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Soil organic phosphorus fraction in pine–oak forest stands in Northeastern Germany
Geoderma 158 (2010) 156–162
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Soil organic phosphorus fraction in pine–oak forest stands in Northeastern Germany Anna Slazak a,⁎, Dirk Freese a, Eduardo da Silva Matos a, Reinhard F. Hüttl a,b a b
Chair of Soil Protection and Recultivation, Brandenburg University of Technology, P.O. Box 10 13 44, 03013 Cottbus, Germany Helmholtz-Centre Potsdam-GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
a r t i c l e
i n f o
Article history: Received 12 July 2009 Received in revised form 14 April 2010 Accepted 26 April 2010 Available online 31 May 2010 Keywords: Sequential P extraction P availability Forest management Pinus sylvestris Quercus petraea
a b s t r a c t Pines (Pinus sylvestris L.) have been widely used for monoculture forest stands in north-eastern Germany. However, several studies have discussed the need to convert monoculture into mixed forest stands such as pine– oak forest. In this study, we evaluated the impact of 102 years-old pine (P. sylvestris L.) in monoculture and mixed forest stands of P. sylvestris + Quercus petraea (Matt.) Liebl. of different ages (10, 35, 106 and 124 years-old) on the dynamics of soil organic phosphorus (Po) pools. The study was carried out in the Northern German Lowlands of Brandenburg. Soil samples were taken from forest floor and two mineral soil layers at the depths of 0–10 and 10– 20 cm. Different P forms of the sandy soil were obtained by sequential P fractionation, using acid and alkaline extractants. The soil total P (STP) ranged from 100 to 183 mg kg− 1 whereas total organic P (TPo) ranged from 77 to 148 mg kg− 1. STP was higher in mixed forest stand than in monoculture and decreased with soil depth. The TPo and labile-P in both soil layers increased significantly with increase in age of oak trees. In addition, TPo content was lower in mineral soil compared to the forest floor and accounted for more than 50% of soil total P in the forest stands. The most available-P fraction–labile-P predominated in the oldest pine–oak forest stand (P + O124), accounting for 29% of STP at the 0–10 cm soil depth. The largest P fraction comprised NaOH–Po and represent 62% of STP. Results showed that forest transformation from pure pine monoculture forest into pine-oak mixed forest stands promoted an increase in the TPo and P available. Furthermore, the forms of labile available P increased with age of oak trees, which are capable of maintaining larger fractions of available P under mixed forest stands. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the middle 19th century, forest conversion has been undertaken in north-eastern Germany, mainly by planting beech trees (Fagus sylvatica L.) in pine (P. sylvestris) stands. Nevertheless, in the continentally influenced regions of the north-eastern German Lowlands (Brandenburg), little effort has been made to convert pine monocultures into mixed forests. This is due to the low precipitation levels and low nutrient status of the sandy soils, which predominate in the region even though, sessile oak (Q. petraea) is suitable for forestry under these conditions. In north-eastern Germany, monoculture pine forest stands can be found in abundance (Müller, 2007). Nevertheless, it has been discussed that monoculture stands can be converted into mixed pine-oak forests (Elmer et al., 2009a). It is known that mixed forest stands are more productive than monoculture forest (Burkhart and A., 1992; Rothe and Binkley, 2001; Rosengren et al., 2005). Since only little is known about nutrient cycling in mixed forest and especially about phosphorus (P), which is commonly known as a limiting factor for plant growth in many soils (Qualls and Haines, 1991; Kaiser et al., 2003), it is therefore interesting
⁎ Corresponding author. Tel.: + 49 355 694329; fax: + 49 355 692323. E-mail address: [email protected] (A. Slazak). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.04.023
to point out the effect of conversion monocultures into mixed forest stands on P dynamics. The P dynamics in forest stands is a relationship between the tree's ability to extract P from soils and cycle absorbed P among ecosystem components, and the soil's ability to supply P in a bioavailable form (Comerford et al., 2002; Richter et al., 2006). P in soil occurs in a wide variety of different inorganic and organic forms (Tate, 1984). Organic P (Po) is the major form of P and its mineralization depend on both microbial activity and extracellular phosphate enzymes produced by plant roots and soil microorganisms (Tate, 1984). Besides, additional recycling of Po is important to the availability of soil P for plants (Cross and Schlesinger, 1995). Thus, to sustain P supplies within the forest ecosystem, the crucial process is the release of inorganic phosphate from the decomposition of Po in the forest floor and soil organic matter (Stewart and Tiessen, 1987; Cross and Schlesinger, 1995). Organic forms of P in soils make a significant contribution to the P for plant nutrition (Firsching and Claassen, 1996; Chen et al., 2002). The forms and dynamics of organic P in the soil depend on biological, chemical and physical factors (Tiessen et al., 1983; Stewart and Tiessen, 1987; Cross and Schlesinger, 1995) as well as environmental factors such as temperature, soil moisture and pH (Tate, 1984; Kaiser et al., 2003). To obtain different inorganic and organic P fractions sequential P fractionation schemes have been broadly used, in which acid and alkaline extractants are used to separate the various fractions, based on the type and strength of Po physicochemical interactions with
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layers. The five soil samples were then combined in the field to form one replicate. Thus, for each plot and soil layer, four replicates were sampled. All samples of a sandy textured soil were collected in September 2007. For chemical analyses, soil samples were air-dried and gently broken to pass through a 2-mm sieve. For microbial P (PMB) fresh and moist soil sample was sieved at 2-mm mesh and stored at 4 °C in plastic bags that preserved moisture and allowed aeration until the final analysis was carried out. Samples from the forest floor were collected from the soil surface without vegetation but covered with pine needles and dry leaves using steel sampling frame (25 × 25 cm). The collected samples describe all organic matter generated by forest vegetation, including litter and unincorporated humus, on the mineral soil surface. Forest floor fraction was separated from mineral soil; however, there was little soil contamination. The sample from the forest floor was milled and stored for final analyses.
other soil components (Bowman and Cole, 1978a; Hedley et al., 1982; Cross and Schlesinger, 1995; Ivanoff et al., 1998). This method separates the Po into four pools: labile P, moderately labile P, moderately stable (fulvic acid-P) and highly stable (humic acid-P) (Bowman and Cole, 1978a). There are few studies on the Po in soils of forest ecosystems (Fahey and Yavitt, 1988; Qualls and Haines, 1991; Qualls et al., 2000; Rothe and Binkley, 2001; Kaiser et al., 2003; Santrucková et al., 2004) and little is known on the effects of the admixture of sessile oak (Q. petraea) into pine monocultures with regard to the Po dynamics of these ecosystems, thus, Po is much important as Pi with regard to P nutrition. We hypothesized that the proportions of soil total P (STP) present in available forms and Po would increase with age-old oak trees. The objective of the study was therefore to investigate the Po forms and dynamics, affected by mixed pine-oak forest stands established through the transformation of monoculture pine to mixed pine-oak forest stages in age-old oak trees. Considering the nutrient perspective it is interesting to focus on long-term changes in the soil nutrient pools when comparing the different forest types.
2.3. Chemical analyses Soil pH was measured in the supernatant suspension of air-dried soil to deionised water of ratio 2.5:1 (weight/volume). Total C (TOC) and N (TN) in the soil and forest floor were measured by dry combustion (Vario El Elementar Analyzer). The STP was determined after digestion of sample in nitric acid (HNO3) using inductively coupled plasma emission spectrometry (ICP). The forest floor organic P (Pof.f.) content was determined after loss on ignition in a muffle furnace at 500 °C for 5 h and digested with HNO3 extractant. The Pof.f. was then quantified as the difference between the amounts HNO3-extractable P for the ignited and unignited samples. The total P content in the forest floor fraction was determined as previously described. Oxalate Al and Fe (Alox and Feox) were determined according to Schwertmann (1964) and McKeague and Day (1966) by the acid-ammonium oxalate extraction.
