Changes in the soil C and N contents,C decomposition and N mineralization potentials in a rice paddy after long-term application of inorganic fertilizers and organic matter

A long-term experiment on combined inorganic fertilizers and organic matter in paddy rice (Oryza sativa L.) cultivation began in May 1982 in Yamagata, northeastern Japan. In 2012, after the 31^st harvest, soil samples were collected from five fertilizer treatments [(1) PK, (2) NPK, (3) NPK + 6 Mg ha^?1 rice straw (RS), (4) NPK + 10 Mg ha^?1 rice straw compost (CM1), and (5) NPK + 30 Mg ha^?1 rice straw compost (CM3)], at five soil depths (0–5, 5–10, 10–15, 15–20 and 20–25 cm), to assess the changes in soil organic carbon (SOC) content and carbon (C) decomposition potential, total nitrogen (TN) content and nitrogen (N) mineralization potential resulting from long-term organic matter addition. The C decomposition potential was assessed based on the methane (CH₄) and carbon dioxide (CO₂) produced, while the N mineralization potential was determined from the potassium chloride (KCl)-extractable ammonium-nitrogen (NH₄⁺-N), after 2, 4, 6 and 8 weeks of anaerobic incubation at 30°C in the laboratory. Compared to NPK treatment, SOC in the total 0–25 cm layer increased by 67.3, 21.0 and10.8%, and TN increased by 64.2, 19.7 and 10.6%, in CM3, RS and CM1, respectively, and SOC and TN showed a slight reduction in the PK treatment by 5.2 and 5.7%, respectively. Applying rice straw compost (10 Mg ha^?1) instead of rice straw (6 Mg ha^?1) to rice paddies reduced methane production by about 19% after the soils were measured under 8 weeks of anaerobic incubation at 30°C. Soil carbon decomposition potential (Co) and nitrogen mineralization potential (No) were highly correlated with the SOC and TN contents. The mean ratio of Co/No was 4.49, lower than the mean ratio of SOC/TN (13.49) for all treatments, which indicated that the easily decomposed organic matter was from soil microbial biomass and soil proteins.     

The influence of compost addition on heavy metal distribution between operationally defined geochemical fractions and on metal accumulation in plant

Purpose

Metal distribution patterns among geochemical fractions are informative for metal phytoavailability. Compost added to polluted soils may adsorb metals on the less phytoavailable fractions. A bioassay experiment was conducted to establish possible correlations between metal concentrations in different soil fractions and metal contents in edible plant parts and to investigate the influence of different compost loads on heavy metal availability to plants.

Materials and methods

Chinese cabbage plants were grown in pots with sandy and clayey soils and soils mixed with different doses of biosolid compost spiked with soluble heavy metal salts (Cd, Cu, and Pb). The metals’ distribution pattern in the soil and mixed samples was determined by sequential extraction procedure (modified BCR protocol). The studied fractions, from most to least bioavailable, were water-extractable (WE), exchangeable-adsorbed (EXC), associated with carbonates and acetic acid-soluble forms (CARB), occluded by reducible (hydro)oxides of Fe and Mn (RO), and associated with organic matter (OM) and a residual fraction (RES). Metal concentrations in soil extracts and in the digested plant tissue were measured by ICP-AES.

Results and discussion

The highest compost doses (72 and 115 Mg ha^?1) enhanced cabbage yield significantly. No excessive phytoaccumulation of metals was observed in plants grown in the clayey soil or its mixtures with compost. The compost dose of 72 Mg ha^?1 was optimal in decreasing Cu accumulation by plants grown in sandy soil, and 28.8 Mg ha^?1 was found to be effective in reducing Cd and Pb uptake. Metals were accumulated in plants primarily from the WE, EXC, and CARB fractions, whereas other fractions decreased phytoaccumulation. Compost addition suppressed heavy metal mobility, but different fractions were active in pollutant sorption, depending on soil type and metal.

Conclusions

Compost addition increased metal proportions in the RO and OM fractions, reducing metal phytoavailability. This is especially important for sandy soils with low adsorption ability and higher vulnerability to metal pollution than clayey soils. A compost dose of 20% v/v (or 28.8 Mg ha^?1) effectively reduced plant accumulation of Cd and Pb. We propose using the first three steps of the modified BCR protocol as a three-step sequential-extraction procedure for the most phytoavailable fractions of heavy metal: WE, EXC, and CARB.     

Ecosystem carbon stock across a chronosequence of spruce plantations established on cutovers of a high-elevation region

Purpose

This study quantified the above- and belowground carbon (C) stocks across a chronosequence of spruce (Picea asperata) plantations established on cutovers and explored the turning point after which the increase in biomass C slowed or biomass C decreased for guiding forest management.

Materials and methods

We assessed above- and belowground plant biomass stocks at 11 sites in three regions, representing 12- to 46-year-old spruce plantations established on clear-cut areas in the eastern Tibetan Plateau, China. Biomass and C stocks of trees, understory vegetation, and forest floor litter were determined from plot-level inventories and destructive sampling. Fine root (<2 mm) biomass and mineral soil organic C (SOC) stock were estimated from soil cores. Tree biomass was quantified using allometric equations based on diameter at breast height (DBH) and height (H).

