Upland Soils Research Articles

Isotopic signals in an agricultural watershed suggest denitrification is locally intensive in riparian areas but extensive in upland soils

Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO3−) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N2) or the greenhouse gas nitrous oxide (N2O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO3− available for downstream denitrification. Here we use patterns in the isotopic composition of NO3− observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ15N and δ18O values of NO3−, as well as increasing δ15N values with decreasing NO3− concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO3− from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO3− mass balance with δ15N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N2O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase.

The Synergetic Effect of Soil Amendments on Reducing Bioavailable Heavy Metals and Greenhouse Gas Emissions from Upland Soil

Heavy metal pollution and greenhouse gas (GHG) emissions from soil are two major detrimental sources in the agriculture environment because of concerns about crop safety and global warming. Applying amendments on site is a common technique used for heavy metal remediation and the reduction in GHG emissions. This study aims to evaluate the effect of different amendments on the reduction in both bioavailable heavy metals and GHG emissions from soil. Four different amendments, namely bottom ash (BA), sintered material (SM), sintered material combined with lime (SM + L), and FeO (SM + FeO) were applied to upland fields, followed by maize (Zea mays L.) cultivation from April to October. Subsequently, SM + FeO treatment demonstrated the highest bioavailability reduction efficiency for As at 79.1%, and SM + L treatment had a high efficiency for the reduction in Cd and Pb by 64.6% and 41.4%, respectively. SM + FeO treatment also reduced bioaccumulated As and Pb in maize grain by 59.3% and 66.2%, respectively. Furthermore, SM + FeO treatment demonstrated the highest reduction efficiency for cumulative N2O emissions by 70.7%, compared to the control soil. Among the four different amendments, the efficiency of heavy metal and GHG emission reduction was in the following order: SM + FeO > SM + L > SM > BA. Overall, SM combined with FeO is a promising amendment for reducing and managing both heavy metal pollution and GHG emissions in soil.

Characteristics of Microbial Utilization for Crop Residue-Derived C in Paddy and Upland Soils

Two typical subtropical agricultural soils, a flooded paddy soil and its adjacent upland, were collected and then incubated with or without 13C-labeled crop residue (maize straw) for 40 days. During the incubation, the mineralization rate of the crop residue was monitored, and the 13C incorporated into fungal and bacterial phospholipid fatty acid (PLFA) was quantified. At the early stage (0.25-1 days), the mineralization rate of crop residue was faster in paddy soil than that in upland soil, whereas the opposite trend was observed from 2 to 20 days. At the late stage (21-40 days), the mineralization rate was similar in both soils. At the end of incubation, 11% of the total crop residue was mineralized in paddy soil, which was about half of that in upland soil (20%). Although paddy soil had a higher amount of microbial biomass (indicated by total PLFA), the total amounts of 13C-PLFA were comparable in both soils, and the enrichment ratio (proportion of 13C to total C in PLFA) was lower in paddy soil than that in upland soil. This indicated that the microbial community in paddy soil was less active in the uptake of crop residue C than that in upland soil. During the incubation, the residue-derived 13C was mainly distributed in bacterial PLFA (up to 86% of total 13C-PLFA, including 59% in gram-positive and 27% in gram-negative bacteria) in paddy soil, and up to 75% of total 13C-PLFA distributed in fungal PLFAs was in upland soil. Thus, bacteria dominated the utilization of crop residue in paddy soil versus fungi in upland soil. Compared with that in upland soil, the microbial activity was suppressed in the anaerobic condition caused by flooding in paddy soil, with a stronger inhibition of fungi than bacteria. Considering the discrepancies of life strategies and necromass turnover between bacteria and fungi, the different dominant microbial groups in the utilization of crop residue in water-logged and well-drained conditions could lead to the distinct accumulation and stabilization of microbial-derived organic matter in paddy and upland soils.

