Phosphatase activity and phosphorus fractions in Karri (Eucalyptus diversicolor F. Muell.) forest soils

Summary Karri forest soils contain negligible concentrations of labile-P, low concentrations of total P and more P in organic forms than inorganic. The ratio of organic P to inorganic P was lowest (1:2) in recently burnt surface soils and greatest (7:1) at depth in soil that had been undisturbed for long periods of time. Phosphomonoesterase and phosphodiesterase activities (to 10 cm depth, phosphomonoesterase 700–1300; phosphodiesterase 2000–2400 g nitrophenol released h^-1 g^-1 fresh weight) were comparable to those in other, organically rich forest soils. The optimum pH for phosphatase activities were within 1–2 units of soil pH (6) and little reduction in activity was observed over the pH range 4–8. Phosphatase activity was reduced by air-drying (up to 20-fold reduction) and was almost entirely absent in soils that were heat-affected as a result of logging/burning operations. Neither phosphomonoesterase nor phosphodiesterase were directly related to soil P fractions or total P. A reduction in P demand is postulated as the cause of reduced phosphatase activity and the increased concentration of organic P with increasing soil depth.     

Influence of long-term residue management on soil enzyme activities in relation to soil chemical properties of a wheat-fallow system

Summary Soil enzyme activities (acid and alkaline phosphatase, arylsulfatase, -glucosidase, urease and amidase) were determined (0- to 20-cm depth) after 55 years of crop-residue and N-fertilization treatment in a winter wheat (Triticum aestivum L.)-fallow system on semiarid soils of the Pacific Northwest. All residues were incorporated and the treatments were: straw (N₀), straw with fall burn (N₀FB), straw with spring burn (N₀SB), straw plus 45 kg N ha^–1 (N₄₅), straw plus 90 kg N ha^–1 (N₉₀), straw burned in spring plus 45 kg N ha^–1 (N₄₅SB), straw burned in spring plus 90 kg N ha^–1 (N₉₀SB), straw plus 2.24 T ha^–1 pea-vine residue and straw plus 22.4 T ha^–1 of straw-manure. Enzyme activities were significantly (P<0.001) affected by residue management. The highest activities were observed in the manure treated soil, ranging from 36% (acid phosphatase) to 190% increase in activity over the control (N₀). The lowest activities occurred in the N₀FB (acid phosphatase, arylsulfatase and -glucosidase) and N₉₀ treated soils (alkaline phosphatase, amidase and urease). Straw-burning had a significant effect only on acid phosphatase activity, which decreased in spring burn treated soil when inorganic N was applied. Urease and amidase activity decreased with long-term addition of inorganic N whereas the pea vine and the manure additions increased urease and amidase activity. There was a highly significant effect from the residue treatments on soil pH. Arylsulfatase, urease, amidase and alkaline phosphatase activities were positively correlated and acid phosphatase activity was negatively correlated with soil pH. Enzyme activities were strongly correlated with soil organic C and total N content. Except for acid phosphatase, there was no significant relationship between enzyme activity and grain yield.Journal Paper No. 8072 of the Agricultural Experimental Station, Oregon State University, Corvallis, OR 97331, USA     

Distribution of two C cycle enzymes in soil aggregates of a prairie chronosequence

Knowledge of the cycling and compartmentalization of soil C that influence C storage may lead to the development of strategies to increase soil C storage potentials. The objective of this study was to use soil hydrolases and soil aggregate fractionation to explore the relationship between C cycling activity and soil aggregate structure. The prairie chronosequence soils were native prairie (NP) and agricultural (AG) and tallgrass prairies restored from agriculture in 1979 (RP-79) and 1993 (RP-93). Assays for -glucosidase (E.C. 3.2.1.21) and N-acetyl--glucosaminidase (NAGase, EC 3.2.1.30) activities were conducted on four aggregate size fractions (>2 mm, 1–2 mm, 250 m–1 mm, and 2–250 m) from each soil. There were significantly greater amounts of >2-mm aggregates in the RP-79 and RP-93 soils compared to the NP and AG soils due to rapid C accumulation from native plant establishment. Activities for both enzymes (g PNP g^–1 soil h^–1) were greatest in the microaggregate (2–250 m) compared to the macroaggregate (>2 mm) fraction; however, microaggregates are a small proportion of each soil (<12%) compared to the macroaggregates (75%). The RP soils have a hierarchical aggregate system with most of the enzyme activity in the largest aggregate fractions. The NP and AG soils show no hierarchical structure based on aggregate C accretion and significant C enzyme activity in smaller aggregates. The distribution of enzyme activity may play a role in the storage of C whereby the aggrading restored soils may be more susceptible to C loss during turnover of macroaggregates compared to the AG and NP soils with less macroaggregates.     

