Land management effects on nitrogen and carbon cycling in an Ultisol

Abstract

Soil carbon (C) content in agro‐ecosystems is important in a global context because of the potential for soil to act as a sink for atmospheric CO₂. However, soil C storage in agro‐ecosystems can be sensitive to land management practices. The objective of this study was to examine the impact of land management systems on C and nitrogen (N) cycling in an Ultisol in Alabama. Soil samples (0–10, 10–20, and 20–30 cm depths) were collected from a Marvyn sandy loam soil (fine‐loamy, siliceous, thermic Typic Hapludults) under five different farm scale management systems for at least 5 years. The five systems were cotton (Gossypium hirsutum L.) production managed with 1) conventional tillage only, 2) conventional tillage with a grazed winter cover crop (wheat, Triticum aestivum L.), 3) conservation tillage with a winter cover crop grown for cover only with strip tillage; or taken out of cotton production with either 4) long‐term fallow (mowed), or 5) Conservation Reserve Program with loblolly pine (Pinus taeda L.) (CRP‐pine). Total N, total organic C (TOC), total P, and soil C:N ratios were determined. Potential C mineralization, N mineralization, C turnover and C:N mineralization ratios were determined on samples during a 30‐day laboratory incubation study. The fallow system had significantly higher TOC concentration (7.7 g kg^‐1 C) while the CRP‐pine system had lower TOC concentration (3.1 g kg^‐1 C) compared with the farmed management systems (=4.7 g kg^‐1 C). The fallow system had a significantly lower C turnover at all three soil depths compared with the other management systems. At the 0–10 cm depth, the highest C:N mineralization ratio levels were observed in management systems receiving the most tillage. Our results indicate that for Ultisols in the Southeast the use of surface tillage in land management systems is a controlling factor which may limit soil C sequestration.     

Tillage effects on plant extractable soil water in a silty clay vertisol in Southern Italy

This paper describes how the CERES-Wheat simulation model can be used to estimate tillage effects on soil water regimes of a silty clay soil in Foggia, Southern Italy. The four tillage treatments compared are traditional mouldboard ploughing, ripper subsoiling, surface disc-harrowing and minimum tillage with rotary hoeing under continuous durum wheat cropping. For each tillage treatment the CERES-Wheat model was used to calculate the water balance for several layers in the root zone.

The water balance routine of the model estimates the water content within saturation and the lower limit at any time. Inputs required by the model are some basic information about the site, weather, genetic parameters and management practices as well as some soil properties, such as albedo, bulk density, organic matter and N contents. The model was calibrated by estimating the genetic parameters for the minimum tillage treatment in the season 1984–1985. The same set of parameters was used for the subsequent validation procedure. Statistical tests proved that the match between measured and simulated soil water content values was quite good. The simulation results also showed some differences among different tillage treatments. The model predicted the lowest plant extractable soil water values and a different water content distribution along the soil profile of the ripper subsoiling in comparison with the other tillage treatments. The soil water content was lower until 20–40 cm depth and higher at 40–60 cm depth in the ripper treatment as compared with the others. In deeper layers differences became non-significant. This might be due to the cracks produced by the ripper through which rainfall infiltrated in deep layers.     

Tillage and cropping sequence impacts on nitrogen cycling in dryland farming in eastern Montana, USA

