Soil Turnover Research Articles

Effects of freeze-thaw and soil moisture on content and spectral structure properties of dissolved organic matter in forest soil leachates.

The contents and stability of soil dissolved organic matter (DOM) can affect key processes of soil carbon and nitrogen cycle. The responses of DOM content and its spectral structure pro-perties in forest soils to climate change remain unclear. We collected soil samples from two temperate forests, i.e., the broadleaf and Korean pine mixed forest (BKPF) and adjacent secondary white birch forest (WBF), in Changbai Mountains, northeastern China. Using a combination of three-dimensional fluorescence spectrum and parallel factor analysis, a simulated freeze-thaw experiment was conducted in the laboratory. We examined the effects of freeze-thaw intensity, freeze-thaw cycle and their interaction on the content, components and spectral properties of DOM leached from the two forest surface soils with different moisture levels. The results showed that DOM content and components of soil leachates varied with forest types, soil moisture, freeze-thaw intensity and freeze-thaw cycle. The DOM content in the leachates was lowest at medium moisture level and was significantly affected by the high freeze-thaw intensity. In addition, the DOM content increased first and then decreased with the increases of freeze-thaw cycles. Three fluorescence components of DOM in the forest soil leachates were identified as humic acid-like DOM, fulvic acid-like DOM and protein-like DOM. The DOM components of BKPF soil leachates were mainly consisted of fulvic acid-like substances with a high humification index. However, the DOM from WBF soil leachates was dominated by humic acid-like substances with low stability, and the three fluorescence components were significantly affected by the freeze-thaw intensity. Results from the redundancy analysis showed that under the experimental conditions, forest type played a leading role in changing DOM properties. The DOM content and its three fluorescence intensities of WBF soil leachates were higher than those of BKPF. Soil moisture significantly affected the aromaticity of DOM in the forest soil leachates, and the DOM aromaticity of soil leachates from the two forest stands ranked as medium moisture > high moisture > low moisture. With the increases of freeze-thaw intensity, the DOM aromaticity of BKPF soil leachates significantly decreased. Furthermore, the increases of freeze-thaw cycles significantly increased the humification degree of DOM in the forest soil leachates. Therefore, upon different freeze-thaw disturbance, the DOM content and bioavailability of soil leachates with low moisture tended to increase, particularly in the WBF soil leachates, which may result in an increased lea-ching of DOM in temperate forest soils during spring freeze-thaw periods. The results provide a refe-rence for further investigating DOM turnover in temperate forest soils during spring freeze-thaw periods.

Effects of burrowing mud lobsters (Thalassina anomala Herbst 1804) on soil macro- and micronutrients in a Malaysian mangrove

The mud lobster, genus Thalassina is the least studied taxon compared to other mud crabs within Thalassinidae family. Burrowing thalassinid populations are considered to be an effective means of bioturbation, affecting the macro- and micronutrient properties in the substrates they occur in through nutrient redistribution. This study assessed the combined effects of fresh mud excavation and old mud deposition on soil turnover and vertical marconutrients including NH4–N, NO3–N, P, K, S, Na, Ca, and Mg and micronutrients including Fe, Cu, Zn and Mn distributions in a Malaysian mangrove, Sibuti Wildlife Sanctuary, Sarawak. The mud lobster Thalassina anomala (Decapoda: Thalassinidae) inhabits and builds mounds both seafront and inland mangroves. Their burrowing activities increased NH4–N, NO3–N, P, and Na concentrations in fresh mud and Fe, Cu and Zn concentrations in the old mud. The nutrient concentrations in excavated soils were greatly affected by habitat type, and depth. Burrowing processes can effectively turnover subsurface soil to the surface, which acidifies strongly upon oxidation. Higher S content in mud lobster soil together with low pH revealed mud lobsters play a strong role in the development of acidity in the study areas. These processes could stimulate formation of acid sulphate soil in the mangroves. Therefore, bioturbation not only affects soil physical and chemical properties but also ecological functions of mangroves, and needs proper attention during restoration and reclamation programs of mangrove forests.