埃塞俄比亚罗毕高原含铬雏形土地区凋落物饲喂的白蚁(Macrotermes spp)的化学性质及其生化活动的研究

Termite(Macrotermes spp.) mounds are complex biological habitats originated by the termite activity and possessing peculiar physical, chemical and biochemical properties. In this study we examined the concentration of nutrients and the biochemical activity of abandoned soil and mounds colonized by termites of the genera Macrotermes located in the Borana District, Ethiopia. To elucidate the magnitude and persistence of the termite-induced effects, we also studied an abandoned mound, previously colonized by termites of the same genera formed on the same soil. Results confirmed that termite-colonized mounds are ‘hot spots' of nutrient concentration and microbial activity in tropical soils. This is due to the termite driven litter input and decomposition. The abandoned mounds showed higher microbial biomass and activity and displayed a nutrient redistribution and a greater microbial activity than the adjacent soils. These findings allowed us to hypothesize a model of nutrient cycling in colonized soils and a partition of the relative roles of termites and soil microorganisms in nutrient location and turnover in tropical soils. These results may be also useful for the optimal management of termite-colonized soils.     

Phosphorus accumulation in savanna termite mound in Venezuela

Nasutitermes ephratae are often abundant in seasonally flooded and dry Venezuelan savannas, where they play an important role in litter decomposition and nutrient cycling. Measurements were made of phosphorus forms and associated properties in 14 mounds and adjacent soils. Mounds contained on average over twice as much total phosphorus as adjacent soils. The difference was differentially distributed, mainly as inorganic forms. Mounds also display a relatively high organic matter content. 'Available' forms of P, especially water-soluble P, increased several times as a result of termite activity. Concomitantly, P-sorption was greatly reduced in mounds compared to surrounding soils.
Mounds did not display higher phosphatase activity. The elevated organic carbon content, high level of soluble P, together with very high C/P₀ ratios found in mounds, probably inhibited phosphatase activity.     

Organic and inorganic soil phosphates and acid phosphatase activity in the rhizosphere of 80-year-old Norway spruce [Picea abies (L.) Karst.] trees

Summary Inorganic and organic phosphates (P) were measured in bulk soil, rhizosphere soil and mycorrhizal rhizoplane soil of Norway spruce. Various methods of P extraction and estimation were compared. In addition, acid phosphatase activity and mycelial hyphae length were determined. In soil solutions from various locations, about 50% (range 35%–65%) of the total P was present as organic P. Compared to the bulk soil, the concentrations of readily hydrolysable organic P were lower in the rhizosphere soil and in the rhizoplane soil; this difference was particularly marked in the humus layer. In contrast, the concentrations of inorganic P either remained unaffected or increased. A 2- to 2.5-fold increase was found in the activity of acid phosphatase in the rhizoplane soil in comparison to the bulk soil. There was a positive correlation (r = 0.83^***) between phosphatase activity and the length of mycelial hyphae. The results stress the role of organic P and of acid phosphatase in the rhizosphere in the P uptake by mycorrhizal roots of spruce trees grown on acid soils.     