Results and discussion

Plant biomass C stocks in spruce plantations rapidly increased from 12 to 20 years at a rate of 7.8 Mg C ha^?1 year^?1, but decreased from 25 to 46 years at a rate of 0.79 Mg C ha^?1 year^?1. SOC stocks in spruce plantations gradually decreased from 12 to 46 years at a rate of 4.4 Mg C ha^?1 year^?1. Total C stock in the ecosystem remained unchanged for the first 20 years after the planting of spruce on cutovers, because the buildup of C stock in spruce biomass during the first 20 years was offset by the decrease in SOC. From 21 to 46 years after the reforestation, ecosystem C stock even decreased at a rate of 5.2 Mg C ha^?1 year^?1. The contribution of the understory vegetation, forest floor litter, and fine root to ecosystem C stock was low (<5.0 %) in the spruce plantations.

Conclusions

Ecosystem C stock in the spruce forest established on the cutover in the eastern Tibetan Plateau was related to stand age. During the first 20 years, this ecosystem was C neutral. However, aged (20–46 years) spruce plantation ecosystem can be a C source if no management was implemented to revitalize tree growth, promote understory vegetation, and enhance SOC accumulation.

     

Impact of flue gas desulfurization gypsum and lignite humic acid application on soil organic matter and physical properties of a saline-sodic farmland soil in Eastern China

Purpose

Appropriate land management is important for improving the soil quality and productivity of the saline-sodic farmland. A recent study has revealed that flue gas desulfurization (FGD) gypsum and lignite humic acid application enhanced the salt leaching and crop production. The purpose of this study was to investigate the effects of applied FGD gypsum and lignite humic acid (powder) on the soil organic matter (SOM) content and physical properties.

Materials and methods

This study was based on a field experiment of five consecutive rapeseed-maize rotations in a saline-sodic farmland soil (Aquic Halaquepts) at coastal area of North Jiangsu Province, China. The soil is sandy clay loam texture with pH of 8.43 and clay content of 185 g kg^?1. Six treatments included three FGD gypsum rates (0, 1.6, and 3.2 Mg ha^?1) and two lignite humic acid rates (0 and 1.5 Mg ha^?1). The amendments were incorporated into 0–20 cm soil depth manually every year. Soil samples were collected from each treatment and analyzed for soil organic matter, water-stable aggregates (wet sieving method), bulk density (clod method), water retention capacity (pressure plate apparatus), total porosity (calculated from bulk density and particle density), and microporosity (calculated from water content at 0.01 MPa).

Results and discussion

After 5 years, the SOM and soil physical properties were significantly (P?<?0.05) affected by the application of FGD gypsum and lignite humic acid, especially at the 0–20 cm soil depth. The highest amount of SOM with best soil physical condition was observed in the field which was treated with FGD gypsum at 3.2 Mg ha^?1 with lignite humic acid, and the SOM, total porosity (TP), microporosity (MP), mean weight diameter (MWD), water-stable macroaggregate (WSMA), and available water content (AWC) were increased by 22.8, 6.34, 23.2, 48.1, 55.5, and 15.8 %, respectively, while the bulk density (BD) was decreased by 5.9 % compared to no amendments applied. The generalized linear regression analysis showed that the SOM explained 42.9, 55.0, 48.5, and 54.2 % of the variability for BD, MWD, WSMA, and MP, respectively.

Conclusions

This study illustrates the benefits of applying FGD gypsum and lignite humic acid for increasing the soil organic matter content and improving the soil physical properties and suggests a great potential for ameliorating saline-sodic farmland soil (Aquic Halaquepts) by using combined amendment of FGD gypsum with lignite humic acid.

     

Soil carbon fractions in response to straw mulching in the Loess Plateau of China

Straw mulching has been used to conserve soil water and sustain dryland crop yields, but the impact of the quantity and time of mulching on soil C fractions are not well documented. We studied the effects of various amounts and times of wheat (Triticum aestivum L.) straw mulching on soil C fractions at 0–10- and 10–20-cm depths from 2009 to 2017 in the Loess Plateau of China. Treatments were no mulching (CK), straw mulching at 9.0 (HSM) and 4.5 Mg ha^?1 (LSM) in the winter wheat growing season, and straw mulching at 9.0 Mg ha^?1 in the summer fallow period (FSM). Soil C fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). All C fractions at 0–10 and 10–20 cm were 8–27% greater with HSM and LSM than FSM and CK. Both SOC and POC at 0–10 cm increased at 0.32 and 0.27 Mg ha^?1 year^?1 with HSM and at 0.40 and 0.30 Mg C ha^?1 year^?1 with LSM, respectively, from 2009 to 2017. Winter wheat grain yield was lower with HSM and LSM, but total aboveground biomass was greater with HSM than other treatments. All C fractions at most depths were correlated with the estimated wheat root residue returned to the soil and PCM at 0–10 and 0–20 cm was correlated with wheat grain yield. Wheat straw mulching during the growing season increased soil C sequestration and microbial biomass and activity compared with mulching during the fallow period or no mulching, regardless of mulching rate, due to increased C input, although it reduced wheat grain yield. Continuous application of straw mulching over time can increase soil C sequestration by increasing nonlabile C fractions while decreasing labile fractions. Straw mulching at higher rate and mulching during the summer fallow period had no additional benefits in soil C sequestration.     

