Rapid incorporation of carbon from fresh residues into newly formed stable microaggregates within earthworm casts

Earthworms play an important role in protecting carbon in the soil, but the exact influence of their activity on the distribution and protection of C is still poorly understood. We investigated the effect of earthworms on the formation of stable microaggregates inside newly formed macroaggregates and the distribution of C in them. We crushed (< 250 µm) soil, and subjected it to three treatments: (i) soil + ¹³C‐labelled residue + earthworms (these added after 8 days' incubation), (ii) soil + ¹³C‐labelled residue, and (iii) control (no additions), and then incubated it for 20 days. At the end, we measured the aggregate size distribution, total C and ¹³C, and we isolated microaggregates (53–250 µm) from macroaggregates (> 250 µm) formed. The ¹³C in fine particulate organic matter between and within the microaggregates was determined. Earthworms helped to form large macroaggregates (> 2000 µm). These large macroaggregates contained four times more stable microaggregates than those from samples without earthworms. There was more particulate organic matter within and between microaggregates in macroaggregates in the presence of earthworms. The larger amounts of organic matter inside stable microaggregates in casts than in bulk soil after 12 days of incubation (140 mg ¹³C kg^?1 soil compared with 20 mg ¹³C kg^?1 soil) indicates that these microaggregates are formed rapidly around freshly incorporated residues within casts. In conclusion, earthworms have a direct impact on the formation of stable microaggregates and the incorporation of organic matter inside these microaggregates, and it seems likely that their activity is of great significance for the long‐term stabilization of organic matter in soils.     

Influence of earthworm activity on aggregate-associated carbon and nitrogen dynamics differs with agroecosystem management

Earthworms are known to be important regulators of soil structure and soil organic matter (SOM) dynamics, however, quantifying their influence on carbon (C) and nitrogen (N) stabilization in agroecosystems remains a pertinent task. We manipulated population densities of the earthworm Aporrectodea rosea in three maize-tomato cropping systems [conventional (i.e., mineral fertilizer), organic (i.e., composted manure and legume cover crop), and an intermediate low-input system (i.e., alternating years of legume cover crop and mineral fertilizer)] to examine their influence on C and N incorporation into soil aggregates. Two treatments, no-earthworm versus the addition of five A. rosea adults, were established in paired microcosms using electro-shocking. A ¹³C and ¹⁵N labeled cover crop was incorporated into the soil of the organic and low-input systems, while ¹⁵N mineral fertilizer was applied in the conventional system. Soil samples were collected during the growing season and wet-sieved to obtain three aggregate size classes: macroaggregates (>250 μm), microaggregates (53-250 μm) and silt and clay fraction (<53 μm). Macroaggregates were further separated into coarse particulate organic matter (cPOM), microaggregates and the silt and clay fraction. Total C, ¹³C, total N and ¹⁵N were measured for all fractions and the bulk soil. Significant earthworm influences were restricted to the low-input and conventional systems on the final sampling date. In the low-input system, earthworms increased the incorporation of new C into microaggregates within macroaggregates by 35% (2.8 g m⁻² increase; P=0.03), compared to the no-earthworm treatment. Within this same cropping system, earthworms increased new N in the cPOM and the silt and clay fractions within macroaggregates, by 49% (0.21 g m⁻²; P<0.01) and 38% (0.19 g m⁻²; P=0.02), respectively. In the conventional system, earthworms appeared to decrease the incorporation of new N into free microaggregates and macroaggregates by 49% (1.38 g m⁻²; P=0.04) and 41% (0.51 g m⁻²; P=0.057), respectively. These results indicate that earthworms can play an important role in C and N dynamics and that agroecosystem management greatly influences the magnitude and direction of their effect.     

