The responses of carbon-and nitrogen-acquiring enzymes to nitrogen and phosphorus additions in two plantations in southern China

In tropical plantations,nutrients such as nitrogen(N)or phosphorus(P)are often applied as management practices.However,the effects of such nutrient additions on topsoil C-and N-acquiring enzymes activities are unclear.In this study,the impacts of fertilization onβ-1,4-glucosidase(BG),β-D-cellobiosidase(CBH),β-1,4-xylosidase(BX),β-1,4-N-acetyl-glucosaminidase(NAG),and leucine amino peptidase(LAP)enzymes activities from topsoil and litter layer of two tropical plantations(Acacia auriculiformis and Eucalyptus urophylla)were measured.The results showed that N addition had neutral impact on topsoil enzymes,while significantly elevating the activities of BG,CBH,BX,and NAG in the litter layer.P fertilization had no impacts except for an elevation of NAG in litter sample.There was no interactions found between N and P additions on these enzyme activities.The clearer impacts of N over P fertilization were unexpected because that the study site receives a high rate of atmospheric N deposition,and has low soil P availability.The impact of P fertilization on hydrolytic enzyme activities may be less important compared with that of N.

Effect of nitrogen addition on DOC leaching and chemical exchanges on canopy leaves in Guangdong Province, China

The impact of nitrogen(N)deposition on dissolved organic carbon(DOC)fractions in throughfall is not well understood.We performed a laboratory experiment and compared DOC leaching from canopy leaves after dipping leaves in pure water(control)and NH4NO3 solution(N-treatment)for 18 h.Net changes of DOC,NH4^+,NO3^-,SO4^2-,K^+,Mg^2+,Ca^2+and H^+contents after dipping leaves were determined by comparing solutions with and without leaves.We recorded no differences of DOC leaching between control and N-treatment,implying that N deposition had minor impacts on canopy DOC production.This confirmed that previous experiments testing the effects of N addition on DOC dynamics without considering the effects of the canopy reaction successfully described the real situation.We also confirmed the previously-reported canopy exchange process in spite of a high background N deposition at our study site.N-treatment significantly increased base cation leaching,especially K^+,and the increase was positively correlated with foliar NH4?retention.Net leaching of H^+and SO4^2-was not affected by the N-treatment.

Testing potassium limitation on soil microbial activity in a sub-tropical forest

Because potassium(K)is a rock-derived essential element that can be depleted in highly-weathered tropical soils,K availability may limit some portion of soil microbial activity in tropical forest ecosystems.In this paper we tested if K limits microbial activity in the condition of sufficient labile C supply.An incubation experiment was performed using surface soil samples(0–10 cm depth)obtained from four permanent ecological research plots in a natural sub-tropical forest in southern China.Soil samples were taken in September 2016.Heterotrophic soil respiration rates and microbial biomass were measured after the addition of glucose(both D and L)with and without K(potassium chloride).We did not observe any effects of K addition on soil microbial respiration,suggesting that K does not limit the microbial activity in the condition of sufficient labile C supply.The lack of microbial response to added K can be attributed to the high mobility of K in forest ecosystems,which may have provided sufficient K to microbes in our soil samples(already provided at the beginning of the incubation).However,at the present stage,we cannot conclude that K is not a limiting factor of soil microbial activity in other tropical forest ecosystems because of the heterogeneity of tropical forest ecosystems and few observations.The hypothesis needs to be tested in larger numbers of tropical forests.

长期磷添加对热带森林土壤磷酸单酯酶与磷酸二酯酶活性比值的影响

热带森林土壤微生物可以通过改变不同类型的磷酸酶活性来适应低磷的环境。以往的研究认为,低磷环境中土壤微生物更倾向于分解有机磷单酯键而非二酯键。因此,磷酸单酯酶(PME)的活性变化对生态系统磷输入的响应可能更敏感,其与磷酸二酯酶(PDE)活性的比值在磷输入后将显著降低。为了检验土壤微生物在低磷的热带森林中对不同磷酸酶的“投资”策略,我们在鼎湖山3种热带森林进行了长达10年的磷添加实验。与以往的研究不同,本研究发现长期磷添加没有改变原生林与次生林的PME:PDE比值,但显著提高了人工林土壤PME:PDE比值。结果表明,长期磷添加可能减少了凋落物或穿透雨来源的PDE,从而使其土壤PDE活性下降较PME降低更快,因而PME:PDE比值未能降低。

Phosphorus limitation on photosynthesis of two dominant understory species in a lowland tropical forest

Aims Elevated nitrogen(N)deposition in tropical regions may accelerate ecosystem phosphorus(P)limitation.However,it is not explicitly addressed that how changes in soil N and P availability affect foliar nutrients and photosynthesis of plants in tropical forests.In this study,we examined the effects of N and P additions on foliar nutrients and net photosynthesis of two dominant understory species,Randia can-thioides(R.canthioides)and Cryptocarya concinna(C.concinna)in an N-saturated old-growth tropical forest(>400-year-old)in south-ern China.Methods A full factorial NP addition experiment(2×2)was established in 2007 and continued through August 2010.Four treatments,including control,N addition(150 kg N ha^(−1) year^(−1)),P addi-tion(150 kg P ha^(−1) year^(−1))and NP addition(150 kg N ha^(−1) year^(−1) plus 150 kg P ha^(−1) year^(−1))were set up in this experiment.Photosynthetic traits maximum photosynthetic CO_(2) assimilation(Amax),stomatal conductance(gs),leaf transpiration(E),light satu-rating point,concentrations of chlorophyll a/b and foliar nutri-ents(N and P)of the two species were measured with standard methods.Important Findings Three years of N addition had no significant effects on any measured photosynthetic parameter of either species.However,N addition significantly elevated foliar N and P concentrations of one species(R.canthioides),resulting in lower photosynthetic nitrogen use effi-ciency(PNUE).N treatments decreased foliar P concentration of the other(C.concinna),resulting in increased photosynthetic phos-phorus use efficiency,which was potentially related to N-induced P shortage.In contrast,positive effects of P treatments on gs of R.can-thioides,Amax and chlorophyll a+b of C.concinna were observed.P treatments also elevated foliar P and PNUE of both species,imply-ing P induced more efficient use of N.Our results suggested a more important role of P than N on influencing photosynthetic traits of these two understory species.Alleviation of P shortage through P addition may enhance photosynthetic performances of some under-story species in N-rich tropical forests.