wet-sieving to obtain four aggregate size classes: large macroaggregates (2000-8000μm), small macroaggregates (250-2000μm), microaggregates (53-250μm) and silt and clay fraction (<53μm). Biochars enhanced aggregation in the finer textured Yolo soil, with 217% and 126% average increases in mean weight diameter for a softwood biochar (pyrolyzed at 600-700°C with algal digestate) and a walnut shell biochar gasified at 900°C), respectively. The increase in aggregate stability was associated with an increase in physically-protected C incorporated into macroaggregates. Both biochars had substantial impacts on microbial community composition in both soils, but only increased microbial biomass in Yolo soil. In the coarser textured Vina soil, neither biochar had an effect on aggregation and the subsequent lack of soil organic matter (SOM) stabilization in macroaggregates was associated with a significant loss of soil C in both biochar treatments over the course of the incubation. Our results suggest that biochar can increase the physical-protection of SOM in Yolo soil by enhancing the proportion of C stored within macroaggregates and thus offers a novel mechanism by which biochar may contribute to soil C sequestration. Better understanding of these drivers and identifying soil conditions that determine whether biochar will physically protect SOM vs. stimulate soil C loss must be considered in managing agroecosystems for both mitigation of, and adaptation to, climate change.

Impact of 47 Years of No Tillage and Stubble Retention on Soil Aggregation and Carbon Distribution in a Vertisol

AbstractAggregation often provides physical protection and stabilisation of soil organic carbon (C). No tillage (NT) coupled with stubble retention (SR) and nitrogen (N) fertiliser application (90 N, 90 kg N ha−1 application) can help improve soil aggregation. However, information is lacking on the effect of long‐term NT, SR and N fertiliser (NT, SR + N) application on soil aggregation and C distribution in different aggregates in vertisols. We analysed the soil samples collected from 0‐ to 30‐cm depth from a long‐term (47 years) experiment for soil aggregation and aggregate‐associated C and N. This long‐term field experiment originally consisted of 12 treatments, having plot size of 61·9 × 6·4 m, and these plots were arranged in a randomised block design with four replications, covering an area of 1·9 ha. Soil organic C concentrations as well as stocks were significantly higher under the treatment of NT, SR + N only in 0–10 cm compared with other treatments such as conventional tillage, stubble burning + 0 N (no N application) and conventional tillage, SR + 0 N. Mineral‐associated organic C (MOC) of &lt;0·053 mm was 5–12 times higher (r = 0·68, p &lt; 0·05, n = 32) compared with particulate organic C (POC) (&gt;0·053 mm) in the 0‐ to 30‐cm layer. We found that NT, SR + N treatment had a positive impact on soil aggregation, as measured by the mean weight diameter (MWD) through wet sieving procedure, but only in the top 0‐ to 10‐cm depth. MWD had significant positive correlation with water stable aggregates (r = 0·67, p &lt; 0·05). Unlike MWD, water stable aggregates were not affected by tillage and stubble management. Large macroaggregates (&gt;2 mm) had significantly higher organic C and N concentrations than small macroaggregates (0·25–2 mm) or microaggregates (0·053–0·25 mm). We also found that N application had a significant effect on MWD and soil organic C in vertisols. It is evident that better soil aggregation was recorded under NTSR90N could have a positive influence on soil C sequestration. Our results further highlight the importance of soil aggregation and aggregate‐associated C in relation to C sequestration. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Effects of tillage and residue managements on organic C accumulation and soil aggregation in a sandy loam soil of the North China Plain

This paper was primarily devoted to reveal the stock of soil organic carbon (C) as well as its lability and to compare their differences existing among tillage and residue practices, aiming to identify the effects on the accumulation process of organic C and its association with macroaggregation. Arable soils following 8-year contrasting managements were collected to determine aggregate size distribution, organic C content and its lability. A wet-sieving method was used to fractionate aggregate fractions including >2000μm large macroaggregates, 2000–250μm small macroaggregates, 250–53μm microaggregates, and <53μm silt+clay fractions. The C amount in physical subfractions was measured using aggregate fractionation techniques. It was found that reduced/no-tillage and straw returning significantly promoted soil macroaggregation and aggregate stability at the 0–10cm depth. The organic C stock was 20.7% higher in 0–5cm soil and 7.5% higher in 5–10cm soil under reduced/no-tillage than continual tillage, whereas straw returning significantly increased organic C stock by 28.8%, 25.1% and 7.7%, respectively at the 0–5, 5–10 and 10–20cm depths. Macroaggregates made a larger contribution to soil organic C accumulation than did other fractions. Both reduced/no-tillage and straw returning significantly increased the contribution of macroaggregates at the expense of microaggregates and silt+clay fractions at the 0–10cm depth. When large and small macroaggregates were further separated into physical subfractions, reduced/no-tillage coupling with straw returning averagely increased the C amount in the intra-particulate organic matter (iPOM) and mineral-associated C (mSOC), but decreased the oxidation stability coefficients (Kos) of organic C in aggregates. Significant and negative relationships were found between the mass proportion of macroaggregates as well as aggregate stability and the Kos values of organic C. The Kos values of macroaggregate-associated C were also significantly negatively correlated with the C amount in subfractions. Overall, the accumulation of organic C in physical subfractions within macroaggregates might contribute to sequester relatively labile organic C in soil following reduced/no-tillage and straw returning, which in turn promoted soil macroaggregation.