Earthworm impacts on soil organic matter and fertilizer dynamics in tropical hillside agroecosystems of Honduras

Earthworms are important processors of soil organic matter (SOM) and nutrient turnover in terrestrial ecosystems. In agroecosystems, they are often seen as beneficial organisms to crop growth and are actively promoted by farmers and extension agents, yet their contribution to agroecosystem services is uncertain and depends largely on management. The Quesungual slash-and-mulch agroforestry system (QSMAS) of western Honduras has been proposed as a viable alternative to traditional slash-and-burn (SB) practices and has been shown to increase earthworm populations, yet the effect of earthworms on soil fertility and SOM in QSMAS is poorly understood. This study examined the role of Pontoscolex corethrurus in QSMAS by comparing their influence on aggregate-associated SOM and fertilizer dynamics with their effects under SB and secondary forest in a replicated field trial. Both the fertilized QSMAS and SB treatments had plots receiving additions of inorganic ¹⁵N and P, as well as plots with no inorganic N additions. Earthworm populations were manipulated in field microcosms at the beginning of the rainy season within each management treatment via additions of P. corethrurus or complete removal of existing earthworm populations. Microcosms were destructively sampled at harvest of Zea mays and soils were wet-sieved (using 53, 250 and 2000 μm mesh sizes) to isolate different aggregate size fractions, which were analyzed for total C, N and ¹⁵N. The effects of management system were smaller than expected, likely due to disturbance associated with the microcosm installation. Contrary to our hypothesis that earthworms would stabilize organic matter in soil aggregates, P. corethrurus decreased total soil C by 3% in the surface layer (0-15 cm), predominantly through a decrease in the C concentration of macroaggregates (>250 μm) and a corresponding depletion of C in coarse particulate organic matter occluded within macroaggregates. Earthworms also decreased bulk density by over 4%, but had no effect on aggregate size distribution. Within the two fertilized treatments, the QSMAS appeared to retain slightly more fertilizer derived N in smaller aggregate fractions (<250 μm) than did SB, while earthworms greatly reduced the recovery of fertilizer N (34% decrease) in both systems. Although management system did not appear to influence the impact of P. corethrurus on SOM or nutrient dynamics, we suggest the lack of differences may be due to artificially low inputs of fresh residue C to microcosms within all management treatments. Our findings highlight the potential for P. corethrurus to have deleterious impacts on soil C and fertilizer N dynamics, and emphasize the need to fully consider the activities of soil fauna when evaluating agroecosystem management options.     

Interactive effects of functionally different earthworm species on aggregation and incorporation and decomposition of newly added residue carbon

The interactive effects of two functionally different earthworm species (Aporrectodea caliginosa (endogeic species) and Lumbricus rubellus (epigeic species)) on the incorporation of fresh residue into large macroaggregates and formation of microaggregates within these large macroaggregates were investigated during a short-term laboratory experiment using ¹³C-labelled sorghum (Sorghum bicolor (L.) Moench) residues. Soil was collected from a long-term no-tillage agricultural field, crushed through a 250-μm sieve and incubated under laboratory conditions. The following earthworm treatments were applied: (i) soil+¹³C-labelled residue+A. caliginosa; (ii) soil+¹³C-labelled residue+L. rubellus; (iii) soil+¹³C-labelled residue+A. caliginosa+L. rubellus and; (iv) soil+¹³C-labelled residue. Two residue placement treatments (i.e. surface and incorporated) were superimposed on the earthworm treatments. Earthworms were added after 8 days of incubation. Aggregate size distribution and total C and ¹³C were measured after 22 days. Microaggregates, fine inter-microaggregate particulate organic matter (inter-POM) and intra-microaggregate POM (intra-POM) were isolated from macroaggregates. Earthworms had a greater stimulating effect on the formation of large macroaggregates (>2000 μm) and microaggregates within large macroaggregates when residue was incorporated in the soil, especially in the presence of A. caliginosa. When residue was placed on the surface, residue-derived intra-POM C was highest when L. rubellus was present and significantly lower in the presence of A. caliginosa. Residue-derived inter-POM C was highest when a mix of both species was present. These results indicate that earthworm species differentially affect incorporation of fresh organic matter into stable microaggregates within macroaggregates, and that interactive effects of earthworm species might have important consequences for the incorporation and protection of C inside of microaggregates within macroaggregates especially when residues are placed on the soil surface.     

Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural C and N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (δ¹³C, δ¹⁵N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C₃ trees/shrubs (δ¹³C=−27‰) have largely replaced C₄ grasslands (δ¹³C=−14‰) over the past 100-200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural δ¹³C and δ¹⁵N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ¹³C with time after woody encroachment. The shortest MRTs (average=30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53-250 μm) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt+clay fractions had the longest MRTs (average=360 years) regardless of whether they were found inside or outside of aggregate structure. δ¹⁵N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ¹⁵N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C₄-derived organic matter by protection within microaggregates and association with silt+clay, and the accumulation of new C₃-derived organic matter in macroaggregates and POM fractions.     

Earthworm populations in relation to soil organic matter dynamics and management in California tomato cropping systems

Earthworms are key regulators of soil structure and soil organic matter (SOM) dynamics in many agroecosystems. They are greatly impacted by agricultural management, yet little is known about how these factors interact to control SOM dynamics. This study sought to explore linkages between agricultural management, earthworms and aggregate associated SOM dynamics through a survey of tomato (Solanum lycopersicum L.) cropping systems in northern California. Earthworms and soil samples were collected between February and April of 2005 from 16 fields under one of three types of residue management: (1) tomato mulch – no postharvest tillage and tomato residues left on the soil surface, (2) cover crop – tomato residues tilled in and leguminous cover crop planted, and (3) bare fallow – tomato residues tilled in and soil surface left exposed throughout the winter. Earthworms were collected via hand-sorting and identified to species, while soils were wet sieved to yield four aggregate size classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the silt and clay fraction (<53 μm). The combined large and small macroaggregate fraction was then fractionated into coarse particulate organic matter (cPOM; 250 μm), microaggregates within macroaggregates (mM; 53–250 μm) and macroaggregate occluded silt and clay (Msc; <53 μm). The earthworms identified in this survey were composed entirely of exotic species and were dominated by Aporrectodea caliginosa. Earthworm abundance was related to residue management, with the tomato mulch systems averaging 4.5 times greater fresh earthworm biomass than bare fallow (P = 0.024). Aggregate stability and total soil C and N also appeared to be influenced by residue management, such that the tomato mulch system displayed significantly greater mean weight diameters than the bare fallow system (P = 0.049), as well as more than 50% greater total soil C and N (P = 0.049 and P = 0.036; respectively). Earthworm biomass was also found to be positively correlated with total soil C (P = 0.009, R² = 0.39) and N (P = 0.010, R² = 0.039) as well as the proportion of macroaggregate C in the cPOM fraction (P = 0.028, R² = 0.30). Our findings suggest that residue handling and the associated management practices (e.g., tillage, organic vs. conventional agriculture) are important for both earthworm populations and SOM storage. Although earthworms are known to influence SOM in many ways, other factors appear to play a more prominent role in governing aggregate associated SOM dynamics.     

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

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( 0.05 mm). Structural indices were higher in the soil receiving organic amendments than in the soil receiving inorganic fertilizer alone. Organically amended soil had a higher proportion of stable macroaggregates than the control and the soil receiving inorganic fertilizer alone, which were rich in microaggregates. Total and available P contents within WSAs were inversely related to the aggregate size, irrespective of treatment. The distribution of available and total P in the soil aggregate size fraction was as follows: silt + clay-size 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) was 8-to 10-times higher than that of the macroaggregates( 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.     