Responses of methane emissions and rice yield to applications of biochar and straw in a paddy field

Purpose

Directly returning straw back to the paddy field would significantly accelerate methane (CH₄) emission, although it may conserve and sustain soil productivity. The application of biochar (biomass-derived charcoal) in soil has been proposed as a sustainable technology to reduce methane (CH₄) emission and increase crop yield. We compared the effects of either biochar or rice straw addition with a paddy field on CH₄ emission and rice yield.

Materials and methods

A 2-year field experiment was conducted to investigate a single application of rice straw biochar (SC) and bamboo biochar (BC) (at 22.5 t ha^?1) in paddy soil on CH₄ emission and rice yield as compared with the successive application (6 t ha^?1) of rice straw (RS). Soil chemical properties and methanogenic and CH₄ oxidation activities in response to the amendment of biochar and rice straw were monitored to explain possible mechanism.

Results and discussion

SC was more efficient in reducing CH₄ emission from paddy field than BC. Incorporating SC into paddy field could decrease CH₄ emission during the rice growing cycle by 47.30 %–86.43 % compared with direct return of RS. This was well supported by the significant decrease of methanogenic activity in paddy field with SC. In comparison to a non-significant increase with BC or RS application, rice yield was significantly raised with SC amendment by 13.5 % in 2010 and 6.1 % in 2011. An enhancement of available K and P and an improvement in soil properties with SC amendment might be the main contributors to the increased crop yield.

Conclusions

These results indicated that conversion of RS into biochar instead of directly returning it to the paddy field would be a promising method to reduce CH₄ emission and increase rice yield.     

Methane and nitrous oxide emissions as affected by organic–inorganic mixed fertilizer from a rice paddy in southeast China

Purpose

The effects of commercial compost fertilizer application on trace gas emissions are not well understood due to a lack of field experiments. The objective of this study was to evaluate the emissions of methane (CH₄) and nitrous oxide (N₂O) along with grain yield from a rice paddy as affected by different organic–inorganic mixed fertilizer (OIMF) treatments.

Materials and methods

A field experiment was initiated in 2006 with chemical compound fertilizer (CF) and three OIMF amendments including pig manure compost (PMC), Chinese medicine residue compost (CMC), and rapeseed cake compost (RCC), from a rice paddy in southeast China. The emissions of CH₄ and N₂O were simultaneously measured using the static opaque chamber method over the entire rice growing season in 2011. Soil biotic parameters were measured in soil collected after the rice was harvested in 2011.

Results and discussion

Relative to the control, the OIMF treatments significantly increased CH₄ emissions by 56–99 %, mainly due to exogenous organic substrate input, whereas no difference was observed in the CF treatment. The N₂O emissions were stimulated substantially by an average of 40 % due to nitrogen fertilization compared with the control. Consecutive OIMF application tended to increase the grain yield, making it marginally higher than that of the CF treatment (7 %, P?=?0.06). Compared with the control, the CF treatment slightly decreased the global warming potential and greenhouse gas (GHG) intensity, while they were remarkably increased in the OIMF treatments. Over the 5-year period of 2006–2011, the annual soil carbon sequestration rate was estimated to be 1.19 t C ha^?1 year^?1 for the control and 1.73–1.98 t C ha^?1 year^?1 for the fertilized treatments.

Conclusions

Our results suggest that despite the beneficial effects of increasing both grain yield and soil organic matter, OIMF application such as PMC, CMC, and RCC may be responsible for increased global warming due mainly to the stimulated CH₄ emissions. This effect should be thus taken into account when balancing agricultural production and GHG mitigation.     

Bulk density and content,density and stock of carbon,nitrogen and heavy metals in vegetable patches and lawns of allotments gardens in the northwestern Ruhr area,Germany

Purpose

In view that soils are bodies and that processes such as storage and release of water, carbon, nutrients and pollutants, and aeration and rooting happen in these bodies, it is of interest to know the density of elements and compounds in soils. On the basis of soil bulk and element density of organic carbon (OC), N, and heavy metals in soils and of horizon thickness, stocks of these elements for garden soils were calculated.

Materials and methods

Fourteen gardens in four allotments of the northwestern part of the Ruhr area, Germany were investigated. The research included 14 vegetable patches, 13 lawns, 2 compost heaps, and 1 meadow. Volume samples were taken. The soil analysis included pH, soil bulk density, and OC, N, Pb, Cd, Zn, Cu, and Ni contents.