2. Materials and methods 2.1. Site description The study was carried out in north-eastern Germany in the Federal State of Brandenburg (51°47′N lat, 13°32′E long). The region experienced a mean annual temperature and rainfall of 9.8 °C and 587 mm, respectively. Five plots were selected to study the transformation of a 120 yearold Scots pine (P. sylvestris) monoculture forest stand into pine-oak mixed forest stands with the ages of Sessile oaks (Q. petraea) raging from 10 to 124 years and, therefore represent the following treatments: 90–100 years-old pine and 10 years-old oak (P + O10), 102 years-old pine and 35 years-old oak (P + O35), 100 years-old pine and 106 years-old oak (P + O106), 120 years-old pine and 124 years-old oak (P + O124) (Table 1). Climatic, geological, soil physical and chemical conditions were similar across all plots. The ground vegetation consisted mainly of Vaccinium myrtillus L., Vaccinium vitis-idea L., Deschampsia flexuosa L., and mosses. Besides, Calluna vulgaris L. is present under the monoculture pine forest. The soil type is Podzols and weakly podzolized Cambisols according to FAO classification. Soil characteristics of all the study sites have been described by Elmer et al. (2009b).
2.4. Microbial analysis The PMB was determined by the irradiation-extraction method, using microwave (Islam and Weil, 1998), and 0.5 M NaHCO3 (pH 8.5) as extractant (Brookes et al., 1982). Approximately, 20 g of fresh soil samples were irradiated using a total of energy exposure of 800 J.g− 1 soil. Irradiated and non-irradiated soil samples were put in centrifuge tubes with 80 mL of 0.5 M NaHCO3 and were shaken at 250 rpm for 1 h. The soil suspension was centrifuged at 1500 ×g for 5 min and filtered. P concentration was quantified by the ammonium molybdate-ascorbic method (Murphy and Riley, 1962) using an atomic absorption spectrophotometer (the absorbance was measured at 880 nm). PMB content was calculated by the difference between P concentration in irradiated and non-irradiated samples (based on dry mass) divided by a conversion factor of 0.40 used to convert the flow of P into microbial biomass P (Brookes et al., 1982).
2.2. Sampling Each plot was divided into four sub-plots, and for each sub-plot, five soil samples were randomly collected from the 0–10 and 10–20 cm soil
Table 1 Vegetation and area of investigated study sites, and particle distribution of the soils. Treatment
Vegetation (age in years)
Area (ha)
Trees (ha− 1)a
0–10 cmb
10–20 cmb
(%)
Pine P + O10 P + O35 P + O106 P + O124
Q. petraea
P. silvestris
Q. petraea
P. sylvestris
Sand
Clay
Sand
Clay
– 10 35 106 (45%)c 50 (55%) 124
102 90–100 102 100
1 0.15 0.25 0.75
– 8700 1424 299
nd 167 268 336
92.4 87.4 83.0 84.0
2.6 3.4 3.7 4.4
93.6 87.2 88.5 91.3
3.8 4.3 3.4 2.8
120
1.00
287
52
92.9
3.0
92.9
3.3
nd: not determined. a After Schröder et al. (2007). b After Matos et al. (in press). c The percentage of oak trees.
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2.5. Soil organic P fractionation
Table 3 C/N and C/P ratio, organic P (Pof.f.) in the forest floor.
Soil Po was fractionated by following the sequential extraction procedure of Bowman and Cole (1978a), which was further modified by Sharpley and Smith (1985) and Ivanoff et al. (1998). Soil Po was sequentially fractionated into: i) a labile P pool, extracted by 0.5 M NaHCO3 (labile–Po); ii) a moderately labile P pool, obtained by extraction with 1.0 M HCl (HCl–Po); and iii) a more resistant Po pool, which is associated with humic compounds and held more strongly by chemisorption to Fe and Al hydrous oxides (McLaughlin et al., 1977; Bowman and Cole, 1978a,b) was extracted with 0.5 M NaOH. The NaOH–Po extract was acidified with concentrated HCl to separate highly stable Po, associated with humic acid (humic acid–Po), from the moderately stable Po, related to fulvic acid (fulvic acid–Po). Total P (Pt) concentration in the NaHCO3 and HCl extracts, was determined by Inductively Coupled Plasma (optical emission spectrometry). Pt in the humic and fulvic acid extracts was obtained by digestion aliquot with 2.5 M H2SO4 and potassium persulfate (K2S2O8), according to the method of Bowman (1989) and modified by Thien and Myers (1992). Pi concentration in all extracts was measured by the ammonium molybdate-ascorbic method (Murphy and Riley, 1962). For each extract, Po was calculated as being the difference between Pt and Pi. Total organic P (TPo) was calculated as the sum of P fractions: labile–Po + HCl–Po + fulvic acid–Po + humic acid–Po. 2.6. Statistical analysis The data presented are mean values of independent field replicates. The effect of the different forest management systems on Po dynamics at each depth was evaluated using one-way analysis of variance. Pearson correlation and multiple linear regression analyses were also performed. All analyses were done using Sigma Plot.8.0 (Jandel Scientific) at significant level of P = 0.01, P = 0.05. 3. Results 3.1. General soil properties Some properties of the soils are given in Table 2. The soils presented low pH values, ranging from 3.58 to 4.61 whereas the TOC content varied between 8.39 and 46.80 g kg− 1 among all treatments. The P+ O10 plots showed the highest TOC content (Table 2). The PMB was higher at the 0–10 cm than at 10–20 cm soil layer, with values ranging from 0.54 to 5.64 mg kg− 1. Among the pine-oak stands, the highest PMB content was observed under P+O124 followed by P+ O10 treatment in the upper soil layer and in the deeper soil layer under P+O35.
Table 2 Chemical characteristics of two soil layers in all treatments. Mean values of total N (TN), total C (TOC), microbial P (PMB), P (STP), oxalate soluble Al (Alox) and Fe (Feox). Treatment
TNa (g kg
TOCa
pHa
C/N
C/P
−1
)
PMB
STP
(mg kg
Aloxa
Feoxa
−1
)
0–10 cm Pine P + O10 P + O35 P + O106 P + O124
1.11 2.52 1.08 1.17 1.36
29.78 46.80 23.97 29.86 29.46
3.58 3.59 3.64 3.64 4.05
29.0 24.4 25.8 27.7 22.0
268 255 167 198 192
5.64 4.94 4.51 2.42 5.29
110.6 183.5 144.3 152.1 152.6
816.6 1162.2 1197.6 910.2 690.8
1410.9 2724.3 2958.5 2677.0 2093.4
10–20 cm Pine P + O10 P + O35 P + O106 P + O124
0.34 0.54 0.46 0.38 0.35
10.61 13.72 12.12 11.77 8.39
4.10 4.05 4.10 4.11 4.61
31.2 25.8 27.5 31.0 24.7
105 122 120 110 85
0.82 0.54 1.81 1.33 0.94
103.5 112.7 102.0 109.1 100.0
1556.6 1433.8 1756.7 1369.3 854.3
2473.2 2146.0 1993.6 2033.7 1433.5
a
After Matos et al. (in press).
Treatment
C/Na
C/P
Pine P + O10 P + O35 P + O106 P + O124 a
28.8 25.9 25.3 25.9 27.7
Pof.f. (mg kg−1)
ratio 613 517 386 484 464
199.8 228.4 414.8 115.9 192.7
After Matos et al. (in press).