Newly depolymerized large organic N contributes directly to amino acid uptake in young maize plants.

The contribution of large molecular size organic nitrogen (N) to plant N uptake is unclear. Soils with and without maize, at three pH levels, were treated with (carbon-14 and -13 (14 C, 13 C), 15 N) triple-labelled >100kDa organic N. After 48h, soil and maize were sampled for bulk and compound specific isotope analysis to study the turnover in soil and plant 13 C and 15 N uptake. Mineralization of >100kDa organic N increased with higher pH only in soil without maize. The >100kDa organic N disappeared rapidly in soils with and without maize, but surprisingly more >100kDa organic N derived amino acids remained in soil with than without maize - most likely in the microbial biomass. Total 15 N uptake in maize increased with higher soil pH. The organic N uptake was estimated to account for 20-30% of the total 15 N uptake. Organic N uptake was confirmed by the presence of 13 C-labelled amino acids in maize roots. The study suggests that the importance of plant organic N uptake increases when N is derived from complex molecules such as proteins compared to studies using single amino acids as N source, and that rhizosphere microorganisms increase anabolic utilization of organic N compared to microorganisms in the bulk soil.

Thermal oxidation of carbon in organic matter rich volcanic soils: insights into SOC age differentiation and mineral stabilization

Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in soils. Four soil horizons (Oa, Bhs, Bs, Bg) were sampled from highly weathered 350 ka Pololu basaltic volcanics on the Island of Hawaii and analyzed by Ramped PyrOX (RPO) in both the pyrolysis (PY) and oxidation (OX) modes to separate a complex mixture of OC into thermally defined fractions. Fractions were characterized for carbon stable isotope and radiocarbon composition. PY and OX modes yielded similar results. Bulk radiocarbon measurements were modern in the Oa horizon (Fm = 1.013) and got progressively older with depth: the Bg horizon had an Fm value of 0.73. Activation energy distributions (p(E)) calculated using the ‘rampedpyrox’ model yielded consistent mean E values of 140 kJ mol−1 below the Oa horizon. The ‘rampedpyrox’ model outputs showed a mostly bimodal distribution in the p(E) below the Oa, with a primary peak at 135 kJ mol−1 and a secondary peak at 148 kJ mol−1, while the Oa was dominated by a single, higher E peak at 157 kJ mol−1. We suggest that mineral-carbon interaction, either through mineral surface-OC or metal-OC interactions, is the stabilization mechanism contributing to the observed mean E of 140 kJ mol−1 below the Oa horizon. In the Oa horizon, within individual RPO analyses, radiocarbon ages in the individual thermal fractions were indistinguishable (p > 0.1). The flat age distributions indicate there is no relationship between age and thermal stability (E) in the upper horizon (> 25 cm). Deeper in the soil profile higher µEf values were associated with older radiocarbon ages, with slopes progressively steepening with depth. In the deepest (Bg) horizon, there was the largest, yet modest change in Fm of 0.06 (626 radiocarbon years), indicating that older OC is slightly more thermally stable.

Freeze-thaw and dry-wet events reduce microbial extracellular enzyme activity, but not organic matter turnover in an agricultural grassland soil

Soils in temperate agroecosystems are frequently exposed to extremes of moisture and temperature during which time soil functioning may be negatively affected. The aim of this study was to directly compare the effects of a single dry-wet or freeze-thaw (−5 °C or −20 °C) cycle on extracellular enzyme activity and soil organic matter turnover. We measured the activity of six enzymes before and after imposing the freeze-thaw or dry-rewet events. Our results showed that drying had a much greater impact on total enzyme activity than a −20 °C freezing event (38 vs. 10% reduction, respectively), while freezing at −5 °C had no appreciable effect. Enzyme activity recovered back to control levels relatively quickly which we ascribe to de novo exoenzyme production (within 3 d for the −20 °C freeze-thaw treatment and 14 d for the dry-wet treatment). We added 14C-labelled plant residues to the soil prior to imposing the same thermal or moisture stress events. Monitoring residue decomposition before and after imposing the treatments indicated that none of the stress regimes greatly affected organic matter turnover rates. Our results did reveal, however, a pulse of 14CO2 which was produced during the drying and freezing events themselves. We ascribe this to a shift in microbial metabolism and the production of stress avoidance metabolites (e.g. osmo- and cryo-protectants, membrane lipids). Our work highlights that extreme weather events may affect exoenzyme activity, however, these responses are transitory and are unlikely to greatly affect soil organic matter cycling unless they occur at high frequency.