Results and discussion

The soils were from sandy loam to loamy sand. The pH was slightly acid and C/N ratio about 20. Soil bulk density was between 0.8 and 1.4 g cm^?3 and mean bulk density was 1.1 g cm^?3. Mean OC content was for compost 7.4 %, vegetable patches 5.2 % (0–30 cm depth), and lawns and meadow 5.8 and 5.2 % (0–5 cm depth). OC density for compost was 76 mg cm^?3, vegetable patches 56 mg cm^?3, and lawns 67 mg cm^?3 (0–5 cm). Mean OC stock in 0–30 cm soil depth in vegetable patches was 16.4 kg m^?2, lawns 15.5 kg m^?2, and meadow 11.1 kg m^?2. N contents were between 0.06 and 0.46 %. For compost, the mean was 0.39 %, vegetable patches 0.27 % (0–30 cm), lawn 0.28 %, and meadow 0.26 % (0–5 cm). Mean stock of N in 0–30 cm depth for vegetable patches was 0.84 kg m^?2, lawn 0.76 kg m^?2, and meadow 0.55 kg m^?2. For heavy metals in compost, vegetable patches, lawn and meadow, Cd contents were in the range of 1.7 to 3.0 mg kg^?1, Pb 49 to 152 mg kg^?1, and Zn 52 to 1830 mg kg^?1. The amounts stored per square meters in 30 cm depth were for Cd 0.6–1.1 g, Pb 15–52 g, Zn 41–440 g, Cu 4–39 g, and Ni 1–8 g.

Conclusions

Allotment gardens have a high capacity to store CO₂ as OC. Roughly, there will be 7–8 million tons of OC stored in the 1.3 million allotment gardens of Germany. The high amount of 8000 kg N ha^?1 could damage the groundwater when released by wrong soil management. Cd, Zn, Pb, Cu, and Ni amounts of 7.8, 1000, 300, 135, and 30 kg ha^?1, respectively, are a lasting burden.

     

Short-term effects of organo-mineral biochar and organic fertilisers on nitrogen cycling,plant photosynthesis,and nitrogen use efficiency

Purpose

Organo-mineral biochar fertiliser has the potential to replace conventional biochar and organic fertiliser to improve soil quality and increase plant photosynthesis. This study explored mechanisms involved in nitrogen (N) cycling in both soil and ginger plants (Zingiber officinale: Zingiberaceae) following different treatments including organic fertiliser, commercial bamboo biochar fertiliser, and organo-mineral biochar fertiliser.

Materials and methods

Soil received four treatments including (1) commercial organic fertiliser (5 t ha^?1) as the control, (2) commercial bamboo biochar fertiliser (5 t ha^?1), (3) organo-mineral biochar fertiliser at a low rate (3 t ha^?1), and (4) organo-mineral biochar fertiliser at a high rate (7.5 t ha^?1). C and N fractions of soil and plant, and gas exchange measurements were analysed.

Results and discussion

Initially, organo-mineral biochar fertiliser applied at the low rate increased leaf N. Organo-mineral biochar fertiliser applied at the high rate significantly increased N use efficiency (NUE) of the aboveground biomass compared with other treatments and improved photosynthesis compared with the control. There was N fractionation during plant N uptake and assimilation since the ¹⁵N enrichment between the root, leaf, and stem were significantly different from zero; however, treatments did not affect this N fractionation.

Conclusions

Organo-mineral biochar fertiliser has agronomic advantages over inorganic and raw organic (manure-based) N fertiliser because it allows farmer to put high concentrations of nutrients into soil without restricting N availability, N uptake, and plant photosynthesis. We recommend applying the low rate of organo-mineral biochar fertiliser as a substitute for commercial organic fertiliser.

     

Effectiveness of crop straws,and swine manure in ameliorating acidic red soils: a laboratory study

Purpose

Crop straws and animal manure have the potential to ameliorate acidic soils, but their effectiveness and the mechanisms involved are not fully understood. The aim of this study was to evaluate the effectiveness of two crop (maize and soybean) straws, swine manure, and their application rates on acidity changes in acidic red soils (Ferralic Cambisol) differing in initial pH.

Materials and methods

Two red soils were collected after 21 years of the (1) no fertilization history (CK soil, pH 5.46) and (2) receiving annual chemical nitrogen (N) fertilization (N soil, pH 4.18). The soils were incubated for 105 days at 25 °C after amending the crop straws or manure at 0, 5, 10, 20, and 40 g kg^?1 (w/w), and examined for changes in pH, exchangeable acidity, N mineralization, and speciation in 2 M KCl extract as ammonium (NH₄⁺) and nitrate plus nitrite (NO₃^??+?NO₂^?).

Results and discussion

All three organic materials significantly decreased soil acidity (dominated by aluminum) as the application rate increased. Soybean straw was as effective (sometimes more effective) as swine manure in raising pH in both soils. Soybean straw and swine manure both significantly reduced exchangeable acidity at amendment rate as low as 10 g kg^?1 in the highly acidic N soil, but swine manure was more effective in reducing the total acidity especially exchangeable aluminum (e.g., in the N soil from initial 5.79 to 0.50 cmol₍₊₎ kg^?1 compared to 2.82 and 4.19 cmol₍₊₎ kg^?1 by soybean straw and maize straw, respectively). Maize straw was less effective than soybean straw in affecting soil pH and the acidity. The exchangeable aluminum decreased at a rate of 4.48 cmol₍₊₎ kg^?1 per pH unit increase for both straws compared to 6.25 cmol₍₊₎ kg^?1 per pH unit from the manure. The NO₃^??+?NO₂^? concentration in soil increased significantly for swine manure amendment, but decreased markedly for straw treatments. The high C/N ratio in the straws led to N immobilization and pH increase.

