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Background and aims: Several lines of evidence indicate that arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) associations can have different effects on soil nutrient dynamics. Some lineages of ECM fungi can extract N from organic matter, with varying impacts on decomposers, soil carbon pools, mineral N availability, and plants that lack ECM. However, these effects are not always observed, and it is not clear how they are mediated by environmental factors. Methods: We used Plant Root Simulator probes to compare soil availability of a wide range of nutrients beneath replicated 30-yr old plantations of Chilean Nothofagus (ECM) and Cupressaceae (Austrocedrus, Fitzroya: AM) on a lowland temperate site. Probes were buried for two 8-week periods in early spring and late summer. We also compared understorey composition beneath plantations, to test for evidence of different successional trajectories beneath Nothofagus and Cupressaceae. Results: Soil organic carbon, total N and total phosphorus did not differ significantly between Nothofagus and Cupressaceae stands. Redundancy analysis revealed significant effects of both plantation type (Nothofagus vs. Cupressaceae) and season on overall mineral nutrient availability. Mineral N availability did not differ significantly between Nothofagus and Cupressaceae plots, but pH and calcium availability were significantly lower beneath Nothofagus. Manganese (Mn) was much more available beneath Nothofagus stands, which might reflect abundant Mn-peroxidase, a key enzyme involved in breakdown of lignin by ECM fungi. Understorey composition varied considerably between individual plantations, but did not differ significantly between Nothofagus and Cupressaceae plantations. Conclusions: Despite an overall effect on the stoichiometry of nutrient availability, we found little evidence of modification of the local N cycle by ECM fungi, or of divergent regeneration patterns beneath AM and ECM plantations. This might reflect the relatively N-rich character of this site, and/or mycorrhizal effects being counteracted by leaf trait differences between Chilean Cupressaceae and Nothofagus species. [ABSTRACT FROM AUTHOR]

Aims: Kura clover living mulch (KCLM) is thought to provide in-season nitrogen (N) to the cash crop. Because of interactions between cropping system and crop performance, the magnitude of in-season N inputs from KCLM cannot be quantified using common N rate research methods. We aim to quantify the timing and magnitude of these N inputs using clover biomass and decomposition data. Methods: This research measured the temporal distribution of clover biomass with intensive sampling, vertical biomass distribution using bead tracer analysis, and residue decomposition using in situ litter bags. These data were used as inputs to a model that estimated the in-season N contributions from KCLM. Results: Our results found that spring management contributed 7–23 kg N ha−1 during the first 6 weeks of the growing season, while shade suppression contributed 59 – 84 kg N ha−1 from mid-June to mid-October. Daily N inputs peaked on 8 August at 1.14 kg N ha−1 d−1. When these estimates were added to fertilizer N inputs and analyzed against crop N uptake, we found that the maize took up 88% of the total N inputs and fertilizer recovery efficiency was 56%. Conclusions: This study provides a methodology which generates estimates that may be used in research related to N cycling and management in KCLM and other legume intercropping systems. [ABSTRACT FROM AUTHOR]

Background and aims: Cover crops are an integral constituent of sustainable subtropical agroecosystems. Using grass/legume mixtures as opposed to monocultures has the potential to maximize their multifunctionality. This project aimed to understand temporal patterns of nitrogen (N) release of cover crop monocultures and mixtures in a subtropical vegetable production system. Methods: A litterbag experiment was established to study N release patterns of two commonly used grasses, two legumes, four two-species mixture each with one grass and one legume, and one four-species mixture. This field experiment was complemented by two laboratory incubations to quantify soil N mineralization after the termination of cover crops in 2020 and 2021. Results: The majority of residue N (> 60%) was lost in the first month of decomposition, suggesting no or minimal N release from cover crops beyond one cropping season. Mixtures enhanced N release relative to monocultures within the first two months; however, the timing of this synergistic effect depended on grass species in the mixture. Initial residue N concentration reasonably predicted the N loss trajectory of all residue types (r = -0.72; p < 0.05). Legumes showed the highest N mineralization rate, followed by mixtures and lastly grasses. Conclusions: Our findings reveal significant temporal variability in N release among different cover crop mixtures, despite the rapid decomposition across all cover crop treatments. Selection of appropriate species in cover crop mixtures helps to facilitate the synchronization of N release and crop update in subtropical systems where N management is extremely time sensitive. [ABSTRACT FROM AUTHOR]

Background and aims: The long-term effects of annual fertilizer nitrogen (N) applications on soil organic carbon (SOC) has important implications for cropping system productivity and climate change mitigation. To understand the impacts of N fertilizer on SOC and other soil properties in a corn-based cropping system, a 10-year (2009–2018) study was initiated in Elora, Ontario, Canada. Methods: Corn was grown every year and crop management was held constant aside from the N rate applied. Plots received one of six long-term N rates (range: 30 to 260 kg-N ha-1). SOC (0–20 cm) was measured in every plot at the start and end of the study. Results: N fertilizer rate increased both grain yield (range: 5.6 to 11.0 Mg ha-1) and residue biomass (range: 5.0 to 7.6 Mg ha-1). Harvest index also increased significantly with N application (range: 0.49 to 0.55). Assuming a fixed harvest index would have overestimated residue returns. SOC increased by a similar amount after 10 years in all N rate treatments (mean: 4.5 Mg-C ha-1). Soil N supply capacity assessed at the end of the experiment was also unaffected by long-term N rate as measured by total soil N (0-20 cm), corn yield planted after trial conclusion (2019) and results from a 10-week incubation study. Conclusions: N rate had no detectable effect on SOC, N supply capacity or aggregate stability after 10 years. While yield and residue C inputs did increase with N rate, assuming a constant harvest index of 0.5 would have overestimated actual residue C returns. [ABSTRACT FROM AUTHOR]

Aims: We investigated the changes in structure and driving factors of soil microbial communities with different habitat specialization and their effects on soil nitrogen mineralization during close-to-nature of plantation. Methods: Close-to-nature forests were classified into Pinus tabulaeformis forest stage, mixed forest stage and near natural forest stage based on the coverage ratio of P. tabulaeformis to tree layer, and natural secondary forests selected for control. Soil physicochemical properties, enzyme activities, and nitrogen mineralization processes were determined. Soil microbial community structures were analyzed using high-throughput sequencing. Soil microorganisms were classified into habitat-generalized, neutral and specialized groups according to the relationship between the actual niche width and the confidence interval of simulated zero distribution. Results: (1) The extreme value of soil properties mainly appeared in the mixed forest stage. (2) The frequency of fungi and bacteria of each group both increased during near-naturalization with the generalized group increased fastest. (3) The bacterial β-diversity of each group differed significantly while the fungi not. (4) The soil bacterial structure of each group was affected by distance and soil properties. The influence of the former increased while the latter decreased with habitat specialization. (5) The structure of dominant genera of each group in the natural secondary forests significantly differed from close-to-nature forests and their structure well explained the soil nitrogen mineralization processes. Conclusion: Close-to-nature has improved soil properties, microbial community structure and nitrogen mineralization rate. Each habitat specialization group of microbial responded differently to near-naturalization and their structure well explained soil nitrogen mineralization processes. [ABSTRACT FROM AUTHOR]

