Soil organic matter assessment in natural regrowth,Pueraria phaseoloides and Mucuna pruriens fallow

Biological and chemical components of soil fertility were quantified under three different fallow types and related to soil quality of an Ultisol in southern Cameroon at the end of a 9-month fallow. Soil organic matter (SOM), soil exchangeable Ca²⁺, Mg²⁺ and K⁺ and available P concentrations, effective cation exchange capacity (ECEC) and, soil acidity in the 0–10 and 10–20 cm layers were evaluated under: natural regrowth mainly composed of Chromolaena odorata and the legume cover crops velvet bean (Mucuna pruriens var. utilis) and kudzu (Pueraria phaseoloides). SOM quality was assessed by C mineralisation during a 4-week incubation at 28°C in the laboratory. In addition, particulate organic matter (POM), the most active part of SOM, was fractionated by wet sieving into coarse (4000–2000 μm), medium (2000–250 μm) and fine (250–53 μm) particle size classes and analysed for C and N contents. Under Mucuna, Ca²⁺, K⁺ and P concentrations, ECEC and soil pH were higher and C mineralisation was lower than under natural regrowth and Pueraria in 0–10 cm depth. Soil under natural regrowth had a higher C mineralisation in 0–10 cm indicating more labile SOM than in Pueraria and Mucuna fallow. There was no difference in weight of total POM, for any of the fractions between the three fallow types. However, both leguminous fallow species increased POM quality through a higher N content. Compared to natural regrowth, Pueraria increased N content in coarse POM by 36% in the 0–10 cm layer and by 19% (coarse POM) and 35% (medium POM) in the 10–20 cm layer. Mucuna increased N content in the 0–10 cm layer by 12% (coarse POM), and by 19% (fine POM), compared to natural regrowth. According to the differences in nutrient concentrations, soil acidity and the biological stability of SOM, the three fallow types ranked: Mucuna≥Pueraria>natural regrowth. However, in terms of POM quality the ranking was: Pueraria>Mucuna>natural regrowth.     

Effect of understory fern (Dicranopteris dichotoma) removal on substrate utilization patterns of culturable soil bacterial communities in subtropical Eucalyptus plantations

Although understory vegetation is known to play an important role in driving the processes and functions of forest ecosystems, little is known about how understory vegetation affects the composition and function of soil microbial communities in forest ecosystems, especially in subtropical and tropical forests. This study used the experimental removal of understory fern (Dicranopteris dichotoma) to investigate the effect of the fern on substrate utilization patterns of culturable soil bacterial communities in two subtropical Eucalyptus plantations. One year after treatment, the removal of understory fern significantly increased soil temperature by 2–3 °C and retarded litter decomposition by 5.6–23.1%. However, understory fern removal did not affect the substrate utilization pattern of soil bacterial communities. Our study provides evidence that, although understory fern removal significantly alters soil temperature and litter decomposition rate, the disturbance caused by understory removal one year after treatment is too weak to cause detectable changes in substrate utilization pattern of culturable soil bacterial communities in subtropical Eucalyptus plantations.     

Soil organic matter in soil depth profiles: Distinct carbon preferences of microbial groups during carbon transformation

This study investigates how carbon sources of soil microbial communities vary with soil depth. Microbial phospholipid fatty acids (PLFA) were extracted from 0–20, 20–40 and 40–60 cm depth intervals from agricultural soils and analysed for their stable carbon isotopes (δ¹³C values). The soils had been subjected to a vegetation change from C3 (δ¹³C≈?29.3‰) to C4 plants (δ¹³C≈?12.5‰) 40 years previously, which allowed us to trace the carbon flow from plant-derived input (litter, roots, and root exudates) into microbial PLFA. While bulk soil organic matter (SOM) reflected ≈12% of the C4-derived carbon in top soil (0–20 cm) and 3% in deeper soil (40–60 cm), the PLFA had a much higher contribution of C4 carbon of about 64% in 0–20 cm and 34% in 40–60 cm. This implies a much faster turnover time of carbon in the microbial biomass compared to bulk SOM. The isotopic signature of bulk SOM and PLFA from C4 cultivated soil decreases with increasing soil depth (?23.7‰ to ?25.0‰ for bulk SOM and ?18.3‰ to ?23.3‰ for PLFA), which demonstrates decreasing influence of the isotopic signature of the new C4 vegetation with soil depth. In terms of soil microbial carbon sources this clearly shows a high percentage of C4 labelled and thus young plant carbon as microbial carbon source in topsoils. With increasing soil depth this percentage decreases and SOM is increasingly used as microbial carbon source. Among all PLFA that were associated to different microbial groups it could be observed that (a) depended on availability, Gram-negative and Gram-positive bacteria prefer plant-derived carbon as carbon source, however, (b) Gram-positive bacteria use more SOM-derived carbon sources while Gram-negative bacteria use more plant biomass. This tendency was observed in all three-depth intervals. However, our results also show that microorganisms maintain their preferred carbon sources independent on soil depth with an isotopic shift of 3–4‰ from 0–20 to 40–60 cm soil depth.     