Conclusions

While swine manure continues to be effective for ameliorating soil acidity, crop straw amendment has also shown a good potential to ameliorate the acidity of the red soil. Thus, after harvest, straws should preferably not be removed from the field, but mixed with the soil to decelerate acidification. The long-term effect of straw return on soil acidity management warrants further determination under field conditions.

     

The impact of different forest types on phytolith-occluded carbon accumulation in subtropical forest soils

Purpose

Occlusion of carbon in phytoliths is an important biogeochemical carbon sequestration mechanism and plays a significant role in the global biogeochemical carbon cycle and atmospheric carbon dioxide (CO₂) concentration regulation at a millennial scale. However, few studies have focused on the storage of phytolith and phytolith-occluded carbon (PhytOC) in subtropical forest soils.

Materials and methods

Soil profiles with 100-cm depth were sampled from subtropical bamboo forest, fir forest, and chestnut forest in China to investigate the variation of phytoliths and PhytOC storage in the soil profiles based on amass-balance assessment.

Results and discussion

The storage of phytoliths in the top 100 cm of the bamboo forest soil (198.13?±?25.08 t ha^?1) was much higher than that in the fir forest (146.76?±?4.53 t ha^?1) and chestnut forest (170.87?±?9.59 t ha^?1). Similarly, the storage of PhytOC in the bamboo forest soil (3.91?±?0.64 t ha^?1) was much higher than that in the fir forest soil (1.18?±?0.22 t ha^?1) and chestnut forest soil (2.67?±?0.23 t ha^?1). The PhytOC percentage in the soil organic carbon pool increased with soil depth and was the highest (4.29 %) in the bamboo forest soil. Our study demonstrated that PhytOC in soil was significantly influenced by forest type and the bamboo forest ecosystem contributed more significantly to phytolith carbon sequestration than other forest ecosystems.

Conclusions

Different forest types have a significant influence on the soil PhytOC storage. Optimization of bamboo afforestation/reforestation in future forest management plans may significantly enhance the biogeochemical carbon sink in the following centuries.

     

Effect of biosolid application to Mollisol Chilean soils on the bioavailability of heavy metals (Cu,Cr, Ni,and Zn) as assessed by bioassays with sunflower (Helianthus annuus) and DGT measurements

Purpose

This study assessed the effect of biosolid application on the bioavailable fraction of some trace elements (Cu, Cr, Ni, and Zn) using a bioassay with sunflower (Helianthus annuus) and a chemical assay, diffusion gradient in thin films (DGT).

Materials and methods

Five surface soil samples (0–20 cm) were collected from an agricultural zone in Central Chile where biosolids are likely to be applied. Municipal biosolids were mixed with the soil at concentrations of 0, 30, 90, and 200 Mg ha^?1. The experiment to determine the bioavailability of metals in the soil using the bioassay was performed using sunflower. The DGT technique and Community Bureau of Reference (BCR) sequential extraction were used to determine the bioavailable fractions of the metals.

Results and discussion

The application of biosolids increased the phytoavailability of Zn, Ni, and Cr in most of the soils, as indicated by the increasing concentrations in sunflower plants as the biosolid application rate increased. In two of the soils, Codigua and Pelvín, this increase peaked at an application rate of 90 Mg ha^?1. Decreases in the bioavailable fractions of Zn, Ni, and Cr were observed with higher biosolid application rates. The bioavailability of metals was estimated through multiple linear regression models between the metals in the sunflower plants and the different chemical fractions of metals in the soils treated with different biosolid rates, which displayed a positive contribution of the labile (water soluble, carbonate, and exchangeable), oxide, and organic metal forms in the soil, particularly with respect to Ni and Zn at application rates of 30 and 90 Mg ha^?1. The bioavailable fraction of metals was determined in soils using the DGT technique. The effective concentration (C _E) results were compared with those in sunflower plants. The DGT technique could effectively predict the bioavailable fractions of Cr, Ni, and Zn in the Taqueral soil but only that of Zn in the Polpaico soil.

Conclusions

The application of biosolids significantly increased the labile fraction of most of the metals in the studied soils, particularly at the highest biosolid application rate. C _E increased as the concentration of biosolids increased for most of the metals. The effectiveness of the DGT technique for predicting the bioavailability of metals was dependent on the soil type and the metal. However, the C _E for soil Cu was not related to plant Cu for all soils studied.     

Phosphorus and carbon status of a paddy soil under different fertilization regimes

Purpose

Phosphorus (P) in soil particulate fraction (PF; >53 μm) is suggested to have a significant importance in soil P cycling. However, the effects of continuous fertilization on P-PF and its association with soil organic carbon (SOC) in paddy soils have not been well studied.

