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1
Abstract.-Plant roots and soil organisms increase phosphorus availability by releasing acid and alkaline phosphatase. Organic phosphorus compounds are hydrolysed into plant-available forms through the action of these enzymes. Phosphatase is of particular importance under conditions of low phosphorus availability which is commonly found in pinon-juniper g-1 soil h-
1
) were studied at two research sites in northern Arizona. At monthly intervals, soil samples were collected underneath Juniperus monosperma canopies and in interspace areas dominated by Hilaria jamesii. In both juniper and interspace soils, alkaline phosphatase activities were significantly higher than acid phosphatase activities. Alkaline phosphatase activity ranged from 57.3 to 167.2 fl9 p-nitrophenol g-1 soil h(1 and was similar to values reported for acid farmfield and grassland soils.
Acid phosphatase activity varied between 9.3 and 29.5 mg p-nitrophenol g-1 soil h
-1 and corresponded to values in Juniperus occidentalis and a range of western coniferous forest soils. The results indicate that soil microbes are the dominant producers of phosphatase in the studied pinon-juniper soils.
INTRODUCTION
Traditionally, most nutrient cycling studies in forests and rangelands have focused on the measurement of nutrient pool sizes and the return of nutrients from plants to soil in litterfall. Only recently haVe we turned our attention to the dynamics of nutrient fluxes and the underlaying processes by which the supply of nutrients to plants is sustained.
Phosphorus availability may be the most limiting factor to plant growth in many terrestrial ecosystems (Attiwill and Adams 1993, McGill and
Christie 1983, Vitousek and Howarth 1991). In natural ecosystems dominated by perennials much of the plant demand for phosphorus' is met by cycling of phosphorus in organic matter (Halm et al. 1972, Attiwill 1980, Attiwill and Adams 1993).
Phosphatase enzymes are a good indicator of the organic phosphorus mineralization potential and biological activity of soils (Cole et al. 1977, Speir and Ross 1978). Phosphatase measured in soils reflects the activity of phosphatase bound to soil colloids and humic substances, free phosphatase in the soil solution, and phosphatase associated with living and dead plant or microbial cells (Nannipieri et al. 1990, Skujins 1976). Phosphatase activity is related to soil and vegetation conditions (Herbien and Neal 1990, Ho 1977, Neal 1973), responds to changes in management (Adams 1992, Clarholm
1993, Pang and Kolenko 1986), and is reasonably correlated with seasonal changes in soil temperature and moisture (Harrison and Pierce
1979, Speir and Cowling 1991).
Phosphatase activity has been studied in various agricultural and forest soils. However, very limited information exits on phosphatase dynamics in semiarid rangeland soils. The present study determined acid and alkaline phosphatase levels in a pifton-juniper woodland of northern Arizona and compares it to values reported for soils from a range of ecosystems. Possible relationships of
1
Graduate Student and Teaching Associate, Department of
Botany, Arizona State University, Tempe, AZ.
24

phosphatase activity levels to various soil and vegetation characteristics are discussed.
MATERIALS AND METHODS
Two study sites were established in a one-seed juniper/galleta Uuniperus nwnospermalHilaria jamesiz) dominated woodland of the Colorado
Plateau in northern Arizona. The sites were located about 30 miles northeast of Flagstaff at Wupatki
National Monument at an elevation of 1650 m. The climate at Wupatki National Monument is semiarid with 54% of the total annual precipitation occurring during thunderstorms in July, August, and September (fig. 1) (Sellers and Hill 1974).
Longterm average precipitation at the visitor center
(1492 m elev.) is 188 mm (Sellers and Hill 1974).
Soils for the analysis of pH, loss on ignition, and texture were collected during August 1993.
Prior to sampling, each study site was subdivided into three equal sized rectangular plots. In each plot, soil subsamples were collected from 5 to 15 cm depth from 10 randomly located areas under both juniper trees and in_ interspace areas. SUbsamples were composited within plots. Soils were air dried and passed through a 2 mm sieve. Standard analysis techniques were used for soil pH (1:1 soiVCaC1
2), loss on ignition, and texture
(Bouyoucos hydrometer) (Klute 1986, Page et al.