Impact of land use change on soil methane fluxes and diffusivity in Pampean plains, Argentina

Upland soils are the main methane (CH4) biological sink, and may be affected by land-use change. Changes in land uses and soil management affect soil properties that control diffusion of gases, which in combination with microbial activity, determine CH4 flux (fCH4) through the soil. Net CH4 fluxes and diffusivity -estimated by the CH4 diffusion coefficient- were measured in three common land uses typical from Pampean plains, South America (natural grassland NG; Eucalyptus globulus Labill. afforestation E; and agricultural land AL: oat, soybean and red clover in successive cultivation) during two years (March 2017–March 2019). Methane fluxes in the soil-atmosphere interface were measured using the static chamber technique, and a diffusion model was applied to estimate soil CH4 diffusivity from soil porosity. We aimed to quantify the effect of land use change (both E and AL vs. NG, the reference system) on fCH4 and gas diffusivity due to changes in the soil parameters. Soils were net sinks in the three land uses, with mean CH4 flux higher in the afforestation, intermediate in the natural grassland and lower in the agricultural land (− 10.99 ± 5.85, − 8.9 ± 5.32 and − 4.58 ± 4.19 ng CH4 m−2 s−1, respectively). CH4 fluxes varied significantly through seasons and space coinciding with variations in water-filled pore space and air-filled pore space variables (ρ > 0.7 and <−0.7 respectively; p < 0.05). Land-use change metric for methane flux ΔfCH4 was − 2.1 ± 3.7 and 4.4 ± 2.5 for NG-E and NG-AL, respectively, indicating a significant increment in net CH4 uptake when the natural grassland is afforested and a decrease when it was converted to agricultural use. This change was mainly explained by changes in soil physical properties (bulk density, soil water content, WFPS and air filled porosity). In relation to this, soil CH4 diffusion coefficient followed the same pattern as fCH4 (0.024 ± 0.011; 0.015 ± 0.007 and 0.008 ± 0.007 cm2 s−1 for E, NG and AL respectively); and allowed us to recalculate mean CH4 fluxes. Theoretical and in situ measured CH4 fluxes were similar and followed the same patterns across land uses, suggesting the possibility to determine CH4 fluxes by means of simple measures of soil properties (bulk density and soil water content) and soil CH4 gradient concentration.

Isolation of a Novel Endophytic Bacillus Strain Capable of Transforming Pentachlorophenol and Structure Determination of Pentachlorophenol Phosphate Using Single-Crystal X-ray Diffraction

A novel approach for the remediation of upland soils contaminated with pentachlorophenol (C6HCl5O; PCP) (1), a fungicide, wood perservative, and herbicide, through the exploitation of plant-endophytic bacteria may overcome the existing issues in bioaugmentaion and phytoremidiation. In this study, we isolated the endophytic Bacillus sp. strain PCP15 and determined its metabolite of PCP (1). This strain degraded 8.03 μmol L-1 PCP (1) within 24 h and generated the novel metabolite PCP phosphate (3). The PCP15 strain showed nearly complete growth inhibition of 20 μmol L-1 PCP (1). In contrast, PCP15 showed resistance to PCP phosphate (3), indicating that the phosphorylation of PCP, which has never previously been reported in organisms, contributed to the detoxification of PCP (1) in bacterial cells. Our results show the potential for practical application of this strain in hybrid remediation of PCP (1)-contaminated soils and reveal a novel PCP (1) detoxification mechanism in organisms.

Complexation and reduction of soil iron minerals by natural polyphenols enhance persulfate activation for the remediation of triphenyl phosphate (TPHP)-contaminated soil

Peroxydisulfate-based in-situ chemical oxidation (PDS-ISCO) is a promising technology for soil remediation. The weak potential of soil constituents for PDS decomposition results in the low degradation efficiency of soil contaminants. Herein, we propose a new soil remediation strategy by accelerating the activation efficiency of PDS using chelating and reducing reagents (CRs) to complex and reduce soil Fe-minerals. Experimental results demonstrated that the introduction of tea polyphenols (TP) into soil-PDS system could effectively improve the reduction of Fe-minerals and the degradation of triphenyl phosphate (TPHP) in soil. With increasing TP concentration from 0 to 40 mM, the degradation efficiency of TPHP increased from 10.8% to 58.6% in an upland soil (T1 soil). TP could directly complex and reduce the solid-phase Fe-minerals, leading to a significant increase of Fe(II) content, especially the insoluble Fe(II) content, which could further induce the PDS activation. Results of radical quenching experiments, electron paramagnetic resonance (EPR) analysis, and kinetic solvent isotope effect (KSIE) evaluation showed that sulfate radicals (SO4•−, especially surface-bound SO4•−), hydroxyl radicals (HO•, especially surface-bound HO•), and singlet oxygen (1O2) played major roles in TPHP degradation, with the estimated contributions of 39.03%, 43.99%, and 16.98%, respectively. The higher content of high active Fe fractions, such as amorphous Fe, the larger yields of Fe(II) species and TPHP degradation. Increased content of soil organic matter (SOM) and pH value from 2.5 to 8.0 may reduce TPHP degradation. The effects of inorganic anions and Mn-minerals, presented in natural soil, on TPHP degradation were very limited. Simulated in-situ soil remediation using soil column confirmed that this strategy is suitable for the large-scale remediation of contaminated soil.