Comparison of forest soil carbon dynamics at five sites along a latitudinal gradient

The aim of this study was to compare the turnover time of labile soil carbon (C), in relation to temperature and soil texture, in several forest ecosystems that are representative of large areas of North America. Carbon and nitrogen (N) stocks, and C:N ratios, were measured in the forest floor, mineral soil, and two mineral soil fractions (particulate and mineral-associated organic matter, POM and MOM, respectively) at five AmeriFlux sites along a latitudinal gradient in the eastern United States. Sampling at four sites was replicated over two consecutive years. With one exception, forest floor and mineral soil C stocks increased from warm, southern sites (with fine-textured soils) to cool, northern sites (with more coarse-textured soils). The exception was a northern site, with less than 10% silt-clay content, that had a soil organic C stock similar to the southern sites. A two-compartment model was used to calculate the turnover time of labile soil organic C (MRT_U) and the annual transfer of labile C to stable C (k₂) at each site. Moving from south to north, MRT_U increased from approximately 5 to 14 years. Carbon-13 enrichment factors (ε), that described the rate of change in δ¹³C through the soil profile, were associated with soil C turnover times. Consistent with its role in stabilization of soil organic C, silt-clay content was positively correlated (r = 0.91; P ≤ 0.001) with parameter k₂. Latitudinal differences in the storage and turnover of soil C were related to mean annual temperature (MAT, °C), but soil texture superseded temperature when there was too little silt and clay to stabilize labile soil C and protect it from decomposition. Each site had a relatively high proportion of labile soil C (nearly 50% to a depth of 20 cm). Depending on unknown temperature sensitivities, large labile pools of forest soil C are at risk of decomposition in a warming climate, and losses could be disproportionately higher from coarse textured forest soils.     

Responses of soil dissolved organic matter to long-term plantations of three coniferous tree species

Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO₂ evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R² = 0.886, P < 0.01) and surface litter C:N ratios (R² = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.     

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.     

Relationships between C respiration and fine particulate organic matter (250–50 μm) weight

Soil organic matter (SOM) status was evaluated using the relationships between two independent soil variables, i.e., C respiration and the weight of particulate organic matter POM (4000–50 μm) under different vegetation covers and ecosystems of central Belgium. A positive relationship was found between the weight of the finest POM fraction, i.e., fine POM fraction (250–50 μm) and C respiration after 1 week (R² = 0.34, n = 120, p < 0.0001) and 2 weeks (R² = 0.28, n = 120, p < 0.0001) of incubation. Therefore, we assumed that the C respiration and the weight of fine POM might be used to evaluate the SOM status under different vegetation covers and ecosystems.     

Stability of soil organic matter under long-term biosolids application

Little is know on the impact of biosolids application on soil organic matter (SOM) stability, which contributes to soil C sequestration. Soil samples were collected in 2006 at plow layer from fields that received liquid and dry municipal biosolids application from 1972 to 2004 at the cumulative rate of 1416 Mg ha⁻¹ in mined soil and 1072 Mg ha⁻¹ in nonmined soil and control fields that received chemical fertilizer at Fulton County, western Illinois. The biosolids application increased the soil microbial biomass C (SMBC) by 5-fold in mined soil and 4-fold in nonmined soil. The biosolids-amended soils showed a high amount of basal respiration and N mineralization, but low metabolic quotient, and low rate of organic C and organic N mineralization. There was a remarkable increase in mineral-associated organic C from 6.9 g kg⁻¹ (fertilizer control) to 26.6 g kg⁻¹ (biosolids-amended) in mined soil and from 8.9 g kg⁻¹ (fertilizer control) to 23.1 g kg⁻¹ (biosolids-amended) in nonmined soil. The amorphous Fe and Al, which can improve SOM stability, were increased by 2–7 folds by the long-term biosolids application. It is evident from this study that the biosolids-modified SOM resists to decomposition more than that in the fertilizer treatment, thus long-term biosolids application could increase SOM stability.     