Chemical,physical and micromorphological properties of termite mounds and adjacent soils along a toposequence in Zona da Mata,Minas Gerais State,Brazil

Little is known about the effects of neotropical mound-building termites in soil chemical and physical properties. The influence of soil termite activity on soil characteristics was studied by assessing chemical, physical and micromorphological properties of a toposequence of Latosols (Oxisols). Soil samples were collected from the walls and inner parts of termite mounds and also from adjacent soil. A high diversity of termite genera was found in the mounds along the toposequence, together with the inquiline termites and other soil-dwelling arthropods. Chemical analyses showed that pH and the contents of organic C and N, P, Ca and Mg were significantly higher in termite mounds compared with adjacent areas, with an inverse trend for Al content. Significant differences in pH and exchangeable Al were observed between soil and mound across the slopes. The mound density across the landscape was higher at the upper slope segment, followed by the hill top, middle slope and lower slope segments. Considering a lifespan of 30 years and dimensions of termite mounds found in the toposequence we conclude that the textural and chemical uniformity of Latosols may be increased, following the pedobiological turnover during mound building, with local rates varying from 2.1 to 7.5 m³ ha^− 1.     

Effects of lead pollution on different soil enzyme activities

We studied the effects of Pb pollution on soil dehydrogenase and phosphatase activity. Samples of four soils (Saxe, Podestà, Porto Teulada, and Sa Xia Manna) were collected from various locations in southwestern Sardinia, Italy. The soils, which differ mainly in heavy metal contents of pedologic origin (Cu, Zn, Cd, and Pb), were treated with Pb (0, 100, 500, 1000, and 5000 g Pb g^-1 soil) and incubated in the laboratory. Samples of the incubated soils were collected periodically (0, 1, 2, 4, 8, and 16 weeks) and the enzymes were measured. Soil dehydrogenase activity was influenced by both the Pb additions and variations in soil moisture content. Only the addition of 5000 g Pb g^-1 soil led to a significant decrease in dehydrogenase activity compared to the controls, while the other doses of Pb did not always result in a clear reduction in enzyme activity. Drying the soil led to a considerable reduction in dehydrogenase activity, sometimes so far as to render the differences found between the various treatments not statistically significant. Soil phosphate activity was also influenced by the Pb additions, but the effect of the variation in soil moisture content was less than that found for the dehydrogenase. After the 2nd week of incubation, the phosphate activity in the Podestà and Saxe soils had decreased proportionally to the increase in Pb content. At the end of the incubation period, in the Porto Teulada and Sa Xia Manna soils, a net reduction in phosphatase activity versus controls was found only at the highest Pb concentration. Although both enzyme activities were influenced by the Pb additions, the phosphate activity was less sensitive to variations in the soil moisture content and may thus be a more suitable indicator for soil pollution by Pb.     

The use of biological methods to determine the microbiological activity of soils under cultivation

Summary In Ap horizons of typical arable soils under cereals in Northwest Germany, biological activity was estimated by measuring microbial activity. Twelve soils on local farms and six soils on a research farm were analysed. Microbial biomass, dehydrogenase activity, and alkaline phosphatase activity were compared with the biological availability of P, an index describing the relationship among several P fractions that has been used in ecological agriculture. The correlation between the microbial biomass and dehydrogenase and alkaline phosphatase activity was strong but the correlation between the biological availability of P and the enzyme activities was weak. In contrast, in the farm fields, there was a significant correlation between the microbial biomass and the biological availability of P. The correlation between the biological availability of P and pH was highly significant (r=0.65–0.93^***). Explanations for these correlations are discussed and proposals for further investigations are made. (1) Is the pH effect a direct chemical one or an indirect biological one? (2) Which soil organisms affect the biological availability of P in contrast to the microbial biomass, dehydrogenase activity, and alkaline phosphatase activity? (3) Is the method suitable for the investigation of all arable soils?     

