Effects of Vegetation Restoration Age on Soil C:N:P Stoichiometry in Yellow River Delta Coastal Wetland of China

Vegetation restoration can alter carbon(C),nitrogen(N),and phosphorus(P)cycles in coastal wetlands affecting C:N:P stoichiometry.However,the effects of restoration age on soil C:N:P stoichiometry are unclear.In this study,we examined the re-sponses of soil C,N,and P contents and their stoichiometric ratios to vegetation restoration age,focusing on below-ground processes and their relationships to aboveground vegetation community characteristics.We conducted an analysis of temporal gradients based on the'space for time'method to synthesize the effects of restoration age on soil C:N:P stoichiometry in the Yellow River Delta wetland of China.The findings suggest that the combined effects of restoration age and soil depth create complex patterns of shifting soil C:N:P stoichiometry.Specifically,restoration age significantly increased all topsoil C:N:P stoichiometries,except for soil total phosphorus(TP)and the C:N ratio,and slightly affected subsoil C:N:P stoichiometry.The effects of restoration age on the soil C:N ratio was well constrained owing to the coupled relationship between soil organic carbon(SOC)and total nitrogen(TN)contents,while soil TP con-tent was closely related to changes in plant species diversity.Importantly,we found that the topsoil C:N:P stoichiometry was signific-antly affected by plant species diversity,whereas the subsoil C:N:P stoichiometry was more easily regulated by pH and electric con-ductivity(EC).Overall,this study shows that vegetation restoration age elevated SOC and N contents and alleviated N limitation,which is useful for further assessing soil C:N:P stoichiometry in coastal restoration wetlands.

Integration of RUSLE Model with Remote Sensing and GIS Tools for Soil Loss Estimation in the Kubanni Drainage Basin, Zaria, Nigeria

The prevalence of unwholesome land use practices and population pressure exacerbates soil loss which is worsening the problem of sedimentation of the Kubanni dam. This study was conducted at the Kubanni drainage basin covering a spatial area of 56.7 Km2 in Samaru, Zaria, Nigeria to estimate annual soil loss using the RUSLE model. Satellite images of Landsat OLI for December 2014, 2016, 2018, February, July and November 2022;soil data, rainfall data from 2010 to 2022, and DEM of 30-meter resolution were utilized for the study. All factors of the RUSLE model were calculated for the basin using assembled data. The erosivity (R-factor) was discovered to be 553.437 MJ∙mm∙ha−1∙h−1∙yr−1. The average erodibility (K-factor) value was 0.1 Mg∙h∙h∙ha−1∙MJ−1∙mm−1∙yr−1. The Slope Length and Steepness factor (LS-factor) in the basin ranged between 0% and 13.47%. The Crop Management Factor (C-factor) values were obtained from a rescaling of the NDVI values derived for the study area and ranged from 0.26 to 0.55. Support practice (P-factors) were computed from the prevalent tillage practice in the basin and ranged from 0.27 to 0.40. The soil loss amount for the Kubanni basin was found to be 28441.482 tons∙ha−1∙yr−1, while the annual soil loss for the entire Kubanni drainage basin was found to be 49780.257 tons∙yr−1. The study has demonstrated the viability of coupling RUSLE model and Remote Sensing and Geographic Information System (GIS) techniques for the estimation of soil loss in the Kubanni drainage basin.

The contribution of root respiration of Pinus koraiensis seedlings to total soil respiration under elevated CO_2 concentrations