Effects of Stalk Incorporation on Soil Carbon Sequestration, Nitrous Oxide Emissions, and Global Warming Potential of a Winter Wheat-Summer Maize Field in Guanzhong Plain

The net greenhouse gas emissions from upland soils, as indicated by global warming potential (GWP), mainly depend on the soil carbon sequestration and nitrous oxide (N2O) emissions. The annual changes in surface (0-20 cm) soil organic carbon (SOC) content from 2010 to 2017 and the N2O emissions from 2014 to 2017 were measured within a long-term fertilization experiment. The objective was to quantify the effect of stalk incorporation on the soil carbon sequestration, annual N2O emissions, and GWP of a winter wheat-summer maize field in the Guanzhong Plain. The field experiment included three treatments:conventional fertilization (CF), conventional fertilization plus maize stalks (CFS), and an unfertilized control (CK). The CF and CFS treatments received the same amount of urea per year, with nitrogen (N) input at 165 kg·hm-2 and 188 kg·hm-2 in the winter wheat season and summer maize season, respectively. The CF treatment retained the stubbles (about 10 cm above ground) when harvesting the winter wheat and summer maize crops. The CFS treatment retained the same wheat stubbles and all maize stalks (containing approximately 40 kg·hm-2 of N). The CK treatment was unfertilized throughout the year, with the stubble management the same as that in the CF treatment. The results showed that the CK treatment displayed few changes in SOC content and low N2O emissions, with GWP varying from 0.04 to 0.11 t·(hm2·a)-1. The SOC contents in the CF and CFS treatments increased linearly with the fertilization years (P<0.001), and their SOC sequestration rates were 0.69 t·(hm2·a)-1 and 0.97 t·(hm2·a)-1, respectively. The N2O emissions from the CF and CFS treatments varied from 1.65 to 5.36 kg·(hm2·a)-1 and from 3.08 to 7.73 kg·(hm2·a)-1, respectively. The annual N2O emissions from the CFS treatment were 43%-94% higher than those from the CF treatment, whereas the difference was only significant between 2015 and 2016 (P<0.05). The GWP of the CF and CFS treatments varied from -1.95 to -0.28 t·(hm2·a)-1 and from -2.59 to -0.35 t·(hm2·a)-1, respectively. The cumulative GWP of the CFS treatment was 42% lower than that of the CF treatment between 2014 and 2017. In summary, the studied winter wheat-summer maize field acted as a sink of greenhouse gases under the conventional fertilization regime. The stalk incorporation further favored greenhouse gas mitigation despite the trade-offs between SOC sequestration and N2O emissions.

Biochar Application Can Mitigate Nh3 Volatilization in Acidic Forest and Upland Soils But Stimulates Gaseous N Losses in Flooded Acidic Paddy Soil

Biochar Application Can Mitigate Nh3 Volatilization in Acidic Forest and Upland Soils But Stimulates Gaseous N Losses in Flooded Acidic Paddy Soil

Bituminous substances in soils of Baikal Rift zone

Parameters of hydrocarbon complex of the soils of lacustrine-alluvial plain and sandy uplands of the Baikal rift zone within the Barguzin Depression have been studied. Data were obtained on organic (non-carbonate) carbon, chloroform and hexane bitumoids, as well as on individual polycyclic aromatic hydrocarbons. The content, composition and properties of bitumoids vary depending on the intensity of endogenous activity. In soils located in the zone of active unload of hydrothermal waters and hydrocarbon fluids, atypical accumulation of organic carbon was noted; the concentrations of bitumoids do not correlate with its content. High values of the bitumoid coefficient, significant level of polycyclic aromatic compounds and their diversity, in combination with salts accumulation, indicate an additional influx of hydrocarbons and instability of the soil hydrocarbon system. The level of the indicators shifts towards background values in the soils of sandy hills with a complex combination of exogenous and endogenous factors of soil forming. Studied parameters of the soil hydrocarbon system in aggregate can serve as markers of the intensity of endogenous processes.