Purpose: Multi-cutting and livestock excrement application are two key agricultural practices affecting nutrient cycling in the plant-soil system of sown grasslands. However, little is known about the combination of multi-cutting and livestock excrement application on nitrogen mineralization and utilization and their mechanisms regulating plant growth. Methods: The experiment was carried out in a salinized grassland at Linze Grassland Agricultural Station, Gansu Province, China. We tested the effects of plant multi-cutting (MC), sheep excrement (SE), multi-cutting and sheep excrement (MC + SE), and plant only (CK) on the soil nitrogen mineralization and plant growth of two forage plants, the spring wheat (Triticum aestivum L., cv. Yongliang 15) and common vetch (Vicia sativa L., cv. Lanjian 3). We measured the plant height, branch number, tiller number and crude protein content, and soil nitrification rate, ammonium rate, microbial nitrogen and microbial carbon, and abundance of soil ammonia-oxidizing bacteria and ammonia-oxidizing archaea. Results: MC and MC + SE significantly increased plant height, dry matter, and branch and tiller number of both forage crops. Although crude protein content and tiller number of spring wheat in SE decreased, these in MC + SE increased. SE and MC + SE significantly promoted nitrification rate in the soil by increasing the soil microbial nitrogen and soil ammonia-oxidizing bacteria abundance of the two forage crops. Conclusions: Combined application of both multi-cutting and sheep excrement could accelerate soil nitrogen mineralization and plant nitrogen uptake, which could be considered in pasture management to improve the sustainable productivity of grass-soil systems of sown grassland. [ABSTRACT FROM AUTHOR]

Background and aims: Masting is a widespread reproductive strategy in perennial plants. Its mechanism remains largely uncertain, although it is generally accepted that internal nutrient dynamics play a role. We aimed to examine whether soil nitrogen (N) supply influences plant N acquisition and drives masting.We measured spatio-temporal variations in soil N mineralization rate and plant N uptake by roots using plant root simulator probes on the periphery of six fruiting and four non-fruiting trees in a mature Fagus crenata Blume forest. In parallel, N concentrations and biomass growth in leaves, current-year branches, cupules, nuts and winter buds were analysed for the same trees.Soil N mineralization rates exhibited significant seasonal variations in both nitrate- and ammonium-N, and varied spatially up to 38-fold between spots sampled on the same date and 39-fold for the same tree in a growing season. Fruiting did not enhance N uptake by roots, and a dramatic decrease in soil N mineralization rate was observed in autumn during nut ripening. Translocation from cupules supplemented this N shortfall in mature nuts. Consequently, the amount of N amount per whole shoot, but not in ripe nuts per shoot, was correlated with the cumulative soil inorganic N.Masting is influenced ultimately by internal resource dynamics, which are decoupled from soil N dynamics. This may be a result of evolutionary selection to insulate trees from the high variation in local soil nutrients to ensure reproductive success.Methods: Masting is a widespread reproductive strategy in perennial plants. Its mechanism remains largely uncertain, although it is generally accepted that internal nutrient dynamics play a role. We aimed to examine whether soil nitrogen (N) supply influences plant N acquisition and drives masting.We measured spatio-temporal variations in soil N mineralization rate and plant N uptake by roots using plant root simulator probes on the periphery of six fruiting and four non-fruiting trees in a mature Fagus crenata Blume forest. In parallel, N concentrations and biomass growth in leaves, current-year branches, cupules, nuts and winter buds were analysed for the same trees.Soil N mineralization rates exhibited significant seasonal variations in both nitrate- and ammonium-N, and varied spatially up to 38-fold between spots sampled on the same date and 39-fold for the same tree in a growing season. Fruiting did not enhance N uptake by roots, and a dramatic decrease in soil N mineralization rate was observed in autumn during nut ripening. Translocation from cupules supplemented this N shortfall in mature nuts. Consequently, the amount of N amount per whole shoot, but not in ripe nuts per shoot, was correlated with the cumulative soil inorganic N.Masting is influenced ultimately by internal resource dynamics, which are decoupled from soil N dynamics. This may be a result of evolutionary selection to insulate trees from the high variation in local soil nutrients to ensure reproductive success.Results: Masting is a widespread reproductive strategy in perennial plants. Its mechanism remains largely uncertain, although it is generally accepted that internal nutrient dynamics play a role. We aimed to examine whether soil nitrogen (N) supply influences plant N acquisition and drives masting.We measured spatio-temporal variations in soil N mineralization rate and plant N uptake by roots using plant root simulator probes on the periphery of six fruiting and four non-fruiting trees in a mature Fagus crenata Blume forest. In parallel, N concentrations and biomass growth in leaves, current-year branches, cupules, nuts and winter buds were analysed for the same trees.Soil N mineralization rates exhibited significant seasonal variations in both nitrate- and ammonium-N, and varied spatially up to 38-fold between spots sampled on the same date and 39-fold for the same tree in a growing season. Fruiting did not enhance N uptake by roots, and a dramatic decrease in soil N mineralization rate was observed in autumn during nut ripening. Translocation from cupules supplemented this N shortfall in mature nuts. Consequently, the amount of N amount per whole shoot, but not in ripe nuts per shoot, was correlated with the cumulative soil inorganic N.Masting is influenced ultimately by internal resource dynamics, which are decoupled from soil N dynamics. This may be a result of evolutionary selection to insulate trees from the high variation in local soil nutrients to ensure reproductive success.Conclusion: Masting is a widespread reproductive strategy in perennial plants. Its mechanism remains largely uncertain, although it is generally accepted that internal nutrient dynamics play a role. We aimed to examine whether soil nitrogen (N) supply influences plant N acquisition and drives masting.We measured spatio-temporal variations in soil N mineralization rate and plant N uptake by roots using plant root simulator probes on the periphery of six fruiting and four non-fruiting trees in a mature Fagus crenata Blume forest. In parallel, N concentrations and biomass growth in leaves, current-year branches, cupules, nuts and winter buds were analysed for the same trees.Soil N mineralization rates exhibited significant seasonal variations in both nitrate- and ammonium-N, and varied spatially up to 38-fold between spots sampled on the same date and 39-fold for the same tree in a growing season. Fruiting did not enhance N uptake by roots, and a dramatic decrease in soil N mineralization rate was observed in autumn during nut ripening. Translocation from cupules supplemented this N shortfall in mature nuts. Consequently, the amount of N amount per whole shoot, but not in ripe nuts per shoot, was correlated with the cumulative soil inorganic N.Masting is influenced ultimately by internal resource dynamics, which are decoupled from soil N dynamics. This may be a result of evolutionary selection to insulate trees from the high variation in local soil nutrients to ensure reproductive success. [ABSTRACT FROM AUTHOR]