The impacts of elevated atmospheric CO2 concentrations (500 靘olmol-1and 700 靘olmol-1) on total soil respiration and the contribution of root respiration of Pinus koraiensis seedlings were investigated from May to October in 2003 at the Research Station of Changbai Mountain Forest Ecosystems, Chinese Academy of Sciences, Jilin Province, China. After four growing seasons in top-open chambers exposed to elevated CO2, the total soil respiration and roots respiration of Pinus koraiensis seedlings were measured by a LI-6400-09 soil CO2 flux chamber. Three PVC cylinders in each chamber were inserted about 30 cm into the soil in-stantaneously to terminate the supply of current photosynthates from the tree canopy to roots for separating the root respiration from total soil respiration. Soil respirations both inside and outside of the cylinders were measured on June 16, August 20 and October 8, respectively. The results indicated that: there was a marked diurnal change in air temperature and soil temperature at depth of 5 cm on June 16, the maximum of soil temperature at depth of 5 cm lagged behind that of air temperature, no differences in temperature between treatments were found (P>0.05). The total soil respiration and soil respiration with roots severed showed strong diurnal and seasonal patterns. There was marked difference in total soil respiration and soil respiration with roots severed between treatments (P<0.01); Mean total soil respiration and contribution of root under different treatments were 3.26, 4.78 and 1.47 靘olm 2s-1, 11.5%, 43.1% and 27.9% on June 16, August 20 and October 8, respectively.

Carbon pool structure and carbon density of soil in Pinus koraiensis plantation ecosystem

The organic carbon contents,carbon density and carbon storage of the soil in the Pinus koraiensis plantation ecosystem were investigated in Maoershan experimental forest farm,Shangzhi County,Heilongjiang,on the west slope of the Zhangguangcai Mountains in northeastern China for providing data to evaluation of the carbon balance in forest ecosystem of northeastern China.These soil carbon indicators were measured in three forest types,pure P.koraiensis plantation,P.koraiensis and Betula platyphylla mixed forest,and the P.koraiensis and Quercus mongolica mixed forest.The soil carbon pool consisted of four compartments,namely L layer,F layer,H layer and B layer.With variance analysis,we found that both organic carbon content and carbon density of the soil were significantly affected by forest types,soil compartments and slope positions.The highest soil carbon density（278.63 Mg·ha^-1）.was observed in the mixed forest of P.koraiensis and Q.mongolica.The B layer had the highest carbon density（212.28 Mg·ha^-1） among all the soil compartments.In terms of slope position,the highest soil carbon density（394.18 Mg·ha^-1） presented in the low slope.Besides,soil carbon content and carbon density had a marked change with the organic matter content and vertical depth of the soil in each compartment.The results of this study implied that in the temperate humid region,the mixed ecosystem of regional Pinus koraiensis plantations and natural forest had relatively high carbon storage capability.

Effects of N,P and K Fertilizers on Growth of Clover Nitrogen-fixing Rhizobia and Soil Fertility after Plantation

In order to develop organic rice and increase paddy soil fertility by cloverorganic rice rotation, the effects of N, P and K fertilizers on growth of clover nitrogen-fixing rhizobia and soil fertility after plantation were investigated, thereby providing certain reference for reasonable cultivation of clover and improvement of soil fertility. A two-year experiment was conducted from 2012 to 2013. The clover was cultivated after rice every year, and different levels of N, P and K fertilizers were applied before winter. In the treatment group, no fertilizer was applied. The effects of different fertilizers and different application amounts on clover yield, nitrogen-fixing rhizobia quantity, nitrogen-fixing rhizobia weight and soil fertility after plantation were analyzed. The results showed that the application of N, P and K fertilizers in the early stage had significantly effect on the growth of clover. When the application amount of N fertilizer was 75 kg/hm^2（N 46%）, the clover yield, nitrogen-fixing rhizobia quantity and nitrogen-fixing rhizobia weight were highest. The soil nitrogen content after plantation increased with the increase of the application amount of N fertilizer, while the P and K content decreased and then increased with the increased application amounts. Soil available P content increased with the increased application amounts of N and P fertilizers, but it did not change significantly with the increased application amount of K fertilizer. Soil available K content increased first and then decreased with the increased application amounts of N and P fertilizers.When the application amounts of N and P fertilizers were 150（N 46%） and 300（P_2O_5 12%） kg/hm^2, soil available K content reached the maximum. Soil organic matter content increased with the increased application amounts of N, P and K fertilizers. Therefore, in the cultivation of clover, appropriate application of N, P and K fertilizers in the early stage can improve clover yield and soil fertility.