Phosphorus enrichment helps increase soil carbon mineralization in vegetation along an urban-to-rural gradient,Nanchang, China

We used four vegetation types located along an urban–suburban–rural gradient in Nanchang, China to study how the deposition of nitrogen (N) and phosphorus (P) in the urban area affected soil carbon (C) cycling. We found that total P, nitrate (NO₃⁻–N), available P, and the abundances of culturable bacteria, actinobacteria, and nitrifying bacteria in soils, collected to 15 cm depth in August of 2008, decreased along the urban-to-rural gradient (P < 0.05); the C/P and N/P ratios, ammonium (NH₄⁺–N), and culturable fungi abundance showed the reverse trends; whereas soil organic C, total N, C/N, mineral N, and the activities of sucrase and neutraland acid phosphatase showed no pattern with gradient and vegetation type. Compared to suburban and rural sites, total and available P in soil increased 168% and 131%, 47% and 139%, respectively in urban sites. The cumulative amount of CO₂ emission per gram of soil (C_min, incubated from 2 to 43 days) varied little along the urban-to-rural gradient, but showed positive correlations with organic C, total N, total P, nitrate, mineral N concentrations, C/N, bacteria and actinobacteria abundances, sucrase and acid phosphatase activities. In contrast, the cumulative amount of CO₂ produced per gram organic C (C_min/OC) within the incubation period was influenced by gradient, vegetation type, and their interaction, and values were about 35% greater in the urban than in suburban and rural sites. The relationship between elevated C_min/OC in urban vegetations and the enrichment of P in organic matter (P/C ratio) suggests that P coming from urban household waste can degrade the stability of organic C in urban soils.     

Determining the distributions of soil carbon and nitrogen in particle size fractions using near-infrared reflectance spectrum of bulk soil samples

This study aimed at assessing the potential of near-infrared reflectance spectroscopy (NIRS) for determining the distribution of soil organic matter (SOM) in particle size fractions, which has rarely been attempted. This was done on sandy soils from Burkina Faso (three sites) and Congo-Brazzaville (one site). Over the total sample set, NIRS accurately predicted carbon (C) and nitrogen (N) concentrations (g kg^?1 fraction) in the fraction <20 μm. When considering Burkina Faso only, predictions were improved in general; those of C and N amounts (g kg^?1 soil) became accurate for the fraction <20 μm but not for the coarser fractions, probably due to heterogeneous SOM repartition. However, most SOM being <20 μm in general, NIRS could be considered promising for determining SOM size distribution.     

Use of palm-mat geotextiles for soil conservation: I. Effects on soil properties

Despite geotextile-mats having the potential for soil conservation, field studies on the effects of geotextiles on soil properties are limited. Hence, the utilization of palm-mat geotextiles as a potential soil conservation technique was investigated at Hilton, east Shropshire, U.K. (52°33′5.7″ N, 2°19′18.3″ W). Geotextile-mats constructed from Borassus aethiopum (Borassus palm of West Africa) and Mauritia flexuosa (Buriti palm of South America) leaves are termed Borassus mats and Buriti mats, respectively. Field experiments were conducted at Hilton during 2007–2009, to study the impacts of Borassus and Buriti mats on selected properties of the topsoil (0–5 cm). Ten fixed plots (10 × 1 m on a 15° slope) were established, with duplicate treatments. The treatments were: (i) bare soil; (ii) permanent grassed; (iii) bare soil with 1 m Borassus-mat buffer strips (area coverage ~ 10%) at the lower end of the plots; (iv) bare soil with 1 m Buriti mat buffer strips (area coverage ~ 10%) at the lower end of the plots; and (v) completely-covered with Borassus mats. Initial and final soil samples of the topsoil were collected and analysed for bulk density, aggregate stability, soil organic matter (SOM), total soil C (TSC), total soil N (TSN) and pH. Results indicate that, apart from Borassus completely-covered plots, soil bulk density increased and aggregate stability decreased in all plots after two years. Despite decreases in SOM contents in bare plots, SOM content did not change after two years in the grassed and geotextile treated plots. Treatments had no effects on changes in pH, TSC or TSN. Both Borassus and Buriti mat-covers within the buffer strip plots had little impact on SOM, TSC and TSN changes compared with bare soils within the same plots. Thus, Borassus buffer strip plots were very effective in maintaining some soil properties (i.e. SOM, TSC, and TSN) after two years of erosion by water. In summary, utilization of Borassus mats as buffer strips was very successful in conserving soil properties on a loamy sand soil.     

