Effect of Long－Term Application of Compost on Some Chemical Properties of Wheat Rhizosphere and NonRhizosphere Soils

Compost of dtherent rates was applied to artificial field plots of a low humic andosol at National Agriculture Research Center (NARC) of Japan for 15 or 28 years, and their effects on the chemical properties of wheat rhisosphere soil and nonrhizosphere soil were measured.Continuous application of compost for 28 years resulted in raise of soil C, N, P, pH and exchangeable bases. The building up of organic matter in the soil occurred slowly A residual effect of the compost on soil chemical properties was still present after 13 years of no application, but this effect was weaker in comparison with that of the continuous application treatments. In the rhizosphere soil, NaHCO3-extracted P and exchangeable Ca were higher than those in the bulk soil. The removal of free organic acid slightly affected the soil pH, especially, the rhizosphere soil pH. The raise of soil pH may result from the increase of exchangeable base by the application of compost.

Rhizosphere organic phosphorus fractions of Simon poplar and Mongolian pine plantations in a semiarid sandy land of northeastern China

The aim of this study was to investigate the role of rhizosphere organic phosphorus （P） in soil P supply in semiarid forests and the effects of tree species on rihizosphere organic P. We examined organic P fractions in rhizosphere and bulk soils of mono-specific Simon poplar （Populus simonii） and Mongolian pine （Pinus sylvestris var. mongolica） plantations in a semiarid sandy soil of Horqin Sandy Land in Northeast China. Total organic P （TPo） accounted for 76% of total P across the two stands. The concentration of organic P （Po） fractions decreased in the order of NaOH-Po〉Res-Po〉HCI-Po〉NaHCO3-Po in both plantations. The concentration of NaHCO3-Po was 38% and 43% lower in rhizosphere soil than in bulk soil in Simon poplar and Mongolian pine plantations, respectively. In contrast, total P, TPo and NaOH-Po significantly accumulated in rhizosphere soil in Simon poplar plantations, but no change in Mongolian pine plantations. Soil recalcitrant organic P fractions were positively correlated with soil organic carbon. The results suggest that rhizosphere labile organic P was an important source of plant-available P in this semiarid region, but the dynamic of rhizosphere recalcitrant organic P fractions varied with tree species and was correlated to organic carbon dynamics.

Nitrogen fertilization has minimal influence on rhizosphere effects of smooth crabgrass (Digitaria ischaemum) and bermudagrass (Cynodon dactylon)

Aims Plants generally respond to nitrogen(N)fertilization with increased growth,but N addition can also suppress rhizosphere effects,which consequently alters soil processes.We quantified the influence of N addition on rhizosphere effects of two C4 grasses:smooth crabgrass(Digitaria ischaemum)and bermudagrass(Cynodon dactylon).Methods Plants were grown in nutrient-poor soil for 80 days with either 20 or 120μg NH4No3-N g dry soil−1.N mineralization rates,microbial biomass,extracellular enzyme activities and bacterial community structure were measured on both rhizosphere and bulk(unplanted)soils after plant harvest.Important Findings Fertilization showed nominal differences in net N mineralization,extracellular enzyme activity and microbial biomass between the rhizosphere and bulk soils,indicating minimal influence of N on rhizosphere effects.Instead,the presence of plant roots showed the strongest impact(up to 80%)on rates of net N mineralization and activities of three soil enzymes indicative of N release from organic matter.Principal component analysis of terminal restriction fragment length polymorphism(t-rFlP)also reflected these trends by highlighting the importance of plant roots in structuring the soil bacterial community,followed by plant species and N fertilization(to a minor extent).overall,the results indicate minor contributions of short-term N fertilization to changes in the magnitude of rhizos-phere effects for both grass species.

The rhizosphere effect on soil gross nitrogen mineralization: A meta-analysis

Rhizosphere effects play crucial roles in determining soil carbon(C)and nitrogen(N)cycling.However,the rhizosphere effect on soil gross nitrogen(N)mineralization(Nmin)has not been quantitatively assessed on the global scale.Here we performed a meta-analysis of compiled data from 24 publications and 37 species to synthesize the rhizosphere effect on soil gross Nmin and its influencing factors.We found that the rhizosphere effect significantly enhanced soil gross Nmin by 81%on average.Such rhizosphere effect was significantly higher in woody species than in nonwoody species,and higher in ECM(ectomycorrhizal)associated species than in AM(arbuscular mycorrhizal)associated species.Moreover,the variations of the rhizosphere effect on soil gross Nmin were correlated with those on soil C mineralization,phenol oxidase activity and root biomass rather than with other plant(growth form and mycorrhizal association)and climatic(mean annual temperature and precipitation)factors.These results support the‘microbial activation’and‘microbial N mining’hypotheses of rhizosphere effects and indicate the coupling of soil C and gross N mineralization in the rhizosphere.Overall,these findings provide novel insights into the rhizosphere effect on soil gross Nmin among plant growth forms and mycorrhizal associations,and improve our mechanistic understanding of soil N dynamics in the rhizosphere.