Phosphorus fractions and speciation in rural and urban calcareous soils in the semiarid region of Sulaimani city, Kurdistan, Iraq

The phosphorus (P) status of soils depends, among other influencing factors such as soil properties, on P content and availability. These vary with the P distribution among species of different mobility and chemical behavior. The P distribution, in turn, depends on the parent rock material and pedoclimate, and may differ between land uses. This study assesses the impact of urbanization on P contents, fractions and speciation in rural versus urban soils in the region of Sulaimani city, Iraq. Topsoil samples from calcareous soils experiencing different human impact intensity were taken in and around Sulaimani city, spanning rural soils from near-natural grassland, riverbanks and arable land, and urban soils from city parks, roadsides and industrial areas. Changes in P status and speciation were determined by combining P fractionation through sequential chemical extraction and P K-edge XANES spectroscopy. The sequence of P contents in the different fractions was (percentage of total P extracted): citrate bicarbonate-P (0.9%) < NaOH-P (1.7%) < NaHCO3-P (2.6%) ≤ citrate bicarbonate dithionate-P (3.8%) < residual P (36.2%) < HCl-P (54.8%). The dominance of stable Ca-phosphates (HCl-P) in all samples is typical for calcareous soils. The P distribution among fractions differed considerably in rural versus urban soils. Poor correlations of P fractions in urban soils with other soil parameters indicated that the P forms and contents were severely altered by human impact. P XANES analyses revealed that P speciation was dominated by P associated with Ca, especially in the urban area, and that irrigated arable land use increased the portions of P associated with pedogenic oxides. Soils of the semiarid regions in and around Sulaimani city exhibit a pattern of P fractions and species typical for calcareous soils. This pattern is clearly altered by human impacts such as agricultural land use and especially urban activities. It is concluded that the P inputs and turnover in urban soils are substantially decoupled from natural processes.

Logging of rainforest and conversion to oil palm reduces bioturbator diversity but not levels of bioturbation

Anthropogenic habitat change is a major driver of species extinctions and altered species communities worldwide. These changes are particularly rapid in the tropics, where logging of rainforests and conversion to agricultural habitats is widespread. Because species have varying effects on their abiotic environment, we expect shifts in species composition to drive changes in ecosystem processes. One important ecosystem process is animal-driven bioturbation: the turnover of soil material by soil-dwelling organisms. We developed a protocol for measuring aboveground bioturbation, and assessed how bioturbation rates and standing amounts of aboveground bioturbated soil change as primary tropical rainforests are logged and converted to oil palm plantation. By identifying the animals that created soil structures, we assigned bioturbation activity to different soil-dwelling groups. Across all habitats, most standing bioturbated soil was generated by termites (97.0%), while short-term, small-scale bioturbation was mainly generated by earthworms (87.3%). The species diversity of social insects (ants and termites) involved in bioturbation was higher in primary forest than in either logged forest or oil palm plantation. However, neither standing bioturbated soil, nor short-term bioturbation rate differed among habitats. Unexpectedly, in primary forest, high levels of bioturbation were associated with low bioturbator diversity. This was because two termite species, where present, conducted nearly all bioturbation. There was no relationship between levels of bioturbation and diversity in the other habitats. Our results emphasize the importance, across all habitats, of termites for generating standing aboveground soil structures, and earthworms for short-term soil turnover. In oil palm plantation, bioturbation relies on a smaller number of species, raising concerns about future environmental change and consequent species loss.

