Trajectories in nitrogen availability during forest secondary succession:illustrated by foliarδ^(15)N

Background:Forest succession is an important ecological process and has been studied for more than a century.However,changes in nitrogen(N)availability during succession remain unclear as they may lead to either N satura-tion or N limitation.Here,we propose a conceptual model to illustrate changes in N availability during four stages of secondary succession using the natural abundance of ^(15) N in plant leaves(foliarδ^(15)N).We predicted that N availability would decline in the early stages of succession and then increase in late stages,coinciding with the changes in foliarδ^(15)N,with the inflection point varying in different climate zones.Data on foliarδ^(15)N from 16 succession sequences were synthesized to explore changes in N availability during forest succession.Results:The compiled data were consistent with the proposed conceptual model.Foliarδ^(15)N in boreal and temperate forests decreased significantly in the first two stages of succession(estimated to last at least 66 years in temperate forests),at a rate of 0.18‰and 0.38‰per decade,respectively,and decreased slightly in tropical forests in the first 23 years.Foliarδ^(15)N is projected to increase in later stages in all forests,which is supported by observations in both temperate and tropical forests.The inflection points of N availability when N limitation peaked during succession were different in different climate zones,implying different ecosystem N turnovers.Conclusions:Our study reconciles the controversies regarding changes in N availability during forest secondary succession.Our findings are also useful for predicting the recovery of N and carbon accumulation during succession.Nonetheless,studies on forest secondary succession using foliarδ^(15)N have thus far been limited,and more research should be conducted to further verify the conceptual model proposed here.

Introduction of Dalbergia odorifera enhances nitrogen absorption on Eucalyptus through stimulating microbially mediated soil nitrogen-cycling

Background:There is substantial evidence that Eucalyptus for nitrogen(N)absorption and increasing the growth benefit from the introduction of N-fixing species,but the underlying mechanisms for microbially mediated soil N cycling remains unclear.Methods:We investigated the changes of soil pH,soil water content(SWC),soil organic carbon(SOC),total N(TN),inorganic N(NH_(4)^(+)-N and NO_(3)^(-)-N),microbial biomass and three N-degrading enzyme activities as well as the biomass and N productivity of Eucalyptus between a pure Eucalyptus urophylla×grandis plantation(PP)and a mixed Dalbergia odorifera and Eucalyptus plantation(MP)in Guangxi Zhuang Autonomous Region,China.Results:Compared with the PP site,soil pH,SWC,SOC and TN in both seasons were significantly higher at the MP site,which in turn enhanced microbial biomass and the activities of soil N-degrading enzymes.The stimulated microbial activity at the MP site likely accelerate soil N mineralization,providing more available N(NH_(4)^(+)-N in both seasons and NO_(3)^(-)-N in the wet-hot season)for Eucalyptus absorption.Overall,the N productivity of Eucalyptus at the MP site was increased by 19.7% and 21.9%,promoting the biomass increases of 15.1% and 19.2% in the drycold season and wet-hot season,respectively.Conclusion:Our results reveal the importance of microbially mediated soil N cycling in the N absorption on Eucalyptus.Introduction of D.odorifera enhances Eucalyptus biomass and N productivity,improve soil N availability and increased soil C and N concentration,which hence can be considered to be an effective sustainable management option of Eucalyptus plantations.

Maize-legume intercropping promote N uptake through changing the root spatial distribution,legume nodulation capacity,and soil N availability

Legume cultivars affect N uptake,component crop growth,and soil physical and chemical characteristics in maize-legume intercropping systems.However,how belowground interactions mediate root growth,N fixation,and nodulation of different legumes to affect N uptake is still unclear.Hence,a two-year experiment was conducted with five planting patterns,i.e.,maize-soybean strip intercropping(IMS),maize-peanut strip intercropping(IMP),and corresponding monocultures(monoculture maize(MM),monoculture soybean(MS),and monoculture peanut(MP)),and two N application rates,i.e.,no N fertilizer(N-)and conventional N fertilizer(N+),to examine relationships between N uptake and root distribution of crops,legume nodulation and soil N availability.Results showed that the averaged N uptake per unit area of intercrops was significantly lower than the corresponding monocultures.Compared with the monoculture system,the N uptake of the intercropping systems increased by 31.7-45.4%in IMS and by 7.4-12.2%in IMP,respectively.The N uptake per plant of intercropped maize and soybean significantly increased by 61.6 and 31.8%,and that of intercropped peanuts significantly decreased by 46.6%compared with the corresponding monocultures.Maize and soybean showed asymmetrical distribution of roots in strip intercropping systems.The root length density(RLD)and root surface area density(RSAD)of intercropped maize and soybean were significantly greater than that of the corresponding monocultures.The roots of intercropped peanuts were confined,which resulted in decreased RLD and RSAD compared with the monoculture.The nodule number and nodule fresh weight of soybean were significantly greater in IMS than in MS,and those of peanut were significantly lower in IMP than in MP.The soil protease,urease,and nitrate reductase activities of maize and soybean were significantly greater in IMS and IMP than in the corresponding monoculture,while the enzyme activities of peanut were significantly lower in IMP than in MP.The soil available N of maize and soybean was significantly greater increased in IMS and IMP than in the corresponding monocultures,while that of IMP was significantly lower than in MP.In summary,the IMS system was more beneficial to N uptake than the IMP system.The intercropping of maize and legumes can promote the N uptake of maize,thus reducing the need for N application and improving agricultural sustainability.