Aims: Topographic positions within a natural forest can considerably influence litter traits, soil microbial characteristics, and nitrogen (N) mineralization, causing plant–soil feedbacks. Despite the high abundance of coarse litter (woody debris and coarse roots) in forest ecosystems, most studies have focused on linkages between fine litter (leaves and fine roots) and N dynamics and/or the soil microbial community. Methods: We investigated the association of fine and coarse litter with soil microbial biomass, community structure, and N mineralization at upper and lower slope positions on sedimentary rocks in a temperate forest dominated by Fagus crenata. Results: Greater coarse litter abundance and litter C-to-N ratio, and lower soil microbial biomass, bacterial abundance, and N mineralization potential were found in upper positions than in lower positions. Among litter traits, coarse litter abundance and litter C-to-N ratio were the best predictors of the microbial biomass and fungal-to-bacterial dominance, possibly due to differences in climatic stress among plant communities. Microbial traits were the best predictors of N mineralization potential. Conclusions: Fine litter traits and coarse litter abundance are likely linked to soil microbial characteristics and N mineralization in natural forests with variable topography. [ABSTRACT FROM AUTHOR]

Background and aims: Plant growth affects soil moisture, mineral N and organic C availability in soil, all of which influence denitrification. With increasing plant growth, root exudation may stimulate denitrification, while N uptake restricts nitrate availability. Methods: We conducted a double labeling pot experiment with either maize (Zea mays L.) or cup plant (Silphium perfoliatum L.) of the same age but differing in size of their shoot and root systems. The 15N gas flux method was applied to directly quantify N2O and N2 fluxes in situ. To link denitrification with available C in the rhizosphere, 13CO2 pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil. Results: Plant water and N uptake were the main factors controlling daily N2O + N2 fluxes, cumulative N emissions, and N2O production pathways. Accordingly, pool-derived N2O + N2 emissions were 30–40 times higher in the treatment with highest soil NO3− content and highest soil moisture. CO2 efflux from soil was positively correlated with root dry matter, but we could not detect any relationship between root-derived C and N2O + N2 emissions. Conclusions: Root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth. [ABSTRACT FROM AUTHOR]

Aims: Cover crops (CC) have been widely used to improve soil quality and sustain agricultural productivity. However, the turnover of carbon (C) and nitrogen (N) from different species and parts of CCs incorporated into the soil under Northern European conditions remains unclear. Methods: In this study, we examined mineralization of C and N from isolated shoots and roots of two leguminous and two non-leguminous CCs labeled with 15N tracers in a 100-day incubation experiment at 10 °C. Results: After 100 days incubation, the average net N mineralization was 33% from the leguminous CCs and 20% from the non-legumes. Net N mineralization from shoots was 1–15% (% of the N input) higher than that from the corresponding roots, and the net C mineralization accounted for 53–62% and 18–39% of the total C applied with CC shoots and roots, respectively. The C/N ratio of plant residues was a good predictor for net N mineralization, while the lignin concentration was a good predictor for C mineralization from both roots and shoots. Roots of both legume and non-legume CCs caused net immobilization of (unlabeled) soil N, whereas no significant soil N immobilization occurred from shoots with low C/N ratios. Conclusions: Our results indicate that different CC characteristics (C/N ratio and lignin) control the turnover of residue N and C, respectively, in the soil. Our results further showed very limited interactions with soil N turnover (no priming effects) when materials with low C/N ratios were applied to soil. [ABSTRACT FROM AUTHOR]

Aims: Corn and soybean crops are often grown in rotation, requiring lower nitrogen (N) inputs than continuous corn. However, soil organic carbon (C) may be declining in corn-soybean systems despite sustained residue inputs. We asked whether corn-soybean rotations increase decomposition of litter and soil C as compared with continuous corn. Methods: We incubated soils from a long-term field experiment including continuous corn and both phases of the corn-soybean rotation. Soils were amended with corn litter, soybean litter, or no litter. We measured natural abundance C stable isotopes (δ13C values) in respiration and microbial biomass to partition C sources. Results: Addition of soybean litter increased microbial biomass while corn did not. However, corn litter addition consistently increased (i.e., primed) soil C decomposition while soybean litter did not. Soils most recently planted to corn following soybeans had the greatest soil C decomposition and N mineralization irrespective of litter addition, and they decomposed corn litter faster and had a faster priming response than the other treatments. Conclusions: Our data support the hypothesis that alternating inputs of N-rich soybean litter and relatively N-poor corn litter could enhance litter and SOC decomposition by driving microbial growth following the soybean phase and stimulating priming following the corn phase. Increased decomposition and N mineralization from litter and SOC in corn-soybean rotations may contribute to the soybean N credit but could also contribute to longer-term soil C and N declines, consistent with field data. [ABSTRACT FROM AUTHOR]

Aims: We tested whether chemical content of individual Tansy plants influences aboveground arthropod and belowground soil microbial community composition. Methods: We use Tansy chemical defence composition as focus for our analysis, given that changes in chemical defence are a dominant mechanism expected to modify aboveground and belowground community composition and nutrient cycling. Detailed assessment of arthropod food webs were made on tansy chemotypes. Next generation soil DNA analyses were used to assess soil bacterial community. Results: Tansy plants defended by Camphor and Borneol had higher total soil bacterial diversity than control plants (soils under non-Tansy plants) and Tansy chemotypes containing high concentrations of Thujon. Furthermore, the frequency of bacterial genera with a significant role in litter mineralization processes decreased with time in soil associated with Camphor and Borneol chemotypes, indicative of complete microbial decomposition and mineralization of plant material. No such changes occurred in soils associated with Thujon chemotypes. Thujon plants had fewer aphids, ladybirds, spiders, and Orius sp. than other chemotypes. Conclusions: Our experiments revealed that Thujon chemotypes had different associated aboveground arthropod and belowground soil microbial communities than Camphor or Borneol chemotypes. The common qualitative response between aboveground and belowground communities to Tansy plant defense is consistent with our hypothesis and extensive literature on plant defense and aboveground and belowground feedbacks. [ABSTRACT FROM AUTHOR]

Background and aims: Due to global warming, it is to be expected that mangroves will move north- and southwards and enter salt marshes, which will then be enriched with mangrove-derived organic compounds. Little is known about the effects that tannins, which are abundantly present in leaf litter of Rhizophora mangle, will have on the biogeochemical cycles in salt marsh soils.Methods: In microcosms, soil from a Distichlis spicata salt marsh was enriched with R. mangle-derived leaf powder (LP), crude condensed tannins (CCT) and purified condensed tannins (PCT), or with commercially available tannic acid (TA). Each treatment received a fixed amount of tannins, i.e. 10 mg /g dry soil. Due to the enrichment with tannins in each fraction, the amount of carbon added increased from C < TA = PCT < CCT < LP. At Day 7, 21, 35 and 42 of the incubation period, CO2 emission rates were determined after closing the microcosms for 24 h. Net nitrogen and phosphorous mineralization rates were also determined for the entire incubation period. Experiments were done at Practical Salinity Units of 20, 28 and 35 to cover the range in salinities observed in the field (20 and 35 PSU).Results: For the whole incubation period, the CO2 emission increased significantly (p < 0.05) in the order of C < PCT < CCT = TA < LP microcosms. The microcosms with LP showed immobilization of mineral nitrogen and phosphorus, while the other microcosms revealed net nitrogen and phosphorus mineralization in the order of CCT < TA < PCT < C for N mineralization, and CCT = TA < C < PCT for P mineralization. The C and N mineralization rates measured at a salinity of 20 PSU were significantly larger than at 28 and 35 PSU.Conclusion: The results showed that mangrove leaf litter stimulated net C mineralization and N and P immobilization. Purified condensed tannins of senescing R. mangle leaves enhanced C and P, and suppressed net N mineralization. [ABSTRACT FROM AUTHOR]