The highest STP content was found in the P+O10 treatment at both soil layers (Table 2). The STP decreased with forest transformation, and the content of STP at the 0–10 cm layer was significantly affected by oaks age (P b 0.05). The C:P ratio decreased from 268 to 85 with increasing soil depth (Table 2). 3.2. Forest floor The total P (TPff) contents in the forest floor fraction increased in relation to forest transformation, with the exception of P + O35 treatment, which has the highest value (Table 3). The Pof.f. was significantly affected by oaks age (P b 0.05), and showed a decreasing trend from P + O10 to P + O124 (except P + O35) with values ranging from 14% to 40% of total P, respectively. The C:P ratio was the highest in the monoculture pine treatments, and the values decreased from 517 to 464 with increasing age of oaks trees. 3.3. Sequential organic P fractionation The distribution of soil P fractions in different pine-oak forest stands is presented in Table 4. The total organic P (TPo) obtained as the sum of labile–Po, HCl–Po, fulvic acid–Po and humic acid–Po, was highest for mixed pine-oak forest stands in comparison to pine stands. Values ranged from 77 mg kg− 1 to 148 mg kg− 1 for 0–10 cm soil layer and from 54 to 83 mg kg− 1 for 10–20 cm soil layer. The content of TPo at the 0–10 cm layer increased with increasing age of oak trees (P b 0.01), with the highest value observed at P + O124 study site. In addition, TPo was as well affected by increasing age of oak trees at the 10–20 cm soil layer (P b 0.05) (Table 4). Furthermore, TPo accounted for more than 50% of STP in both layer and reached the highest values under P + O124 forest stands. The labile–Po increased significantly at both soil layers with increasing age of oak trees. The proportion of soil P held in a labile forms (labile–Pi and labile–Po) decreased from the upper (0–10 cm) to the lower (10–20 cm) layer in soil profiles for all study sites with the exception of pine forest stand at 0–10 cm layer (Table 4). The moderately labile Po pool (HCl–Po), accounted for 3% of TPo for both soil layers and was not significantly affected by increasing age of oak trees. The moderately stable Po pool (fulvic acid–Po) decreased in mixed pine–oak forest stands but increased with respect to the reference plot (Table 4), and the age of oaks had no influence on it (P = 0.40). The humic acid–Po increased with increasing age-old oak trees (r2 = 0.23, P b 0.05). Additionally, humic acid–Po represented, on the average, 26% of TPo at the 0–10 cm soil layer in the mixed stands, and was significantly affected by age of oak trees. In the 10–20 cm soil layer, the percentage of TPo increased from 35% to about 41% for P + O10 and P + O124 study site, respectively. The distribution of the organic (labile–Po + HCl–Po + NaOH–Po) and inorganic P pools (labile–Pi + HCl–Pi + NaOH–Pi) is shown in Fig. 1. The Po forms were higher at the 0–10 than at the 10–20 cm soil layer, and increased gradually with age of oaks, reaching the highest P content under P + O124 treatment. The inorganic forms of P increased with depth across the study sites.
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Table 4 Concentration of phosphorus (P) in the various extracts with the standard errors of pine forest and different mixed pine–oak forest study sites at two depths: 0–10 cm and 10–20 cm. Treatment
Labile–Pi
Labile–Po
NaOH–Po
HCl–Po
Fulvic acid–Po
Humic acid–Po
TPo
(mg kg−1) 0–10 cm Pine P + O10 P + O35 P + O106 P + O124
1.58 ± 0.15 0.98 ± 0.05 1.15 ± 0.07 2.35 ± 0.41 3.82 ± 0.51
16.24 ± 0.58 23.80 ± 1.16 23.94 ± 1.64 31.87 ± 7.79 40.15 ± 3.34
48.39 ± 2.57 81.05 ± 1.46 64.74 ± 2.80 79.16 ± 2.66 95.63 ± 3.68
2.19 ± 0.15 4.50 ± 0.92 1.31 ± 0.34 1.78 ± 0.69 6.64 ± 0.92
35.48 ± 2.74 62.27 ± 2.03 47.11 ± 1.54 47.94 ± 1.93 65.58 ± 5.21
23.20 ± 6.16 23.18 ± 2.19 25.54 ± 2.04 35.93 ± 1.45 35.96 ± 5.14
77.12 ± 6.14 113.76 ± 1.30 97.91 ± 3.52 117.52 ± 9.91 148.33 ± 7.22
10–20 cm Pine P + O10 P + O35 P + O106 P + O124
2.45 ± 0.25 0.67 ± 0.02 0.73 ± 0.08 1.50 ± 0.19 2.10 ± 0.58
10.58 ± 0.32 13.12 ± 0.88 12.42 ± 0.99 12.93 ± 0.91 16.21 ± 2.16
26.61 ± 2.49 52.42 ± 1.66 40.30 ± 1.52 47.13 ± 1.73 48.11 ± 0.84
2.79 ± 0.60 1.82 ± 0.32 0.81 ± 1.14 3.15 ± 0.61 2.84 ± 0.86
20.09 ± 0.83 39.44 ± 4.49 30.30 ± 2.68 30.51 ± 3.09 25.42 ± 2.81
20.62 ± 2.14 28.96 ± 4.08 23.41 ± 2.03 32.80 ± 3.42 31.38 ± 2.94
54.08 ± 3.03 83.34 ± 3.17 66.95 ± 0.29 79.39 ± 2.53 75.85 ± 1.76
Labile–Pi, labile–Po extracted with 0.5 M NaHCO3. TPo total organic P as a sum of organic P fractions: labile–Po, HCl–Po, fulvic acid–Po, humic acid–Po.
Labile–Pi and labile–Po forms constitute only a small proportion of the STP, and account for 1.3 and 15% of STP, respectively. Labile–P, which is often used as an index of P availability (Olsen et al., 1954; Bowman and Cole, 1978a) indicated an increase in available P as a percentage of STP, with increasing age of oak trees in both soil layers. Visible trend of increase was observed at the 0–10 cm soil depth. The highest available P content was found under P + O124 treatment; 28.8% and 18.3% of STP for 0–10 cm and 10–20 cm, respectively (Fig. 2).
3.4. Relationship among soil properties and Po fractions Table 5 shows the Pearson's correlation values between some soil properties and different P forms. Significant positive correlation was found between TOC and NaOH–Po at the 0–10 cm soil layer (P b 0.05) and between TOC and NaOH–Pi (P b 0.05) as well between Alox, Feox and NaOH–Pi (P b 0.01) at 10–20 cm soil layer. There was also a strong negative correlation between Alox and Feox and labile forms of P, NaOH–Po, HCl–Pi at 0–10 cm soil layer. At the 10–20 cm soil layer strong negative correlation was found between Alox and Feox and labile–Pi, labile–Po, HCl–Pi and between TOC and labile–Pi, HCl–Pi (P b 0.01). 4. Discussion 4.1. General soil properties Our results showed that, soil C:P ratio decreased with increasing soil depth from 268 to 85, probably due to preferential mineralization of C. The pine-oak mixed forest stands may have influenced the distribution of TOC and TN in the forest floor and the underlying soils. The distribution of TOC and TN, which was lower in the forest floor and higher in the mineral layer could be explained by the difference in root distribution patterns between coniferous forest and mixed deciduous forest (Oostra et al., 2006). Moreover, deciduous forest e.g. oak forest can store C over a longer period within deeper soil horizon compared to coniferous forest soils (Koch and Makeschin,
Fig. 1. Organically and inorganically bound phosphorus in 0–10 cm depth and 10–20 cm depth for all forest treatments (standard errors shown by horizontal bars).