Anthropogenic Pedofeature in Andosols in Santome Shinden, One of the Representative Sites of the Satoyama Environment in Japan

Santome Shinden, a representative site of the Satoyama landscape in Japan, has its origins in 1696 when the common land of wild grass on the Musashino plateau was developed into a strip-shaped land division, which consists of residential area, upland field, and secondary forest. We investigated soils with different land use over 300 years; they are under the secondary forest, and in the upland field where fallen leaves compost derived from secondary forest has been applied continuously for about 300 years since the development. The measured exchangeable cation values suggested that the nutrients in the secondary forest were taken out to the field as fallen leaves. On the surface layer of the upland field soil, characteristic granules of dark brown coated peds (DBC peds) were developed strongly. However, on the surface layer of the secondary forest soil, the DBC peds were not found. Electron probe micro analyzer analysis showed that the outside (dark brown part) of the DBC peds, which was observed only in the upland fields was rich in carbon. The dark brown coated pedofeature was suggested to have an anthropogenic effect due to the long-term application of fallen leaves compost. The anthropogenic activities, which were fallen leaves compost application and cultivation for about 300 years, were thought to affect the soil micromorphology. Therefore, the peds newly found in the Santome shinden field were considered to be a soil microstructure that symbolizes anthropogenic activities.

Vertical Distribution and Microscopic Sequestration Mechanisms of Soil Organic Carbon in Upland and Paddy Soils

Vertical Distribution and Microscopic Sequestration Mechanisms of Soil Organic Carbon in Upland and Paddy Soils

Metagenome-assembled Genomes of Six Novel Ammonia-oxidizing Archaea (AOA) from Agricultural Upland Soil.

Ammonia-oxidizing archaea (AOA), key players in agricultural upland soil nitrification, convert soil ammonium to nitrite. The microbial oxidation of ammonia to nitrite is an important part of the global biogeochemical nitrogen cycle. In the present study, we recovered six novel AOA metagenome-assembled genomes (MAGs) containing genes for carbon (C) fixation and nitrogen (N) metabolism by using a deep shotgun metagenomic sequencing strategy. We also found that these AOA MAGs possessed cobalamin synthesis genes, suggesting that AOA are vitamin suppliers in agricultural upland soil. Collectively, the present results deepen our understanding of the metabolic potential and phylogeny of AOA in agroecosystems.

Presence, sources, and risk assessment of heavy metals in the upland soils of northern China using Monte Carlo simulation

The spatial dynamics of heavy metal contamination in the upland soils of northern China are relatively unknown, despite the region’s high contribution to the national grain output. In this study, the concentrations of As, Cd, Co, Cr, Cu, Mn, Pb, Sb, Sc, Ti, and Zn and subsequent ecological and human health risks were investigated in three major grain producing areas (Hexi Corridor, L1; Hetao irrigation area, L2; and eastern Inner Mongolia, L3) of northern China. Among the heavy metals, Ti had the highest average concentration of 3.02 g/kg, followed by Mn (470 mg/kg), Cr (56.6 mg/kg), Zn (34.3 mg/kg), Pb (19.4 mg/kg), Cu (17.8 mg/kg), Co (9.66 mg/kg), Sc (7.26 mg/kg), As (5.35 mg/kg), Sb (0.73 mg/kg), and Cd (0.17 mg/kg). Generally, the heavy metal concentrations decreased from west to east (L1 > L2 > L3) across northern China. Moreover, three potential sources of the heavy metal were distinguished, including natural process, anthropogenic activities (industrial development and agricultural cultivation), and atmospheric deposition. Although the contamination of the single metal (including Cd, Cr, Cu, and Pb) was moderate in L1 and L2, the combined contamination was low in the upland soils. It was noted that Cd posed a moderate to considerable ecological risk on the upland soils in northern China. This metal was the most sensitive factor in assessing the combined ecological risk, with a contribution rate of 91.56–94.84%. Considering the ingestion exposure, the current concentrations of the metals posed minimal risks to human health. Furthermore, children experienced higher health risks than adults. Present study analyzed the probabilistic distribution of contamination, ecological, and health risk of heavy metals in upland soils of northern China, providing fundamental information for better agricultural heavy metal pollution assessment in China.