Background and aims: The addition of biochar to soil may offer a chance to mitigate climate change by increasing soil carbon stocks, improving soil fertility and enhancing plant growth. The impacts of biochar in cold environments with limited microbial activity are still poorly known.Methods: In order to understand to what extent different types and application rates of biochar affect carbon (C) and nitrogen (N) fluxes in boreal forests, we conducted a field experiment where two different spruce biochars (pyrolysis temperatures 500 °C and 650 °C) were applied at the rate of 0, 5 and 10 t ha−1 to Pinus sylvestris forests in Finland.Results: During the second summer after treatment, soil CO2 effluxes showed no clear response to biochar addition. Only in June, the 10 t ha−1 biochar (650 °C) plots had significantly higher CO2 effluxes compared to the control plots. The pyrolysis temperature of biochar did not affect soil CO2 effluxes. Soil pH increased in the plots receiving 10 t ha−1 biochar additions. Biochar treatments had no significant effect on soil microbial biomass and biological N fixation. Nitrogen mineralization rates in the organic layer tended to increase with the amount of biochar, but no statistically significant effect was detected.Conclusions: The results suggest that wood biochar amendment rates of 5-10 t ha−1 to boreal forest soil do not cause large or long-term changes in soil CO2 effluxes or reduction in native soil C stocks. Furthermore, the results imply that biochar does not adversely affect soil microbial biomass or key N cycling processes in boreal xeric forests, at least within this time frame. Thus, it seems that biochar is a promising tool to mitigate climate change and sequester additional C in boreal forest soils. [ABSTRACT FROM AUTHOR]

Background and aims: Rhizosphere priming occurs when plant belowground carbon (C) allocation influences the rate of soil organic matter (SOM) decomposition. We investigated the effects of priming and plant-mediated cover crop decomposition on agroecosystem C and nitrogen (N) dynamics. Methods: Using C stable isotopes, we tracked C and N from corn, clover ( Trifolium pratense) and rye ( Secale cereale) cover crop litter, and background SOM in plots following clover, rye, or no cover crop (fallow) in 2013 and 2014. Results: Corn enhanced the decomposition of N-rich clover cover crop litter in 2013, but there was little evidence of priming of bulk SOM decomposition. There was no corn effect on litterbag decomposition in 2014, likely due to greater soil moisture and temperature in no-corn plots. Corn N uptake per unit of corn-derived CO respiration was consistently lower following rye than clover and fallow, suggesting a higher C cost for corn to access N following a rye cover crop. Conclusions: This is one of the first field-based studies to provide evidence that plant-mediated litter decomposition potentially provides an important source of plant-available N. Climate and residue quality influence the extent to which corn mediates its own N supply with implications for agroecosystem C and N cycling. [ABSTRACT FROM AUTHOR]

Background and aims: The rhizosphere priming effect is caused by root carbon (C) exudation into the rhizosphere; the role of this effect in nutrient cycling and ecosystem recovery of natural grasslands as affected by different grazing intensities is still unknown. The objective of the present study was to investigate the relationships among root C exudation, rhizospheric microbial activity, and their influences on plant nutrient uptake during grazing and recovery periods . Methods: Field experiments were conducted in the Hongyuan Alpine Meadow to measure root exudation rate and nutrient cycling processes of the dominant species Elymus nutans. Three grazing intensities (an ungrazed control, moderate grazing and heavy grazing) were introduced for two months, following which all treatments received a recovery practice (no grazing for 21 days). Results: Heavy grazing significantly decreased root exudation rate, soil nitrogen (N) mineralization rate, β-1,4-glucosidase (BG) activity, and foliar C concentration, while moderate grazing had no influence on these parameters compared to the control. After the 21 days of recovery, all these parameters, except N mineralization rate and foliar C concentrations in the heavy grazing treatment, returned to similar levels as in the control, whereas root exudation rate and BG activity rose to even higher levels. Meanwhile, moderate grazing significantly promoted root exudation rate, soil inorganic N concentration, net soil N mineralization rate, and β- N-acetylglucosaminidase (NAG) activity during the recovery stage as compared to the control. Foliar quality was also improved by the recovery practice, indicating that the high availability of N and P is a consequence of the positive root-microbe feedback and will ultimately benefit grazers. Conclusions: The flush of labile C released to the rhizosphere by grazed plants stimulated extracellular enzyme activities, enhanced soil N mineralization, and increased plant nutrient uptake. These results imply that reasonable (i.e. moderate) grazing followed by a recovery practice can effectively restore and strengthen grassland vegetation, and contribute to the sustainable use of alpine meadows such as Hongyuan. [ABSTRACT FROM AUTHOR]

Aims: High-yielding maize-based crop systems require maize to take up large quantities of nitrogen over short periods of time. Nitrogen management in conventional crop systems assumes that soil N mineralization alone cannot meet rapid rates of crop N uptake, and thus large pools of inorganic N, typically supplied as fertilizer, are required to meet crop N demand. Net soil N mineralization data support this assumption; net N mineralization rates are typically lower than maize N uptake rates. However, net N mineralization does not fully capture the flux of N from organic to inorganic forms. Gross ammonification may better represent the absolute flux of inorganic N produced by soil N mineralization. Methods: Here we utilize a long-term cropping systems experiment in Iowa, USA to compare the peak rate of N accumulation in maize biomass to the rate of inorganic N production through gross ammonification of soil organic N. Results: Peak maize N uptake rates averaged 4.4 kg N ha d, while gross ammonification rates over the 0-80 cm depth averaged 23 kg N ha d. Gross ammonification was highly stratified, with 63% occurring in the 0-20 cm depth and 37% in the 20-80 cm depth. Neither peak maize N uptake rate nor gross ammonification rate differed significantly among three cropping systems with varied rotation lengths and fertilizer inputs. Conclusions: Gross ammonification rate was 3.4 to 4.5 times greater than peak maize N uptake across the cropping systems, indicating that inorganic N mineralized from soil organic matter may be able to satisfy a large portion of crop N demand, and that explicit consideration of gross N mineralization may contribute to development of strategies that reduce crop reliance on large soil inorganic N pools that are easily lost to the environment. [ABSTRACT FROM AUTHOR]