Information on N cycling in dryland crops and soils as influenced by long-term tillage and cropping sequence is needed to quantify soil N sequestration, mineralization, and N balance to reduce N fertilization rate and N losses through soil processes. The 21-yr effects of the combinations of tillage and cropping sequences was evaluated on dryland crop grain and biomass (stems + leaves) N, soil surface residue N, soil N fractions, and N balance at the 0–20 cm depth in Dooley sandy loam (fine-loamy, mixed, frigid, Typic Argiboroll) in eastern Montana, USA. Treatments were no-tilled continuous spring wheat (Triticum aestivum L.) (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat–barley (Hordeum vulgare L.) (1984–1999) followed by spring wheat–pea (Pisum sativum L.) (2000–2004) (FSTW-B/P), and spring-tilled spring wheat–fallow (STW-F). Nitrogen fractions were soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), potential N mineralization (PNM), NH₄-N, and NO₃-N. Annualized crop grain and biomass N varied with treatments and years and mean grain and biomass N from 1984 to 2004 were 14.3–21.2 kg N ha⁻¹ greater in NTCW, STCW, FSTCW, and FSTW-B/P than in STW-F. Soil surface residue N was 9.1–15.2 kg N ha⁻¹ greater in other treatments than in STW-F in 2004. The STN at 0–20 cm was 0.39–0.96 Mg N ha⁻¹, PON 0.10–0.30 Mg N ha⁻¹, and PNM 4.6–9.4 kg N ha⁻¹ greater in other treatments than in STW-F. At 0–5 cm, STN, PON, and MBN were greater in STCW than in FSTW-B/P and STW-F. At 5–20 cm, STN and PON were greater in NTCW and STCW than in STW-F, PNM and MBN were greater in STCW than in NTCW and STW-F, and NO₃-N was greater in FSTW-B/P than in NTCW and FSTCW. Estimated N loss through leaching, volatilization, or denitrification at 0–20 cm depth increased with increasing tillage frequency or greater with fallow than with continuous cropping and ranged from 9 kg N ha⁻¹ yr⁻¹ in NTCW to 46 kg N ha⁻¹ yr⁻¹ in STW-F. Long-term no-till or spring till with continuous cropping increased dryland crop grain and biomass N, soil surface residue N, N storage, and potential N mineralization, and reduced N loss compared with the conventional system, such as STW-F, at the surface 20 cm layer. Greater tillage frequency, followed by pea inclusion in the last 5 out of 21 yr in FSTW-B/P, however, increased N availability at the subsurface layer in 2004.     

Cattle grazing drives nitrogen and carbon cycling in a temperate salt marsh

We examined the impact of long-term cattle grazing on soil processes and microbial activity in a temperate salt marsh. Soil conditions, microbial biomass and respiration, mineralization and denitrification rates were measured in upper salt marsh that had been ungrazed or cattle grazed for several decades. Increased microbial biomass and soil respiration were observed in grazed marsh, most likely stimulated by enhanced rates of root turnover and root exudation. We found a significant positive effect of grazing on potential N mineralization rates measured in the laboratory, but this difference did not translate to in situ net mineralization measured monthly from May to September. Rates of denitrification were lowest in the grazed marsh and appeared to be limited by nitrate availability, possibly due to more anoxic conditions and lower rates of nitrification. The major effect of grazing on N cycling therefore appeared to be in limiting losses of N through denitrification, which may lead to enhanced nutrient availability to saltmarsh plants, but a reduced ability of the marsh to act as a buffer for land-derived nutrients to adjacent coastal areas. Additionally, we investigated if grazing influences the rates of turnover of labile and refractory C in saltmarsh soils by adding ¹⁴C-labelled leaf litter or root exudates to soil samples and monitoring the evolution of ¹⁴CO₂. Grazing had little effect on the rates of mineralization of ¹⁴C used as a respiratory substrate, but a larger proportion of ¹⁴C was partitioned into microbial biomass and immobilized in long- and medium-term storage pools in the grazed treatment. Grazing slowed down the turnover of the microbial biomass, which resulted in longer turnover times for both leaf litter and root exudates. Grazing may therefore affect the longevity of C in the soil and alter C storage and utilization pathways in the microbial community.     

Soil fertility breakdown in a subtropical South African vertisol site used as a home garden

Soil samples were taken at three depths (0–10, 10–20, 20–30 cm) from an old home garden site, subtropical South Africa (KwaZulu Natal Province), exhibiting symptoms of soil fertility breakdown, and from a new garden site, used formerly as extensive grassland. The aims of our study were to investigate (1) whether soil fertility breakdown is reflected by soil biological indices and (2) whether these indices provide information for improving the situation. The bulk density was significantly larger at the old garden site, with an average of 1.25 g cm^-3 in comparison to 1.04 g cm^-3 at the new garden site. Also the soil pH was significantly higher at the old garden site, with an average of 5.6 in comparison to 4.0 at the new garden site. Soil organic C and all soil microbial indices declined markedly with depth. If the concentrations were converted to the total amounts stored per hectare at 0- to 30-cm depth, soil organic C was only 10% greater (18.9 versus 17.2 t) at the new garden site, but microbial biomass C was 110% (1,680 vs 790 kg) and ergosterol even 220% greater (5.1 vs 1.6 kg). The microbial biomass C to soil organic C ratio ranged from 0.25% to 1.10%, declined markedly with depth and was almost twice as high at the new garden site. The metabolic quotient qCO₂ revealed strong depth-specific variations of between 18 and 38 mg CO₂-C day^-1 g^-1 microbial biomass C at the old garden site on a two to three times higher level than at the new garden site. Soil micro-organisms are much more strongly affected by compaction than by acidification at the present two sites. Whether land use practices led to the increase in bulk density at the old garden site or to the decrease in soil pH at the new garden site is still not fully understood. For these reasons, it is very difficult to recommend a feasible melioration plan.     