Atmospheric Methane Consumption and Methanotroph Communities in West Siberian Boreal Upland Forest Ecosystems

Upland forest ecosystems are recognized as net sinks for atmospheric methane (CH4), one of the most impactful greenhouse gases. Biological methane uptake in these ecosystems occurs due to the activity of aerobic methanotrophic bacteria. Russia hosts one-fifth of the global forest area, with the most extensive forest landscapes located in West Siberia. Here, we report seasonal CH4 flux measurements conducted in 2018 in three types of stands in West Siberian middle taiga–Siberian pine, Aspen, and mixed forests. High rates of methane uptake of up to −0.184 mg CH4 m−2 h−1 were measured by a static chamber method, with an estimated total growing season consumption of 4.5 ± 0.5 kg CH4 ha−1. Forest type had little to no effect on methane fluxes within each season. Soil methane oxidation rate ranged from 0 to 8.1 ng CH4 gDW−1 h−1 and was negatively related to water-filled pore space. The microbial soil communities were dominated by the Alpha- and Gammaproteobacteria, Acidobacteriota and Actinobacteriota. The major group of 16S rRNA gene reads from methanotrophs belonged to uncultivated Beijerinckiaceae bacteria. Molecular identification of methanotrophs based on retrieval of the pmoA gene confirmed that Upland Soil Cluster Alpha was the major bacterial group responsible for CH4 oxidation.

Vertical profiles of methane concentration above and within the canopy of a temperate Japanese cypress forest

Methane (CH 4 ) is one of the major greenhouse gases, and therefore its source identification and quantification are quite important. So far water-unsaturated soils of upland forests are usually recognized as sinks, but recent increasing evidence supports tree stems in certain environments can be a source of CH 4 , and thereby research on the dynamics of CH 4 in forest ecosystems has entered a new phase. In this study, we measured vertical distributions of CH 4 concentrations within and above a forest canopy to examine how they are influenced by local ecological processes and synoptic-to local-scale atmospheric processes. Profile measurements were conducted using a meteorological tower in a temperate Japanese cypress forest with a 30-min time resolution. Time series data of CH 4 concentrations above the canopy at our site showed occasional spikes relative to background levels, and these spikes corresponded with temporal variations in CO 2 concentrations. Backward trajectory analyses showed that most of the air masses associated with the spikes in CH 4 and CO 2 concentrations had continental origins. Seasonal-mean diurnal properties in CH 4 concentrations above and within the canopy were weaker than those for CO 2 concentrations. It was considered likely that nighttime accumulation of CH 4 in a stable surface layer and its convective diffusion into a mixed layer accounted for the diurnal properties in CH 4 , while ecological processes played a major role in the diurnal properties observed in CO 2 levels. Vertical distributions in CH 4 concentration within the canopy showed a monotonic decrease from the canopy to the forest floor, with variations depending on the season. The profiles agreed with soil-atmosphere CH 4 fluxes measured using a closed-chamber method at our site, indicating that the surface soil in upland areas plays a major role for determining the within-canopy distributions of CH 4 . Also, it was inferred that at least at our site, CH 4 emissions from stems and leaves, which has become a hot topic over the recent years, was not significant for cypress trees in upland area enough to influence the within-canopy distributions of CH 4 during the study period. • CH 4 concentrations above and within a cypress forest were measured for one year. • CH 4 and CO 2 concentrations are often affected by continental outflow. • Vertical CH 4 profiles are coherent with CH 4 uptake rates in upland soil. • Trunk and leaves of Japanese cypress do not emit or absorb CH 4 significantly.

Characterizing copper and zinc content in forested wetland soils of North Carolina, USA