Fig. 2. The proportion of plant available P (labile–P) to soil total P (STP) in mineral soil at 0–10 and 10–20 cm depth.
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Table 5 Pearson's correlations between soil physicochemical parameters and the different forms of P fraction for mixed pine–oak forest stands from 10 year old oak trees (P + O10) to 124 year old (P + O124) for two soil layers: 0–10 cm. 10–20 cm. Soil properties
Labile–Pi 0–10 cm
10–20 cm
Labile–Po 0–10 cm
10–20 cm
NaOH–Pi 0–10 cm
10–20 cm
NaOH–Po 0–10 cm
10–20 cm
HCl–Pi 0–10 cm
10–20 cm
HCl–Po 0–10 cm
10–20 cm
TOC Alox Feox
− 0.356 − 0.990** − 0.917**
− 0.912** − 0.886** − 0.849*
− 0.315 − 0.996* − 0.924*
− 0.873 − 0.949* − 0.934
− 0.722** 0.311 0.347
0.911* 0.907** 0.969**
0.266* − 0.845* − 0.956**
0.133 − 0.501 0.043
− 0.186 − 0.980** − 0.977**
− 0.958** − 0.880* − 0.992*
0.379 − 0.612 − 0.868**
− 0.482 − 0.752** − 0.389
Labile–Pi, labile–Po extracted with 0.5 M NaHCO3. TOC — total organic carbon, Alox — oxalate soluble Al, Feox — oxalate soluble Fe. *, ** significant at 0.05 and 0.01, respectively.
2004). More details on TOC and TN distribution on the study plots can be found in Matos et al. (in press). Concerning the P transformation and redistribution into different forms, microbial activity plays a significant role (Stewart and Tiessen, 1987) since microorganisms can solubilise and immobilise P. The importance of mycorrhiza in increasing the nutrient uptake by plants, especially P, is well documented (Bolan, 1991; Schneider et al., 2001; Göransson et al., 2006). According to Tate (1984) PMB usually represents a small fraction of the total P in soil, and rapidly turns over to provide inorganic P to plant roots. This seems to be confirmed by the low PMB ranging from 1.6 to 5.1% of STP and 0.5 to 1.8% at the 0–10 and 10–20 cm soil layers, respectively observed in this study. Similar results were reported for Podzols under spruce forest in Czech Republic (Santrucková et al., 2004). The upper soil layer under pine stand, has the highest percentage of PMB/TP (5.1%) compared to mixed pine–oak forest stands and may presumably due to the higher root density of the mixed pine–oak forests having more mycorrhizal fungal spores than the monoculture pine forest (unpublished data). This is in agreement with Göransson et al. (2006), who reported that coniferous trees (spruce) produce less external ectomycorrhiza mycelia than deciduous trees (oak) and have their fine roots more concentrated at the top soil layer than deciduous trees. Moreover, the study of Rosengren et al. (2005) showed that Norway spruce growing in a mixture of deciduous trees has a better nutrient status than spruce growing in pure stands. 4.2. Forest floor The TPff content in pure pine forest stand was around 0.75 g kg− 1. Similar results were found in sandy soil under coniferous forest in Poland, where TPff content was 0.9 g kg− 1 and the C:N ratio was 26 (Małachowska, 2007). Studies at other forest sites reported a C:P ratio of 305 for flowering dogwood forest floor (Blair, 1988) and between 360 and 480 for forest floor in the Hubbard Brook forest (Gosz et al., 1973). The substrate C:P ratio is considered to be an important indicator of whether P will be mineralised or immobilised as the forest floor litter decomposes (Blair, 1988). In our study, the forest floor under monoculture pine forests had the highest C:P ratio and decreased with increasing age of oak trees from 517 to 464, with the exception of P + O35 study sites with C:P ratio of 386, suggesting that P was immobilized with increased mineralization of C. Forms of P present in forest floor are important source of P for trees in many forest ecosystems (Pritchett and Fisher, 1987). The results indicated that the highest content of Pof.f. was at the P + O10 and P + O35 study sites. This may be due to higher organic matter input, as a result of greater litter fall. According to Rosengren et al. (2005) mixed litter fall (from deciduous and coniferous trees) naturally increase forest floor diversity and enhance forest floor quality with regard to decomposition. Besides, study by Binkley (1992) shows that P content in forest floor was significantly higher in mixed forest stands compared to conifer monoculture. The P release from decomposition of litter material varies considerably depending on the types of trees from which the litter originates, environmental conditions and the way in which the litter material is incorporated into the soil (Di et al.,
1996). In forest soil the organic horizon is the richest layer of macronutrients (Małachowska, 2007) and consequently, the main source of nutrients for plant (Wall and Hytönen, 2005). The quantity and quality of the litter layer build-up on the forest floor and the rate of decomposition of organic matter are important factors for forest ecosystems (Rothe and Binkley, 2001). 4.3. Sequential organic P fractionation The amount of Po in the soil was higher in mixed forest stands than in monoculture and this could be due to the higher organic matter input (leaves, roots etc.). A study by Göransson et al. (2006) found that deciduous trees have relatively higher nutrient uptake capacity thus, confirming that root distribution may be an indicator of the distribution of nutrient uptake. In addition, Po accounted for more than 50% of STP in investigated forest stands. This suggests that the organic P form plays an essential role in P cycling and plant nutrient availability in sandy soil. The greater P preserved in organic forms suggests that P cycling is dependent on the decomposition of the organic matter (Xavier et al., 2009). Soil P content held in labile forms generally decrease with soil depth, partly as a result of a decrease in root density and soil microbial activity with increasing depth (Trasar-Cepeda and Gil-Sotres, 1987; Rothe and Binkley, 2001; Rosengren et al., 2005). The higher Po values under oak forest stand in comparison with pine monoculture forest concurred with similar results of higher labile–Po under oak than under pine forests reported by TrasarCepeda and Gil-Sotres (1987). Moreover, the highest Po values observed in the upper soil layer may be due to a decline in soil microbial activity with increasing soil depth. Labile-P forms are considered to be readily available to plants in the short term (Bowman and Cole, 1978b). Our results suggest that labile–Po increased with age of oak trees that are capable of maintaining larger fractions of available P under mixed forest stands. Additionally, the availability and transformation of soil P, which depends on the interactions among soil properties, plants and microbial community (Novais and Smyth, 1981), may be a plausible explanation for the higher amounts of available P under mixed pineoak forest stands than in monoculture. Our results on STP and available P are comparable to that reported by Smal and Olszewska (2008) in coniferous forest, with STP and available P values approximately 170 and 8 mg kg− 1, respectively. In soils with low labile–Pi content, NaOH–Po was found to be a source for microbial uptake (Saá et al., 1998; Kramer and Green, 1999). Our study revealed that the NaOH–Po was the largest fraction of Po in soil. Other studies have also shown NaOH–Po to be the largest fraction of Po in the soil (Roberts et al., 1985; McDowell and Stewart, 2006; Richter et al., 2006; Xavier et al., 2009). This fraction accounted for three folds the amount of labile–Po in all treatments. The percentage of NaOH–Po in relation to STP decreased with soil depth in all treatments, with the exception of P + O10, and increased significantly with increasing age of oak trees (P b 0.01, P b 0.05 for 0–10 and 10–20 cm layer, respectively). This can be explained by the fact that Pi is released from organic forms in biologically active surface
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horizons and by the accelerated decomposition of Po. This is probably due to greater litter fall inputs in forests where the oldest oaks exist (unpublished data). The HCl–Po increased with soil depth and age of oaks and accounted for 5 to 16% of STP at the 0–10 cm soil layer and 6 to 37% of STP in the 10– 20 cm soil layer. Similar trend was found in gray Luvisols under native aspen forest catena where HCl extractable P increases with increasing soil depth (Schoenau et al., 1989). The fulvic acid–Po represented 47 and 40% of TPo in the 0–10 and 10–20 cm soil layers, respectively. Fulvic acid associated P constitutes a large fraction of the Po in most soils, and can be attributed to recent organic inputs (Grindel and Zyrin, 1965). The soil P distribution during the forest development can be illustrated by the separation into organic and inorganic P forms. The difference between organic and inorganic forms was most pronounced in pine-oak forest stands. Beck and Elsenbeer (1999) found similar trend of increasing organically bound P with increasing age of beech forest as well as an increasing inorganic bound P and decreasing organic forms with soil depth.