Biological and physical effects of non-native earthworms on nitrogen cycling in riparian soils

Global nitrogen cycling is being altered by anthropogenic disturbances including invasion by non-native species. European and Asian earthworms have invaded northern temperate forests in North America with dramatic consequences for litter thickness, forest floor plant diversity, and soil nitrogen cycling. Invasive earthworms present at the boundary of terrestrial and aquatic ecosystems (i.e., riparian zones) may alter the flux of nitrogen into adjacent aquatic ecosystems. We examined how nitrogen cycling in riparian soil responds to amendments of invasive earthworms or artificial earthworm burrows. In earthworm-free riparian plots (0.25 m²), we established treatments of invasive earthworms (60 g fresh mass·m⁻²), artificial burrows (120 m⁻²), or control plots and sampled the plots after 30 days. Before and after treatment application we measured major soil characteristics (water-filled pore space, organic matter, and pH), nitrogen pools (exchangeable NH₄⁺ and NO₃⁻), and nitrogen transformation rates (net N-mineralization, net nitrification, and denitrification). Exchangeable NH₄⁺ and NO₃⁻ changed through time but did not differ among treatments. Net N-mineralization and net nitrification rates did not change through time and were similar across all treatments. However, denitrification rates in plots with added earthworms were 4 times greater than rates in control and burrow-only plots, which represents a large rapid increase in gaseous nitrogen flux out of these riparian soils. For all response variables, artificial burrows responded similarly to control plots, suggesting that earthworm biological activity (i.e., feeding, excretion, and mucus production) rather than physical effects (i.e., burrowing and soil aeration) drove the changes in nitrogen cycling. Examination of soil nitrogen pool and flux measurements suggest that this increase in denitrification was coupled with NH₄⁺ consumption by nitrifying bacteria, but future studies are needed to confirm this hypothesis. We conclude that the activity of invasive earthworms in riparian zones can increase the flux of N out of riparian zones, but the hydrologic context of the riparian zone (e.g., pore-water residence time) ultimately controls whether denitrification or nitrate leaching is the dominant flux of N.     

Soil fauna alter the effects of litter composition on nitrogen cycling in a mineral soil

Plant chemical composition and the soil community are known to influence litter and soil organic matter decomposition. Although these two factors are likely to interact, their mechanisms and outcomes of interaction are not well understood. Studies of their interactive effects are rare and usually focus on carbon dynamics of litter, while nutrient dynamics in the underlying soil have been ignored. A potential mechanism of interaction stems from the role fauna plays in regulating availability of litter-derived materials in the mineral soil. We investigated the role of soil fauna (meso, macro) in determining the effect of surface-litter chemical composition on nitrogen mineralization and on the micro-food web in mineral soils. In a field setting we exposed mineral soil to six types of surface-applied litter spanning wide ranges of multiple quality parameters and restricted the access of larger soil animals to the soils underlying these litters. Over six months we assessed litter mass and nitrogen loss, nitrogen mineralization rates in the mineral soils, and soil microbes and microfauna. We found evidence that the structure of the soil community can alter the effect of surface-litter chemical composition on nitrogen dynamics in the mineral soil. In particular, we found that the presence of members of the meso- and macrofauna can magnify the control of nitrogen mineralization by litter quality and that this effect is time dependent. While fauna were able to affect the size of the micro-food web they did not impact the effect of litter composition on the abundance of the members of the micro-food web. By enhancing the strength of the impact of litter quality on nitrogen dynamics, the larger fauna can alter nitrogen availability and its temporal dynamics which, in turn, can have important implications for ecosystem productivity. These findings contribute to evidence demonstrating that soil fauna shape plant litter effects on ecosystem function.     

