Phosphorus fertilizer induced changes in the soil available P,the P nutrition and the growth of Pinus radiata seedlings grown in association with understory

A study was carried out to investigate changes in the soil plant-available P,the P nutrition and the growth of Pinus radiata seedlings grown in association with understory,broom（Cytisus scoparius L.） or ryegrass（Lolium multiflorum） on Orthic Allophanic Soil,following the application of three rates of triple superphosphate（TSP）（0,50,and 100 mg·kg^-1P） under a glasshouse condition.The application of P fertilizer enhanced P availability in the rhizospheric of radiata seedlings and the bulk soils in a P-deficient site.P availability in the rhizospheric soils of ryegrass and broom,grown in association with radiata,were also increased by the presence of radiata roots.P concentrations in new shoot needles,old shoot needles,stem and roots of radiata pine increased with increase rates of TSP application,but the effects of ryegrass and broom on P nutrition of radiata seedlings depended on the soil P status.In the absence of P fertilizer addition（control treatment）,P concentrations in new shoot needles,old shoot needles,stem,and roots of radiata grown in association with broom were higher than those with ryegrass,whereas,when P fertilizer was added（50 and 100 mg·kg^-1） the P concentration was lower.This is probably related to the growth of broom that may have removed much of the plant-available P in the soil as indicated by the consistently lower Bray-2 P concentration in the rhizosphere soil of radiata in association with broom than that in the rhizosphere soil of radiata in association with grass at the two high P rates.Furthermore,in the high P fertile soil（application rate of 100 mg·kg^-1）,the dry matter yield of radiata was lower when it was grown with broom than with ryegrass.This result suggests that in moderate to high P fertile soils,P.radiata seedlings grow better with ryegrass than with broom,because broom grows vigorously in high P fertile soil and competes with P.radiata for P and perhaps for other nutrients as well.

Relationship between acid phosphatase activity and P concentration in organs of Cyclopia and Aspalathus species,and a non-legume of the Cape Floristic Region

Aims The role of tissue acid phosphatase(APase)activity of legumes and non-legumes in their P nutrition and adaptation to low-P soils is not well understood.To better understand this,a relationship between APase activ-ity and P concentration in leaves,stems,roots and nodules of legumes,Cyclopia and Aspalathus and a non-legume,Leucadendron strictum,all native to the P-poor soils of the Cape fynbos biome,was assessed.Methods Plants were collected and each separated into leaves,stems and roots.Phosphatase enzyme activity was assayed in soil using the p-nitrophenol method,while soil P and shoot P were measured using ICP-MS.To measure tissue APase activity,an acetate buffer was added into ground plant material and contents filtered.An ace-tate buffer and a p-nitrophenyl solution were added to the superna-tant and contents incubated.After incubation,NaOH(0.5 M)was added and absorbance read at 405 nm.Important Findings At Koksrivier,Cyclopia genistoides exhibited the highest leaf enzyme activity whilst Aspalathus aspalathoides showed the highest enzyme activity in the stems.At both Kleinberg and Kanetberg,Cyclopia subternata and Cyclopia longifolia showed the highest APase activity in leaves,followed by stems and low-est in roots.P concentration closely mirrored enzyme activity in organs of all test species from each site.APase activity posi-tively correlated with P concentration in organs of all the test Cyclopia and Leucadendron species,indicating that intracellular APase activity is directly linked to P mobilization and transloca-tion in these species.Percentage of N derived from fixation was positively correlated with tissue APase activity in C.genistoides(r=0.911^(*)),A.aspalathoides(r=0.868^(*))and Aspalathus cal-edonensis(r=0.957^(*)),suggesting that APase activity could be directly or indirectly linked to symbiotic functioning in these fynbos legumes,possibly via increased P supply to sites of N2 fixation.