4.4. Relationship among soil properties and Po fractions The significant positive correlation between NaOH–Pi and Fe- and Al-oxides (Table 5) indicate that this P form is geochemically fixed on the soil matrix, thus become less available for plants. This is in line with findings of Sharpley and Smith (1985) who studied highly weathered soils.
5. Conclusion The focus of this study was to investigate the effect of the transformation of dominant coniferous monocultures into more stable mixed pine–oak forest stand on the Po dynamics. Our hypothesis that P availability and organic P pool (sum of all organic fraction) should increase with age-old oak trees was proved. The forest transformation promoted an increase in the most labile P pool. Moreover, the content of STP, TPo and labile–P were higher in soils under mixed pine–oak forest stands than under a pure pine monoculture. Following the method of P fractionation we observed that the organic forms of P occurred in the topsoil and reached the highest value under the oldest pine–oak forest stand (P + O124). The amount of TPo in the soil increased with the conversion from monoculture into mixed pine–oak, suggesting that mixed forests have a positive influence on the dynamics of organic P in the sandy soil. In addition TPo accounted for more than 50% of STP in the investigated forest stands, indicating that the organic P form plays an essential role in the P cycling and plant nutrient in a nutrient poor sandy soil. Furthermore, the forms of labile available P increased with age of oak trees, which are capable of maintaining larger fractions of available P in mixed forest stands. Nevertheless, further research is necessary to better understand the interactions among different tree species in mixed forest stands and their effects on soil development and nutrient supply.
Acknowledgements We would like to thank the Forest Brandenburg for allowing us to use the forest plots, G. Franke and R. Müller for the technical assistance in the P analyses, E. Khumbah for the help in soil sampling and laboratory work, M. Elmer and U. Bachmann for their comments, C. Corfield and S. Nii-Annang for language correction, and anonymous reviewers for their precious suggestions. This research was partly supported by the International Graduate School (IGS) of Brandenburg University of Technology.
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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a
Soil organic phosphorus fraction in pine–oak forest stands in Northeastern Germany Anna Slazak a,⁎, Dirk Freese a, Eduardo da Silva Matos a, Reinhard F. Hüttl a,b a b
Chair of Soil Protection and Recultivation, Brandenburg University of Technology, P.O. Box 10 13 44, 03013 Cottbus, Germany Helmholtz-Centre Potsdam-GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
a r t i c l e
i n f o
Article history: Received 12 July 2009 Received in revised form 14 April 2010 Accepted 26 April 2010 Available online 31 May 2010 Keywords: Sequential P extraction P availability Forest management Pinus sylvestris Quercus petraea
a b s t r a c t Pines (Pinus sylvestris L.) have been widely used for monoculture forest stands in north-eastern Germany. However, several studies have discussed the need to convert monoculture into mixed forest stands such as pine– oak forest. In this study, we evaluated the impact of 102 years-old pine (P. sylvestris L.) in monoculture and mixed forest stands of P. sylvestris + Quercus petraea (Matt.) Liebl. of different ages (10, 35, 106 and 124 years-old) on the dynamics of soil organic phosphorus (Po) pools. The study was carried out in the Northern German Lowlands of Brandenburg. Soil samples were taken from forest floor and two mineral soil layers at the depths of 0–10 and 10– 20 cm. Different P forms of the sandy soil were obtained by sequential P fractionation, using acid and alkaline extractants. The soil total P (STP) ranged from 100 to 183 mg kg− 1 whereas total organic P (TPo) ranged from 77 to 148 mg kg− 1. STP was higher in mixed forest stand than in monoculture and decreased with soil depth. The TPo and labile-P in both soil layers increased significantly with increase in age of oak trees. In addition, TPo content was lower in mineral soil compared to the forest floor and accounted for more than 50% of soil total P in the forest stands. The most available-P fraction–labile-P predominated in the oldest pine–oak forest stand (P + O124), accounting for 29% of STP at the 0–10 cm soil depth. The largest P fraction comprised NaOH–Po and represent 62% of STP. Results showed that forest transformation from pure pine monoculture forest into pine-oak mixed forest stands promoted an increase in the TPo and P available. Furthermore, the forms of labile available P increased with age of oak trees, which are capable of maintaining larger fractions of available P under mixed forest stands. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the middle 19th century, forest conversion has been undertaken in north-eastern Germany, mainly by planting beech trees (Fagus sylvatica L.) in pine (P. sylvestris) stands. Nevertheless, in the continentally influenced regions of the north-eastern German Lowlands (Brandenburg), little effort has been made to convert pine monocultures into mixed forests. This is due to the low precipitation levels and low nutrient status of the sandy soils, which predominate in the region even though, sessile oak (Q. petraea) is suitable for forestry under these conditions. In north-eastern Germany, monoculture pine forest stands can be found in abundance (Müller, 2007). Nevertheless, it has been discussed that monoculture stands can be converted into mixed pine-oak forests (Elmer et al., 2009a). It is known that mixed forest stands are more productive than monoculture forest (Burkhart and A., 1992; Rothe and Binkley, 2001; Rosengren et al., 2005). Since only little is known about nutrient cycling in mixed forest and especially about phosphorus (P), which is commonly known as a limiting factor for plant growth in many soils (Qualls and Haines, 1991; Kaiser et al., 2003), it is therefore interesting
⁎ Corresponding author. Tel.: + 49 355 694329; fax: + 49 355 692323. E-mail address: [email protected] (A. Slazak). 0016-7061/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.04.023
to point out the effect of conversion monocultures into mixed forest stands on P dynamics. The P dynamics in forest stands is a relationship between the tree's ability to extract P from soils and cycle absorbed P among ecosystem components, and the soil's ability to supply P in a bioavailable form (Comerford et al., 2002; Richter et al., 2006). P in soil occurs in a wide variety of different inorganic and organic forms (Tate, 1984). Organic P (Po) is the major form of P and its mineralization depend on both microbial activity and extracellular phosphate enzymes produced by plant roots and soil microorganisms (Tate, 1984). Besides, additional recycling of Po is important to the availability of soil P for plants (Cross and Schlesinger, 1995). Thus, to sustain P supplies within the forest ecosystem, the crucial process is the release of inorganic phosphate from the decomposition of Po in the forest floor and soil organic matter (Stewart and Tiessen, 1987; Cross and Schlesinger, 1995). Organic forms of P in soils make a significant contribution to the P for plant nutrition (Firsching and Claassen, 1996; Chen et al., 2002). The forms and dynamics of organic P in the soil depend on biological, chemical and physical factors (Tiessen et al., 1983; Stewart and Tiessen, 1987; Cross and Schlesinger, 1995) as well as environmental factors such as temperature, soil moisture and pH (Tate, 1984; Kaiser et al., 2003). To obtain different inorganic and organic P fractions sequential P fractionation schemes have been broadly used, in which acid and alkaline extractants are used to separate the various fractions, based on the type and strength of Po physicochemical interactions with
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layers. The five soil samples were then combined in the field to form one replicate. Thus, for each plot and soil layer, four replicates were sampled. All samples of a sandy textured soil were collected in September 2007. For chemical analyses, soil samples were air-dried and gently broken to pass through a 2-mm sieve. For microbial P (PMB) fresh and moist soil sample was sieved at 2-mm mesh and stored at 4 °C in plastic bags that preserved moisture and allowed aeration until the final analysis was carried out. Samples from the forest floor were collected from the soil surface without vegetation but covered with pine needles and dry leaves using steel sampling frame (25 × 25 cm). The collected samples describe all organic matter generated by forest vegetation, including litter and unincorporated humus, on the mineral soil surface. Forest floor fraction was separated from mineral soil; however, there was little soil contamination. The sample from the forest floor was milled and stored for final analyses.