Tillage effect on organic carbon in a purple paddy soil

The distribution and storage of soil organic carbon （SOC） based on a long-term experiment with various tillage systems were studied in a paddy soil derived from purple soil in Chongqing, China. Organic carbon storage in the 0-20 and 0-40 cm soil layers under different tillage systems were in an order： ridge tillage with rice-rape rotation （RT-rr） 〉 conventional tillage with rice only （CT-r） 〉 ridge tillage with rice only （RT-r） 〉 conventional tillage with rice-rape rotation （CT-rr）. The RT-rr system had significantly higher levels of soil organic carbon in the 0-40 cm topsoil, while the proportion of the total remaining organic carbon in the total soil organic carbon in the 0-10 cm layer was greatest in the RT-rr system. This was the reason why the RT-rr system enhanced soil organic carbon storage. These showed that tillage system type was crucial for carbon storage. Carbon levels in soil humus and crop-yield results showed that the RT-rr system enhanced soil fertility and crop productivity. Adoption of this tillage system would be beneficial both for environmental protection and economic development.     

Free amino sugar reactions in soil in relation to soil carbon and nitrogen cycling

Amino sugars represent a major constituent of microbial cell walls and hydrolyzed soil organic matter. Despite their potential importance in soil nitrogen cycling, comparatively little is known about their dynamics in soil. The aim of this study was therefore to quantify the behaviour of glucosamine in two contrasting grassland soil profiles. Our results show that both free amino sugars and amino acids represented only a small proportion of dissolved organic N and C pool in soil. Based upon our findings we hypothesize that the low concentrations of free amino sugars found in soils is due to rapid removal from the soil solution rather than slow rates of production. Further, we showed that glucosamine removal from solution was a predominantly biotic process and that its half-life in soil solution ranged from 1 to 3 h. The rates of turnover were similar to those of glucose at low substrate concentrations, however, at higher glucosamine concentrations its microbial use was much less than for glucose. We hypothesized that this was due to the lack of expression of a low affinity transport systems in the microbial community. Glucosamine was only weakly sorbed to the soil's solid phase (K_d=6.4±1.0) and our results suggest that this did not limit its bioavailability in soil. Here we showed that glucosamine addition to soil resulted in rapid N mineralization and subsequent NO₃⁻ production. In contrast to some previous reports, our results suggest that free amino sugars turn over rapidly in soil and provide a suitable substrate for both microbial respiration and new biomass formation.     

Chemical composition and diversity influence non-additive effects of litter mixtures on soil carbon and nitrogen cycling: Implications for plant species loss

Diverse plant litter mixtures frequently decompose differently than expected compared to the average of the component species decomposing alone, and it remains unclear why decomposition may respond non-additively to diversity. Here, we hypothesized that litter chemical composition and chemical diversity would be important determinants of the strength and direction (synergistic versus antagonistic) of non-additive soil carbon (C) and nitrogen (N) cycling responses to litter mixtures. To test this, we performed a soil incubation experiment using litter mixtures comprised of up to four plant species, and we measured three components of decomposition: respiration, net N mineralization, and microbial biomass N accumulation. We used nine chemical traits to calculate the chemical composition and diversity of the litter mixtures. First, we found that respiration responded as the average of the individual species in the mixture (i.e. additively), rather than non-additively as initially predicted. Second, litter mixtures stimulated significantly more net N immobilization than expected in 64% of cases, and non-additive responses were highly dependent on mixture chemical composition, and were influenced to a lesser degree by chemical diversity. Specifically, concentrations of tannins and certain low molecular weight phenolics in the mixtures were positively correlated with greater N immobilization than expected. Non-additive N mineralization responses were poorly correlated with traditional measures of litter chemistry like N concentration, C:N, lignin:N, and phenolic:N. Our results also show that non-additive N mineralization responses were affected by loss of some species significantly more than others, and the effects of species loss could depend on 1) whether a species contains compounds with strong effects on non-additive responses; and 2) whether those compounds are also found in other species. Finally, litter mixtures stimulated more microbial biomass N than expected in 45% of cases, but non-additive responses were only weakly dependent on the litter chemistry variables that we measured.     