施磷对吊兰修复镉污染土壤及土壤酶活性的影响

选用吊兰进行室内盆栽,研究在Cd污染土壤中,施用不同剂量磷肥对吊兰生理指标、土壤酶活性等的影响,为合理施用磷肥,提高Cd污染土壤的植物修复效果提供参考依据。结果表明:吊兰在100mg/kg的Cd污染土壤中能正常生长,施P对吊兰的叶绿素、细胞膜透性、抗氧化酶活性影响显著(p0.01),表现为低浓度促进,高浓度抑制作用。当施磷量为200mg/kg时,叶绿素含量、CAT酶活性最大,外渗电导率和MDA含量最低,POD酶活性开始上升,说明施磷能使吊兰的细胞膜受害程度最低,抗氧化作用增强。同时,施P有利于改善Cd污染土壤的酶系统,对过氧化氢酶、脲酶的活性表现为低浓度促进、高浓度抑制,对蔗糖酶的活性表现相反,对磷酸酶的活性表现抑制作用;且200mg/kg的磷处理时,过氧化氢酶、脲酶的活性最高,蔗糖酶的活性最低。4种土壤酶活性均与P浓度呈高度相关性(p0.05),相关性为脲酶过氧化氢酶蔗糖酶磷酸酶。     

Ant-induced soil modification and its effect on plant below-ground biomass

Lasius flavus is a dominant mound-building ant species of temperate grasslands that significantly modifies soil parameters. These modifications are usually the result of workers’ activities such as food accumulation and nest construction. An alternative hypothesis that could explain changes in soil is colony founding in areas of higher soil fertility.In our study we investigated several soil parameters sampled in 10 ant nests and adjacent (control) plots in mountain grassland in Slovakia. The alternative hypothesis was tested by comparing occupied and abandoned mounds. While we found increased concentrations of available P and K in the nests, concentrations of total C, total N, Ca²⁺ and Mg²⁺ were lower there. We propose that differences found between the soil of nests and control plots are entirely a product of ant activity during mound occupancy and not due to initial soil differences during nest establishment. This was confirmed by the comparison of occupied and abandoned nests in which the soil fertility of abandoned nests was similar to conditions in the surrounding soil.Along with the modification of soil chemistry, we recorded changes in soil physical properties and the vertical distribution of nutrients. Ant nests were characterized by the dominance of 0.02–0.1 mm particles and lower bulk density. In the same habitat, nutrient concentrations did not change along the vertical gradient in contrast to control plots where soil nutrients decreased and bulk density increased with depth. Root biomass followed the vertical pattern observed with nutrients: in control plots, most roots were concentrated in the uppermost layer (0–3 cm), whereas they were evenly distributed along the vertical gradient in the nests. We also found that rhizome internodes of Agrostis capillaris were thinner and longer in plants from the mounds. Changes in soil physical properties, vertical distribution of nutrients and root biomass in the nests are most probably a consequence of mounding and soil mixing (bioturbation), which has been less reported on in ant-soil studies.     

Effects of termites on the physical and chemical properties of the acid sandy soils of southern Nigeria

Abstract

This study was aimed at characterizing the effects of the activity of termites of the genus Nasutitermes on the physico‐chemical properties of the acid sandy soils of southern Nigeria. Selected morphological properties of the termite mounds were measured in the field. Outside portions of the termite mound and surface (0–15 cm) soil were collected and analyzed for some physical and chemical properties. Results obtained showed a density of 112 mounds ha^‐1 with average height of 0.85 m. There were significantly higher proportions of clay, silt, and organic carbon, and higher pH, exchangeable potassium (K), calcium (Ca), magnesium (Mg), available phosphorus (P), effective cation exchange capacity and base saturation in the mounds of the Nasutitermes than in the surrounding topsoil. Mounds of Nasutitermes termites, if returned to the soil, could improve the properties of the soil in areas where termites occur in large numbers.     