Effects of substituting chemical fertilizers with manure on rice yield and soil labile nitrogen in paddy fields of China: A meta-analysis

Substituting chemical fertilizers with manure is an important method for efficient nutrient management in rice cropping systems of China.Labile nitrogen(N)is the most active component of the soil N pool and plays an essential role in soil fertility.However,the effects of manure substitution on soil labile N in rice cropping systems and their relationships with soil properties,fertilization practices,and climatic conditions remain unclear and should be systematically quantified.Here,we investigated rice grain yield and four types of soil labile N that have been widely reported,including available nitrogen(AN),ammonium nitrogen(NH_(4)^(+)-N),nitrate nitrogen(NO_(3)^(−)-N),and microbial biomass nitrogen(MBN).We reviewed 187 published articles and performed a meta-analysis to quantify the effects of manure substitution on yield and soil labile N.The results showed that manure substitution increased AN,MBN,NH+4-N,and NO−3-N by 11.3%,38.5%,5.9%,and 8.1%,respectively.Partial substitution significantly increased the yield by 1.4%–5.9%,but full substitution significantly decreased the yield by 2.9%.The positive effects of manure substitution on yield and AN were stronger with long-term fertilization.The differences in responses varied across specific manure types,N application rates,soil properties,and climatic factors.In conclusion,manure substitution can increase soil labile N and is regarded as an efficient strategy for improving soil N fertility and a recommended measure for applying both chemical and organic fertilizers in rice systems.This study provides evidence of the effects of manure substitution on yield enhancement by increasing soil labile N.

How silicon fertilizer improves nitrogen and phosphorus nutrient availability in paddy soil?

In order to reveal the mechanism of silicon(Si)fertilizer in improving nitrogen(N)and phosphorus(P)nutrient availability in paddy soil,we designed a series of soil culture experiments by combining application of varying Si fertilizer concentrations with fixed N and P fertilizer concentrations.Following the recommendations of fertilizer manufacturers and local farmers,we applied Si in concentrations of 0,5.2,10.4,15.6,and 20.8μg/kg.At each concentration of added Si,the availability of soil N and P nutrients,soil microbial activity,numbers of ammonia-oxidizing bacteria and P-decomposing bacteria which means that the organic P is decomposed into inorganic nutrients which can be absorbed and utilized by plants,and urease and phosphatase activity first increased,and then decreased,as Si was added to the soil.These indicators reached their highest levels with a Si application rate of 15.6μg/kg,showing values respectively 19.78%,105.09%,8.34%,73.12%,130.36%,28.12%,and 20.15%higher than those of the controls.Appropriate Si application(10.4 to 15.6µg/kg)could significantly increase the richness of the soil microbial community involved in cycling of N and P nutrients in the soil.When the Si application rate was 15.6μg/kg,parameters for characterizing microbial abundance such as sequence numbers,operational taxonomic unit(OTU)number,and correlation indices of microbial community richness such as Chao1 index,the adaptive coherence estimator(ACE)index,Shannon index,and Simpson index all reached maximum values,with amounts increased by 14.46%,10.01%,23.80%,30.54%,0.18%,and 2.64%,respectively,compared with the control group.There is also a good correlation between N and P mineralization and addition of Si fertilizer.The correlation coefficients between the ratio of available P/total P(AP/TP)and the number of ammonia-oxidizing bacteria,AP/TP and acid phosphatase activity(AcPA),AP/TP and the Shannon index,the ratio of available N/total amount of N(AN/TN)and the number of ammoniated bacteria,and AN/TN and AcPA were 0.9290,0.9508,0.9202,0.9140,and 0.9366,respectively.In summary,these results revealed that enhancement of soil microbial community structure diversity and soil microbial activity by appropriate application of Si is the key ecological mechanism by which application of Si fertilizer improves N and P nutrient availability.

