Distribution of persistent organic pollutants in aggregate fractions of a temperate forest and semi-rural soil

The fate of persistent organic pollutants (POPs) and their interactions with aggregates of forest soils are not completely understood. Our objectives here were to quantify the distribution of different POPs in water-stable aggregate fractions and to study their influence on soil organic carbon (Corg) content. Soil samples were taken from a forest-site, Gogerddan (G) and a semi-rural site, Hazelrigg (H) in Great Britain, from 0-2 and 2-5 cm and 0-4 and 8-12 cm soil depth, respectively. POPs analyzed were PAHs, PCBs, total DDT, PBDEs and HCB. The bulk soil analysis showed that the concentration of POPs was significantly higher (p ≤ 0.05) in forest site G than in semi-rural site H, particularly at the surface soil levels compared to the subsurface soil depths in both sites. Total concentrations of PCBs and PAHs of both sites were positively correlated with Corg contents. POPs concentrations and Corg, Nt contents of forest site G were significantly higher (p ≤ 0.05) in water-stable macro aggregates (>0.25, >1, >2 mm) than the micro aggregates (>0.053 mm). The POP concentrations of all aggregate fractions after normalizing to their respective Corg content were increased due higher contamination and strong sorption by Corg. These results showed a strong effect of Corg on the partitioning of organic pollutants to soil aggregate size fractions. The present study affirms the ecological significance of forest soils act as a potential sink of POPs. In summary, our results suggest that aggregate fractions may promote soil C storage and act as a potential POP sink in surface soil without increasing their concentration in the aggregate fraction of subsoil.

Organic amendment effects on nematode distribution within aggregate fractions in agricultural soils

To evaluate the effect of organic amendments on soil nematode community composition and diversity within aggregate fractions,a study was initiated in agricultural soils with four-year organic amendments.Soil samples were collected from the plow layer(0-20 cm)under three cornfield management scenarios:1)conventional cropping(CK,corn straw removal and no organic manure application);2)straw retention(SR,incorporation of chopped corn stalk);and 3)manure application(MA,chicken manure input).The soil samples were fractionated into four aggregate sizes,i.e.,>2 mm(large macroaggregates),1-2 mm(macroaggregates),0.25-1 mm(small macroaggregates),and<0.25 mm(microaggregates,silt and clay fractions).The composition and diversity of soil nematode communities were determined within each aggregate fraction.The results showed that both SR and MA treatments significantly increased the percentage of macroaggregates(>1 mm)and only MA treatment strongly increased the mean weight diameter compared to the CK(P<0.05).The abundance of total nematodes and four trophic groups were affected significantly by the aggregate fractions and their higher abundance occurred in the larger aggregates.The effects of aggregate size on most nematode genera were significant.Bacterivores in the small macroaggregates and microaggregates,and fungivores in the large macroaggregates were significantly different among treatments.The percentage of bacterivores increased after the application of organic materials,while that of fungivores decreased.It can be concluded that organic management significantly affects soil aggregation and soil characteristics within aggregates,and the aggregate size subsequently influences the distribution of nematode communities.

Effects of the combined application of organic and chemical nitrogen fertilizer on soil aggregate carbon and nitrogen:A 30-year study

To understand the long-term effects of combined organic and chemical nitrogen fertilization on soil organic C(SOC) and total N(TN), we conducted a 30-year field experiment with a wheat–maize rotation system on the Huang-HuaiHai Plain during 1990–2019. The experimental treatments consisted of five fertilizer regimes: no fertilizer(control), chemical fertilizer only(NPK), chemical fertilizer with straw(NPKS), chemical fertilizer with manure(NPKM), and 1.5 times the rate of NPKM(1.5NPKM). The NPK, NPKS, and NPKM treatments had equal N inputs. The crop yields were measured over the whole experimental duration. Soil samples were collected from the topsoil(0–10 and 10–20 cm) and subsoil(20–40 cm) layers for assessing soil aggregates and taking SOC and TN measurements. Compared with the NPK treatment, the SOC and TN contents increased significantly in both the topsoil(24.1–44.4% for SOC and 22.8–47.7% for TN) and subsoil layers(22.0–47.9% for SOC and 19.8–41.8% for TN) for the organically amended treatments(NPKS, NPKM and 1.5NPKM) after 30 years, while no significant differences were found for the average annual crop yields over the 30 years of the experiment. The 0–10 cm layer of the NPKS treatment and the 20–40 cm layer of the NPKM treatment had significantly higher macroaggregate fraction mass proportions(19.8 and 27.0%) than the NPK treatment. However, the 0–10 and 20–40 cm layers of the 1.5NPKM treatment had significantly lower macroaggregate fraction mass proportions(–19.2 and –29.1%) than the control. The analysis showed that the higher SOC and TN in the soil of organically amended treatments compared to the NPK treatment were related to the increases in SOC and TN protected in the stable fractions(i.e., free microaggregates and microaggregates within macroaggregates), in which the contributions of the stable fractions were 81.1–91.7% of the increase in SOC and 83.3–94.0% of the increase in TN, respectively. The relationships between average C inputs and both stable SOC and TN stocks were significantly positive with R2 values of 0.74 and 0.72(P<0.01) for the whole 40 cm soil profile, which indicates the importance of N for soil C storage. The results of our study provide key evidence that long-term combined organic and chemical nitrogen fertilization, while maintaining reasonable total N inputs, benefited soil C and N storage in both the topsoil and subsoil layers.