Chemical structure of humic acids in biosolids-amended soils as revealed by NMR spectroscopy

We used NMR spectroscopy to characterize humid acids extracted from soils that had received long-term application of 2 levels of biosolids to evaluate the soil organic matter (SOM) stability in biosolids-amended soils. The study also quantified fulvic acids (FAs), humic acids (HAs) and Fe/Al oxides. The soils were collected in 2004 from 7 fields, in Fulton County, southwestern Illinois, which received biosolids at a cumulative rate of 0 (control), 554 (low biosolids) and 1,066 (high biosolids) Mg ha⁻¹. The application of biosolids increased both FA and HA contents, but biosolids-amended soil and control soil did not differ in FA/HA ratio. Biosolids application had no effect on water-soluble organic carbon content. Biosolids application increased the presence of Fe/Al in the SOM complex and lowered its C/Fe and C/Al ratios. ¹³C NMR spectra showed increased alkyl C and decreased aromatic C content in soil HAs with the application of biosolids, and the extent of such changes was higher with high than low biosolids treatment. Under biosolids application, the soil HAs’ C structure shifts from O-alkyl-dominant to alkyl-dominant. Biosolids application does not decrease SOM stability but rather increases the stability of soil humic substances.     

Characterizing the relationships between soil organic matter components and microbial function and composition along a tillage disturbance gradient

We studied the effect of no-till (disc seeder), conventional-till (tine scarifier+disc seeder) and rotary-till (rotary hoe+disc seeder) management on soil organic matter (SOM) components, rates of carbon (C) and nitrogen (N) cycling, substrate utilization and microbial community composition. We hypothesized that labile SOM fractions are sensitive to changes in tillage techniques and, in turn mediate any tillage-induced changes in microbial function and composition. A replicated field site was established in May 1998 in the semi-arid agricultural region of Western Australia and soils were collected in September 2004. We found soil pH varied between different tillage techniques as an initial lime application was mixed to deeper soil depths in rotary-till soil than no-till and conventional-till soil. Total-C was greater in surface soil and lower in subsurface soil from no-till and conventional-till plots than from rotary-till plots, but there was no effect of tillage technique on total-C when averaged across soil depths. Light (specific density <1.0 g cm^?3) fraction organic matter (LFOM), dissolved organic matter (DOM) and microbial biomass (MB) C and N pools, and rates of C and N cycling all tended to decrease with soil depth. In general, LFOM-C and N, dissolved organic C (DOC) and microbial biomass carbon (MB-C), soil respiration, cellulase activity, gross immobilization rates were positively correlated (r>0.50) and were greater in no-till and conventional-till soil than rotary-till soil both within, and across soil depths. These soil variables generally increased (r>0.5) with increasing soil pH. Dissolved organic N and gross N mineralization were positively correlated (r>0.90) but neither was affected by tillage techniques. No-till soil had greater utilization of carboxylic acids and lower utilization of amino acids and carbohydrates than conventional-till and rotary-till soil; surface soil also had greater utilization of carboxylic acids than subsurface soil. In turn, substrate utilization differed between soil depths, and between no-till soil and conventional-till and rotary-till soil; these differences were correlated with soil pH, total-N, DOC, LFOM-N and microbial biomass nitrogen (MB-N). Bacterial and fungal biomasses generally decreased with soil depth and were greater in no-till and conventional-till soil than rotary-till soil. Microbial community composition differed between all tillage techniques and soil depths; these differences were correlated with soil textural classes, soil pH, and total, LFOM, DOM and microbial C and N pools. These results indicate that most tillage-induced changes to soil properties were associated with the greater soil disturbance under rotary-till than under no-till or conventional-till management. Our results indicate that tillage-induced changes to soil pH, and LFOM, DOM and microbial biomass pools are likely to be important regulators of the rates of C and N cycling, substrate utilization and microbial community composition in this coarse textured soil.     