Background and scope: Mangroves provide several ecosystem goods and services to society. However, mangroves are frequently subjected to land conversion, overharvesting, and pollution including increased nitrogen (N) availability. Aiming to provide useful information to predict effects of N enrichment on mangroves, we evaluated literature data on N transformation rates in sediment, dissolved N (DN) fluxes across the sediment-water interface, and natural abundance of N stable isotopes (δN) in the sediment-plant system in conserved mangroves and those subjected to anthropogenic N enrichment. Conclusions: Mangroves and terrestrial tropical forests exhibit a great overlap in rates of biological N fixation (BNF) and denitrification and nitrous oxide flux rates. Mangroves can be highly efficient users of DN forms from tidal waters, related to rapid plant uptake and an efficient conservation of DN in sediment by microbial activity. The main factors limiting N transformation rates in mangrove sediment are inorganic DN availability and microbial immobilization. The δN data indicated that fringe forests exhibit higher N transformation rates in sediment and higher N losses to atmosphere, compared to other mangrove types. Except for BNF, all other N transformation and flux rates seem to be intensified by increasing N availability. [ABSTRACT FROM AUTHOR]

Background and aims: Invasive plants have been associated with alterations to soil properties, functions, and organisms, with the potential to impact ecosystem processes. An observational study was conducted to determine how the invasive plant Frangula alnus affects soil microbial communities and biogeochemical processes in Wisconsin forests. Methods: Paired invaded/non-invaded sites ( n = 10), including high ( n = 5) and low ( n = 5) density invasions, were sampled in spring, summer, and fall. Soil was analyzed for extractable and total nitrogen (N), N mineralization rate, total carbon, microbial biomass carbon and N, and microbial community structure using terminal restriction fragment length polymorphisms. Results: Linear regression analysis with robust variance estimation revealed higher N mineralization rates in invaded sites than non-invaded sites in summer, and in high density invaded sites than non-invaded sites overall ( p < 0.05). There was not a corresponding increase in extractable N. No differences between invaded and non-invaded sites were observed for other variables. Conclusions: Nitrogen-rich F. alnus leaf litter (3.2 % of dry mass) may contribute to elevated N mineralization at these sites, though pre-existing conditions may be responsible. Results suggest that F. alnus alters N cycling but has little impact on soil carbon pools and microbial communities. [ABSTRACT FROM AUTHOR]

Background and aims: Further understanding the factors which influence cover crop decomposition is needed to tailor management practices to achieve production and environmental stewardship goals. Methods: Short-term N from warm-season cover crops in organic farming systems was assessed with a variety of field and lab-based measures including: residue biochemical composition, extractable inorganic N, ion-exchange resin membranes, potential C and N. Results: Residue biochemical composition and environmental conditions appeared to be the primary factors affecting short-term N in our study. Legume-dominated cover crops demonstrated greater potential N than grass cover crops with higher C:N ratios. However even cover crops with C:N ratios >40:1 were predicted to result in mean net N rather than immobilization. Sorghum-sudangrass demonstrated a greater ability to moderate nitrate loss from the top 0-15 cm of soil over the fall season compared to the other cover crops evaluated. Conclusions: The difference in potential N of low and high C:N ratio cover crops was not as appreciable as expected. Our data suggests that the microbial community reduced its carbon use efficiency (CUE) due to suboptimal soil moisture. Further investigation into the impacts of soil moisture on residue decomposition in agroecosystems is needed to better understand and manage short-term C and N cycling. [ABSTRACT FROM AUTHOR]

Aims: The Birch effect is a pulse in soil C and N mineralization caused by the wetting of dry soils, but the role of the soil moisture increment ( ΔSWC) is still poorly understood. We quantified the relationship between ΔSWC and the Birch effect, and its interactions with pre-wetting soil moisture ( preSWC) and substrate supply. Methods: Two soils (clay loam and sandy loam) under a Pinus halepensis forest were subjected to rewetting in laboratory treatments combining different ΔSWC and preSWC values, with or without additional substrate (5 mg g P. halepensis needles). Respiration flush ( ΔR), changes in microbial biomass C ( MBC) and net N mineralization ( NMIN) were measured. Results: Overall, we found a relationship with the form: ΔR = a ΔSWC + b, where the slope ( a) was significant only when pre-wetting water potential was below a threshold value in the range of −100 to −1,200 kPa. However, the threshold alone does not fully describe the role of preSWC in slope variability. Substrate addition modified the ΔSWC sensitivity of Birch effect, enhancing it in the clay loam and suppressing it in the sandy loam. Conclusions: The intensity of the wetting is a dominant factor regulating Birch effect, and ΔSWC is useful for its quantification. [ABSTRACT FROM AUTHOR]

Background and aims: Climate warming, nitrogen (N) deposition and land use change are some of the drivers affecting ecosystem processes such as soil carbon (C) and N dynamics, yet the interactive effects of those drivers on ecosystem processes are poorly understood. This study aimed to understand mechanisms of interactive effects of temperature, form of N deposition and land use type on soil C and N mineralization. Methods: We studied, in a laboratory incubation experiment, the effects of temperature (15 vs. 25 °C) and species of N deposition (NH-N vs. NO-N) on soil CO efflux, dissolved organic C (DOC) and N (DON), NH-N, and NO-N concentrations using intact soil columns collected from adjacent forest and grassland ecosystems in north-central Alberta. Results: Temperature and land use type interacted to affect soil CO efflux, concentrations of DON, NH-N and NO-N in most measurement times, with the higher incubation temperature resulted in the higher CO efflux and NH-N concentrations in forest soils and higher DON and NO-N concentrations in grassland soils. Temperature and land use type affected the cumulative soil CO efflux, and DOC, DON, NH-N and NO-N concentrations. The form of N added or its interaction with the other two factors did not affect any of the C and N cycling parameters. Conclusions: Temperature and land use type were dominant factors affecting soil C loss, with the soil C in grassland soils more stable and resistant to temperature changes. The lack of short-term effects of the deposition of different N species on soil C and N mineralization suggest that maybe there was a threshold for the N effect to kick in and long-term experiments should be conducted to further elucidate the species of N deposition effects on soil C and N cycling in the studied systems. [ABSTRACT FROM AUTHOR]

Background and aims: Nitrate leaching from intensively cropped soils represents a huge environmental problem. In order to diversify the range of nitrogen management strategies, this investigation is focused on the effects of ribwort plantain, Plantago lanceolata L., and its allelochemicals on soil N mineralization. Methods: High-performance liquid chromatography was used in this study for phytochemical analysis of the major allelochemicals aucubin, catalpol, and verbascoside. Soil incubation experiments demonstrated a significant suppression of soil N mineralization caused by the incorporation of the iridoid glycoside (IG) aucubin, leaf material of two varieties ( P. lanceolata cv. Libor and cv. Arterner), and an aqueous extract of P. lanceolata leaves. Results: Throughout the growing season, the two varieties conspicuously differed in aucubin and verbascoside contents as well as in leaf dry weight. In soil incubation experiments, incorporated leaf material of both varieties affected long-term low soil nitrate concentrations. Experimental aucubin application resulted in an inhibitory effect on soil N mineralization. This was not true for the IG catalpol. Furthermore, we observed a negative relationship between IG concentrations and inorganic soil nitrogen concentrations when the soil was incubated with aqueous P. lanceolata leaf extract of different concentrations. Conclusion: This study enforced the hypothesis that allelochemicals of P. lanceolata have an active role in a suppression effect on soil N mineralization. Further research may be necessary to investigate the specific effects of P. lanceolata allelochemicals on the nitrogen cycle. [ABSTRACT FROM AUTHOR]