Effect of grazing intensity and soil characteristics on soil organic carbon and nitrogen stocks in a temperate long-term grassland

The effects of different grazing pressures (GPs) on soil properties are not sufficiently understood. The objectives were to analyse the effects of three different extensive GPs on stocks of soil organic C and total N, soil microbial biomass C, basal respiration and mineral N in three different soil depths of a long-term pasture in Central Germany (FORBIOBEN field trial). No significant (p ≤ 0.05) effects of GP on weighted stocks of soil organic C, total N, soil microbial biomass C, mineral N and basal respiration rate were observed, suggesting that the C and N cycles are coupled in the three grazing treatments. Oxalate soluble Fe contents explained a marked part of the variation of soil organic C (multiple linear regression: R² = 0.64) and total N contents (R² = 0.64) in the soils, whereas almost all of the variability of soil microbial biomass C contents and basal respiration was explained by soil organic C contents. Overall, variabilities of soil organic C and N contents were largely explained by oxalate soluble Fe contents, whereas grazing intensity did not affect the C and N dynamics.     

Irrigation and dryland cultivation effects on the surface properties and erodibility of a Zimbabwe vertisol

Abstract. Changes in chemical and physical properties and erodibility of a vertisol were studied in relation to land use. The vertisol, which occurs extensively in the semi-arid south-east of Zimbabwe, is derived from basalt and has a self mulching surface layer. Irrigated crops show static yields despite introduction of improved varieties. Four uncultivated sites were selected as controls and compared with five irrigated and four dryland sites. Surface soils were analysed for a range of chemical and physical properties, and laboratory rainfall simulation was used to measure soil erodibility under high intensity rain. The irrigated soils had greater exchangeable sodium and available phosphorus than the uncultivated soils. In contrast, dryland soils showed no such changes apart from a decrease in the amount of small water-stable aggregates. The soils are very erodible under high intensity rain but no significant differences were found between sites. We conclude that, although soil chemical changes have taken place in the irrigated soils, significant soil degradation has not occurred at the sites examined. The static yields probably result from management problems. However, immediate measures should be taken to improve drainage and irrigation management in the irrigated soils to avoid further sodium increases.     

Soil–plant nitrogen cycling modulated carbon exchanges in a western temperate conifer forest in Canada

Nitrogen controls, on the seasonal and inter-annual variability of net ecosystem productivity (NEP) in a western temperate conifer forest in British Columbia, Canada, were simulated by a coupled carbon and nitrogen (C&N) model. The model was developed by incorporating plant–soil nitrogen algorithms in the Carbon-Canadian Land Surface Scheme (C-CLASS). In the coupled C&N-CLASS, the maximum carboxylation rate of Rubisco (V_cmax) is determined non-linearly from the modelled leaf Rubisco-nitrogen, rather than being prescribed. Hence, variations in canopy assimilation and stomatal conductance are sensitive to leaf nitrogen status through the Rubisco enzyme. The plant–soil nitrogen cycle includes nitrogen pools from photosynthetic enzymes, leaves and roots, as well as organic and mineral reservoirs from soil, which are generated, exchanged, and lost by biological fixation, atmospheric deposition, fertilization, mineralization, nitrification, root uptake, denitrification, and leaching. Model output was compared with eddy covariance flux measurements made over a 5-year period (1998–2002). The model performed very well in simulating half-hourly and monthly mean NEP values for a range of environmental conditions observed during the 5 years. C&N-CLASS simulated NEP values were 274, 437, 354, 352 and 253 g C m⁻² for 1998–2002, compared to observed NEP values of 269, 360, 381, 418 and 264 g C m⁻², for the respective years. Compared to the default C-CLASS, the coupled C&N model showed improvements in simulating the seasonal and annual dynamics of carbon fluxes in this forest. The nitrogen transformation to soil organic forms, mineralization, plant nitrogen uptake and leaf Rubisco-nitrogen concentration patterns were strongly influenced by seasonal and annual temperature variations. In contrast, the impact of precipitation was insignificant on the overall forest nitrogen budget. The coupled C&N modelling framework will help to evaluate the impact of nitrogen cycle on terrestrial ecosystems and its feedbacks on Earth's climate system.     