other soil components (Bowman and Cole, 1978a; Hedley et al., 1982; Cross and Schlesinger, 1995; Ivanoff et al., 1998). This method separates the Po into four pools: labile P, moderately labile P, moderately stable (fulvic acid-P) and highly stable (humic acid-P) (Bowman and Cole, 1978a). There are few studies on the Po in soils of forest ecosystems (Fahey and Yavitt, 1988; Qualls and Haines, 1991; Qualls et al., 2000; Rothe and Binkley, 2001; Kaiser et al., 2003; Santrucková et al., 2004) and little is known on the effects of the admixture of sessile oak (Q. petraea) into pine monocultures with regard to the Po dynamics of these ecosystems, thus, Po is much important as Pi with regard to P nutrition. We hypothesized that the proportions of soil total P (STP) present in available forms and Po would increase with age-old oak trees. The objective of the study was therefore to investigate the Po forms and dynamics, affected by mixed pine-oak forest stands established through the transformation of monoculture pine to mixed pine-oak forest stages in age-old oak trees. Considering the nutrient perspective it is interesting to focus on long-term changes in the soil nutrient pools when comparing the different forest types.
2.3. Chemical analyses Soil pH was measured in the supernatant suspension of air-dried soil to deionised water of ratio 2.5:1 (weight/volume). Total C (TOC) and N (TN) in the soil and forest floor were measured by dry combustion (Vario El Elementar Analyzer). The STP was determined after digestion of sample in nitric acid (HNO3) using inductively coupled plasma emission spectrometry (ICP). The forest floor organic P (Pof.f.) content was determined after loss on ignition in a muffle furnace at 500 °C for 5 h and digested with HNO3 extractant. The Pof.f. was then quantified as the difference between the amounts HNO3-extractable P for the ignited and unignited samples. The total P content in the forest floor fraction was determined as previously described. Oxalate Al and Fe (Alox and Feox) were determined according to Schwertmann (1964) and McKeague and Day (1966) by the acid-ammonium oxalate extraction.
2. Materials and methods 2.1. Site description The study was carried out in north-eastern Germany in the Federal State of Brandenburg (51°47′N lat, 13°32′E long). The region experienced a mean annual temperature and rainfall of 9.8 °C and 587 mm, respectively. Five plots were selected to study the transformation of a 120 yearold Scots pine (P. sylvestris) monoculture forest stand into pine-oak mixed forest stands with the ages of Sessile oaks (Q. petraea) raging from 10 to 124 years and, therefore represent the following treatments: 90–100 years-old pine and 10 years-old oak (P + O10), 102 years-old pine and 35 years-old oak (P + O35), 100 years-old pine and 106 years-old oak (P + O106), 120 years-old pine and 124 years-old oak (P + O124) (Table 1). Climatic, geological, soil physical and chemical conditions were similar across all plots. The ground vegetation consisted mainly of Vaccinium myrtillus L., Vaccinium vitis-idea L., Deschampsia flexuosa L., and mosses. Besides, Calluna vulgaris L. is present under the monoculture pine forest. The soil type is Podzols and weakly podzolized Cambisols according to FAO classification. Soil characteristics of all the study sites have been described by Elmer et al. (2009b).
2.4. Microbial analysis The PMB was determined by the irradiation-extraction method, using microwave (Islam and Weil, 1998), and 0.5 M NaHCO3 (pH 8.5) as extractant (Brookes et al., 1982). Approximately, 20 g of fresh soil samples were irradiated using a total of energy exposure of 800 J.g− 1 soil. Irradiated and non-irradiated soil samples were put in centrifuge tubes with 80 mL of 0.5 M NaHCO3 and were shaken at 250 rpm for 1 h. The soil suspension was centrifuged at 1500 ×g for 5 min and filtered. P concentration was quantified by the ammonium molybdate-ascorbic method (Murphy and Riley, 1962) using an atomic absorption spectrophotometer (the absorbance was measured at 880 nm). PMB content was calculated by the difference between P concentration in irradiated and non-irradiated samples (based on dry mass) divided by a conversion factor of 0.40 used to convert the flow of P into microbial biomass P (Brookes et al., 1982).
2.2. Sampling Each plot was divided into four sub-plots, and for each sub-plot, five soil samples were randomly collected from the 0–10 and 10–20 cm soil
Table 1 Vegetation and area of investigated study sites, and particle distribution of the soils. Treatment
Vegetation (age in years)
Area (ha)
Trees (ha− 1)a
0–10 cmb
10–20 cmb
(%)
Pine P + O10 P + O35 P + O106 P + O124
Q. petraea
P. silvestris
Q. petraea
P. sylvestris
Sand
Clay
Sand
Clay
– 10 35 106 (45%)c 50 (55%) 124
102 90–100 102 100
1 0.15 0.25 0.75
– 8700 1424 299
nd 167 268 336
92.4 87.4 83.0 84.0
2.6 3.4 3.7 4.4
93.6 87.2 88.5 91.3
3.8 4.3 3.4 2.8
120
1.00
287
52
92.9
3.0
92.9
3.3
nd: not determined. a After Schröder et al. (2007). b After Matos et al. (in press). c The percentage of oak trees.
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2.5. Soil organic P fractionation
Table 3 C/N and C/P ratio, organic P (Pof.f.) in the forest floor.