The availability of key plant nutrients may change with elevation in tropical mountains due to altitudinal gradients in temperature and moisture which affect pedogenesis and nutrient cycling. In a transect from upper lowland to montane forests in NE Ecuador, we tested the hypotheses that (1) the availability of P is low in low-elevation forests but increases upslope, while the availability of N is relatively high at low elevations but decreases with elevation, and (2) increasing amounts of calcium, magnesium and potassium are stored on top of the soil with progressive humus accumulation toward higher elevations, likely to improve nutrient availability. In each 20 plots in undisturbed natural forest at 500, 1000, 1500 and 2000 m a.s.l., we measured in situ N net mineralization and nitrification rate (N and N, buried bag method), plant-available phosphorus (P, resin-bag method), and salt-exchangeable calcium, potassium and magnesium concentrations (Ca, K, Mg) in the organic and mineral topsoils. N and N, and the Ca, K and Mg concentrations were much more variable at the plot level than across the four elevations, while P varied equally at small and large spatial scales. P increased more than 10fold from 500 to 2000 m. The net release of nitrate dominated over ammonium at all elevations. While mass-related N and N rates and also organic matter C/N ratio in the topsoil remained invariant along the slope, N and N rates per ground area decreased by about 40% from 500 to 2000 m. Thus, the N/P ratio decreased markedly with elevation proving our first hypothesis. In support of the second hypothesis, the pools of Ca, Mg and K in the organic layers increased with elevation, demonstrating the key role that organic topsoil horizons are playing for forest nutrition at high elevations. We suggest that the large difference in N versus P availability of tropical (upper) lowland and montane forests is likely to be a key factor influencing the species composition and productivity along tropical mountain slopes. [ABSTRACT FROM AUTHOR]

Arctic plant communities vary greatly over short distances due to heterogeneities in topography and hydrological conditions across the landscape. Recent evidence suggests substantial changes in vegetation including increasing shrub cover and density in the Arctic over the past three decades that may be in response to climate change. We investigated soil microbial biomass, nutrient availability, nitrogen (N) mineralization potential and nitrification potential in four of the principal vegetation-types across the low Arctic: dry heath, birch hummock, tall birch and wet sedge. Soil total carbon (C) and N contents, microbial biomass C, dissolved organic C (DOC) and N (DON), mineral N, and N mineralization potential differed considerably among vegetation-types. Tall birch and wet sedge soils had significantly higher DON, mineral N, and N mineralization potential than birch hummock or dry heath soils. Soil N mineralization potential across all soils was positively correlated with soil available C and N, and negatively correlated with soil total C:N ratios. Nitrification potential was negligible in all soils. These results demonstrate close relationships between soil biogeochemical properties, mineral N supply rates, and vegetation-types across an arctic landscape. Our soil N mineralization data suggest that climate warming may enhance N availability in tall birch soils more than in birch hummock soils, and therefore that increases in shrub densities across the landscape are most likely within and directly around current tall shrub patches. [ABSTRACT FROM AUTHOR]

We assessed direct and indirect effects of snow cover on litter decomposition and litter nitrogen release in alpine tundra. Direct effects are driven by the direct influence of snow cover on edaphoclimatic conditions, whereas indirect effects result from the filtering effect of snow cover on species’ abundance and traits. We compared the in situ decomposition of leaf litter from four dominant plant species (two graminoids, two shrubs) at early and late snowmelt locations using a two-year litter-bag experiment. A seasonal experiment was also performed to estimate the relative importance of winter and summer decomposition. We found that growth form (graminoids vs. shrubs) are the main determinants of decomposition rate. Direct effect of snow cover exerted only a secondary influence. Whatever the species, early snowmelt locations showed consistently reduced decomposition rates and delayed final stages of N mineralization. This lower decomposition rate was associated with freezing soil temperatures during winter. The results suggest that a reduced snow cover may have a weak and immediate direct effect on litter decomposition rates and N availability in alpine tundra. A much larger impact on nutrient cycling is likely to be mediated by longer term changes in the relative abundance of lignin-rich dwarf shrubs. [ABSTRACT FROM AUTHOR]

Soils are spatially heterogeneous. To better understand the effects of sub-plant scale soil resource heterogeneity on primary productivity we aggregated plant litter ( Trifolium pratense L. shoots and Secale cereale L. leaves) into 8, 24, 32, and 72 patches and uniformly distributed in 50-L containers (0.15 m2) of soil and grew Zea mays L. throughout two seasons. On average aggregated T. pratense litter enhanced final aboveground maize biomass by 14% relative to uniformly distributed T. pratense litter. This effect may be related to the reduction in root carbon allocation observed as lower apparent root respiration rates compared to uniform litter distribution. In contrast, the spatial distribution of S. cereale litter did not affect productivity. The common experimental approach of uniformly distributing resources to understand their influence on soil-plant processes likely oversimplifies field conditions because our results indicate that the spatial distribution of plant litter alone can influence productivity and plant carbon allocation. [ABSTRACT FROM AUTHOR]

We investigated soil fertility status under shifting cultivation in East Kalimantan with special reference to mineralization patterns of labile soil organic matter (SOM). The soils in this region were generally strongly acidic with high Al, low bases and low pH values. A 133-day incubation experiment using fresh soils revealed that NH4 + often accumulated during the course of N mineralization, indicating a delay of nitrification relative to N mineralization in these soils. Principal component analysis followed by stepwise multiple linear regression showed the contribution of soil physicochemical properties to mineralization patterns of SOM. Those results indicated that the overall SOM level positively contributed to the amount of readily mineralizable C and N, NH4 at day 133, and NO3 at day 133. The results also showed that the factors relating to soil acidity and P and K depletion, as well as accumulation of readily mineralizable C, contributed to suppress nitrification and accelerate NH4 + accumulation and possibly subsequent N immobilization. Our results suggest that it will be difficult to establish a cropping system without a long period of fallow unless very high amounts of fertilizer as well as liming are applied in these regions. [ABSTRACT FROM AUTHOR]