Effects of long-term phosphorus fertilization and straw incorporation on phosphorus fractions in subtropical paddy soil

Study on soil phosphorus（P） fraction is an important aspect in probing the mechanisms of soil P accumulation in farmland and mitigating its losing risk to the environment. We used a sequential extraction method to evaluate the impacts of long-term fertilization and straw incorporation on inorganic, organic, and residual P（Pi, Po, and Pre） fractions in the plow layer（0–20 cm） of acidic paddy soil in southern China. The experiment comprised of six treatments：（i） no fertilizer control（CK）;（ii） straw incorporation and green manure（SG）;（iii） nitrogen and P fertilizer（NP）;（iv） NP＋SG;（v） NP＋K fertilizer（NPK）; and（vi） NPK＋SG. The results showed that, compared to the initial total soil P content（TSP, 600 mg kg^–1 in 1990）, long-term（20 years） combined continuous P fertilizer and SG significantly increased P accumulation（by 13–20%） while single fertilization（39.3 kg P ha^–1 yr^–1） could maintain soil P status at the most. The average soil P fractions comprised of extractable Pi, Po, and Pre by 51.7, 33.4, and 14.9% in total soil P, respectively. With comparison of no fertilizer addition（CK）, long-term single fertilization significantly（P〈0.05） increased the accumulation of Na HCO3^–, Na OH^–, and HCl^– extractable Pi fractions accounting for two- to three-fold, while SG increased the accumulation of Na HCO3^– and Na OH^– extractable Piand Po accounting for 12–60%. Though the mobilization of Pre fractions was not significant（P〉0.05）, our data indicate that SG may partially substitute for fertilizer P input and minimizing soil P accumulation and subsequent environmental risk in the subtropical paddy soil.

Trends of Yield and Soil Fertility in a Long-Term Wheat-Maize System

The sustainability of the wheat-maize rotation is important to China＇s food security. Intensive cropping without recycling crop residues or other organic inputs results in the loss of soil organic matter （SOM） and nutrients, and is assumed to be non- sustainable. We evaluated the effects of nine different treatments on yields, nitrogen use efficiency, P and K balances, and soil fertility in a wheat-maize rotation system （1991-2010） on silt clay loam in Shaanxi, China. The treatments involved the application of recommended dose of nitrogen （N）, nitrogen and phosphorus （NP）, nitrogen and potassium （NK）, phosphorus and potassium （PK）, combined NPK, wheat or maize straw （S） with NPK （SNPK）, or dairy manure （M） with NPK （M1NPK and M2NPK）, along with an un-treated control treatment （CK）. The mean yields of wheat and maize ranged from 992 and 2 235 kg ha-1 under CK to 5 962 and 6 894 kg ha-1 under M2NPK treatment, respectively. Treatments in which either N or P was omitted （N, NK and PK） gave significantly lower crop yields than those in which both were applied. The crop yields obtained under NP, NPK and SNPK treatments were statistically identical, as were those obtained under SNPK and MNPK. However, M2NPK gave a significant higher wheat yield than NP, and MNPK gave significant higher maize yield than both NP and NPK. Wheat yields increased significantly （by 86 to 155 kg ha-1 yr-1） in treatments where NP was applied, but maize yields did not. In general, the nitrogen use efficiency of wheat was the highest under the NP and NPK treatments; for maize, it was the highest under MNPK treatment. The P balance was highly positive under MNPK treatment, increasing by 136 to 213 kg ha-1 annually. While the K balance was negative in most treatments, ranging from 31 to 217 kg ha^-1 yr^-1, levels of soil available K remained unchanged or increased over the 20 yr. SOM levels increased significantly in all treatments. Overall, the results indicated that combinations of organic manure and inorganic nitrogen, or retuming straw with NP is likely to improve soil fertility, increasing the yields achievable with wheat-maize system in a way which is environmentally and agronomically beneficial on the tested soil.