Competition between invasive earthworms (Amynthas corticis, Megascolecidae) and native North American millipedes (Pseudopolydesmus erasus, Polydesmidae): Effects on carbon cycling and soil structure

Invasive earthworms can have significant impacts on C dynamics through their feeding, burrowing, and casting activities, including the protection of C in microaggregates and alteration of soil respiration. European earthworm invasion is known to affect soil micro- and mesofauna, but little is known about impacts of invasive earthworms on other soil macrofauna. Asian earthworms (Amynthas spp.) are increasingly being reported in the southern Appalachian Mountains in southeastern North America. This region is home to a diverse assemblage of native millipedes, many of which share niches with earthworm species. This situation indicates potential for earthworm-millipede competition in areas subject to Amynthas invasion.In a laboratory microcosm experiment, we used two ¹³C enriched food sources (red oak, Quercus rubra, and eastern hemlock, Tsuga canadensis) to assess food preferences of millipedes (Pseudopolydesmus erasus), to determine the effects of millipedes and earthworms (Amynthas corticis) on soil structure, and to ascertain the nature and extent of the interactions between earthworms and millipedes. Millipedes consumed both litter species and preferred red oak litter over eastern hemlock litter. Mortality and growth of millipedes were not affected by earthworm presence during the course of the experiment, but millipedes assimilated much less litter-derived C when earthworms were present.Fauna and litter treatments had significant effects on soil respiration. Millipedes alone reduced CO₂ efflux from microcosms relative to no fauna controls, whereas earthworms alone and together with millipedes increased respiration, relative to the no fauna treatment. CO₂ derived from fresh litter was repressed by the presence of macrofauna. The presence of red oak litter increased CO₂ efflux considerably, compared to hemlock litter treatments.Millipedes, earthworms, and both together reduced particulate organic matter. Additionally, earthworms created significant shifts in soil aggregates from the 2000-250 and 250-53 μm fractions to the >2000 μm size class. Earthworm-induced soil aggregation was lessened in the 0-2 cm layer in the presence of millipedes. Earthworms translocated litter-derived C to soil throughout the microcosm.Our results suggest that invasion of ecosystems by A. corticis in the southern Appalachian Mountains is unlikely to be limited by litter species and these earthworms are likely to compete directly for food resources with native millipedes. Widespread invasion could cause a net loss of C due to increased respiration rates, but this may be offset by C protected in water-stable soil aggregates.     

Earthworm activity affects soil aggregation and organic matter dynamics according to the quality and localization of crop residues—An experimental study (Madagascar)

Soil organic matter (SOM) plays a central role in the functioning of ecosystems, and is beneficial from agronomic and from environmental point of view. Alternative cultural systems, like direct seeding mulch-based cropping (DMC) systems, enhance carbon (C) sequestration in agricultural soils and lead to an increase in soil macrofauna. This study aimed at evaluating in field mesocosms the effects of earthworms on SOM dynamics and aggregation, as influenced by residue quality and management.In the highlands of Madagascar, buckets were filled with 2 mm-sieved clayey Inceptisol. The effects of earthworm addition (Pontoscolex corethrurus), residue addition (rice, soybean, and no addition), and localization of the residues (mulched or buried) were studied. After 5 months, soil from mesocosms with earthworms had significantly lower C concentration and higher proportion of large water-stable macroaggregates (>2000 μm) than those without earthworms, because of the production of large macroaggregates by earthworms. Earthworm effect on soil aggregation was greater with rice than with soybean residues. Casts (extracted from mesocosms with earthworms) were slightly enriched in C and showed significantly higher mineralization than the non-ingested soil (NIS), showing that at the time scale of our study, the carbon contained in the casts was not protected against mineralization. No difference in microbial biomass was found between casts and NIS.Complementary investigations are necessary to assess long-term effects of earthworm addition on SOM dynamics, the conditions of occurrence of physical protection, and the impact of earthworms on the structure of the microbial community.     

Particulate organic matter in water stable aggregates formed after the addition of C-labeled maize residues and wetting and drying cycles in vertisols