Soil microbial biomass and organic matter fractions during transition from conventional to organic farming systems

The aim of this study was to investigate the response of soil microbial biomass and organic matter fractions during the transition from conventional to organic farming in a tropical soil. Soil samples were collected from three different plots planted with Malpighia glaba: conventional plot with 10 years (CON); transitional plot with 2 years under organic farming system (TRA); organic plot with 5 years under organic farming system (ORG). A plot under native vegetation (NV) was used as a reference. Soil microbial biomass C (MBC) and N (MBN), soil organic carbon (SOC) and total N (TN), soil organic matter fractioning and microbial indices were evaluated in soil samples collected at 0–5, 5–10, 10–20 and 20–40 cm depth. SOC and fulvic acids fraction contents were higher in the ORG system at 0–5 cm and 5–10 cm depths. Soil MBC was highest in the ORG, in all depths, than in others plots. Soil MBN was similar between ORG, TRA and NV in the surface layer. The lowest values for soil MBC and MBN were observed in CON plot. Soil microbial biomass increased gradually from conventional to organic farming, leading to consistent and distinct differences from the conventional control by the end of the second year.     

Changes in chemical and biochemical soil properties induced by 11-yr repeated additions of different organic materials in maize-based forage systems

The repeated addition of organic materials to the soil greatly affects the physical, chemical and biological characteristics. In the present work, we analyzed changes in soil quality properties of the tilled layer caused by different agronomic managements of maize which supply different amounts of carbon (C) and nitrogen (N) through the addition of slurry, farmyard manure or plant residues. The agronomic history of the analyzed soils, which derived from a medium-term (11 yr) field experiment located in NW Italy, represents typical managements of maize for this region. The area is characterized by highly intensive agriculture, with consequent risks to soil degradation that could be limited by the efficient utilization of organic inputs and by recycling within cropping systems, the large amounts of manure that are produced from the many animal breeding farms in this region. We used a combination of both different chemical (soil organic C and total N) and biochemical indicators (potential soil respiration, potentially mineralizable N (PMN) and potential soil microbial biomass (SMB)). We considered the suitability of the selected biochemical indicators to describe the changes in soil characteristics resulting from the past management.The results showed that the application of the different organic materials, in addition to urea-N fertilizer, increased SOM contents and altered the selected soil biochemical properties compared with the unfertilized treatment, especially in the upper 15 cm of the 0?30 cm tilled soil layer. Farmyard manure applications caused the greatest increase in SOM content, PMN and potential SMB, whilst return of maize straw produced the largest increase in potential soil respiration, but had less effect on total soil organic C and SMB. The use of slurry only caused a moderate increase in SOM and showed intermediate changes in biochemical properties. Also, the rate of C accumulation in the soil per unit of C applied was higher for farmyard manure application than for slurry and straw incorporation in the soil. Fertilization with only mineral N did not induce an increase in C_org and N_tot and even reduces soil N mineralization potential.Because of the high variability in the data, potential SMB carbon could be considered as a less successful indicator for differentiating between past agronomic histories and effects on soil quality, whilst microbial activity (measured by potential soil respiration) and PMN, gave a more reliable and useful indication of the amount of easily decomposable organic carbon.     