Magnitude,direction,and drivers of rhizosphere effect on soil nitrogen and phosphorus in global agroecosystem

The rhizosphere is the most active soil area for material transformation and energy flow of soil,root,and microorganism,which plays an important role in soil biochemical cycling.Although the rhizospheric nitrogen(N)and phosphorous(P)were easily disturbed in the agroecosystem,the effects of rhizosphere on the dynamics of soil N and P cycling have not yet been systematically quantified globally.We summarized the magnitude,direction,and driving forces of rhizosphere effects on agroecosystem's N and P dynamics by 1063 observations and 15 variables from 122 literature.Rhizosphere effects increased available N(AN,9%),available P(AP,11%),and total P(TP,5%),and decreased nitrate N(NO_(3)-N,18%)and ammonia N(NH_(4)-N,16%).The effect of rhizosphere on total N(TN)was not significant.These effects improved AN in tropical(12%)and subtropical(14%)regions.The effect of rhizosphere on TP was greater under subtropical conditions than in other climates.The most substantial effects of the rhizosphere on TP and AP were observed under humid conditions.Rhizosphere effects increased AN and AP in vegetables more than in other crop systems.Application of N>30o kg ha^(-1) had the most significant and positive rhizosphere effects on TN and AN.P application of 100-150 kg ha^(-1) had the greatest rhizosphere effects on TP and AP.These effects also improved the microbial(biomass N and P)and enzymatic aspects(urease,acid phosphatase,and alkaline phosphatase)of soil P and N cycling.Structural equation modeling suggested that aridity indices,fertilizer application rate,soil pH,microbial biomass,and soil enzymes strongly influence the magnitude and direction of the rhizosphere's effect on the P and N cycles.Overall,these findings are critical for improving soil nutrient utilization efficiency and modeling nutrient cycling in the rhizosphereforagricultural systems.

Influencing factors and partitioning methods of carbonate contribution to CO_(2)emissions from calcareous soils

In calcareous soils,recent studies have shown that soil-derived CO_(2)originates from both soil organic carbon(SOC)decomposition and soil inorganic carbon(SIC)dissolution,a fact often ignored in earlier studies.This may lead to overestimation of the CO_(2)emissions from SOC decomposition.In calcareous soils,there is a chemical balance between precipitation and dissolution of CaCO_(3)-CO_(2)-HCO_(3),which is affected by soil environmental factors(moisture,temperature,pH and depth),root growth(rhizosphere effect)and agricultural measures(organic materials input,nitrogen fertilization and straw removal).In this paper,we first introduced the contribution of SIC dissolution to CO_(2)emissions from calcareous soils and their driving factors.Second,we reviewed the methods to distinguish two CO_(2)sources released from calcareous soils and quantify the 13C fractionation coefficient between SIC and SIC-derived CO_(2)and between SOC and SOC-derived CO_(2),and to partition three CO_(2)sources released from soils with plants and organic materials input.Finally,we proposed methods for accurately distinguishing three CO_(2)sources released from calcareous soils.This review helps to improve the accuracy of soil C balance assessment in calcareous soils,and also proposes the direction of further investigations on SIC-derived CO_(2)emissions responses to abiotic factors and agricultural measures.

Microbial community and functional genes in the rhizosphere of alfalfa in crude oil-contaminated soil

A rhizobox system constructed with crude oil- contaminated soil was vegetated with alfalfa （Medicago sativa L.） to evaluate the rhizosphere effects on the soil microbial population and functional structure, and to explore the potential mechanisms by which plants enhance the removal of crude oil in soil. During the 80-day experiment, 31.6% of oil was removed from the adjacent rhizosphere （AR）; this value was 27% and 53% higher than the percentage of oil removed from the far rhizosphere （FR） and from the non-rhizosphere （NR）, respectively. The populations of heterotrophic bacteria and hydrocarbon- degrading bacteria were higher in the AR and FR than in the NR. However, the removal rate of crude oil was positively correlated with the proportion of hydrocarbon- degrading bacteria in the rhizosphere. In total, 796, 731, and 379 functional genes were detected by microarray in the AR, FR, and NR, respectively. Higher proportions of functional genes related to carbon degradation and organic remediation, were found in rhizosphere soil compared with NR soil, suggesting that the rhizosphere selectively increased the abundance of these specific functional genes. The increase in water-holding capacity and decrease in pH as well as salinity of the soil all followed the order of AR 〉 FR 〉 NR. Canonical component analysis showed that salinity was the most important environmental factor influencing the microbial functional structure in the rhizosphere and that salinity was negatively correlated with the abundance of carbon and organic degradation genes.