Effects of free iron oxyhydrates and soil organic matter on copper sorption-desorption behavior by size fractions of aggregates from two paddy soils

Effects of free iron oxyhydrates （Fed） and soil organic matter （SOM） on copper （Cu^2＋） sorption-desorption behavior by size fractions of aggregates from two typical paddy soils （Ferric-Accumulic Stagnic Anthrosol （Soil H） and Gleyic Stagnic Anthrosol （Soil W）） were investigated with and without treatments of dithionite-citrate-bicarbonate and of H2O2. The size fractions of aggregates were obtained from the undisturbed bulk topsoil using a low energy ultrasonic dispersion procedure. Experiments of equilibrium sorption and subsequent desorption were conducted at soil water ratio of 1：20, 25℃. For Soil H, Cu^2＋ sorption capacity of the DCB-treated size fractions was decreased by 5.9% for fine sand fraction, by 40.4% for coarse sand fraction, in comparison to 2.9% for the bnlk sample. However, Cu^2＋ sorption capacities of the H2O2-treated fractions were decreased by over 80% for the coarse sand fraction and by 15% for the clay-sized fraction in comparison to 88% for bulk soil. For Soil W, Cu^2＋ sorption capacity of the DCB-treated size fraction was decreased by 30% for the coarse sand fraction and by over 75% for silt sand fraction in comparison to 44.5% for the bulk sample. Cu^2＋ sorption capacities of the H2O2-treated fractions were decreased by only 2.0% for the coarse sand fraction and by 15% for the fine sand fraction in comparison to by 3.4% for bulk soil. However, Cu^2＋ desorption rates were increased much in H2O2-treated samples by over 80% except the clay-sized fraction （only 9.5%） for Soil H. While removal of SOM with H2O2 tendend to increase the desorption rate, DCB- and H2O2-treatments caused decrease in Cu^2＋ retention capacity of size fractions, Particularly, there hardly remained Cu^2＋ retention capacity by size fractions from Soil H after H2O2 treatment except for clay-sized fraction. These findings supported again the dominance of the coarse sand fraction in sorption of metals and the preference of absorbed metals bound to SOM in differently stabilized status among the size fractions. Thus, enrichment and turnover of SOM in paddy soils may have great effects on metal retention and chemical mobility in paddy soils.

Effects of slope position and land use on the stability of aggregateassociated organic carbon in calcareous soils

Topography and land use affect soil organic carbon(SOC) storage, stabilization, and turnover, through several biogeochemical processes. This study investigated the aggregate composition and SOC content of bulk soils and aggregates at different slope positions under different land uses in a typical karst catchment of southwestern China. Our results show that the proportion of macro-aggregates and the SOC content of bulk soils and aggregates at different slope positions decreased from the upper to the lower slope. The SOC content generally increased with an increase in the mean weight diameter and proportion of macro-aggregates under different land uses. Our results indicate that macro-aggregates in forest and grassland soils make a greater contribution to both aggregate composition and SOC content than that in arable land soils. Therefore,converting farmland to forest or grassland can facilitate the accumulation of macro-aggregates as well as the storage of SOC.

Influence of the Aggregate Volume on the Electrical Resistivity and Properties of Portland Cement Concretes

The electrical resistivity of concretes with various aggregate volume fractions （Va） of 0%-70% at water/cement （W/C） ratios of 0.4 and 0.5 during 1 day was monitored.It is found that the addition of normal aggregate to cement paste leads to a regular increase in concrete resistivity at each hydration stage and the electrical resistivity has a deeper increase for the lower W/C at a fixed aggregate volume fraction.The number of normalized resistivity （NR） of concrete to its paste matrix was introduced,which is only a function of aggregate volume fraction （Va）.The quantitative relationships give an alternative method for the prediction of aggregate volume in the concrete.A logarithmic relation is established between the elastic modulus of concrete at 7 days or 28 days and the electrical resistivity of concrete at 1 day.The equations are obtained,the compressive strength of concrete at 7 days or 28 days can be determined by the electrical resistivity of concrete at 1 day and the used aggregate content in the concrete.The quantitative relationships give a non-destructive test （NDT） method for prediction of concrete elastic modulus and compressive strength.