Understanding mulching influences on nitrogen (N) availability is important for developing N management strategies in plantations at the upland sites of the southwestern China. Dynamics of biomass loss and nutrient release of mulching material, N availability in the soil and N mineralization in situ were evaluated for the treatments with different mulch quantity in degraded agricultural soil. The time taken for 95% decomposition of the initial biomass of Cogon grass ( Imperata cylindrical L. Beauv. var. major) was 17 months with a half-life ( t 1/2) of about 4.8 months. During the first 4 months about 55.2% of N was released, and after 1-year decomposition about 71.6% of N was released from the mulch material. The fresh grass mulch increased the available N in the soil as they decomposed. Compared to no mulch treatment, mulch treatments with 2.5, 5.0 and 7.5 kg m−2 mulching grass increased available N by about 13.1, 40.8 and 56.4% in the top soil (0–5 cm), and about 23.6, 78.0 and 139.3% in the middle layer (5–20 cm), respectively. The mean annual net N mineralization in the mulched plots had 9.0–40.9% higher cumulative rate than that in no-mulch plots, and the majority of the accumulated N in the incubated soils existed as NO3–N. There was a positive relationship between the rate of N mineralization and the available N in both the top soil and the middle layer. Mulch improves soil nutrients and this improvement increased with increasing mulching quantity. The increment of net N mineralization was approximately 69, 161 and 322 kg N ha−1 year−1 in the soil of 0–20 cm depth for the 2.5, 5.0 and 7.5 kg m−2 grass mulch treatments, respectively. The results from this study will provide a basis to optimize mulching techniques for poplar plantations in degraded agricultural soils of southwestern China. [ABSTRACT FROM AUTHOR]

We examined the influence of treefall gaps on soil properties and processes in old growth northern hardwood-hemlock forests in the upper Great Lakes region, USA. We found significantly greater solar radiation, soil moisture contents and soil temperatures in gaps compared to adjacent closed canopy plots. Gaps had significantly less exchangeable base cations (K, Ca, and Mg) compared to forest plots in the upper mineral soil (0–25 cm). Gaps also had significantly more dissolved organic N and extractable nitrate at depth (25–50 cm), indicating increased nutrient leaching in gaps. In-situ N mineralization was significantly greater in gaps and edge plots compared to forest plots. We found significantly greater potential N mineralization (measured in the laboratory at 25°C and 40% water holding capacity) in forest compared to gap plots. Microbial biomass N was significantly greater (ca. two-fold) in the gap edge compared to both gaps and closed forest. Using principal component analyses we found that edge plots were positively correlated with all principal components, indicating increased in-situ and potential N mineralization, microbial biomass N, soil NO and NH, and soil organic matter. The gap edge may be a region of optimal microclimate and substrate to enhance microbial biomass and activity within these forest ecosystems. [ABSTRACT FROM AUTHOR]

The global net terrestrial carbon sink was estimated to range between 0.5 and 0.7 Pg C y−1 for the early 1990s. FACE (free atmospheric CO2 enrichment) studies conducted at the whole-tree and community scale indicate that there is a marked increase of primary production, mainly allocated into below-ground biomass. The enhanced carbon transfer to the root system may result in enhanced rhizodeposition and subsequent transfer to soil C pools. During the first rotation of the POP/EuroFACE experiment in a short-rotation Poplar plantation, total soil C content increased more under ambient CO2 treatment than under FACE, while under FACE more new C was incorporated than under ambient CO2. These unexpected and opposite effects may have been caused by a priming effect, where priming effect is defined as the stimulation of SOM decomposition caused by the addition of labile substrates. In order to gain insight into these processes affecting SOM decomposition, we obtained the labile, refractory and stable pools of soil C and N by chemical fractionation (acid hydrolysis) and measured rates of N-mineralization. Results of the first 2 years of the second rotation show a larger increase of total soil C% under FACE than under ambient CO2. In contrast to the first rotation, total C% is now increasing faster under FACE than under ambient CO2. Based on these observations we infer that the priming effect ceased during the second rotation. FACE treatment increased the labile C fraction at 0–10 cm depth, which is in agreement with the larger input of plant litter and root exudates under FACE. N-mineralization rates were not affected by FACE. We infer that the system switched from a state where extra labile C and sufficient N-availability (due to the former agricultural use of the soil) caused a priming effect (first rotation), to a state where extra C input is accumulating due to limited N-availability (second rotation). Our results on N-mineralization (second rotation) are in agreement with observations made at three forest FACE sites (Duke Forest, Oak Ridge, and Rhinelander), but our finding of increasing mineral soil C content contrasted with results at the Duke Forest where no significant increase in C content of the mineral soil occurred. However, the FACE induced increase in total C content occurred within the fraction with the shortest turnover time, i.e. the labile fraction. The refractory and stable fractions were not affected. The question remains whether the currently observed larger increase of total soil C and the increase of labile C under FACE will eventually result in long-term C storage in refractory and stable organic matter fractions. [ABSTRACT FROM AUTHOR]

Characterization of decomposition characteristics is important for sound management of organic residues for both soils and livestock, but routine residue quality analysis is hindered by slow and costly laboratory methods. This study tested the accuracy and repeatability of near-infrared spectroscopy (NIR) for direct prediction of in vitro dry matter digestibility (IVDMD) and C and N mineralization for a diverse range of organic materials (mostly crop and tree residues) of varying quality ( n = 32). The residue samples were aerobically incubated in a sandy soil and amounts of C and N mineralized determined after 28 days. IVDMD and quality attributes were determined using wet chemistry methods. Repeatability was higher with NIR than the original wet chemistry methods: on average NIR halved the measurement standard deviation. NIR predicted IVDMD and C and N mineralization more accurately than models based on wet chemical analysis of residue quality attributes: reduction in root mean square error of prediction with NIR, compared with using quality attributes, was IVDMD, 6%; C mineralization after 28 days, 8%; and N mineralization after 28 days, 8%. Cross-validated r 2 values for measured wet chemistry vs. NIR-predicted values were: IVDMD, 0.88; C mineralization, 0.82; and N mineralization, 0.87. Direct prediction of decomposition and mineralization from NIR is faster, more accurate and more repeatable than prediction from residue quality attributes determined using wet chemistry. Further research should be directed towards establishment of diverse NIR calibration libraries under controlled conditions and direct calibration of soil quality, crop and livestock responses in the field to NIR characteristics of residues. [ABSTRACT FROM AUTHOR]