1982). Soil textural classes follow the USDA classification scheme (Gee and Bauder 1982). The depth of cinders and litter covering the soil surface was measured to the nearest half centimeter at three random locations in each interspace and tree plot used for phosphatase analysis. Variations in soil properties between sites, trees, and interspace areas were analyzed with t-tests at the 0.05 level.
Tree canopy cover was determined at 10 ra,ndom locations with a spherical densiometer
(Lemmon 1956). Understory cover of dominant herbaceous species was estimated in 20x50 cm plots. At each site, 30 plots were read at 1 m intervals along 5 transects. Cover was estimated based on Daubenmire cover classes (Daubenmire
1968). Canopy and understory cover differences between sites were compared with
0.05 level. t-tests at the
Within each study site, four juniper and four interspace plots were randomly selected for the study of soil phosphatase. Soil was collected from 0 to 20 cm depth at monthly intervals from
September 1993 to June 1994. At each plot, 3 randomly located subsamples were obtained with a core sampler lined with a 2.5 cm diameter butyrate sampling tube. Samples in juniper plots were collected at approximately half the distance between the tree trunk and canopy edge. Sampling tubes were capped and transported to the laboratory on ice. At the laboratory, the 5 to 15 cm core sections were passed through a 2 nun sieve and composited within sample plots. Sieyed samples were stored in polyethylene bottles at 4 C.
Field moist soil samples were analyzed for acid and alkaline phosphomonoesterase activity with a buffered disodium p-nitrophenyl phosphate tetrahydrate solution (Tabatabai 1982, Schinner et al. 1991). Results were calculated in J..lg pnip-ophenol released per gram dry soil per hour at
37 C. A hierarchical AN OVA (Zar 1984) was used to test differences in acid and alkaline phosphatase activity, juniper and interspace plots, and between sites Within sampling dates. .,
Wupatki Nat. Monument (1492 m) 14,1
°c
188 mm
60
-
Temperature
30
50
E
E
40 c:::
0 ....
30
.8
'a.
'u
20
CLl
'-
0...
10
0
J F M A M J J A S 0 N
Month
D
0
-i
20 CD
3
10
"0
..,
,....
..,
CD
0
(")
Figure 1.-Longterm precipitation and temperature averages
(1941-1970) at Wupatki National Monument (Sellers and Hill
1974).
25
RESULTS AND DISCUSSION
Soil and vegetation characteristics varied considerably between study sites (table 1 and 2) although the sites were located within 500 m of each other. Soils were alkaline and ranged from pH
7.4 to 7.9 in interspace plots, and 7.9 to 8.0 in juniper plots (table 1). At site 1, soil pH was significantly lower in interspace than juniper/lots
(P ~ 0.01). Both interspace and juniper so' pH levels were significantly lower (P ~ 0.01) at site 1 than site 2.
Soil organic ma~-qstimated by percent loss on ignition was between ~.4 and 4.1 % and did not differ significantly within or between study sites
(table 1).

Site 1 soils were of a sandy loam texture with significantly less sand (P S 0.05) in interspace
(54.3%) than juniper plots (63.6%) (table 1). Soils at site 2 were finer textured and classified as loam.
Both study sites are covered with black cinders from the eruption of Sunset Crater. Cinder cover was significantly deeper at site 1 than site 2, and under juniper trees as compared to interspace areas (P £ 0.05). Interspace plot cinder cover was 3.2 cm at site 1 and 1.6 cm at site 2 (table 1). Under juniper trees, cinders are mixed with tree litter.
Average cinder/litter depth for juniper plots was
8.6 and 4.3 cm for site 1 and 2, respectively (table 1).
Rocks and plant residue on the soil surface act as a mulch and can significantly decrease evaporative loss of soil moisture, and decrease soil temperature fluctuations (Brady 1984). These factors may be critical in determining aboveground vegetation and belowground soil phosphatase characteristics associated with the study sites.