Effects of Organic Amendments on Soil Physical Attributes and Aggregate-Associated Phosphorus Under Long-Term Rice-Wheat Cropping

The quantification of phosphorus(P) in bulk soil and P distribution in different size fractions of water-stable aggregates(WSAs)are important for assessing potential P loss through runoff. We evaluated available and total P distribution within WSAs of a sitty clay to clay soil in a long-term fertility experiment of a rice-wheat cropping system in India. Surface soil samples were collected from seven plots amended with NPK fertilizers in combination with or without organic amendments, farmyard manure(FYM), green manure(GM), and paddy straw(PS). The plot with no NPK fertilizers or organic amendments was set as a control. The soil samples were separated by wet sieving into four soil aggregate size fractions: large macroaggregates(> 2.0 mm), small macroaggregates(0.25–2.0 mm), fine microaggregates(0.05–0.25 mm), and a silt + clay-sized fraction(< 0.05 mm). Structural indices were higher in the soil receiving organic amendments than in the soil receiving inorganic fertilizer alone. Organically amended soil had a higher proportion of stable macroaggregates than the control and the soil receiving inorganic fertilizer alone, which were rich in microaggregates. Total and available P contents within WSAs were inversely related to the aggregate size, irrespective of treatment. The distribution of available and total P in the soil aggregate size fraction was as follows: silt + clay-size fraction > small macroaggregates > fine microaggregates> large macroaggregates. Within a size class, aggregate-associated available and total P contents in the organically amended soil were in the following order: FYM > PS ≥ GM. The available P content of the microaggregates(< 0.25 mm) was 8-to 10-times higher than that of the macroaggregates(> 0.25 mm), and the total P content of the microaggregates was 4-to 5-times higher than that of the macroaggregates. Cultivation without organic amendments resulted in more microaggregates that could be checked by the application of organic amendments such as FYM and GM, which increased the proportion of water-stable macroaggregates by consolidating microaggregates into macroaggregates.

Changes in the soil microbial communities of different soil aggregations after vegetation restoration in a semiarid grassland,China

Soil aggregate fractions can regulate microbial community composition and structure after vegetation restoration.However,there has been less focus on the effects of soil aggregate fractions on the distributions of microbial communities.Here,we used phospholipid fatty acid(PLFA)analysis to explore the effects of different years of vegetation restoration(a 35-year-old Thymus mongolicus community(Re-35yrs)and a 2-year-old nongrazing grassland(Ug-2yrs))on microbial communities within different soil aggregate sizes(<0.25 mm,0.25–1 mm,1–2 mm,2–3 mm,3–5 mm and>5 mm).The results indicated that the amount of total PLFA in Re-35yrs was 10 times greater than that in Ug-2yrs.The soil aggregate stability increased with increasing duration of vegetation restoration.In Re-35yrs,the total PLFA shown an increase as the soil aggregate size increased,and the highest values were observed in 3–5 mm.Ug-2yrs differed from Re-35yrs,the soil microbial diversity was higher in medium particle sizes(1–2 mm and 2–3 mm)and lower in microaggregates(<0.25 mm and 0.25–1 mm)and macroaggregates(3–5 mm and>5 mm).Soil microbial diversity was highest in large particle size aggregates,which resulted in low environmental stress and strong stability.The same tendency was observed in the high values of cyc/prec,S/M and soil organic matter,which indicated a lower turnover speed(F/B)of fungal energy utilization and a higher fixation rate.After years of natural restoration,the soil microbial community generally transformed from nutrient-rich to heterotrophdominant,especially in microaggregates(reflected in the G^(+)/G^(–)ratio).

Effects of Integrated Soil-Crop System Management on Soil Organic Carbon Characteristics in a Primosol in Northeast China

A synchronous increase in crop productivity, nutrient use efficiency, and soil carbon(C) sequestration is important from the point of view of food security and environmental protection. In recent years, integrated soil-crop system management(ISSM), which uses crop models and advanced nutrient management to redesign cropping systems, has been successfully demonstrated to achieve both high crop productivity and high nutrient use efficiency in China, but the effects of ISSM on soil organic C(SOC) characteristics remain unknown. In this study, the effects of current farmers' practice(FP), high-yielding practice(HY), which maximizes yields without considering costs, and ISSM on the content and chemical composition of SOC were studied in a 4-year(2009–2013) field plot experiment with maize(Zea mays L.) monoculture in an Alluvic Primosol in Northeast China. The ISSM resulted in higher soil total organic C(TOC), water-soluble organic C, easily-oxidizable organic C, particulate organic C, and humic acid C compared with HY and FP in the region. The SOC contents in aggregate size fractions generally followed a similar pattern to TOC. Compared with FP,HY decreased the mean weight diameter, geometric mean diameter, percentage of > 0.25-mm water-stable aggregates, and the stability ratio of water-stable aggregates, and increased the structure-deterioration rate and index of unstable aggregates. The opposite trend was observed between ISSM and HY. Solid-state ^(13)C nuclear magnetic resonance spectra of bulk soil showed that ISSM had higher O-alkyl C and aliphatic C/aromatic C ratio, but lower aromatic C, carbonyl C, and alkyl C/O-alkyl C and hydrophobic C/hydrophilic C ratios than HY and FP. Our results suggest that ISSM improves the quantity and quality of SOC and has a positive effect on soil aggregation and aggregate stability.