Effect of N and P addition on soil organic C potential mineralization in forest soils in South China

Atmospheric nitrogen deposition is at a high level in some forests of South China. The effects of addition of exogenous N and P on soil organic carbon mineralization were studied to address: (1) if the atmospheric N deposition promotes soil C storage through decreasing mineralization; (2) if the soil available P is a limitation to organic carbon mineralization. Soils (0–10 cm) was sampled from monsoon evergreen broad-leaved forest (MEBF), coniferous and broad-leaved mixed forest (CBMF), and Pinus massoniana...

Response of soil Olsen-P to P budget under different long-term fertilization treatments in a fluvo-aquic soil

The concentration of soil Olsen-P is rapidly increasing in many parts of China, where P budget(P input minus P output) is the main factor influencing soil Olsen-P. Understanding the relationship between soil Olsen-P and P budget is useful in estimating soil Olsen-P content and conducting P management strategies. To address this, a long-term experiment(1991–2011) was performed on a fluvo-aquic soil in Beijing, China, where seven fertilization treatments were used to study the response of soil Olsen-P to P budget. The results showed that the relationship between the decrease in soil Olsen-P and P deficit could be simulated by a simple linear model. In treatments without P fertilization(CK, N, and NK), soil Olsen-P decreased by 2.4, 1.9, and 1.4 mg kg^(–1) for every 100 kg ha^(–1) of P deficit, respectively. Under conditions of P addition, the relationship between the increase in soil Olsen-P and P surplus could be divided into two stages. When P surplus was lower than the range of 729–884 kg ha^(–1), soil Olsen-P fluctuated over the course of the experimental period with chemical fertilizers(NP and NPK), and increased by 5.0 and 2.0 mg kg^(–1), respectively, when treated with chemical fertilizers combined with manure(NPKM and 1.5 NPKM) for every 100 kg ha^(–1) of P surplus. When P surplus was higher than the range of 729–884 kg ha^(–1), soil Olsen-P increased by 49.0 and 37.0 mg kg^(–1) in NPKM and 1.5 NPKM treatments, respectively, for every 100 kg ha^(–1) P surplus. The relationship between the increase in soil Olsen-P and P surplus could be simulated by two-segment linear models. The cumulative P budget at the turning point was defined as the "storage threshold" of a fluvo-aquic soil in Beijing, and the storage thresholds under NPKM and 1.5 NPKM were 729 and 884 kg ha^(–1)P for more adsorption sites. According to the critical soil P values(CPVs) and the relationship between soil Olsen-P and P budget, the quantity of P fertilizers for winter wheat could be increased and that of summer maize could be decreased based on the results of treatments in chemical fertilization. Additionally, when chemical fertilizers are combined with manures(NPKM and 1.5 NPKM), it could take approximately 9–11 years for soil Olsen-P to decrease to the critical soil P values of crops grown in the absence of P fertilizer.

Degradation induces changes in the soil C:N:P stoichiometry of alpine steppe on the Tibetan Plateau

Due to the Tibetan Plateau’s unique high altitude and low temperature climate conditions,the region’s alpine steppe ecosystem is highly fragile and is suffering from severe degradation under the stress of increasing population,overgrazing,and climate change.The soil stoichiometry,a crucial part of ecological stoichiometry,provides a fundamental approach for understanding ecosystem processes by examining the relative proportions and balance of the three elements.Understanding the impact of degradation on the soil stoichiometry is vital for conservation and management in the alpine steppe on the Tibetan Plateau.This study aims to examine the response of soil stoichiometry to degradation and explore the underlying biotic and abiotic mechanisms in the alpine steppe.We conducted a field survey in a sequent degraded alpine steppe with seven levels inNorthern Tibet.The plant species,aboveground biomass,and physical and chemical soil properties such as the moisture content,temperature,pH,compactness,total carbon(C),total nitrogen(N),and total phosphorus(P)were measured and recorded.The results showed that the contents of soil C/N,C/P,and N/P consistently decreased along intensifying degradation gradients.Using regression analysis and a structural equation model(SEM),we found that the C/N,C/P,and N/P ratios were positively affected by the soil compactness,soil moisture content and species richness of graminoids but negatively affected by soil pH and the proportion of aboveground biomass of forbs.The soil temperature had a negative effect on the C/N ratio but showed positive effect on the C/P and N/P ratios.The current study shows that degradation-induced changes in abiotic and biotic conditions such as soil warming and drying,which accelerated the soil organic carbon mineralization,as well as the increase in the proportion of forbs,whichwere difficult to decompose and input less organic carbon into soil,resulted in the decreases in soil C/N,C/P,and N/P contents to a great extent.Our results provide a sound basis for sustainable conservation and management of the alpine steppe.