Soil Po was fractionated by following the sequential extraction procedure of Bowman and Cole (1978a), which was further modified by Sharpley and Smith (1985) and Ivanoff et al. (1998). Soil Po was sequentially fractionated into: i) a labile P pool, extracted by 0.5 M NaHCO3 (labile–Po); ii) a moderately labile P pool, obtained by extraction with 1.0 M HCl (HCl–Po); and iii) a more resistant Po pool, which is associated with humic compounds and held more strongly by chemisorption to Fe and Al hydrous oxides (McLaughlin et al., 1977; Bowman and Cole, 1978a,b) was extracted with 0.5 M NaOH. The NaOH–Po extract was acidified with concentrated HCl to separate highly stable Po, associated with humic acid (humic acid–Po), from the moderately stable Po, related to fulvic acid (fulvic acid–Po). Total P (Pt) concentration in the NaHCO3 and HCl extracts, was determined by Inductively Coupled Plasma (optical emission spectrometry). Pt in the humic and fulvic acid extracts was obtained by digestion aliquot with 2.5 M H2SO4 and potassium persulfate (K2S2O8), according to the method of Bowman (1989) and modified by Thien and Myers (1992). Pi concentration in all extracts was measured by the ammonium molybdate-ascorbic method (Murphy and Riley, 1962). For each extract, Po was calculated as being the difference between Pt and Pi. Total organic P (TPo) was calculated as the sum of P fractions: labile–Po + HCl–Po + fulvic acid–Po + humic acid–Po. 2.6. Statistical analysis The data presented are mean values of independent field replicates. The effect of the different forest management systems on Po dynamics at each depth was evaluated using one-way analysis of variance. Pearson correlation and multiple linear regression analyses were also performed. All analyses were done using Sigma Plot.8.0 (Jandel Scientific) at significant level of P = 0.01, P = 0.05. 3. Results 3.1. General soil properties Some properties of the soils are given in Table 2. The soils presented low pH values, ranging from 3.58 to 4.61 whereas the TOC content varied between 8.39 and 46.80 g kg− 1 among all treatments. The P+ O10 plots showed the highest TOC content (Table 2). The PMB was higher at the 0–10 cm than at 10–20 cm soil layer, with values ranging from 0.54 to 5.64 mg kg− 1. Among the pine-oak stands, the highest PMB content was observed under P+O124 followed by P+ O10 treatment in the upper soil layer and in the deeper soil layer under P+O35.
Table 2 Chemical characteristics of two soil layers in all treatments. Mean values of total N (TN), total C (TOC), microbial P (PMB), P (STP), oxalate soluble Al (Alox) and Fe (Feox). Treatment
TNa (g kg
TOCa
pHa
C/N
C/P
−1
)
PMB
STP
(mg kg
Aloxa
Feoxa
−1
)
0–10 cm Pine P + O10 P + O35 P + O106 P + O124
1.11 2.52 1.08 1.17 1.36
29.78 46.80 23.97 29.86 29.46
3.58 3.59 3.64 3.64 4.05
29.0 24.4 25.8 27.7 22.0
268 255 167 198 192
5.64 4.94 4.51 2.42 5.29
110.6 183.5 144.3 152.1 152.6
816.6 1162.2 1197.6 910.2 690.8
1410.9 2724.3 2958.5 2677.0 2093.4
10–20 cm Pine P + O10 P + O35 P + O106 P + O124
0.34 0.54 0.46 0.38 0.35
10.61 13.72 12.12 11.77 8.39
4.10 4.05 4.10 4.11 4.61
31.2 25.8 27.5 31.0 24.7
105 122 120 110 85
0.82 0.54 1.81 1.33 0.94
103.5 112.7 102.0 109.1 100.0
1556.6 1433.8 1756.7 1369.3 854.3
2473.2 2146.0 1993.6 2033.7 1433.5
a
After Matos et al. (in press).
Treatment
C/Na
C/P
Pine P + O10 P + O35 P + O106 P + O124 a
28.8 25.9 25.3 25.9 27.7
Pof.f. (mg kg−1)
ratio 613 517 386 484 464
199.8 228.4 414.8 115.9 192.7
After Matos et al. (in press).
The highest STP content was found in the P+O10 treatment at both soil layers (Table 2). The STP decreased with forest transformation, and the content of STP at the 0–10 cm layer was significantly affected by oaks age (P b 0.05). The C:P ratio decreased from 268 to 85 with increasing soil depth (Table 2). 3.2. Forest floor The total P (TPff) contents in the forest floor fraction increased in relation to forest transformation, with the exception of P + O35 treatment, which has the highest value (Table 3). The Pof.f. was significantly affected by oaks age (P b 0.05), and showed a decreasing trend from P + O10 to P + O124 (except P + O35) with values ranging from 14% to 40% of total P, respectively. The C:P ratio was the highest in the monoculture pine treatments, and the values decreased from 517 to 464 with increasing age of oaks trees. 3.3. Sequential organic P fractionation The distribution of soil P fractions in different pine-oak forest stands is presented in Table 4. The total organic P (TPo) obtained as the sum of labile–Po, HCl–Po, fulvic acid–Po and humic acid–Po, was highest for mixed pine-oak forest stands in comparison to pine stands. Values ranged from 77 mg kg− 1 to 148 mg kg− 1 for 0–10 cm soil layer and from 54 to 83 mg kg− 1 for 10–20 cm soil layer. The content of TPo at the 0–10 cm layer increased with increasing age of oak trees (P b 0.01), with the highest value observed at P + O124 study site. In addition, TPo was as well affected by increasing age of oak trees at the 10–20 cm soil layer (P b 0.05) (Table 4). Furthermore, TPo accounted for more than 50% of STP in both layer and reached the highest values under P + O124 forest stands. The labile–Po increased significantly at both soil layers with increasing age of oak trees. The proportion of soil P held in a labile forms (labile–Pi and labile–Po) decreased from the upper (0–10 cm) to the lower (10–20 cm) layer in soil profiles for all study sites with the exception of pine forest stand at 0–10 cm layer (Table 4). The moderately labile Po pool (HCl–Po), accounted for 3% of TPo for both soil layers and was not significantly affected by increasing age of oak trees. The moderately stable Po pool (fulvic acid–Po) decreased in mixed pine–oak forest stands but increased with respect to the reference plot (Table 4), and the age of oaks had no influence on it (P = 0.40). The humic acid–Po increased with increasing age-old oak trees (r2 = 0.23, P b 0.05). Additionally, humic acid–Po represented, on the average, 26% of TPo at the 0–10 cm soil layer in the mixed stands, and was significantly affected by age of oak trees. In the 10–20 cm soil layer, the percentage of TPo increased from 35% to about 41% for P + O10 and P + O124 study site, respectively. The distribution of the organic (labile–Po + HCl–Po + NaOH–Po) and inorganic P pools (labile–Pi + HCl–Pi + NaOH–Pi) is shown in Fig. 1. The Po forms were higher at the 0–10 than at the 10–20 cm soil layer, and increased gradually with age of oaks, reaching the highest P content under P + O124 treatment. The inorganic forms of P increased with depth across the study sites.
A. Slazak et al. / Geoderma 158 (2010) 156–162
159
Table 4 Concentration of phosphorus (P) in the various extracts with the standard errors of pine forest and different mixed pine–oak forest study sites at two depths: 0–10 cm and 10–20 cm. Treatment
Labile–Pi
Labile–Po
NaOH–Po
HCl–Po
Fulvic acid–Po
Humic acid–Po
TPo
(mg kg−1) 0–10 cm Pine P + O10 P + O35 P + O106 P + O124
1.58 ± 0.15 0.98 ± 0.05 1.15 ± 0.07 2.35 ± 0.41 3.82 ± 0.51
16.24 ± 0.58 23.80 ± 1.16 23.94 ± 1.64 31.87 ± 7.79 40.15 ± 3.34
48.39 ± 2.57 81.05 ± 1.46 64.74 ± 2.80 79.16 ± 2.66 95.63 ± 3.68
2.19 ± 0.15 4.50 ± 0.92 1.31 ± 0.34 1.78 ± 0.69 6.64 ± 0.92
35.48 ± 2.74 62.27 ± 2.03 47.11 ± 1.54 47.94 ± 1.93 65.58 ± 5.21
23.20 ± 6.16 23.18 ± 2.19 25.54 ± 2.04 35.93 ± 1.45 35.96 ± 5.14
77.12 ± 6.14 113.76 ± 1.30 97.91 ± 3.52 117.52 ± 9.91 148.33 ± 7.22
10–20 cm Pine P + O10 P + O35 P + O106 P + O124
2.45 ± 0.25 0.67 ± 0.02 0.73 ± 0.08 1.50 ± 0.19 2.10 ± 0.58
10.58 ± 0.32 13.12 ± 0.88 12.42 ± 0.99 12.93 ± 0.91 16.21 ± 2.16
26.61 ± 2.49 52.42 ± 1.66 40.30 ± 1.52 47.13 ± 1.73 48.11 ± 0.84
2.79 ± 0.60 1.82 ± 0.32 0.81 ± 1.14 3.15 ± 0.61 2.84 ± 0.86
20.09 ± 0.83 39.44 ± 4.49 30.30 ± 2.68 30.51 ± 3.09 25.42 ± 2.81
20.62 ± 2.14 28.96 ± 4.08 23.41 ± 2.03 32.80 ± 3.42 31.38 ± 2.94
54.08 ± 3.03 83.34 ± 3.17 66.95 ± 0.29 79.39 ± 2.53 75.85 ± 1.76
Labile–Pi, labile–Po extracted with 0.5 M NaHCO3. TPo total organic P as a sum of organic P fractions: labile–Po, HCl–Po, fulvic acid–Po, humic acid–Po.