Organic phosphorus utilization by wheat plants under sterile conditions

An experiment was set up to make a critical assessment of the role of organic P (Po) in soil solution in the nutrition of wheat plants under sterile conditions. Three concentration gradients of Po (17.08, 27.31 and 37.61 M l^-1) were created in a sterilized Oxisol by fertilization with a soil solution prepared by a dry freezing technique and containing antibiotics to minimize microbial growth. Due to the high P fixing capacity of the Oxisol and high buffering capacity of inorganic P (Pi), as compared to Po, a negligible change in Pi concentration occurred, due to fertilization after equilibrium in soil solution. Phosphorus supply had a positive effect on dry matter and P concentration of the plants. Acid phosphatase secretion by plant roots was 5–11 times higher in Po treatments than in the respective Pi treatments. No alkaline phosphatase activity was detected, confirming the absence of microbial activity in our system. Net P inflow into plants was significantly higher in the Pi + Po treatments at all three Po concentrations than in the respective Pi (control) treatment, providing evidence for the role of Po in the P nutrition of plants. It was hypothesized that plants secrete phosphatases in response to the presence of Po in soil solution and Po might be responsible for the increase in P influx to wheat plants.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Activity and biomass of soil microorganisms at different depths

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 g g^-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 g g^-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 g g^-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO₂ values increased with depth and were especially large at 70–90 cm in depth.     

Response of phosphomonoesterases in soils to chloroform fumigation

The chloroform fumigation technique has been successfully employed to quantify intracellular and extracellular urease and arylsulfatase activities in soil. In this study, the same approach was evaluated for its ability to differentiate between various pools of phosphomonoesterase activities in soils and reference proteins purified from plant and microbial sources. The activities of acid and alkaline phosphatases were assayed in 10 surface soils and reference proteins at their optimal pH values before and after chloroform fumigation and in the presence and absence of toluene. Chloroform fumigation decreased the activities of acid and alkaline phosphatases in soils, on average, by 6 and 8%, respectively. Similarly, the activities of two purified reference enzyme proteins were decreased after fumigation, with acid and alkaline phosphatase activities exhibiting a reduction of 17 and 8%, respectively. Toluene treatment caused an increase in the activities of acid and alkaline phosphatases by 8 to 18% in nonfumigated soils, but showed no effect in the fumigated soils. Average enzyme protein concentrations, calculated for the 10 soils based on the activity values of the soils and the specific activity of the purified enzymes (i.e., activity values per mg protein), were 22.5 and 2.1 mg protein (kg soil)^—1 for acid and alkaline phosphatase, respectively. The decrease in enzyme activity by the fumigant was either by direct denaturing of the periplasmic and extracellular portion of the particular protein after lysis of the microbial cell membrane, by absorption and/or inhibition of the released phosphomonoesterases by organic and inorganic constituents or by degradation of the protein by soil proteases. The ratios of acid phosphatase protein concentrations relative to organic C in six soils were significantly, but negatively correlated with soil organic C, suggesting differences in organic C quality. Comparison of the activity values of soil phosphatases with those of the protein concentrations present in soils indicated that alkaline phosphatase has greater catalytic efficiency than does acid phosphatase.     

Microbial biomass phosphorus in soils of beech (Fagus sylvatica L.) forests

Thirty-eight soils from forest sites in central Germany dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of the overlying organic layers. Microbial biomass P was estimated by the fumigation — extraction method, measuring the increase in NaHCO₃-extractable phosphate. The size of the microbial P pool varied between 17.7 and 174.3 g P g^-1 soil and was on average more than seven times larger than NaHCO₃-extractable phosphate. Microbial P was positively correlated with soil organic C and total P, reflecting the importance of soil organic matter as a P source. The mean microbial P concentration was 13.1% of total P, varying in most soils between 6 and 18. Microbial P and microbial C were significantly correlated with each other and had a mean ratio of 14.3. A wide (5.1–26.3) microbial C: P ratio indicates that there is no simple relatinship between these two parameters. The microbial C: P ratio showed strong and positive correlations with soil pH and cation exchange capacity.     