Orchard floor management effects on nitrogen fertility and soil biological activity in a newly established organic apple orchard

This study addresses the often-competing goals of organic fertility and weed control by evaluating alternative orchard floor management strategies for their impact on N cycling, tree performance, and soil biological activity in a newly established apple (Malus domestica Borkh.) orchard. The standard tillage weed control practice resulted in satisfactory tree growth with desirable levels of leaf N and most other nutrients; however, soil biological activity did not improve. Maintenance of a living cover understory increased soil N concentration and availability and improved soil biological activity; however, tree growth was less than in other treatments likely in response to competition with the living cover understory for space and water. Application of wood chip mulch resulted in exceptional tree growth which may have resulted from greater water availability, but available soil N was lower, and consequently, tree leaf N concentration was low; in addition, soil biological activity was not improved. Clove oil organic herbicide provided poor weed control resulting in lower leaf N and tree growth and did not improve soil biological activity. Brassicaceae seed meal applications enhanced N availability and soil nematode abundance, but leaf N and many other nutrients were below desirable levels, and additional research is needed to optimize this treatment. We conclude that meeting the multiple objectives of weed control, optimal tree health, and increased soil biological activity may require employment of different orchard floor management strategies at different times during the life of the orchard.     

Effects of NPK fertilizer combinations on yield and nitrogen balance in sorghum or pigeonpea on a vertisol in the semi-arid tropics

A long-term experiment was carried out on a Vertisol from 1986 to 1992 to examine the combined effects of NPK fertilizers on yield using sorghum (Sorghum bicolor L. Moench cv. CSH 5) and short-duration pigeonpea (Cajanus cajan L. Millsp. cv. ICPL 87). The fertilizer treatments were as follows: 0 (no fertilization), N (150 kg N ha^-1 ), P (65.5 kg P₂O₅ ha^-1), K (124.5 kg K₂O ha^-1), and all possible combinations (NP, NK, PK, and NPK). In this study we continued this experiment during the period 1993 to 1994 and analyzed the crop yield response to fertilizers and the N balance. The amount of N derived from the atmosphere and fertilizer was estimated by the ¹⁵N natural abundance method and ^l5N isotope dilution method, respectively. A combined application of Nand P fertilizers gave the highest grain yield for the two crops under the 8th and 9th continuous croppings, unlike the application of K fertilizer. The values of total N for the two crops were significantly higher in the NP and NPK plots. These crops took up N mainly from soil. There was a significant positive relationship between the uptake of N_dff and N_dfs by each crop. Pigeonpea or sorghum took up more N from the soil in the N fertilizer plots than in the plots without N, suggesting that soil N fertility was enhanced and the amount of N supplied from soil increased in the plots with consecutive application of N fertilizer for 7 y. Even pigeonpea, which fixes atmospheric N inherently, needed N fertilizer to achieve high grain yield, suggesting that N fixation by the nodules was not always sufficient to meet the N requirements of the crop under these conditions. Although fertilizer N exerted a beneficial effect on plant growth and yield in the two crops, the values of fertilizer N recovery (FNR) by the two crops were considerably low. Therefore, it is suggested that the development of N fertilizer management which could maximize FNR of each crop should be promoted.     

Hydrological effects on the diversity of phenolic degrading bacteria in a peatland: implications for carbon cycling

Northern peatlands store ca. 1/3 of the world's soil organic carbon and this is attributed to low decomposition rates as a result of waterlogged, anaerobic conditions and high levels of phenolic substances. Climate change models predict both an increase in summer droughts and increased rainfall, depending on region, but information on the effect of these changes on the microbial population that mediate phenolic degradation is sparse. Temporal temperature gradient gel electrophoresis (TTGGE) was therefore used to assess the effect of simulated summer drought and increased rainfall on the diversity of phenolic degrading bacteria in a northern peatland using the gene XylE, encoding for the enzyme Catechol 2,3-dioxygenase (C23O), as an indicator. Under simulated drought, a greater diversity (129.4%, P<0.05) and abundance of phenolic catabolising bacterial species was found. Concurrent increased total phenol oxidase activities (83.3%) and β-glucosidase activities (157.6%, P<0.01) were found with consistently lower concentrations of phenolic compounds, DOC and increased CO₂ fluxes. This increased mineralisation is likely to lower carbon storage capacity and increase climate forcing. Conversely, the increased rainfall simulation suppressed diversity (62.2%, P<0.05), abundance and phenol oxidase activities (103.3%, P<0.001), giving increased phenolic compound (424.8%, P<0.1 only) and DOC concentrations (201.3%, P<0.001), along with increased anaerobic trace gas fluxes. These hugely increased aquatic carbon concentrations available for export are of serious concern due to their deleterious effect on drinking water quality.     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