One-seed juniper and galleta were the dominant plant species on both study sites. Juniper canopy cover was 8.2% at site 1 and 3.5% at site 2
(table 2). Galleta cover was significantly higher (P
:S;
0.05) at site 1 (7.0%) than site 2 (4.5%) (table 2).
29.5
Acid phosphatase activities ranged from 13.3 to f.Lg p-nitrophenol g-l h-
1 in juniper soil and from 9.3 to 33.4 f.Lg p-nitrophenol g-l h-
1 in interspace soil (fig. 2). Acid phosphatase levels found in this study are similar to values reported for western juniper (Juniperus occidentalis), a range of coniferous forest soils, and fescuelryegrass
(Festuca sppjLolium spp.) pasture (fig. 3) (Ho 1979).
Acid phosphatase activity in low pH soils of eucalyptus (Eucalyptus diversicolor), Douglas firlred alder (Pseudotsuga menzies iii Alnus rubra), red alder,
..-..
"i
200
..I:
I
C\ 150
"0 c::
~ c.. o
~
~ c::
I c..
100
50
Site 1
,
I .
H
1 ........ 1
Month
... Interspace pH 11 to Interspace pH 6.5
I
I
·······'1.
.. I "
~
250
--~--~~~~--~--~~ ~~~--~
......... f
200
:;::; lJ
<C
150
QJ
CII o
C
100
..t: a.
~
50
..t: a.. o
, Site 2 t&
S
• Juniper pH 11
0
Juniper pH 6.5
, & ........
1...
I 1
~~
"'e-~a .~
..
~ fill o N D J F A M J
Figure 1.-Longterm precipitation and temperature averages
(1941-1970) at Wupatkl National Monument (Sellers and Hili
, 1974). mixed oak (Quercus spp.) forest soils, and ad· ... farmfields was considerably higher than levels found in this study (fig. 3) (Adams 1992, Eivazi and
Tabatabai 1977, Herbien and Neal 1990, Ho 1979).
57.3
Alkaline phosphatase activity varied between and 145.4 f.Lg p-nitrophenol g-l h-
1 in1unifer soil and 75.9 and 167.2 f.Lg p-nitrophenol g- h- in interspace soil (fig. 2). These values are similar to alkaline phosphatase activities reported for grassland and acid farmfield soils (fig. 3) (Eivazi
Table 1.--chemlcal and physical soli characteristics of research sites. Lower case letters Indicate significant differences between
Interspace and Juniper solis within study sites. Numbers Indicate significant differences In Interspace or Juniper soils between study sites. pH
Loss on ig n ition (%)
Texture
% Sand
% Clay
Cinder/litter depth (em)
Interspace
7.4 a1
3.6 a1 sandy loam'
54.3 a1
20.0 a1
3.2 a1
Juniper
7.9 b1
3.4a1 sandy loam
63.6 b1
14.1 a1
8.6 b1
Interspace
7.982
3.6 a1 loam
48.2 a1
21.4 a1
1.682
Table 2.--cover of dominant plant species at research sites. Values are means ± 8E. Lower case letters Indicate significant differences between sites. TR
= trace ( < 0.5%).
Cover C%l
Species Site 1 81te2
Juniperus monosperma
Hilaria jamesii
Sporobolus cryptandrus
Aristida arizoniea
Stipe. neomexicana
Sitanion hystrix
8.2 (±22.7) a
7.0 (±2.1) a
TR
TR
TR
3.5 (±S.7) a
4.S(±1.1)b
TR
TR
Juniper
8.082
4.1 a1 loam
50.682
20.2 a1
4.3b2
26

• Acid Phosphatase
0
Alkaline Phosphatase and Tabatabai 1977, Herbien and Neal 1990).
Alkaline phosphatase levels in alkaline farmfields and a cornfield were much higher than activities found in this study (Eivazi and Tabatabai 1977,
Herbien and Neal 1990). However, much less information exits on alkaline soil phosphatase compared to acid phosphatase and comparisons are therefore limited.