Using digital polymerase chain reaction to characterize microbial communities in wetland mesocosm soils under different vegetation and seasonal nutrient loadings

Constructed wetlands are multi-functional systems that can effectively store and transform pollutants primarily through natural processes. However, the removal of nitrogen pollutant by wetlands is highly variable, likely due to a combination of factors such as plant species-specific assimilation behavior, the effects of soil microbial diversity, and variable nitrogen inputs. In this study, the effects of plant species richness (i.e., number of plant species in a system) and seasonal nutrient loading (i.e., nitrogen fertilization) on the microbial community responsible for regulating nitrogen turnover in wetland mesocosm soils was investigated. Digital polymerase chain reaction was used to quantify bacterial abundance. Principal component analysis was employed to identify dominant patterns within the data, and resampling-based analysis of variance was used to assess statistical significance of any observed differences caused by fertilization, season, and/or plant species richness. Results indicated that fertilization or season, which was convolved with fertilization, was the dominant factor influencing the microbial community in the study environment. The effects of plant species richness were more nuanced. Its greater richness significantly impacted the abundance of only a subset of bacterial groups (i.e., the ammonia oxidizing bacteria, Nitrospira spp. of nitrite-oxidizing bacteria, and comammox, but not the denitrifying bacteria).

Phosphorus forms affect the hyphosphere bacterial community involved in soil organic phosphorus turnover.

Interactions between bacteria and arbuscular mycorrhizal (AM) fungi play a significant role in mediating organic phosphorus (P) transformations and turnover in soil. The bacterial community in soil is largely responsible for mobilization of the soil organic P pool, and the released P is taken up by extraradical AM hyphae, which mediate its use for plant growth. However, the functional microbiome involved in organic P mineralization in the hyphosphere remains poorly understood. The aim of this study was to determine how AM hyphae-associated bacterial communities related to P turnover in the hyphosphere of leek (Allium porrum) respond to different forms of soil P. Using a compartmented microcosm, leek was grown with the AM fungus Funneliformis mosseae, and the extraradical mycelium of F. mosseae was allowed to grow into a separate hyphal compartment containing either no added P, or P as KH2PO4 or phytin. High-throughput sequencing showed that the alkaline phosphatase (ALP)-harboring bacterial community associated with the AM hyphae was dominated by Sinorhizobium, Bradyrhizobium, Pseudomonas, and Ralstonia and was significantly changed in response to different P treatments, with Pseudomonas showing higher relative abundance in organic P treatments than in control and inorganic P treatments. Pseudomonas was also the major genus harboring the β-propeller phytase (BPP) gene in the hyphosphere, but the BPP-harboring community structure was not affected by the presence of different P forms. These results demonstrate the profound differences in ALP- and BPP-harboring bacterial communities in the hyphosphere at bacterial genus level, providing new insights to link bacteria and biogeochemical P cycling driven in association with mycorrhizal hyphae.

Responses of soil carbon and nitrogen mineralization to nitrogen deposition in tundra zone of the Changbai Mountain, China.

The alpine tundra ecosystem, with low soil inorganic nitrogen (N) availability, has a weak buffer against nitrogen and is susceptible to exogenous N enrichment. Here, with a laboratory incubation experiment, we investigated the response of soil carbon and nitrogen mineralization to N deposition with soil samples from the tundra zone on the northern slope of the Changbai Mountain. We set three N levels, control (CK, 0 kg·hm-2), low N (N1, 25 kg·hm-2), and high N (N2, 50 kg·hm-2), with N being added as NH4NO3. The results showed that N addition had no significant effect on soil C mineralization rate, but significantly affected the accumulation of soil C minera-lization. The N2 treatment inhibited soil C mineralization. After the 40 d incubation, soil inorganic N content increased with increasing N addition. After the 80 d incubation, soil inorganic N content in the N2 and N1 was similar and significantly higher than that of CK. Those results indicated that N addition promoted soil N mineralization. The soil microbial biomass C and N in the N1 was higher than that in the N2 and CK, indicating that low N input had stronger effects on soil microbial activity. Increasing N deposition might accelerate C and N turnover in the tundra soils and enhance the soil inorganic N content. While it could provide more N for plants, it may increase the risk of N loss.