Residue cover and rainfall intensity effects on runoff soil organic carbon losses

Soil cover and rainfall intensity (RI) are recognized to have severe impacts on soil erosion and an interaction exists between them. This study investigates the effect of rainfall intensity (RI) and soil surface cover on losses of sediment and the selective enrichment of soil organic carbon (SOC) in the sediment by surface runoff. A field rainfall simulator was used in the laboratory to produce 90 min rainfall events of three rainfall intensities (65, 85 and 105 mm h^− 1) and four cover percentages (0%, 25%, 50% and 75%) on soil material at 9% slope. A strong negative exponential relation was observed between cover percentage and RI on sediment loss under 85 and 105 mm h^− 1 of rain, while under RI of 65 mm h^− 1, the highest sediment loss was observed under 25% cover. Overall, higher RI and lower cover produced higher sediment and consequently higher nutrient loss, but resulted in a lower SOC enrichment ratio (ER_SOC) in the sediment. The amount of runoff sediment rather than the ER_SOC in the sediment was the determinant factor for the amount of nutrients lost. The values of ER_SOC were high and positively correlated with the ER values of particles smaller than 20 µm (p < 0.01). Although the sediment contained substantially more fine fractions (fine silt and clay, < 20 µm), the original soil and runoff sediment were still of the same texture class, i.e. silt clay loam.     

Arbuscular mycorrhiza and soil nitrogen cycling

Nitrogen is a major nutrient that frequently limits primary productivity in terrestrial ecosystems. Therefore, the physiological responses of plants to soil nitrogen (N) availability have been extensively investigated, and the study of the soil N-cycle has become an important component of ecosystem ecology and biogeochemistry. The bulk of the literature in these areas has, however, overlooked the fact that most plants form mycorrhizal associations, and that nutrient uptake is therefore mediated by mycorrhizal fungi. It is well established that ecto- and ericoid mycorrhizas influence N nutrition of plants, but roles of arbuscular mycorrhizas in N nutrition are less well established; perhaps even more importantly, current conceptual models ignore possible influences of arbuscular mycorrhizal (AM) fungi on N-cycling processes. We review evidence for the interaction between the AM symbiosis with microbes and processes involved in soil N-cycling. We show that to date investigations have rather poorly addressed such interactions and discuss possible reasons for this. We outline mechanisms that could potentially operate with regards to AM fungal – N-cycling interactions, discuss experimental designs aimed at studying these, and conclude by pointing out priorities for future research.     

Mycorrhiza and soil fertility effects with growth,nodulation and nitrogen fixation of leucaena grown on a typic eutrustox

Abstract

The rapidly growing, woody perennial legume, Leucaena (Leucaena leucocephala (Lam.) de Wit), is adaptable to a wide range of neotropical soil conditions. Effective Rhizobium inoculation and endophyte mycorrhizal colonization are essential for high levels of production and symbiotic N₂ fixation. The objective of this study was to determine growth, nodulation, nitrogenase activity and nodule composition of inoculated Leucaena as affected by mycorrhizal colonization and factorial soil fertility treatments of a Typic Eutrustox. Highly significant increases in top growth, nodule fresh wt. and nitrogenase activity resulted with Glomus fasciculatum colonization, soil K and linear increases with low‐soluble P fertilization to 300 mg P kg^‐1 soil. Highly significant interactions for increased nodulation and nitrogenase activity resulted with K × mycorrhiza. Interactions of all three factors P, K and mycorrhiza were highly significant for nodule fresh wt. However, responses comparing inoculation with G. fasciculatum and with combined G. fasciculatum, G. microcarpus and G. clavium were not significant. Highly significant increases with applied K levels to 300 mg K kg^‐1 soil resulted with top and root growth, nodulation and nitrogenase when applied with soluble P at 100 mg kg^‐1 soil and 500 mg Ca kg^‐1 soil. Significant and highly significant interactions of P, Ca and K level resulted for all parameters. Plant nutrient element composition of nodules increased with the fertilization treatments for P, Ca and increased K levels. A highly significant inverse relation was apparent with decreased Na resulting with increased K levels. Half or more of total nodule K, P and Mg but less than 20% of Ca and Na were within the nodule cytosol. Sodium, Mg, P, and Ca decreased in the cytosol fraction with increased K content.