Development of soil structure and the dynamics of water stable aggregates (WSA) in many soils are known to be closely related to the cycling of soil organic matter. In some fine and medium textured soils particulate organic matter (POM) has been found to act as a nucleus for macroaggregate formation. However, this role of POM in aggregate formation has not been demonstrated in soils dominated by smectitic clay minerals. This study explored aggregation processes in a Vertisol from a semi-arid region in Northeastern Mexico in relation to the addition of ¹⁴C-labeled maize residues and application of wetting and drying cycles during 105 days of incubation. Fractionation of the WSA formed showed that labeled residues were preferentially accumulated in large macroaggregates (>2000 μm). Treatments with addition of organic residues had three to four times more intra-aggregate particulate organic matter (iPOM) in large macroaggregates than the control after 14 days of incubation. Residue-derived carbon accounted for 53% and 41% of the total carbon stored in the iPOM fraction in amended treatments with and without wetting and drying cycles, respectively. Conversely, residue-derived carbon represented <20% of the total carbon in the iPOM fraction from small macroaggregates (250-2000 μm) and microaggregates (53-250 μm). Results also showed that the amount and concentration of carbon per large macroaggregate did not differ between the large macroaggregates formed under wetting and drying and those formed in continuous moist conditions. However, due to formation of higher number of large macroaggregates per kg of soil, more carbon could be stored in amended soils under wetting and drying than in constantly wet soil: 1.4, 1.8 and 2.7 times more ¹⁴C kg⁻¹ soil after 14, 58 and 105 incubation days, respectively. The results in this study suggest that wetting and drying enhanced protection of the added maize residues inside large macroaggregates by forming more aggregates, rather than by increasing the amount of POM entrapped per aggregate. Therefore, after the addition of organic residues, this soil could accumulate more C than continuous moist soil through the influence that wetting and drying has on soil aggregation.     

Pasture degradation impacts soil phosphorus storage via changes to aggregate-associated soil organic matter in highly weathered tropical soils

Maintaining the productivity of tropical pastures is a major challenge for the sustainable management of tropical landscapes around the globe. To address this issue, we examined linkages between soil organic matter (SOM), aggregation, and phosphorus (P) dynamics by comparing productive vs. degraded pastures in the deforested Amazon Basin of Colombia. Paired plots of productive (dominated by planted Brachiaria spp.) vs. degraded pasture were identified on nine farms in the Department of Caquetá and sampled during the rainy season of 2011. Aboveground pasture biomass production and nutrient content were measured. Surface soils (0–10 cm) were also fractionated by wet sieving, and C, ¹³C, N and P contents were analyzed for the bulk soil and various aggregate size classes. Productive pastures yielded more than double the aboveground biomass compared to degraded pastures (during a 35 day regrowth period following cutting), with over 60% higher N and P contents in this material. Similar trends were observed for the standing litter biomass and nutrient contents. Soil aggregate stability was found to differ between pasture types, with a mean weight diameter of 3590 vs. 3230 μm in productive vs. degraded pastures, respectively. Productive pastures were found to have 20% higher total soil C and N contents than degraded pastures. While there was no difference in total P content between pasture types, organic P was found to be nearly 40% higher in soils of productive vs. degraded pastures. Differences in total SOM between pasture types were largely explained by a higher C content in the large macroaggregate fraction (>2000 μm), and more specifically in the microaggregates (53–250 μm) occluded within this fraction. These findings confirm the role of microaggregates within macroaggregates as a preferential site for the physical stabilization of SOM, and furthermore, suggest that it may serve as a useful diagnostic fraction for evaluating management impacts on SOM in tropical pasture systems. Similar to trends observed for C and N, total P content was 25% higher in the microaggregates within large macroaggregates of productive vs. degraded pasture soils. This correspondence between C and total P contents in large macroaggregate fractions, along with elevated levels of organic P in productive pastures, suggests that this P is likely in an organic form and that there is a close link between soil structure, SOM dynamics and the maintenance of organic P in these soils. Given the potential relevance of organic P for efficient P cycling in these soils, our findings offer critical new insight for the management of SOM and aggregate-associated P pools in tropical pasture systems.     