中国四川盆地地区侵蚀斜坡土壤酶活性研究

Determining how soil erosion affects enzyme activity may enhance our understanding of soil degradation on eroded agricultural landscapes. This study assessed the changes in enzyme activity with slope position and erosion type by selecting water and tillage erosion-dominated slopes and performing analyses using the ¹³⁷Cs technique. The ¹³⁷Cs data revealed that soil loss occurred in the upper section of the two eroded slope types, while soil accumulation occurred in the lower section. The invertase activity increased downslope and exhibited a pattern similar to the ¹³⁷Cs data. The spatial patterns of urease and alkaline phosphatase activities were similar to the ¹³⁷Cs inventories on the water and tillage erosion-dominated slopes, respectively. On both the eroded slope types, the invertase activity and soil organic carbon content were correlated, but no correlation was observed between the alkaline phosphatase activity and total phosphorus content. Nevertheless, the urease activity was correlated with the total nitrogen content only on the water erosion-dominated slopes. The enzyme activity-to-microbial biomass carbon ratios indicated high activities of invertase and urease but low activity of phosphatase on the water erosion-dominated slopes compared with the tillage erosion-dominated slopes. Both the invertase activity and the invertase activity-to-microbial biomass carbon ratio varied with the slope position. Changes in the urease activity-to-microbial biomass carbon ratio were significantly affected by the erosion type. These suggested that the dynamics of the invertase activity were linked to soil redistribution on the two eroded slope types, whereas the dynamics of the urease and alkaline phosphatase activities were associated with soil redistribution only on the water or tillage erosion-dominated slopes, respectively. The erosion type had an obvious effect on the activities of invertase, urease and alkaline phosphatase. Soil redistribution might influence the involvement of urease in the N cycle and alkaline phosphatase in the P cycle. Thus, enzyme activity-to-microbial biomass ratios may be used to better evaluate microbiological activity in eroded soils.     

Using the spermosphere model technique to describe the dominant nitrogen-fixing microflora associated with wetland rice in two Egyptian soils

Summary This study is an attempt to describe the dominant N₂-fixing microflora associated with the roots of wetland rice. Rice cultivar Giza 171 was grown in a phytotron on two alluvial Egyptian soils for 8 days, a stage when the nitrogenase activity of undisturbed plants reached a level of 245 × 10^–6 mol C₂H₄ h^–1 g^–1 dry weight of leaf. The roots and rhizosphere soils were then used for counting and isolating dominant diazotrophs. Counts and initial enrichment steps were carried out on a selective medium made of an axenic rice plantlet, the spermosphere model, incubated under 1 % acetylene. The counts were very high, exceeding 10⁸ bacteria g^–1 dry weight of rhizosphere soil. Enterobacteriaceae were dominant; most isolates were Enterobacter cloacae belonging to different biotypes in the two soils. Enterobacter agglomerans, Citrobacter freundii and Klebsiella planticola were also present as members of the dominant microflora. Azospirillum brasilense and Azospirillum lipoferum were present as well, but less abundant.     

Dynamics and availability of phosphorus in the rhizosphere of a temperate silvopastoral system

A field rhizosphere study was carried out over a period of 12 months on a 6-year-old silvopastoral trial in New Zealand. The trial comprised radiata pine (Pinus radiata) with lucerne (Medicago sativa) and perennial ryegrass (Lolium perenne) understoreys. The study was initiated because of the unique interrelationships between roots in silvopastoral systems and a paucity of understanding about the processes involved in phosphorus (P) dynamics in temperate silvopastoral systems. Improving our understanding in this area has important implications for nutrient management in silvopastoral systems. Rhizosphere soils were analysed to determine inorganic (Pi) and organic (Po) P fractions, macroporous resin Pi and Po, phosphatase enzyme activity, microbial biomass carbon and pH. Concentrations of labile Pi were consistently greater and Po lower in tree rhizosphere soil compared to the companion understorey, indicating that radiata pine when grown with a productive understorey mineralised Po to a greater extent than either understorey species. Tree rhizosphere soil from under lucerne and lucerne rhizosphere soil contained the lowest concentrations of total Pi and Po compared with tree under ryegrass and ryegrass rhizosphere soils. This was partly attributed to higher levels of phosphatase enzyme activity in the lucerne rhizosphere soils. The results suggest the combination of lucerne with radiata pine may enhance greater utilisation of soil P, although this requires further investigation. Lower levels of labile Po, and higher levels of labile Pi and phosphatase enzyme activity, were determined in tree and understorey lucerne and ryegrass rhizosphere soils in spring compared with autumn. This data confirmed that overall rates of soil organic P mineralisation are greatest in spring.