Proton accumulation accelerated by heavy chemical nitrogen fertilization and its long-term impact on acidifying rate in a typical arable soil in the Huang-Huai-Hai Plain

Cropland productivity has been significantly impacted by soil acidification resulted from nitrogen （N） fertilization, especially as a result of excess ammoniacal N input. With decades＇ intensive agricultural cultivation and heavy chemical N input in the Huang-Huai-Hai Plain, the impact extent of induced proton input on soil pH in the long term was not yet clear. In this study, acidification rates of different soil layers in the soil profile （0-120 cm） were calculated by pH buffer capacity （pHBC） and net input of protons due to chemical N incorporation. Topsoil （0-20 cm） pH changes of a long-term fertilization field （from 1989） were determined to validate the predicted values. The results showed that the acid and alkali buffer capacities varied significantly in the soil profile, averaged 692 and 39.8 mmolc kg-1 pH-1, respectively. A significant （P〈0.05） correlation was found between pHRC and the content of calcium carbonate. Based on the commonly used application rate of urea （500 kg N ha-1 yr-1）, the induced proton input in this region was predicted to be 16.1 kmol ha-1 yr-1, and nitrification and plant uptake of nitrate were the most important mechanisms for proton producing and consuming, respectively. The acidification rate of topsoil （0-20 cm） was estimated to be 0.01 unit pH yr-1 at the assumed N fertilization level. From 1989 to 2009, topsoil pH （0-20 cm） of the long-term fertilization field decreased from 8.65 to 8.50 for the PK （phosphorus, 150 kg P205 ha-1 yr-1; potassium, 300 kg K20 ha-1 yr-1; without N fertilization）, and 8.30 for NPK （nitrogen, 300 kg N ha-1 yr-1; phosphorus, 150 kg P2Os ha-1 yr-1; potassium, 300 kg K20 ha -1 yr-1）, respectively. Therefore, the apparent soil acidification rate induced by N fertilization equaled to 0.01 unit pH yr-1, which can be a reference to the estimated result, considering the effect of atmospheric N deposition, crop biomass, field management and plant uptake of other nutrients and cations. As protons could be consumed by some field practices, such as stubble return and coupled water and nutrient management, soil pH would maintain relatively stable if proper management practices can be adopted in this region.

Effect of Phopshate Fertilizer and Manure on Crop Yield, Soil P Accumulation, and the Environmental Risk Assessment

Phosphorus （P） applied from fertilizer and manure is important in increasing crop yield and soil fertility; however, excessive uses of phosphate fertilizer and manure may also increase P loss from agricultural soils, posing environmental impact. A long term experiment was conducted on a calcareous soil （meadow cinnamon） in Hebei Province, China, from 2003 to 2006 to investigate the effects of phosphate fertilizer and manure on the yield of Chinese cabbage, soil P accumulation, P sorption saturation, soluble P in runoff water, and P leaching. P fertilizer （P2O5） application at a rate of 360 kg ha^-1 or manure of 150 t ha^-1 significantly increased Chinese cabbage yield as compared to the unfertilized control. However, no significant yield response was found with excessive phosphate or manure application. Soil Olsen-P, soluble P, bioavailable P, the degree of phosphorus sorption saturation in top soil layer （0-20 cm）, and soluble P in runoff water increased significantly with the increase of phosphate fertilizer and manure application rates, whereas the maximum phosphorus sorption capacity （Qm） decreased with the phosphate fertilizer and manure application rates. Soil Olsen-P and soluble P also increased significantly in the sub soil layer （20-40 cm） with the high P fertilizer and manure rates. It indicates that excessive P application over crop demand can lead to a high environmental risk owing to the enrichment of soil Olsen-P, soluble P, bioavailable P, and the degree of phosphorus sorption saturation in agricultural soils.