Wetlands are often located in landscape positions where they receive runoff or floodwaters, which may contain toxic trace metals and other pollutants from anthropogenic sources. Over time, this can lead to the accumulation of potentially harmful levels of metals in wetlands soils. To assess the potential risk of Cu and Zn buildup in wetland soils in North Carolina, soil data from 88 wetlands were analyzed. In a subset of 16 wetlands, more intensive sampling was conducted. Samples were analyzed for Mehlich 3 Cu and Zn, and a subset of the samples was analyzed for total Cu and Zn. Overall, Mehlich 3 Cu and Zn were low, with mean values of 0.9mg/kg for Cu and 3.2mg/kg for Zn. Warning levels for Mehlich 3 Zn were only exceeded in three of the 88 sites; elevated Mehlich Cu was not observed. Total Cu and Zn were also low, with only a few sites having elevated levels; however, there was not a strong linear relationship between Mehlich 3 and total metals. Mean levels of Mehlich 3 Cu and Zn in wetlands were much lower than for human-impacted upland soils and background threshold concentrations that might be indicative of disturbance were much lower than warning levels for agricultural soils. The very low mobile Zn and Cu in most of these wetlands indicated that these metals do not pose a risk to the biota in most North Carolina wetlands, but wetlands with a direct and significant anthropogenic source of metal contamination could be exceptions.

Paddy soils have a much higher microbial biomass content than upland soils: A review of the origin, mechanisms, and drivers

Many studies have shown that the microbial biomass content in paddy soils is much higher than that in upland soils, but a comprehensive review of the underlying mechanisms and processes is lacking. We conducted a meta-analysis of published literature on the microbial biomass content in continuous paddy soils (>1700 data pairs) and paddy-upland rotation soils (>1100 data pairs) as compared to that in adjacent upland soils (>360 data pairs), measured by the fumigation extraction or fumigation incubation method. The microbial biomass carbon (MBC) content in paddy soils was double that in upland soils. This MBC surplus in paddy soils compared to upland soils was explained by (1) higher input of root C and rhizodeposits by rice plants compared with upland crops; (2) lower oxygen availability and consequently slower microbial turnover; (3) higher microbial C assimilation efficiency in paddy soils; and (4) additional C stabilization on iron (oxyhydr)oxides in paddy soils. The proportion of MBC in total soil organic C in paddy-upland rotation, paddy, and upland soils was 3.5%, 2.5%, and 2.1%, respectively. The higher microbial biomass C/N ratio in paddy soils (12.4 ± 0.11) compared to upland soils (9.9 ± 0.21) reflects greater N losses (through nitrate leaching and denitrification) in relation to slower C losses under anoxic conditions. Despite higher temperature and better water availability, microbial biomass turnover was 1.1–1.6 times slower in paddy soils than in upland soils because of oxygen limitation. Multiple stepwise regression and redundancy analyses showed that microbial biomass in continuous paddy and paddy-upland rotation soils was affected by similar soil factors (such as total N and organic C), whereas microbial biomass in upland soils was mainly affected by pH and the organic C content. Paddy-upland rotation soils undergo oxic–anoxic cycles and consequently can absorb and coprecipitate organic compounds with iron (oxyhydr)oxides as an additional advantage for C stabilization. We conclude that the reduced microbial activity and slower microbial turnover under oxygen-limited conditions lead to nearly two times higher microbial biomass content in paddy than in upland soils.

Soil C, N, P stocks and stoichiometry as related to land use types and erosion conditions in lateritic red soil region, south China

Irrational land-use can accelerate soil erosion by deteriorating soil structure and depleting soil nutrients, especially profoundly alter soil organic carbon (C), total nitrogen (N), total phosphorus (P) contents and stocks. However, it remains unknown how soil C, N, P stoichiometry patterns are affected by land-use type and soil erosion. This study was carried out to examine the soil C:N:P stoichiometry patterns in woodland (WL), upland (UL), and paddy (PD) at soil depths of 0–10, 10–20, and 20–30 cm under collapsed gully erosion (CGE) and sheet erosion (SE) conditions in lateritic red soil region. Compared to WL, C content and stock decreased by 19.1–31.9 g kg−1 and 12.8–31.7 Mg ha−1, but P content and stock increased by 4.9–7.4 g kg−1 and 6.7–9.9 Mg ha−1 in UL soil at 0–30 cm, resulting in a decrease in C:N, C:P and N:P ratios by 8.6–14.8, 23.0–28.6 and 0.7–1.0, respectively. In addition, N, P contents and stocks increased by −0.04–1.8 g kg−1, 4.1–10.5 g kg−1 and 1.8–1.1 Mg ha−1, 7.0–8.2 Mg ha−1 in PD soil at 0–30 cm, resulting in a decrease in the ratio of C:N, C:P and N:P by 8.1–10.6, 20.7–26.0 and 0.6–0.8, respectively. Furthermore, C, N, P contents, stocks and C:N ratio in UL and PD soils were more significant under SE than CGE condition with, while it was opposite trend in WL at 0–30 cm depth. Soil C, N, P contents and stocks significantly decreased with soil depth, while no significant changes were detected on their stoichiometric ratio. Soil porosity and texture had significant contribution to C, N and P stoichiometry by PCA and RDA analysis. Our results demonstrate that the intensive agricultural practices changing soil porosity and texture under SE condition led to more pronounced differences of soil C, N, P and their stoichiometric ratios in lateritic red soil region.