Reciprocal transfer of carbon and nitrogen by decomposer fungi at the soil-litter interface

We have investigated whether decomposer fungi translocate litter-derived C into the underlying soil while simultaneously translocating soil-derived inorganic N up into the litter layer. We also located and quantified where the translocated C is deposited within the soil aggregate structure. When ¹³C-labeled wheat straw was decomposed on the surface of soil amended with ¹⁵N-labeled inorganic N, we found that C and N were reciprocally transferred by fungi, with a significant quantity (121-151 μg C g⁻¹ whole soil) of litter-derived C being deposited into newly formed macroaggregates (>250 μm sized aggregates). Fungal inhibition reduced fungal biomass and the bidirectional C and N flux by approximately 50%. The amount of litter-derived C found in macroaggregates was positively correlated with litter-associated fungal biomass. This fungal-mediated litter-to-soil C transfer, which to our knowledge has not been demonstrated before for saprophytic fungi, may represent an important mechanism by which litter C enters the soil and becomes stabilized as soil organic matter within the macroaggregate structure.     

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

Soil 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 (>2.0 mm), small macroaggregates (0.25–2.0 mm), microaggregates (0.053–0.25 mm) and mineral fraction (<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 > shrubland > natural grassland > orchard > blue pine > broadleaf > mixed conifer > dry land > 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.     

Sensitivities to nitrogen and water addition vary among microbial groups within soil aggregates in a semiarid grassland

We investigated whether enhanced nitrogen (N) and water inputs would redistribute the microbial community within different soil aggregate size classes in a field manipulation experiment initiated in 2005. Distribution of microbial groups was monitored in large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), and microaggregates (<250 μm) in a semiarid grassland. Both arbuscular mycorrhizal (AM) fungi and saprophytic fungi were the most abundant in soil macroaggregates. The gram-negative bacteria were more abundant in soil microaggregates. Total phospholipid fatty acid (PLFA) concentration in general and actinomycetes in particular decreased with N addition under ambient precipitation but was unaffected by combined additions of N and water within the three soil aggregate fractions as compared to control plots. In contrast, the abundance of saprophytic fungi decreased with combined N and water addition, but it was not affected by N addition under ambient precipitation. The abundance of gram-positive bacteria increased with N addition under both ambient and elevated water conditions for all soil aggregate fractions. In summary, the higher short-term nutrient and water availabilities provoked a shift in soil microbial community composition and increased total PLFA abundance irrespectively of the level of soil aggregation. In the long term, this could destabilize soil carbon pools and influence the nutrient limitation of soil biota within different soil aggregate size classes under future global change scenarios.     

Soil aggregation and organic carbon in short-term stockpiles

Surface mining is known to drastically reduce soil organic carbon (OC) pools through various mechanisms associated with topsoil salvage, stockpiling and respreading. Stockpiling is an important management practice; however, the effects of this practice on reductions and recovery of soil aggregation and aggregate OC are poorly understood. Objectives of this research were to monitor soil aggregation and aggregate OC in the surface of a short‐term stockpile (<3 yr) followed by a second movement of stockpiled soils to a temporary location. Samples were analysed for aggregate size distribution, aggregate fractions, OC, and organic matter turnover using ¹³C natural abundance. Macroaggregate proportions increased and microaggregate proportions decreased after 3 yr of storage, possibly indicating recovery of soil structure. Following the removal of the stockpile and placement in a temporary pile, macroaggregation decreased and free silt and clay fractions increased relative to initially stockpiled soils. The second disturbance resulted in greater destruction of aggregate structure than the initial disturbance during topsoil salvage. Aggregate organic matter (as indicated by OC) increased significantly between the early sampling of the stockpiled soils (<1 yr in storage) and the placement of the topsoil in a temporary pile in macroaggregates and remained the same for microaggregates. Organic matter not protected within aggregates decreased with storage time as this material was available for utilization by microbes while aggregate protected organic matter (OM) remained unchanged or slightly increased for macro‐ and microaggregates with stockpile storage time. Aggregate δ¹³C values did not indicate inclusion of new OM within soil aggregates after 3 yr of topsoil stockpiling. Short‐term stockpiling was beneficial for aggregation in the surface layers where plant roots and microbial communities were active; however, subsequent movement of the topsoil resulted in a greater loss of soil aggregation relative to the initial topsoil salvage without impacting soil OC.     