Phosphorus Changes and Sorption Characteristics in a Calcareous Soil Under Long-Term Fertilization

Knowledge of phosphorus （P） behavior in long-term fertilized soils is essential for programming fertilization practices and for sustaining environmental quality. The long-term （1984-1997） effects of various fertilization treatments on P changes and sorption isotherms as well as the relationship of soil properties to P sorption and P forms were evaluated in an Ustic Isohumisol, a calcareous soil, on the Loess Plateau, China. Compared to 1984, after 13 years of crop production, total soil P in the no-P treatments （control and N treatment） decreased by 5%-7%, but in the phosphorus fertilizer alone （P）, nitrogen and phosphorus fertilizers in combination （NP）, manure alone （M）, and nitrogen and phosphorus fertilizers and manure in combination （NPM） treatments, it increased by 22%, 19%, 28%, and 58%, respectively. Residual fertilizer P was found mainly in NH4Ac-soluble P （Cas-P）, followed by NaHCO3-soluble P （NaHCO3-P）, and NH4F-soluble P （Al-P）. Phosphorus sorption in the soils with different fertilization practices fit the Langmuir equations. Phosphorus sorption capacity in the no-P treatments increased, whereas it decreased in the P-included treatments （P, NP, and NPM treatments）. Phosphorus sorption maximum （Qm） was significantly and negatively correlated to inorganic P including NaHCO3-P, Cas-P, NaOH-Na2CO3-soluble P （Fe-P）, and Al-P （P ≤ 0.01）. Moreover, long-term fertilization increased soil organic carbon in the NP, M, and NPM treatments and decreased pH in the NP and NPM treatments. Thus, the ability of the soil to release sorbed P to the environment increased under long-term P fertilization.

Long-term phosphorus accumulation and agronomic and environmtal critical phosphorus levels in Haplic Luvisol soil, northern China

Sufficient soil phosphorus （P） content is essential for achieving optimal crop yields, but accumulation of P in the soil due to excessive P applications can cause a risk of P loss and contribute to eutrophication of surface waters. Determination of a critical soil P value is fundamental for making appropriate P fertilization recommendations to ensure safety of both environment and crop production. In this study, agronomic and environmental critical P levels were determined by using linear-linear and linear-plateau models, and two segment linear model, for a maize （Zea mays L.）-winter wheat （Triticum aestivum L.） rotation system based on a 22-yr field experiment on a Haplic Luvisol soil in northern China. This study included six treatments： control （unfertilized）, no P （NoP）, application of mineral P fertilizer （MinP）, MinP plus return of maize straw （MinP＋StrP）, MinP plus low rate of farmyard swine manure （MinP＋L.Man） and MinP plus high rate of manure （MinP＋ H.Man）. Based on the two models, the mean agronomic critical levels of soil Olsen-P for optimal maize and wheat yields were 12.3 and 12.8 mg kg-1, respectively. The environmental critical P value as an indicator for P leaching was 30.6 mg Olsen-P kg-1, which was 2.4 times higher than the agronomic critical P value （on average 12.5 mg P kg-1）. It was calculated that soil OIsen-P content would reach the environmental critical P value in 41 years in the MinP treatment, but in only 5-6 years in the two manure treatments. Application of manure could significantly raise soil Olsen-P content and cause an obvious risk of P leaching. In conclusion, the threshold range of soil Olsen-P is from 12.5 to 30.6 mg P kg-1 to optimize crop yields and meanwhile maintain relatively low risk of P leaching in Haplic Luvisol soil, northern China.