Grazing and enclosure alter the vertical distribution of organic nitrogen pools and bacterial communities in semiarid grassland soils

Different grazing management practices have a significant impact on the sustainability of grassland ecosystems. This study invested the vertical distribution of soil nitrogen (N) forms and soil microbial community structures in a semiarid grassland ecosystem under different grazing management practices in Inner Mongolia. Soil samples were collected from three semiarid grassland plots subjected to different long-term management practices namely, free grazing (FG) and two different periods of enclosure (E83, enclosed since 1983 and E97, enclosed since 1997). The soil organic nitrogen (N) pools were analyzed by classical methods, and the bacterial community was assessed by PCR-DGGE and high-throughput sequencing. The surface soil N-supplying capacity was in the order of E97 ≥ E83 ≥ FG. The soil ammonium N, amino N, and N-supplying capacity were greater in the enclosed plots than in the FG plot. Additionally, the 0–40-cm soil layer showed the influence of different management practices on the soil properties. The structure and diversity of the soil microbial community also varied with the management type. The soil organic N composition was significantly related to the soil bacterial community structure and microbial categories. An appropriate number of years of fencing helps to improve the soil surface nutrient status, whereas overgrazing and prolonged enclosure are not conducive to the restoration of soil nutrients. Different grazing management practices can affect the microbial community structure and turnover of soil N in grasslands.

Changing precipitation exerts greater influence on soil heterotrophic than autotrophic respiration in a semiarid steppe

Future precipitation change is anticipated to have a profound influence on ecosystem carbon (C) cycling, especially soil respiration, the largest C flux from the terrestrial ecosystem to the atmosphere. Due to different substrate sources and biological processes, the heterotrophic (SRh) and autotrophic (SRa) components of soil respiration (SRtot) may respond to changing precipitation in different ways. Determining the differential responses of SRh and SRa to precipitation will facilitate our evaluation of soil C storage and stability under future precipitation change. Here, a 3-year precipitation manipulation experiment with 5 levels of precipitation (±60%, ±30% and ambient growing season precipitation) was conducted in a semiarid steppe to determine the influence of precipitation on soil respiration. Results showed that SRtot increased nonlinearly with increasing water supply, in which, SRh increased much more than SRa. Consequently, the ratio of SRh to SRtot was enhanced significantly with increasing precipitation, i.e., from 41% to 62% as precipitation increased from 122 mm to 408 mm. Structural equation modeling analysis indicated that changes in SRh and SRa were predominated by the soil water content (SWC) and plant growth, respectively. The stronger effects exerted by changes in precipitation on SWC compared with plant growth contributed to the greater change in SRh than in SRa. Additionally, SRa exhibited a higher temperature sensitivity than SRh. Thus, the increased soil temperature in the drought treatments had greater influence on SRa than SRh, and greatly offset the influence of drought stress on SRa, leading to the smaller change in SRa compared with SRh. Our study highlights the different responses of the two soil respiration components to changing precipitation. These results indicate that future increases in precipitation in semiarid regions will increase soil heterotrophic respiration which may accelerate the turnover of soil C, and further affect the stability of soil C stock.

Welcome to the Atta world: A framework for understanding the effects of leaf‐cutter ants on ecosystem functions