Effect of litter quality and soil fungi on macroaggregate dynamics and associated partitioning of litter carbon and nitrogen

We investigated the effect of plant residue decomposability and fungal biomass on the dynamics of macroaggregate (250–2000 μm) formation in a three months' incubation experiment and determined the distribution of residue-derived C and N in the microbial biomass and in aggregate size fractions (250–2000 μm, 53–250 μm and <53 μm) using ¹³C and ¹⁵N data. A silty loam soil (sieved <250 μm) was incubated with and without addition of ¹⁵N labelled maize leaves (C/N = 27.4) and roots (C/N = 86.4). Each treatment was carried out with and without fungicide application. The addition of maize residues enhanced soil respiration and microbial biomass C and N and resulted in increased macroaggregate formation with a higher and more rapid maximum macroaggregation in the soil amended with maize leaves than in that with addition of roots. Fungicide application led to a significant decline of microbial biomass C and mineralization of the added residues compared to untreated soils, which demonstrates a successful suppression of part of the active microbial biomass by the fungicide. However, this was not confirmed by a generally lower ergosterol concentration. Consequently, ergosterol was no reliable fungal biomarker in periods of rapid decline of the fungal biomass. A single addition of fungicide was insufficient for continued inhibition of the fungal biomass. Yet, a significant delay (28–42 days) in macroaggregation in fungicide treated compared to untreated samples highlighted the importance of the fungal biomass in macroaggregate formation. Macroaggregates were enriched in maize-derived ¹³C and ¹⁵N compared to microaggregates or the fraction < 53 μm. They turned over rapidly with decreasing substrate availability, which entailed a transfer of maize-derived C and N stored within macroaggregates during the first weeks of incubation to microaggregates with proceeding incubation time. Our results indicate that this transfer happened within macroaggregates, because no considerable amount of free particulate organic matter (POM) was released upon macroaggregate breakdown. We conclude that substrate decomposability and fungal activity are key factors determining extent and dynamics of macroaggregation during decomposition processes. Macroaggregate formation implied rapid incorporation and thereby short-term protection of maize-derived C and N. Moreover, macroaggregates allowed a transfer of maize-derived organic matter into microaggregates within macroaggregates, which prevented the release of significant amounts of free POM upon macroaggregate breakdown. Consequently, macroaggregates constitute to the transfer of recently added C into more stable soil organic matter fractions.     

Responses of enzymatic activities within soil aggregates to 9-year nitrogen and water addition in a semi-arid grassland

Soil microorganisms secrete enzymes used to metabolize carbon (C), nitrogen (N), and phosphorus (P) from the organic materials typically found in soil. Because of the connection with the active microbial biomass, soil enzyme activities can be used to investigate microbial nutrient cycling including the microbial response to environmental changes, transformation rates and to address the location of the most active biomass. In a 9-year field study on global change scenarios related to increasing N inputs (ambient to 15 g N m⁻² yr⁻¹) and precipitation (ambient to 180 mm yr⁻¹), we tested the activities of soil β-glucosidase (BG), N-acetyl-glucosaminidase (NAG) and acid phosphomonoesterase (PME) for three soil aggregate classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm) and microaggregates (<250 μm). Results showed higher BG and PME activities in micro-vs. small macroaggregates whereas the highest NAG activity was found in the large macroaggregates. This distribution of enzyme activity suggests a higher contribution of fast-growing microorganisms in the micro-compared with the macroaggregates size fractions. The responses of BG and PME were different from NAG activity under N addition, as BG and PME decreased as much as 47.1% and 36.3%, respectively, while the NAG increased by as much as 80.8%, which could imply better adaption of fungi than bacteria to lower soil pH conditions developed under increased N. Significant increases in BG and PME activities by as much as 103.4 and 75.4%, respectively, were found under water addition. Lower ratio of BG:NAG and higher NAG:PME underlined enhanced microbial N limitation relative to both C and P, suggesting the repression of microbial activity and the accompanied decline in their ability to compete for N with plants and/or the accelerated proliferation of soil fungi under elevated N inputs. We conclude that changes in microbial activities under increased N input and greater water availability in arid- and semi-arid grassland ecosystems where NPP is co-limited by N and water may result in substantial redistribution of microbial activity in different-sized soil particles. This shift will influence the stability of SOM in the soil aggregates and the nutrient limitation of soil biota.     