Phosphorus Status and Risk of Phosphate Leaching Loss From Vegetable Soils of Different Planting Years in Suburbs of Changsha, China

The aim of the study was to develop an index to assess the environmental risk of P loss potential in vegetable soils with chronic difference of plantation in the suburbs of Changsha, Hunan Province, China. Chemical methodology was used to study soil phosphorus status and the relationships between available P in soil and potential soil leaching P. The results showed that there was a significant linear relationship between Olsen P and CaC12-P or P concentration in soil solution. Olsen P increased sharply when either CaCI2-P or P concentration in soil solution reached a certain level. It was confirmed that 80 mg kg-t of Olsen P was the critical value of soil P leaching in the vegetable soils. P leaching probability over the critical was assessed by GIS and indicator Kriging and four secondary risks of phosphorus leaching loss were defined. In the area with vegetable cropping for over 30 yr （Chenjiadu） and 10-15 planting years （Huangxingzhen）, the indices of phosphorus leaching loss risk were 3 and 2.93, respectively. These two areas belonged to strong secondary of risk of phosphate leaching loss. In the new vegetable planting field less than 2 yr （Ningxiang）, the index was 0.06, which had almost no risk of phosphorus leaching. In vegetable soils in the suburban region of Changsha, the phosphorus leaching peotential is high and the phosphorus leaching loss is related to chronic length of vegetable cropping.

Comparisons of extraction and purification methods of soil microorganism DNA from rhizosphere soil

Microorganism DNA of rhizosphere soil from Pinus koraiensis and Pinus sylvestriformis were extracted by proteinase K based on SDS method, CTAB method, PVP （polyvinylpolypyrrolidone） method, and freezing and thawing method and the crude DNA from rhizosphere soil were purified by dialysis method, silver beads absorption method, and squeezing DNA gel method. The results of different extracting and purifying methods were compared and evaluated. Results indicated that the best method of extraction for microorganism DNA in rhizosphere soil was proteinse K based on SDS method with high salt concentration of 1.0% （w/v） NaCl, which could effectively eliminate humic acids and other impurities. The dialysis method was suitable to purify DNA from rhizosphere soil because of effectively removing brown matters and humic acids and the purified products were suited to PCR amplification. Squeezing DNA gel method was also a good purification method with the advantage of inexpensive in cost and efficient in use.

Effect of Nitrogen Fertilizers on Movement and Transformation of Phosphorus in an Acid Soil

The effects of two different nitrogen fertilizers （urea and NH4C1） with monocalcium phosphate （MCP） on the movement and transformation of fertilizer P in soil microsites along with soil pH changes at different distances from the fertilizer application site were studied in an incubation experiment. A highly acidic red soil （Ultisol, pH 4.57） from south China with MCP fertilizer alone or in combination with NH4C1 or urea was added to the surface of soil cylinders and packed in wax blocks. After 7 and 28 days, the extraction and analysis of each 2 mm layer from the interface of the soil and fertilizer showed that added NH4C1 or urea did not change the movement distance of fertilizer P. However, P transformation was significantly affected （P 〈 0.05）. After 7 days, at 0-8 mm distance from the fertilizer site the addition of urea significantly decreased the water-extractable P concentration; however, after 28 days the effect of N addition had disappeared. Also,at limited distances close to the fertilizer site NH4Cl application with MCP significantly increased acid-extractable P and available P, while with the addition of urea they significantly decreased. Compared with application of MCP alone,addition of urea significantly increased soil pH in fertilizer microsites, whereas the addition of NH4Cl significantly decreased soil pH.