Abstract Leaf‐cutter ants are a prominent feature in Neotropical ecosystems, but a comprehensive assessment of their effects on ecosystem functions is lacking. We reviewed the literature and used our own recent findings to identify knowledge gaps and develop a framework to quantify the effects of leaf‐cutter ants on ecosystem processes. Leaf‐cutter ants disturb the soil structure during nest excavation changing soil aeration and temperature. They mix relatively nutrient‐poor soil from deeper layers with the upper organic‐rich layers increasing the heterogeneity of carbon and nutrients within nest soils. Leaf‐cutter ants account for about 25% of all herbivory in Neotropical forest ecosystems, moving 10%–15% of leaves in their foraging range to their nests. Fungal symbionts transform the fresh, nutrient‐rich vegetative material to produce hyphal nodules to feed the ants. Organic material from roots and arbuscular mycorrhizal fungi enhances carbon and nutrient turnover in nest soils and creates biogeochemical hot spots. Breakdown of organic matter, microbial and ant respiration, and nest waste material decomposition result in increased CO2, CH4, and N2O production, but the build‐up of gases and heat within the nest is mitigated by the tunnel network ventilation system. Nest ventilation dynamics are challenging to measure without bias, and improved sensor systems would likely solve this problem. Canopy gaps above leaf‐cutter ant nests change the light, wind and temperature regimes, which affects ecosystem processes. Nests differ in density and size depending on colony age, forest type and disturbance level and change over time resulting in spatial and temporal changes of ecosystem processes. These characteristics remain a challenge to evaluate rapidly and non‐destructively. Addressing the knowledge gaps identified in this synthesis will bring insights into physical and biological processes driving biogeochemical cycles at the nest and ecosystem scale and will improve our understanding of ecosystem biogeochemical heterogeneity and larger scale ecological phenomena. A plain language summary is available for this article.

Impact of wetting and drying cycles on soil structure dynamics

Soil structure is not static but undergoes continuous changes due to a wide range of biotic and abiotic drivers such as bioturbation and the mechanical disturbance by tillage. This continuous alteration of soil structure beyond the pure swelling and shrinking of some stable structure is what we refer to as soil structure dynamics. It has important consequences for carbon turnover in soil as it controls how quickly soil organic matter gets occluded from or exposed to mineralization. So far there are hardly any direct observations of the rate at which soil pores are formed and destroyed.Here we employ are recently introduced labeling approach for soil structure that measures how quickly the locations of small garnet particles get randomized in soil as a measure for soil structure dynamics. We investigate the effect of desiccation crack dynamics on pore space attributes in general and soils structure turnover in particular using X-ray microtomography for repeated wetting-drying cycles. This is explored for three different soils with a range of soil organic matter content, clay content and different clay mineralogy that were sieved to a certain aggregate size fraction (0.63–2 mm) and repacked at two different bulk density levels.The total magnitude of desiccation crack formation mainly depended on the clay content and clay mineralogy. Higher soil organic matter content led to a denser crack pattern with smaller aperture. Wetting-drying cycles did not only effect visible macroporosity (>8 μm), but also unresolved mesoporosity. The changes in macroporosity were higher at lower bulk density. Most importantly, repeated wetting-drying cycles did not lead to a randomization of distances between garnet particles and pores. This demonstrates that former failure zones are reactivated during subsequent drying cycles. Hence, wetting-drying resulted in reversible particle displacement and therefore would not have triggered the exposure of occluded carbon that was not already exposed during the previous drying event.

Metagenomic exploration of the interactions between N and P cycling and SOM turnover in an apple orchard with a cover crop fertilized for 9 years

We investigated the types of metabolism and genes involved with N and P reactions, and SOM turnover as well as their interaction was monitored in an apple orchard by metagenome sequencing. The field experiment included plots with and without a cover crop but with weed control, and two subplots with or without chemical fertilizer. The relative abundances of genes involved with N reactions (nirB, nirD, amoB, gltB, and GDH2), P reactions (phoA, pqqC, and E3.1.3.8), and plant degradation were higher in the cover cropped soil after treated with chemical fertilizer compared with the other treatments. The cover crop had a greater effect on the gene network than the chemical fertilizer and it improved the number of links in the network compared with no cover crop. The betweenness centrality scores showed that the genes involved in N and P reactions were important for SOM turnover as well as for N and P reactions. Chemical fertilizer provided more available N and P for SOM turnover in the cover-cropped soil, thereby improving the soil fertility. Finally, the available N and P contents, SOM, and apple yield were increased by the application of chemical fertilizer and cropping with cover plant due to the promotion of interactions among the N and P reactions (as hypothesized by considering abundances of genes involved in both nutrient cycles), and SOM. Our results provide new insights into the interactions between N and P reactions in soil and SOM turnover from the genetic level in an apple orchard with a cover crop and when subjected to chemical fertilization.