Earthworms increase soil microbial biomass carrying capacity and nitrogen retention in northern hardwood forests

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of ¹³C and ¹⁵N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more ¹³C and ¹⁵N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions.     

引黄灌溉耕作对土壤团聚体有机碳的影响

为研究灌溉耕作影响下土壤团聚体及有机碳的特征情况,以宁夏引黄灌区为研究对象,选取对照土壤与耕作土壤,通过干、湿筛结合的方法,得到大团聚体(2mm)、中间团聚体(2~0.25mm)、微团聚体(0.25~0.053mm)和粉+黏团聚体(0.053mm),并测定团聚体有机碳含量,分析团聚体有机碳与总有机碳之间的关系。结果表明,灌溉耕作对团聚体分布具有极显著影响(P0.01),其中大团聚体和中间团聚体质量分数上升,微团聚体和粉+黏团聚体质量分数下降,灌溉土壤团聚体分布趋势为微团聚体粉+黏团聚体中间团聚体大团聚体。经灌溉耕作后土壤团聚体稳定性大于对照土壤,不同类型的灌溉土壤稳定性基本一致,对照土壤间差异明显。除0.053mm外,团聚体有机碳分布在经过灌溉耕作后有显著性差异(P0.05),团聚体有机碳分布随粒级大小基本呈"V"形分布。团聚体有机碳含量均表现出灌溉土壤高于对照土壤,其中灌溉土壤中灌淤土和潮土团聚体有机碳总量较高。未受人为灌溉耕作影响的自然土壤团聚体有机碳与总有机碳间具有显著的正相关性,土壤总有机碳增加主要依赖0.053mm团聚体有机碳增加。引黄灌溉耕作有利于增加大粒级团聚体的比例,提升团聚体稳定,显著增加有机碳含量。     

Soil aggregation and distribution of carbon and nitrogen in different fractions under long-term application of compost in rice–wheat system

Soil organic matter improves the physical, chemical and biological properties of soil, and crop residue recycling is an important factor influencing soil organic matter levels. We studied the impact of continuous application of rice straw compost either alone or in conjunction with inorganic fertilizers on aggregate stability and distribution of carbon (C) and nitrogen (N) in different aggregate fractions after 10 cycles of rice–wheat cropping on a sandy loam soil at Punjab Agricultural University research farm, Ludhiana, India. Changes in water stable aggregates (WSA), mean weight diameter (MWD), aggregate-associated C and N, total soil C and N, relative to control and inorganically fertilized soil were measured. Total WSA were significantly (p = 0.05) higher for soils when rice straw compost either alone or in combination with inorganic fertilizers was applied as compared to control. The application of rice straw compost either alone or in combination with inorganic fertilizers increased the macroaggregate size fractions except for 0.25–0.50 mm fraction. The MWD was significantly (p = 0.05) higher in plots receiving rice straw compost either alone at 8 tonnes ha⁻¹ (0.51 mm at wheat harvest and 0.41 mm at rice harvest) or at 2 tonnes ha⁻¹ in combination with inorganic fertilizers (0.43 and 0.38 mm) as compared to control (0.34 and 0.33 mm) or inorganically fertilized plots (0.33 and 0.31 mm). The macroaggregates had higher C and N density compared to microaggregates. Application of rice straw compost at 2 tonnes ha⁻¹ along with inorganic fertilizers (IN + 2RSC) increased C and N concentration significantly over control. The C and N concentration increased further when rice straw compost at 8 tonnes ha⁻¹ (8RSC) was added. It is concluded that soils can be rehabilitated and can sustain the soil C and N levels with the continuous application of rice straw compost either alone or in combination with inorganic fertilizers. This will also help in controlling the rising levels of atmospheric carbon dioxide.