Effects of two soil reclamation techniques on the distribution of the organic N compounds in a 15N labelled burnt soil

The evolution of the soil organic-N forms and their bio-availability was studied in a ¹⁵N labelled and burnt soil (BLS) after two successive reclamation steps under greenhouse conditions: a 3-month growing period of Lolium, without (BLS-L) or with poultry manure addition (4 and 8 Mg ha^− 1: BLS + PM4-L and BLS + PM8-L), followed by a 12-month growing phase of pine seedlings (BLS-P, BLS + PM4-P and BLS + PM8-P). The results were compared with those obtained for the homologous labelled unburnt soil (LS, LS-L and LS-P) to evaluate the efficacy of these reclamation techniques in the mitigation of the drastic post-fire changes exhibited by the major biologically available N pool in terrestrial ecosystems: the soil organic N. The significant and steady decrease of the ¹⁵N enrichment observed in the unburnt soil during the successive plant growth cycles (LS > LS-L > LS-P) contrasts with the lack of significant changes, in both the content of total organic ¹⁵N and the atom % ¹⁵N in excess, among the treatments with the burnt soil (BLS ≥ BLS-L ≥ BLS-P). These results showed that: a) in LS, N mineralization proceeds faster for the recently incorporated N (¹⁵N enriched) than for the native N, supplying the growing vegetation with inorganic N more ¹⁵N enriched than the bulk soil N; and b) in BLS, soil combustion has reduced the usually higher biological availability of the recently added N to levels similar to those of the endogenous N.The re-vegetation with Lolium and Pinus and the addition of poultry manure mitigated the high differences observed in the size of the amino acid and the organic derived NH₄⁺–N pools due to the combustion process, which are usual between burnt and unburnt soils. Conversely, these burnt soil reclamation techniques (re-vegetation and poultry manure addition), even jointly used, were unable to reduce the huge differences observed between the burnt and the unburnt soils for the other N fractions considered (amides, amino sugars, hydrolysable unidentified-N, hydrolysable organic N and un-hydrolysable N) that accounted for more than 80% of the soil organic N. Consequently, it seems that without the introduction of N₂-fixing microorganisms or plants in the burnt soils the recovery of the natural soil organic N composition will take place slowly.     

Long-term effect of straw and farmyard manure on soil organic matter in field experiment in the Czech Republic

The effect of long-term (45 years) mineral and organic fertilization on soil organic matter (SOM) quantity (organic C and N content) and quality (hot-water-soluble C content, microbial biomass C content, hydrophobic organic components of SOM, soil enzyme activities) was determined in a field experiment established in Trutnov (North Bohemia, sandy loam, Eutric Cambisol). Six treatments were chosen for investigation: unfertilized control, mineral fertilization (NPK), straw N, farmyard manure (FYM) and straw and FYM completed with mineral NPK. Soil samples were taken from the arable layer (0–20 cm) in spring over the period of 2004–2010. The positive effect of FYM on the total organic C and N content, hot-water-soluble C content and hydrophobic organic components of SOM was more than 50% higher than that of straw and mineral N fertilization. Application of straw N increased microbial biomass C content in soil and generated invertase activity above the level of FYM. Hot-water-soluble C content, hydrophobic organic components of SOM and urease activity were positively correlated with total organic C and N content (R = 0.58–0.98; p < 0.05). Addition of mineral NPK to both the straw and FYM emphasized the effect of organic fertilization in most of monitored characteristics.     

Influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol

In this study, the influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol from north east Scotland was investigated. Firstly, soil cores were incubated at 5, 10 and 15°C for up to 8 weeks. Net mineralisation was observed at all temperatures with larger rates observed at higher temperatures. In contrast, water extractable dissolved organic nitrogen (DON) displayed no clear trend with time and showed little response to temperature. Secondly, intact cores of the same soil, with and without vegetation, were leached with artificial rain for 6 weeks at 6.5 and 15°C. Temperature and the presence of vegetation interacted to have a significant (P<0.01) effect on the concentration of NO₃⁻ in leachates; highest concentrations were observed in leachates from cores without vegetation at 15°C, whereas lowest concentrations were observed in leachates from cores with vegetation at 6.5°C. In contrast, concentrations of DON and dissolved organic carbon (DOC) were significantly (P<0.001) higher in leachates from cores with vegetation than without vegetation and were not affected by temperature. The cumulative amounts of DON and DOC leached from the cores with vegetation were 4 and 2.5 times greater, respectively, than those leached from the cores without vegetation. Comparison of soil solution (extracted by centrifugation at 0–5 and 5–10 cm depth) after leaching for 6 weeks, showed that the upper layer contained more than twice the amount of DON than the 5–10 cm layer and that the difference in concentration between the two depths was enhanced in the presence of vegetation. The results indicate that vegetation is an important source of DON and DOC. However, the removal of vegetation did not lead to an increase in the quantity of total dissolved nitrogen (TDN) in soil water, but resulted in a change in the dominant N fraction from DON to NO₃⁻. In addition, the results show that DON, in both the incubated and leached cores, did not change as inorganic N was mineralised. This suggests that if water extractable DON was acting as a source of NH₄⁺ or NO₃⁻, then it was being replenished by, and in equilibrium with, a large reserve of organic N. Evidence of such a pool was indirect in the form of additional DON (equivalent to 2 g N m⁻²) being extracted by 0.5 M K₂SO₄.     