Crop straw incorporation interacts with N fertilizer on N2O emissions in an intensively cropped farmland

Nitrogen (N) fertilization and straw incorporation strongly influence nitrous oxide (N2O) emissions from agricultural fields. An in-situ micro-plot experiment on intensively farmed winter wheat (Triticum aestivum L.) was conducted to investigate the source and rate of N2O emissions from soils following labeled 15N fertilization with and without straw incorporation. Four treatments, i.e., no N fertilizer and no straw incorporation (N0S0), straw incorporation only (N0S1), N fertilizer only (N1S0), and N fertilization plus straw incorporation (N1S1), were established in the experiment. The N2O emissions mainly occurred after N fertilization and lasted for approximately 1–2 weeks, accounting for 60%–67% of the wheat seasonal N2O emissions. Within the 6 days after basal fertilization and 2–4 days after top-dressing, most of the N2O fluxes (>50%) were derived from fertilizer. Thereafter, soil-derived N2O dominated the total N2O emissions and within 10–20 days after N fertilization, fertilizer-derived N2O became negligible. Fertilizer N and soil N both accounted for 40%–60% of the seasonal N2O emissions, which may be explained by the high soil N stock due to long-term high N fertilization in the region. This implies the similar roles of soil N pool and fertilizer N in N2O generation under intensively farmed soils. The N fertilization had a significant priming effect on the turnover of soil N, which contributed 21.02%–50.47% of the total N2O emissions. During the basal fertilization/first irrigation event, straw incorporation significantly (P < 0.05) stimulated CO2 fluxes both in N-fertilized and non-N-fertilized plots; however, after the top-dressing/second irrigation event, the significant increase of CO2 fluxes induced by straw incorporation was only observed in the N-fertilized treatment. Straw incorporation interacted with N fertilization, and tended to enhance N2O emissions in the basal fertilization and lower N2O emissions in the top-dressing period. In N-fertilized plots, the seasonal N2O emissions from straw-incorporated and straw-removed treatments were similar, indicating that straw incorporation enhanced the N supply without increasing the N2O emissions. Our study highlights that there are significant benefits of straw incorporation to soil fertility improvement; however, the long-term impacts of straw incorporation on greenhouse gas emissions should be further examined.

The oxygen isotope composition of bioavailable phosphate in soil reflects the oxygen isotope composition in soil water driven by plant diversity effects on evaporation

Abstract Despite the importance of phosphorus (P) for organisms, the contributions of different processes delivering P into the bioavailable pool in soil have not yet been unraveled mainly due to methodological constraints in separating physico-/geochemical from biological processes. Stable O isotope analyses of phosphate might help to disentangle these processes and recent method improvements now allow an application in terrestrial ecosystems. Plant diversity influences not only plant P uptake but also biological P release mechanisms in soil thereby potentially affecting stable isotope ratios of bioavailable P (δ18OPi). We calculated the degree of enzyme-mediated O incorporation into phosphate, determined the contributions of different biological processes (Pi mobilization from soil organic matter, microbial P turnover), and tested for plant diversity effects on δ18O values of phosphate and of soil water as well as on the processes involved in O incorporation. We added 18O-enriched and 18O-depleted water to separated subplots of 27 study sites differing in plant species richness and plant functional group composition. δ18O values of phosphate and soil water were measured before and six days after labeling. We calculated mixing models based on isotope fractionation published so far. The O incorporation factor a reflecting the proportional incorporation of O atoms from labeled water into phosphate molecules under field conditions averaged 0.33 (0.15–0.70). δ18O values of bioavailable phosphate indicated microbial P processing i.e., turnover and subsequent release of P in soil. Plant species richness was related to δ18O values of soil water via evaporation. Because of the complete exchange of O atoms in a phosphate molecule with ambient water during microbial P turnover in soil, plant species richness effects were also visible in δ18O values of bioavailable phosphate. In conclusion, our approach yielded new insights into the role of microorganisms in supplying bioavailable P. For the first time we could show that plant-community induced microclimate shapes δ18O values of bioavailable phosphate in soil indirectly by affecting δ18O values of soil water with which phosphate exchanges O. Therefore, stable O isotope analyses in phosphate represent a useful tool for unraveling complex plant-soil-microorganism interactions in the P cycle.