Alkaline phosphatase activity was significantly higher (P ~ 0.01) than acid phosphatase in both juniper and interspace soil throughout the study period (fig. 2). This agrees with the conclusion of
Eivazi and Tabatabai (1977) that alkaline phosphatase predominates over acid phosphatase in high pH soils. Plant roots can be a major source of acid phosphatase (Dinkelaker and Marschner
1992, Juma and Tabatabai 1988, Neal 1973, Speir and Cowling 1991), but do not produce any alkaline phosphatase (Nakas et al. 1987, Tarafdar and Claassen 1988). Alkaline phosphatase originates from soil bacteria, fungi, and fauna (Ho and Zak 1979, Nakas et al. 1987, Pang and Kolenko
1986, Tarafadar and Claassen 1988). Soil microbes appear to be the main producers of phosphatase in the studied pifton-juniper soils.
Phosphatase activities were not significantly different between juniper and interspace soils.
However, with the exception of the June 1994 sampling date, alkaline phosphatase activity was consistently higher in interspace than in juniper soil (fig. 2). Soil water potential measurements at the study sites indicate that juniper soils are drier than interspace soils (S. Kramer, unpubl. data).
Since soil microbial activity is positively correlated with soil water potential (Griffin 1981, Wilson and
Griffin 1975), higher microbial activity and therefore higher alkaline phosphatase levels can be expected in interspace soils. This explanation agrees with Hoffmann and Elias-Azar (1965) and
Tarafdar and Claassen (1988), who observed higher phosphatase activity in soils with higher microbial populations. Physical and chemical soil properties
(table 1) may further influence the measured alkaline phosphatase levels in juniper and interspace soils (Skujins 1976, Speir and Ross 1978).
Phosphatase activities at site 1 were generally higher than those at site 2 (fig. 2). Values were significantly different (P ~ 0.1) during September,
November, and December 1993, and during
January and June 1994. Differences in phosphatase activities between sites are likely to be caused by dissimilar soil moisture and temperature regimes due to variations in soil texture and .cinder/litter cover.
One-Seed Juniper
Western Juniper'
.... D········· ······0
•
Lodgepole Pine'
Ponderosa Pine'
Douglas Fir'
Douglas Fir/Red Alder'
•
•
•
•
Red Alder'
Mixed Oak
2
Eucalyptus
J
Galleta
Fescue/Ryegrass'
Grassland
2 o
•
.......
o
D······· ·····0
•
Farm Fields (acid)4
Farm Fields (alk.) 4
Cornfield
2
D············~
________
DO o o
100 200 300 400 600
-1 -1
Phosphatase Activity (}J-g p-nitrophenol 9 h )
Figure 3.-Phosphatase activities (J.1g e-nltrophenol g01h01) In solis of various ecosystems. Sources: Ho 1979, 2Herblen and Neal
1990, 3Adams 1992, 4Eivazl and Tabatabal1977. Data for oneseed Juniper and galleta are from this study.
Alkaline phosphatase activities were highest during November and December 1993 (fig. 2).
Significant precipitation (28.7 mm) during October
1993 and possibly favorable soil temperatures may have caused a quick proliferation of soil organisms that synthesize alkaline phosphatase. In lab incubations, a sudden increase in phosphatase activity was observed after 21 days of rewetting soils to field capacity (Pang and Kolenko 1986). The subsequent slow decrease in phosphatase activity suggests that phosphatase was being degraded and not synthesized fast enough by soil organisms to maintain the enzyme pool measured in November and December.
CONCLUSIONS
•
•
Phosphatase levels reported in this study are comparable to those of a range of soils with similar pH values. Variations in activities observed appear to be related to site specific soil and vegetation conditions. However, further studies are necessary to establish more detailed relationships between phosphatase activity and soil conditions.
Research is presently underway to examine soil moisture and temperature changes relative to organic phosphorus cycling and relate them to levels of observed phosphatase activities reported in this study. To more completely understand phosphorus dynamics in pifton-juniper soils, we must look at phosphatase as part of an environmental matrix that consists of many individual, yet interacting factors.
27

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