Extractability of global fallout Pu from agricultural soils and its potential indication of bioavailability

We present the first investigation of the extractability of global fallout Pu from agricultural soils under natural conditions and its potential for indication of Pu bioavailability. On average, the nominal water soluble and exchangeable Pu fractions in sampled upland field soils were 0.07 ± 0.07% and 0.02 ± 0.01%, respectively, while those fractions in the rice paddy soils were 0.10 ± 0.06% and 0.03 ± 0.02%, respectively. Sequential extraction results indicated that the dominant sinks of Pu in both upland and paddy soils were the immobile organic matter fraction and the immobile residual fraction. Furthermore, both the water soluble and NH4OAc exchangeable Pu percentages showed a significant negative correlation with the exchangeable Ca2+ concentration in the soils. Regarding the potential plant bioavailability of Pu, we observed a positive correlation (r = 0.549, p = 0.034) between the Pu concentration in rice and the water soluble Pu concentration in the corresponding rice paddy soils, and that illustrated the water soluble Pu fraction was likely to be available for rice plant uptake. This bioavailable fraction was further applied in evaluating the available transfer factor (ATF) of Pu by dividing the Pu concentration in rice with the bioavailable Pu concentration in the corresponding soil. The ATFs of Pu from soil to rice plant ranged from 6.9 × 10−3 to 1.7 × 10−1. Statistical analysis indicated the ATF data were less discrete compared with the conventional transfer factor (TF). Therefore, we consider the ATF to be more representative than the TF to illustrate the transfer ability of Pu from soil to rice plant.

Land use conversion and soil moisture affect the magnitude and pattern of soil-borne N2, NO, and N2O emissions

In this study, soil-borne N2, NO, and N2O emissions induced by land use conversion and water management were investigated in intact soil cores under a helium/oxygen atmosphere by a robotized incubation system in combination with the N2O 15N site preference signature and molecular-based microbial analysis. The experiment consisted of five treatments: i) paddy-flooded (PF); ii) orchard-wet (OW, 70% WFPS); iii) orchard-dry (OD, 43% WFPS); iv) vegetable-wet (VW, 70% WFPS); and v) vegetable-dry (VD, 43% WFPS). The vessels of each treatment received 200 mg urea-nitrogen (N) (equivalent to 210 kg of urea-N ha−1), and soil moisture in the OW and VW treatments was adjusted to a higher constant moisture to simulate a scenario after irrigation or rainfall. The results showed that total gaseous N losses during the incubation period were 25.33 ± 0.33 kg N ha−1 in the PF treatment, whereas smaller losses were recorded in the OD and VD treatments (4.28 ± 2.04 and 9.75 ± 3.75 kg N ha−1, respectively). The potential contribution of bacterial denitrification to N2O emissions in the OD and VD treatments was 11.1% and 15.4% higher, respectively, than that in the PF treatment (58.8% ± 0.5%). Furthermore, the corresponding N2O/(N2O + N2) ratio in the OD and VD treatments decreased by 50% and 73.8%, respectively, relative to the ratio in the PF treatment (0.42 ± 0.01). Such changes indicated the crucial role of altered soil properties caused by land use conversion in regulating the production and consumption of N2O. Relative to the normal moisture (dry) condition, enhanced soil moisture increased total gaseous N losses in the orchard and vegetable soils by 386.9% and 67.4%, accompanied by a higher N2O/(N2O + N2) product ratio, but decreased the share of bacterial N2O by 11.1% and 15.4%, respectively. The changes in the abundance and community composition of soil denitrifiers caused by land use conversion from rice paddies to orchards and vegetable fields could partly explain the differences in gaseous N loss therein. These findings highlight the influence of land use conversion on soil gaseous N emissions and demonstrate that increased moisture in upland soils reduced the dominance of bacterial denitrification in N2O production.