The shift of bacterial community composition magnifies over time in response to different sources of soybean residues

The transformation of plant residues by soil microorganisms contributes significantly to soil carbon (C) sequestration. Although the contribution of different soybean residue sources (leaf, stalk and root) to soil organic C have been quantified in the highly productive Mollisols, the temporal dynamics of microbial community composition during the transformation has not. This study examined the effect of the soybean residues and incubation time on the bacterial community in a Mollisol during 150 days of incubation. Compared to the non-residue control, the presence of soybean residues increased abundances of Bacteroidetes, Actinobacteria and Firmicutes, but decreased those of Proteobacteria and Acidobacteria 15 days after residue amendment. The bacterial community composition differed between the residue sources after 60 days of incubation with the abundance of Niastella increasing in the leaf (from 4.5% to 19.1%) and stalk (from 6.5% to 25.7%) but not root treatments. The abundance of Bacillus significantly decreased in the stalk (from 12.2% to 2.2%) and root (from 4.0% to 1.6%) but not leaf-residue treatments. The abundance of bacterial genera was significantly associated with soil chemical variables including soil C, N, and pH. Overall, chemical variables drove the temporal response of the bacterial community succession, which provides insight into bacterial contribution to C turnover in soils.

Boreal forest biomass accumulation is not increased by two decades of soil warming

Increased soil organic matter decomposition with increasing temperature has been hypothesized to enhance soil nitrogen availability, consequently stimulating forest biomass production and offsetting decomposition-induced soil carbon losses1–5. This projection, however, is based on evidence gathered from short-term studies (≤10 years)2,3,5. The key question for carbon sequestration is whether such responses are transient or persist over long forest rotation periods. Here we report on biomass production in a typical nitrogen-limited boreal Picea abies forest, exposed to 18 years of soil warming manipulation (+5 °C) at a plot scale (100 m2). We show that two decades of soil warming elicited only short-duration growth responses, thus not significantly increasing aboveground biomass accumulation. Furthermore, in combination with published work from this forest, our results suggest that increased decomposition is slight and ephemeral, and increased fine root production and turnover in deeper soil may be greater than increased decomposition, netting slightly more biomass, perhaps conserving the soil carbon stock. Thus, this long-term study does not support the notion that the projected increase in soil temperatures will cause either an increased carbon loss with decomposition or a compensatory growth increase from nitrogen mineralization. Nearly two decades of data from a boreal forest soil warming experiment (+5 °C) show no significant increase in aboveground biomass accumulation beyond an initial transitory response.

Effect of carbon and nitrogen mobilization from livestock mortalities on nitrogen dynamics in soil

Carcass decomposition in the soil can be an important source of nutrients such as nitrogen (N) by affecting N turnover in soils. The objective of this research was to estimate N input from decaying swine carcasses, thereby evaluating the impact of carrion decomposition on N dynamics in soil. Carcass decomposition using recently culled (<6 h) swine carcasses was carried out in a reactor filled with agricultural soil. Soil samples, collected four times (at 0, 10, 30 and 60 days after carcass placement) from the reactor were used in a tracer experiment to quantify the changes in soil nutrients and N dynamics. Tracer incubation experiments were carried out for seven days using 14C-labelled L-alanine (C3H7NO2) to investigate key N cycling processes in the soil. Mortalities were a significant source of N and carbon (C), providing an average of 42 and 236 g/kg, respectively, to the soil directly below the decomposing carcasses. There was also a significant and long-term input of amino acids (ca. 11 mg/kg) into the soil. The abundance of N increased the microbial turnover of labile N substances in the tracer experiment. Based on results from this study, it has been demonstrated that decaying carcasses provide a significant and long-lasting localized resource with the potential to contribute to soil N cycling. Therefore, it is important to develop guidelines on the management of carcass burial farmland using soil from burial pits as a nutrient supplement where biosecurity is assured.