Materials and methods

We sampled paddy soils at 0–20 cm from a long-term field experiment (initiated in 1981) conducted under humid subtropical conditions in China, which has five fertilization treatments with equivalent P input (135 kg P₂O₅?ha^?1 year^?1) except the control treatment (CK). Changes in total P (Pt), inorganic P (Pi), organic P (Po), and SOC under different fertilization managements were evaluated in the whole soil, in the PF, and in the mineral-associated fraction (MAF; <53 μm).

Results and discussion

Continuous fertilization increased the contents of SOC and P in all soil fractions. Both Po and organic carbon in PF were the most sensitive variables to fertilization, indicating that they constitute a useful tool to detect the effects of management practices. Among the fertilization treatments, organic amendments significantly increased Po-PF contents more than chemical fertilizer applied only (p?<?0.05), although they had equivalent P input. The paddy soil without fertilization showed a more significant decrease in Pi compared with Po. The SOC/Po ratios were significantly lower in fertilization treatments (especially those with manure or straw incorporation) than in CK and decreased from PF to MAF. A significant relationship was found between Po-PF contents and rice P uptake during the growing season.

Conclusions

These results demonstrate that Po-PF may also play a significant role in P cycling of paddy soil, and thus, it would be better to consider Po-PF in soil diagnosis to promote P management of paddy soil, especially for that under long-term organic amendments.

     

Effect of rice straw mulching on migration and transportation of Cd,Cu, Zn,and Ni in surface runoff under simulated rainfall

Purpose

Heavy metals in runoff from contaminated land are becoming a major environmental problem. The presented paper considers the effects of mulching with rice straw on the migration and transportation of heavy metals from the soil into runoff under conditions of simulated rainfall.

Materials and methods

A simulated rainfall experiment was conducted to investigate the impact of rice straw mulching on emissions of sediment and heavy metals in runoff. The soil box was in 20-cm depth with a surface area of 1 m² and the slope was set to 10°. The rainfall intensity was 90 mm h^?1with a 60-min rainfall duration. The study involved samples with different treatments of rice straw mulching: bare soil (BS), low mulching (LM), and high mulching (HM), which had straw contents of 0, 200, and 500 g m^?2, respectively.

Results and discussion

The results showed that compared with BS, the cumulative runoff volume declined by 31 and 50 % and cumulative sediment declined significantly by 93 and 97 % for the LM and HM treatments, respectively. Additionally, with an increase of straw mulching, the concentrations of total heavy metals in the LM and HM treatments declined by 79.90–82.84 and 81.90–90.07 %, and the cumulative total heavy metals decreased significantly by 86.5–87.0 and 90.3–94.6 %, respectively. Particulate-bound heavy metals decreased by 88.1–88.9 % for the LM and 94.5–97.1 % for the HM. Furthermore, Cd, Cu, Zn, and Ni migrated and transported mainly in particulate-bound form and had high enrichment in sediments.

Conclusions

Therefore, straw mulching on soil could reduce the sediment yields, and the loss of both particulate-bound heavy metals, especially for Cd and Ni, and cumulative total heavy metals in runoff. Accordingly, it can be used as an effective measure to control heavy-metal-contaminated soil posing pollution risk to environment through surface runoff.

     

Superabsorbent polymers influence soil physical properties and increase potato tuber yield in a dry-farming region

Purpose

Superabsorbent polymers, new water-saving materials and soil conditioners, are used widely in dry-farming agriculture. However, little is known about their effects on the soil physical properties under dry-farming conditions. To elucidate the effects of two SAPs (Wote and microbe) at different doses on the soil bulk density, water status, potato growth, yield, and economic benefit in a dry-farming region, we conducted a 2-year fixed field position experiment in the semiarid drought-prone area of Ningxia, China.

Materials and methods

The two SAPs were diluted 1:10 (product:soil) and applied at different rates before planting, i.e., Wote SAP 30 kg ha^?1, Wote SAP 60 kg ha^?1, Wote SAP 90 kg ha^?1, microbe SAP 30 kg ha^?1, microbe SAP 60 kg ha^?1, and microbe SAP 90 kg ha^?1. The treatment without SAP was used as the control.

Results and discussion

The tilth soil bulk density decreased under different SAP doses compared with the control, and the soil total porosity improved greatly, where the Wote SAP treatments had the greatest effects. The soil bulk density (0–60 cm) under Wote SAP 90 kg ha^?1 was significantly decreased by 6.4% compared with the control. The Wote SAP treatments had the greatest effects on water conservation during the critical potato growth stage, where the soil water storage (0–100 cm) was significantly higher than the control. The Wote SAP treatments promoted potato growth in the later period, where the plant height and stem diameter were higher than the control. Higher yield and commodity rate improvements were achieved by the application of Wote and microbe SAP compared with the control, where the optimum dose was 60–90 kg ha^?1 for Wote SAP. The application of Wote SAP 90 kg ha^?1 significantly increased crop water use efficiency compared with no SAP, and the commodity rate was highest with Wote SAP 60 kg ha^?1. The mean potato yield, commodity rate, and net income increased significantly using Wote SAP at 60 and 90 kg ha^?1, i.e., by 38.2 and 50.5%, 18.5 and 14.1%, and 28.5 and 35.0%, respectively, compared with no SAP.