The effects of clearcut and partial harvesting of early-seral trembling aspen plots were compared to conventional clearcut harvesting in mid-seral mixedwood and late-seral conifer plots. Twice a year, for three consecutive years, we assessed mineral N and microbial dynamics in the forest floor of these plots to test three hypotheses related to the higher litter quality of aspen leaves and to the sustained inputs of available C on partially harvested plots: (1) the post-clearcutting mineral N flush and the net [(NO): (NO+ NH)] production ratio (RNI) are higher in aspen plots than in black spruce plots, with intermediate values occurring in mixedwood plots; (2) net N mineralization rates in aspen plots are higher in spring than in autumn; and (3) compared to clearcutting, partial harvesting reduces potential ammonification and nitrification rates. Initial NHand NOconcentrations respectively ranged between 1.7-4.4 and 0.2-1.5 g N kg-1 Ntotal, net ammonification and nitrification rates (30 d incubations) respectively ranged between 5.3-17.8 and 0.1-27.6 g N kg-1 Ntotal, basal respiration ranged between 20.9-38.9 mg CO2-C kg-1 h-1, and microbial biomass ranged between 6.1-8.7 g Cmic kg-1. Although clearcutting increased NOconcentrations in aspen plots, the balance of our results did not support our first hypothesis, because NHconcentrations increased in conifer plots only, potential ammonification was unaffected by clearcutting, potential nitrification increased in mixedwood plots only, and RNI increased in all plots. In each seral stage, basal respiration, microbial biomass, and metabolic quotient either increased or were unaffected by clearcutting, suggesting that increases in RNI after disturbance were not related to lower microbial immobilisation of NOdue to lower available C. Forest floors in mid-seral mixedwood plots exhibited a distinct combination of mineral N and microbial properties, suggesting that the functional richness of the forest is enhanced not only by the number of species, but also by the diversity of assemblages that are present. Results supported our second hypothesis and showed, furthermore, that net N mineralization in conifer stands is greater in autumn than in spring. Partial harvesting in aspen stands resulted in lower potential mineralization of N and lower RNI, compared to clearcutting. Further lysimetry studies are needed to confirm whether partial harvesting mitigates NOleaching following disturbance. [ABSTRACT FROM AUTHOR]

Rising concentrations of atmospheric [CO2] in a multi-species ecosystem can influence species composition and increase plant productivity, but have a less predictable effect on soil C storage and nutrient availability. Using a free-air [CO2]-enriched (FACE) system and seasonal sampling over a 5-year period, we examined the influence of elevated atmospheric [CO2] (475 μL L-1) on soil C and N pools and mineralization in a fertilized (P, K, S), sheep-grazed pasture of mixed grass, clover, and forb species on a seasonally dry sand (Mollic Psammaquent). Annual yields of herbage dry matter ranged from about 300 to 1600 g m-2. Total yields did not increase significantly under elevated [CO2], but the proportions of clovers and forbs increased markedly. Most properties in 0–50 mm-depth soil differed significantly (P<0.05) with year of sampling, but [CO2]-treatment effects were non-significant (P>0.10) for moisture, pH, total C and N, extractable C and organic N, microbial C, and mineral-N. However, microbial N, CO2-C production (0–14 days) in field-moist soil, and net mineral-N production (14–56 days) in soil at 60% of water-holding capacity were significantly higher (per unit weight of soil) in the elevated-[CO2] treatment (P=0.071, 0.063, 0.003, respectively); the degree of these treatment differences was roughly similar when values were also expressed on a total C or N basis. Relationships with soil moisture were mainly non-significant for microbial C and N, but mainly significant (P<0.05) for net mineral-N production in field-moist soil, and highly significant (P<0.001) for CO2-C production. Overall, the data tend to suggest greater soil metabolic activity, but little if any change in soil C pools, after 5 years' exposure of the pasture to elevated [CO2]. They do, however, suggest increased availability of N, probably because of increased inputs from N-fixing clovers. [ABSTRACT FROM AUTHOR]

Positive effects of legumes and actinorhizal plants on N-poor soils have been observed in many studies but few have been done at high latitudes, which was the location of our study. We measured N2 fixation and several indices of soil N at a site near the Arctic Circle in northern Sweden. More than 20 years ago lupine (Lupinus nootkatensis Donn) and gray alder (Alnus incana L. Moench) were planted on this degraded forest site. We measured total soil N, net N mineralization and nitrification with a buried bag technique, and fluxes of NH+4 and NO-3 as collected on ion exchange membranes. We also estimated N2 fixation activity of the N2-fixing plants by the natural abundance of 15N of leaves with Betula pendula Roth. as reference species. Foliar nitrogen in the N2-fixing plants was almost totally derived from N2 fixation. Plots containing N2-fixing species generally had significantly higher soil N and N availability than a control plot without N2-fixing plants. Taken together, all measurements indicated that N2-fixing plants can be used to effectively improve soil fertility at high latitudes in northern Sweden. [ABSTRACT FROM AUTHOR]

Many soils of southern Africa are severely N deficient, but inorganic fertilizers are unaffordable for most subsistence farmers. Rotations and intercrops of legumes with crops may alleviate N deficiency through biological N2 fixation and redistribution of subsoil N to the surface. We monitored soil inorganic N dynamics for two seasons in a gliricidia [Gliricidia sepium (Jacq.) Walp.] - maize (Zea mays L.) intercrop in the unimodal rainfall area of southern Malawi. One maize crop per year was grown with or without interplanted gliricidia, in factorial combination with three rates of N (0, 24 or 48 kg N ha-1). Application of gliricidia prunings increased (p < 0.001) topsoil (0 to 20 cm) inorganic N at the end of the dry season and during the early rains. Differences between plus and minus gliricidia treatments were less when total inorganic N to 1-m depth was summed. A greater proportion of the total inorganic N to 1-m depth occurred in the topsoil (0 to 20 cm) when gliricidia was present, suggesting that redistribution of subsoil N to the surface accounted for part of the N increase by gliricidia. Gliricidia lowered (p < 0.05) subsoil water content during drier periods. Gliricidia plots accumulated more (p < 0.01) ammonium-N during the dry season. Nitrate-N remained constant during the dry season but rose rapidly in gliricidia plots after the onset of rains. A 2-factor model including preseason inorganicN and anaerobic N mineralization potential accounted for 84% of the variability in maize yields for the two seasons' data combined. The combination of preseason inorganic N and potential N mineralization appears to provide a good estimate of N supply to maize in systems receiving both organic and inorganic sources of N. [ABSTRACT FROM AUTHOR]

We investigated plant and soil nitrogen pools and soil processes in monospecific stands of the C3 sedge Scirpus olneyi and theC4 grass Spartina patens grown in the field in open top chambers in a brackish marsh on the Chesapeake Bay. Stands of S. olneyi responded to eight years of elevated CO2, by increased rates of net ecosystem gas exchange and a large stimulation of net ecosystem production. We conducted our study in the summer of 1994 and 1995 when soil cores were collected and aboveground biomass was estimated. Nitrogen concentration in elevated CO2 treatments wasreduced 15% in stems of S. olneyi and 8% in the upper 10 cm of the soil profile. While total plant nitrogen per unit of land area remained the same between treatments, total soil nitrogen showed a non-significant tendency to decrease in the upper 10 cm of the soil profile inelevated CO2 both years of study. A significant decrease in soil bulk density largely contributed to the observed decrease in soil nitrogen. Exchangeable nitrogen and potential denitrification rates were also reduced in elevated CO2, but net nitrogen mineralization was unchanged by elevated CO2 treatment in S.olneyi both years. Plants and soils in a pure stand of the C4 grass, S. patens, showed none of these effects of elevated CO2 treatment. Our data provides evidence of changes in nitrogen dynamics of an ecosystem exposed to elevated CO2 for eight years; however due to the variability in these data, we cannot sayif or how these changes are likely to impact the effect of rising CO2 on primary production or carbon accumulation in this ecosystem in the future. [ABSTRACT FROM AUTHOR]