Relationships between soil pH and microbial properties in a UK arable soil

Effects of changing pH along a natural continuous gradient of a UK silty-loam soil were investigated. The site was a 200 m soil transect of the Hoosfield acid strip (Rothamsted Research, UK) which has grown continuous barley for more than 100 years. This experiment provides a remarkably uniform soil pH gradient, ranging from about pH 8.3 to 3.7. Soil total and organic C and the ratio: (soil organic C)/(soil total N) decreased due to decreasing plant C inputs as the soil pH declined. As expected, the CaCO₃ concentration was greatest at very high pH values (pH > 7.5). In contrast, extractable Al concentrations increased linearly (R² = 0.94, p < 0.001) from below about pH 5.4, while extractable Mn concentrations were largest at pH 4.4 and decreased at lower pHs. Biomass C and biomass ninhydrin-N were greatest above pH 7. There were statistically significant relationships between soil pH and biomass C (R² = 0.80, p < 0.001), biomass ninhydrin-N (R² = 0.90, p < 0.001), organic C (R² = 0.83, p < 0.001) and total N (R² = 0.83, p < 0.001), confirming the importance of soil organic matter and pH in stimulating microbial biomass growth. Soil CO₂ evolution increased as pH increased (R² = 0.97, p < 0.001). In contrast, the respiratory quotient (qCO₂) had the greatest values at either end of the pH range. This is almost certainly a response to stress caused by the low p. At the highest pH, both abiotic (from CaCO₃) and biotic Co₂ will be involved so the effects of high pH on biomass activity are confounded. Microbial biomass and microbial activity tended to stabilise at pH values between about 5 and 7 because the differences in organic C, total N and Al concentrations within this pH range were small. This work has established clear relationships between microbial biomass and microbial activity over an extremely wide soil pH range and within a single soil type. In contrast, most other studies have used soils of both different pH and soil type to make similar comparisons. In the latter case, the effects of soil pH on microbial properties are confounded with effects of different soil types, vegetation cover and local climatic conditions.     

Diversity and abundance of earthworms in land use systems in central-western Colombia

The neotropical landscapes of Colombia's Andean region are characterized by a mosaic of agroecosystems presenting a range of vegetational cover for which soil fauna adaptation is still unknown. To analyze the diversity and abundance of earthworm species in relation to changes in selected soil physical and chemical parameters (bulk density, C stock, N stock, %C, %N, ¹³C, ¹⁵N, C:N ratio), these systems were classified into five categories according to land use intensity: non-intensive (NI), low intensity (LI), medium intensity (MI), high intensity (HI), and maximum intensity (IN). The influence of livestock production was confirmed in the significant differences observed in bulk density and ¹⁵N between NI, HI, and IN (P < 0.05). The C and N contents of IN systems (pastures) differed significantly (P < 0.05) in comparison with the other categories. Average δ ¹³C at the three soil depths evaluated (0–10, 10–20, and 20–30 cm) ranged between ?24.9 for LI and ?22.87 for IN, indicating that soil organic carbon was related to C3-type vegetation for all land uses and vegetation covers. Overall, 26 earthworm species were recorded, of which 16 showed a high capacity to adapt to natural, NI, LI, and MI systems. Depending on land use intensity, significant differences were also observed (P < 0.05) in the origin and ecological category of earthworms, mainly in terms of the following variables: diversity of native species, diversity of endogeic species, abundance of native species, biomass of native species, and abundance of exotic species. Based on the results obtained, the most favorable mosaic systems for agrobiodiversity conservation were identified as well as those subsystems that require special management to solve problems of habitat degradation.