Rhizosphere soil bacterial community composition in soybean genotypes and feedback to soil P availability

Soil with low phosphorus (P) availability and organic matter contents exists in large area of southwest of China, but some soybean genotypes still show well adaptations to this low yield farmland. However, to date, the underlying mechanisms of how soybean regulates soil P availability still remains unclear, like microbe-induced changes. The objective of the present study was to compare the differences of rhizosphere bacterial community composition between E311 and E109 in P-sufficiency (10.2 mg kg^-1) and P-insufficiency (5.5 mg kg^-1), respectively, which then feedback to soil P availability. In P-sufficiency, significant differences of the bacterial community composition were observed, with fast-growth bacterial phylum Proteobacteria, genus Dechloromonas, Pseudomonas, Massilia, and Propionibacterium that showed greater relative abundances in E311 compared to E109, while in P-insufficiency were not. A similar result was obtained that E311 and E109 were clustered together in P-insufficiency rather than in P-sufficiency by using principal component analysis and hierarchical clustering analysis. The quadratic relationships between bacterial diversity and soil P availability in rhizosphere were analyzed, confirming that bacterial diversity enhanced the soil P availability. Moreover, the high abundance of Pseudomonas and Massilia in the rhizosphere of E311 might increased the P availability. In the present study, the soybean E311 showed capability of shaping rhizosphere bacterial diversity, and subsequently, increasing soil P availability. This study provided a strategy for rhizosphere management through soybean genotype selection and breeding to increase P use efficiency, or upgrade middle or low yield farmland.

Response of Tomato on Calcareous Soils to Different Seedbed Phosphorus Application Rates

Field experiments were conducted with five rates （0, 75, 150, 225, and 450 kg P205 ha^-1） of seedbed P fertilizer application to investigate the yield of tomato in response to fertilizer P rate on calcareous soils with widely different levels of Olsen P （13-142 mg kg^-1） at 15 sites in some suburban counties of Beijing in 1999. Under the condition of no P fertilizer application, tomato yield generally increased with an increase in soil test P levels, and the agronomic level for soil testing P measured with Olsen method was 50 or 82 mg kg^-1 soil to achieve 85% or 95% of maximum tomato yield, respectively. With regard to marketable yield, in the fields where Olsen-P levels were 〈 50 mg kg^-1, noticeable responses to applied P were observed. On the basis of a linear plateau regression, the optimum seedbed P application rate in the P-insufflcient fields was 125 kg P205 ha^-1 or about 1.5-2 times the P removal from harvested tomato plants. In contrast, in fields with moderate （50 〈 Olsen P 〈 90 mg kg^-1） or high （Olsen P 〉 90 mg kg^-1） available P, there was no marked effect on tomato fruit yield. Field survey data indicated that in most fields with conventional P management, a P surplus typically occurred. Thus, once the soil test P level reached the optimum for crop yield, it was recommended that P fertilizer application be restricted or eliminated to minimize negative environmental effects.

Effects of Soil Properties on Phosphorus Subsurface Migration in Sandy Soils

The soil factors influencing the potential migration of dissolved and particulate phosphorus(P) from structurally-weak sandy subsoils were evaluated by means of soil column leaching experiments. Soil colloids were extracted from two types of soils to make the colloid-bound forms of P solution. Eight sandy soils with diverse properties were collected for packing soil columns. The effects of influent solutions varying in concentrations of colloids,P,and electrolyte,on the transport of P and quality of leachates were characterized. P migration in the soils was soil property-dependent. High soil electrical conductivity values retarded the mobility of colloids and transportability of colloid-associated P(particulate P) . Soil electrical conductivity was negatively correlated with colloids and reactive particulate P(RPP) concentrations in the leachates,whereas,the total reactive P(TRP) and dissolved reactive P(DRP) concentrations in the leachates were mainly controlled by the P adsorption capacity and the P levels in the subsoil. The reactive particulate P in the leachates was positively correlated with the colloidal concentration. Increased colloidal concentration in the influent could significantly increase the colloidal concentration in the leachates. Elevated P concentration in the influent had little effect on P recovery in the leachates,but it resulted in significant increases in the absolute P concentration in the leachates.