Conclusions

The application of SAPs can decrease soil bulk density and significantly improve soil porosity and soil water conservation capacity, thereby promoting potato growth. The application of Wote SAP 60–90 kg ha^?1 significantly increased potato yield and net income in a dry-farming region of Ningxia, China.

     

Soil carbon and nitrogen storage in alluvial wet meadows of the Southern Sierra Nevada Mountains,USA

Purpose

Wet meadows formed on alluvial deposits potentially store large amounts of soil carbon (C) but its stability is subject to the impacts of management practices. The objective of this study was to quantify and characterize soil organic carbon (SOC) and nitrogen (N) in mountain wet meadows across ranges of meadow hydrology and livestock utilization.

Materials and methods

Eighteen wetlands in the southern Sierra Nevada Mountains representing a range of wetness and livestock utilization levels were selected for soil sampling. In each wetland meadow, whole-solum soil cores delineated by horizon were analyzed for total and dissolved organic C (DOC) total (TN) and mineral nitrogen and soil water content (SWC). Multiple regression and GIS analysis was used to estimate the role of wet meadows in C storage across the study area landscape.

Results and discussion

Average solum SOC contents by wetland ranged from 130 to 805 Mg ha^?1. All SOC and TN components were highly correlated with SWC. Regression analyses indicated subtle impacts of forage utilization level on SOC and TN concentrations, but not on whole-solum, mass-per-area stocks of SOC and TN. Proportions of DOC and TN under seasonally wet meadows increased with increasing utilization. GIS analysis indicated that the montane landscape contains about 54.3 Mg SOC ha^?1, with wet meadows covering about 1.7% of the area and containing about 12.3% of the SOC.

Conclusions

Results indicate that soil organic C and N content of meadows we sampled are resilient to current light to moderate levels of grazing. In seasonally wet meadows, higher proportions of DOC and N with increasing utilization indicate vulnerability to loss. Partial drying of the wettest and seasonally wet meadows could result in losses of over five % of landscape SOC.     

Effects of biochar and polyacrylamide on decomposition of soil organic matter and <Superscript>14</Superscript>C-labeled alfalfa residues

Purpose

Various soil conditioners, such as biochar (BC) and anionic polyacrylamide (PAM), improve soil fertility and susceptibility to erosion, and may alter microbial accessibility and decomposition of soil organic matter (SOM) and plant residues. To date, no attempts have been made to study the effects of BC in combination with PAM on the decomposition of soil SOM and plant residues. The objective of this study was to evaluate the effects of BC, PAM, and their combination on the decomposition of SOM and alfalfa residues.

Materials and methods

An 80-day incubation experiment was carried out to investigate the effects of oak wood biochar (BC; 10 Mg ha^?1), PAM (80 kg ha^?1), and their combination (BC?+?PAM) on decomposition of SOM and ¹⁴C-labeled alfalfa (Medicago sativa L.) residues by measuring CO₂ efflux, microbial biomass, and specific respiration activity.

Results and discussion

No conditioner exerted a significant effect on SOM decomposition over the 80 days of incubation. PAM increased cumulative CO₂ efflux at 55–80 days of incubation on average of 6.7 % compared to the soil with plant residue. This was confirmed by the increased MBN and MB¹⁴C at 80 days of incubation in PAM-treated soil with plant residue compared to the control. In contrast, BC and BC?+?PAM decreased plant residue decomposition compared to that in PAM-treated soil and the respective control soil during the 80 days. BC and BC?+?PAM decreased MBC in soil at 2 days of incubation indicated that BC suppressed soil microorganisms and, therefore, decreased the decomposition of plant residue.

Conclusions

The addition of oak wood BC alone or in combination with PAM to soil decreased the decomposition of plant residue.

     

Effects of biogas slurry application on peanut yield,soil nutrients,carbon storage,and microbial activity in an Ultisol soil in southern China

Purpose

Biogas slurry (BS) was known to influence soil–plant ecosystems when applied as a fertilizer, especially in combination with a chemical fertilizer (CF). Limited information was available regarding how this combination of BS–CF actually affected the soil–plant ecosystems. The purpose of this study was to evaluate the effects of BS–CF combinations on peanut yield, soil properties, and carbon (C) storage in a red soil (Ultisol) in southern China.

Materials and methods

The soil was fertilized with five treatments, including a control (T1), CF-only (T2) treatment, and three treatments with different BS–CF combinations (T3–T5). The final quantities of N/P₂O₅/K₂O applied in T2–T5 were 120:90:135 kg ha^?1. In T3–T5, 15 % (18 kg ha^?1), 30 % (36 kg ha^?1), and 45 % (54 kg ha^?1) of total N (TN), respectively, were applied with BS and the remaining TN was applied with CF. Crop yield, soil nutrients, C storage, and microbial activity were determined through field and laboratory experiments.