Effect of nitrogen limitation on antioxidant qualities is highly associated with genotypes of lettuce(Lactuca sativa L.)

Antioxidants are highly beneficial to human health, and their accumulation in lettuce, one of the most popular leafy vegetables, depends on both genetic and environmental factors. Nitrogen(N) availability plays an essential role in regulating antioxidant accumulation, but the influence of genotype × N interactions on the antioxidant qualities of lettuce is poorly understood. Therefore, the present study investigated the variation of growth and antioxidant qualities of 20 lettuce(Lactuca sativa L.) genotypes(10 green lettuce genotypes and 10 red lettuce genotypes) under limited N(low N, LN) conditions and standard N(high N, HN) conditions. For all 20 genotypes, LN conditions reduced shoot(i.e., leaf) growth, but increased plant concentrations of vitamin C,glutathione, and phenolic compounds, with the exception of carotenoids, compared with HN conditions. Because of reduced biomass under LN conditions, not all lettuce genotypes exhibited increased antioxidant yields or total antioxidant capacity yield. The variation in antioxidant quality was primarily genetically determined. Generally, the green lettuce genotypes exhibited more pronounced increases in antioxidant yields and total antioxidant capacity yield than the red lettuce genotypes under LN conditions. These results suggest that even though LN conditions generally tend to improve the antioxidant qualities of lettuce, the extent of this effect is highly dependent on genotype. Therefore, genotype should be given priority in future studies that aim to improve antioxidant qualities in lettuce through N management.

Alkaline hydrolyzable nitrogen and properties that dictate its distribution in paddy soil profiles

Nitrogen(N) is a key nutrient for rice production, and its bioavailability in paddy soils is strongly coupled to soil organic matter(SOM) cycling. A better understanding of potentially available N forms in soil, such as alkaline hydrolyzable N(AH-N), and their depth distribution will support the development of best management practices to improve the N use efficiency of rice while minimizing adverse environmental effects. Fifteen rice(Oryza sativa L.) fields from Southern Brazil were selected, and stratified soil samples were taken to a depth of 60 cm before crop establishment. Selected soil physical and chemical properties were analyzed to evaluate their relationships with AH-N contents in the soil profile. The AH-N contents below 20 cm varied extensively(increased,reduced, or constant) compared with that above 20 cm. Although clay and clay + silt contents were highly correlated to AH-N for some soils, the major property dictating AH-N distribution by depth was total N(TN), as the correlation between TN and AH-N was mainly by direct effect. The proportion of TN recovered as AH-N across sites and depths presented high amplitude, and thus AH-N was not a constant N pool across depths, indicating that AH-N can be affected by soil management practices even when TN showed no major changes. The distinct distribution of AH-N across soil sampling sites and depths indicates that depths greater than 20 cm should be considered when calibrating the AH-N index for N fertilizer recommendations for flooded rice in Southern Brazil.

Comparative analysis of planted and unplanted controls for assessment of rhizosphere priming effect

The rhizosphere priming effect(RPE)is increasingly being considered to be an important regulator of soil organic matter(SOM)decomposition and nutrient turnover,with potential importance for the global CO_(2) budget.As a result,studies on the RPE have rapidly increased in number over the last few years.Most of these experiments have been performed using unplanted soil as the control,which could potentially lead to incorrect assessment of the RPE.Therefore,we performed a greenhouse experiment to investigate how the choice of control(i.e.,unplanted control and planted control)influenced the quantification of RPE on SOM decomposition and gross nitrogen(N)mineralization,and to link this to differences in microbial and abiotic soil properties between the two controls.In the planted control,planted seedlings were cut at soil surface 5 d before measurement of the RPE.The RPE on SOM decomposition was positive in pine soil and almost 2-fold higher when calculated from the planted control than from the unplanted control.In spruce soil,a negative RPE on SOM decomposition was found when calculated from the planted control,while the RPE was positive when calculated from the unplanted control.No RPE on gross N mineralization was found when calculated from the planted control,while a positive RPE of more than 100%was found when calculated from the unplanted control.The microbial biomass and growth rate were lower,while the inorganic N content was higher in the unplanted control than in the planted control.The microbial community composition and potential enzyme activity in the planted treatment and planted control were similar,but they differed significantly from those in the unplanted control.The results showed that the RPE varied widely depending on the choice of control;thus,we suggest that a planted control,in which the aboveground plant parts are removed only a few days before the measurement of RPE,should be used as the control when elucidating the RPE on belowground C and N cycling responses to environmental change.