Results and discussion

In the field experiment, peanut grain yields of T3–T5 were higher than those of T1 (44.5–55.7 %) and T2 (10.8–19.4 %), with the highest yield from T4 (3588 kg ha^?1). The relationship between BS–TN inputs and peanut grain yield conformed to the linear-quadratic equation: y?=??1.14x ²?+?59.1x?+?2988 (R ²?=?0.98). The biomasses of peanut plants, at the flowering, pod production, and harvesting stages, were higher in T4 compared with those in T1 and T2. Moreover, T4 produced higher soil N and P (total and available) concentrations at the pod production and harvesting stages relative to other treatments, with increased soil microbial biomass C and N, and enhanced dehydrogenase and urease activities, at the flowering, pod production, and harvesting stages. Data from the incubation experiment were fitted to a first-order kinetic model, which showed that although the application of BS increased potentially mineralizable C, the additional C seemed to slowly degrade, and so would be retained in the soil for a longer period.

Conclusions

A BS–CF combination increased peanut grain yield and biomass, due to increases in soil N and P availability, microbial biomass C and N concentrations, and urease and dehydrogenase activities. Moreover, the organic C retention time in the red soil was extended. Combined application of BS–CF at a suitable ratio (36 kg BS–TN ha^?1), together with proper management practices, could be effective to improve the quality and nutrient balance of amended soils.

     

Changes of labile and recalcitrant carbon pools under nitrogen addition in a city lawn soil

Purpose

Carbon (C) flux is largely controlled by the highly bio-reactive labile C (LC) pool, while long-term C storage is determined by the recalcitrant C (RC) pool. Soil nitrogen (N) availability may considerably affect changes of these pools. The aim of this study was to investigate the effects of N treatments on soil LC and RC pools.

Materials and methods

A field experiment was conducted in a city lawn soil for 600 days with three N treatments, i.e., the control (0 kg N ha^?1 year^?1), low-N (100 kg N ha^?1 year^?1), and high-N (200 kg N ha^?1 year^?1) treatments. As the N source, NH₄NO₃ solution was added to soil surface monthly. Measurements of LC, RC, and other soil biochemical properties, including pH, soil respiration rates, microbial biomass, and enzymes activities, were taken during the experiment period.

Results and discussion

The low-N and high-N treatments increased 6.3 and 13% of the LC pool, respectively, which was caused by decreased microbial biomass and soil respiration rates under the N treatments. By contrary, the low-N and high-N treatments decreased 5.9 and 12% of the RC pool, respectively. The N addition treatments enhanced phenol oxidase activities. The enhanced oxidase activities decreased new RC input and the increased dissolved organic C stimulated RC pool decomposition. The LC and RC pools were highly influenced by the N treatments, whereas effect of the N treatments on soil organic C was not significant. The N addition treatments also caused soil acidification and reduced bacterial biomass proportion in the soil microbial composition.

Conclusions

The N addition increased the LC pool but decreased the RC pool in the soil. These changes should greatly impact soil long-term C storage.     

Relationships between microbial biomass nitrogen,nitrate leaching and nitrogen uptake by corn in a compost and chemical fertilizer-amended regosol

Abstract

To determine the relationships between microbial biomass nitrogen (N), nitrate–nitrogen leaching (NO₃-N leaching) and N uptake by plants, a field experiment and a soil column experiment were conducted. In the field experiment, microbial biomass N, 0.5 mol L^?1 K₂SO₄ extractable N (extractable N), NO₃-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha^?1 containing 158 kg N ha^?1 of total N [approximately 50% is easily decomposable organic N]), chemical fertilizer (CF) and no fertilizer (NF) treatments from May 2000 to September 2002. In the soil column experiment, microbial biomass N, extractable N and NO₃-N leaching were monitored in soil treated with SDC (20 Mg ha^?1) + rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha^?1 containing 0, 15, 29, 44 and 59 kg N ha^?1) and in soil treated with CF in 2001. Nitrogen was applied as (NH₄)₂SO₄ at rates of 220 kg N ha^?1 for SDC and SDC + RS treatments and at a rate of 300 kg N ha^?1 for the CF treatment in both experiments. In the field experiment, microbial biomass N in the SDC treatment increased to 147 kg N ha^?1 at 7 days after treatment (DAT) and was maintained at 60–70 kg N ha^?1 after 30 days. Conversely, microbial biomass N in the CF treatment did not increase significantly. Extractable N in the surface soil increased immediately after treatment, but was found at lower levels in the SDC treatment compared to the CF treatment until 7 DAT. A small amount of NO₃-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO₃-N leaching in the CF treatment was 146 kg N ha^?1, which was equal to half of the applied N, but only 53 kg N ha^?1 in the SDC treatment. In contrast, there was no significant difference between N uptake by corn in the SDC and CF treatments. In the soil column experiment, microbial biomass N in the SDC + RS treatment at 7 DAT increased with increased RS application. Conversely, extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT decreased with increased RS application. In both experiments, microbial biomass N was negatively correlated with extractable N at 7 DAT and cumulative NO₃-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO₃-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO₃-N leaching without impacting negatively on N uptake by plants.