Labile–Pi and labile–Po forms constitute only a small proportion of the STP, and account for 1.3 and 15% of STP, respectively. Labile–P, which is often used as an index of P availability (Olsen et al., 1954; Bowman and Cole, 1978a) indicated an increase in available P as a percentage of STP, with increasing age of oak trees in both soil layers. Visible trend of increase was observed at the 0–10 cm soil depth. The highest available P content was found under P + O124 treatment; 28.8% and 18.3% of STP for 0–10 cm and 10–20 cm, respectively (Fig. 2).
3.4. Relationship among soil properties and Po fractions Table 5 shows the Pearson's correlation values between some soil properties and different P forms. Significant positive correlation was found between TOC and NaOH–Po at the 0–10 cm soil layer (P b 0.05) and between TOC and NaOH–Pi (P b 0.05) as well between Alox, Feox and NaOH–Pi (P b 0.01) at 10–20 cm soil layer. There was also a strong negative correlation between Alox and Feox and labile forms of P, NaOH–Po, HCl–Pi at 0–10 cm soil layer. At the 10–20 cm soil layer strong negative correlation was found between Alox and Feox and labile–Pi, labile–Po, HCl–Pi and between TOC and labile–Pi, HCl–Pi (P b 0.01). 4. Discussion 4.1. General soil properties Our results showed that, soil C:P ratio decreased with increasing soil depth from 268 to 85, probably due to preferential mineralization of C. The pine-oak mixed forest stands may have influenced the distribution of TOC and TN in the forest floor and the underlying soils. The distribution of TOC and TN, which was lower in the forest floor and higher in the mineral layer could be explained by the difference in root distribution patterns between coniferous forest and mixed deciduous forest (Oostra et al., 2006). Moreover, deciduous forest e.g. oak forest can store C over a longer period within deeper soil horizon compared to coniferous forest soils (Koch and Makeschin,
Fig. 1. Organically and inorganically bound phosphorus in 0–10 cm depth and 10–20 cm depth for all forest treatments (standard errors shown by horizontal bars).
Fig. 2. The proportion of plant available P (labile–P) to soil total P (STP) in mineral soil at 0–10 and 10–20 cm depth.
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Table 5 Pearson's correlations between soil physicochemical parameters and the different forms of P fraction for mixed pine–oak forest stands from 10 year old oak trees (P + O10) to 124 year old (P + O124) for two soil layers: 0–10 cm. 10–20 cm. Soil properties
Labile–Pi 0–10 cm
10–20 cm
Labile–Po 0–10 cm
10–20 cm
NaOH–Pi 0–10 cm
10–20 cm
NaOH–Po 0–10 cm
10–20 cm
HCl–Pi 0–10 cm
10–20 cm
HCl–Po 0–10 cm
10–20 cm
TOC Alox Feox
− 0.356 − 0.990** − 0.917**
− 0.912** − 0.886** − 0.849*
− 0.315 − 0.996* − 0.924*
− 0.873 − 0.949* − 0.934
− 0.722** 0.311 0.347
0.911* 0.907** 0.969**
0.266* − 0.845* − 0.956**
0.133 − 0.501 0.043
− 0.186 − 0.980** − 0.977**
− 0.958** − 0.880* − 0.992*
0.379 − 0.612 − 0.868**
− 0.482 − 0.752** − 0.389
Labile–Pi, labile–Po extracted with 0.5 M NaHCO3. TOC — total organic carbon, Alox — oxalate soluble Al, Feox — oxalate soluble Fe. *, ** significant at 0.05 and 0.01, respectively.
2004). More details on TOC and TN distribution on the study plots can be found in Matos et al. (in press). Concerning the P transformation and redistribution into different forms, microbial activity plays a significant role (Stewart and Tiessen, 1987) since microorganisms can solubilise and immobilise P. The importance of mycorrhiza in increasing the nutrient uptake by plants, especially P, is well documented (Bolan, 1991; Schneider et al., 2001; Göransson et al., 2006). According to Tate (1984) PMB usually represents a small fraction of the total P in soil, and rapidly turns over to provide inorganic P to plant roots. This seems to be confirmed by the low PMB ranging from 1.6 to 5.1% of STP and 0.5 to 1.8% at the 0–10 and 10–20 cm soil layers, respectively observed in this study. Similar results were reported for Podzols under spruce forest in Czech Republic (Santrucková et al., 2004). The upper soil layer under pine stand, has the highest percentage of PMB/TP (5.1%) compared to mixed pine–oak forest stands and may presumably due to the higher root density of the mixed pine–oak forests having more mycorrhizal fungal spores than the monoculture pine forest (unpublished data). This is in agreement with Göransson et al. (2006), who reported that coniferous trees (spruce) produce less external ectomycorrhiza mycelia than deciduous trees (oak) and have their fine roots more concentrated at the top soil layer than deciduous trees. Moreover, the study of Rosengren et al. (2005) showed that Norway spruce growing in a mixture of deciduous trees has a better nutrient status than spruce growing in pure stands. 4.2. Forest floor The TPff content in pure pine forest stand was around 0.75 g kg− 1. Similar results were found in sandy soil under coniferous forest in Poland, where TPff content was 0.9 g kg− 1 and the C:N ratio was 26 (Małachowska, 2007). Studies at other forest sites reported a C:P ratio of 305 for flowering dogwood forest floor (Blair, 1988) and between 360 and 480 for forest floor in the Hubbard Brook forest (Gosz et al., 1973). The substrate C:P ratio is considered to be an important indicator of whether P will be mineralised or immobilised as the forest floor litter decomposes (Blair, 1988). In our study, the forest floor under monoculture pine forests had the highest C:P ratio and decreased with increasing age of oak trees from 517 to 464, with the exception of P + O35 study sites with C:P ratio of 386, suggesting that P was immobilized with increased mineralization of C. Forms of P present in forest floor are important source of P for trees in many forest ecosystems (Pritchett and Fisher, 1987). The results indicated that the highest content of Pof.f. was at the P + O10 and P + O35 study sites. This may be due to higher organic matter input, as a result of greater litter fall. According to Rosengren et al. (2005) mixed litter fall (from deciduous and coniferous trees) naturally increase forest floor diversity and enhance forest floor quality with regard to decomposition. Besides, study by Binkley (1992) shows that P content in forest floor was significantly higher in mixed forest stands compared to conifer monoculture. The P release from decomposition of litter material varies considerably depending on the types of trees from which the litter originates, environmental conditions and the way in which the litter material is incorporated into the soil (Di et al.,
1996). In forest soil the organic horizon is the richest layer of macronutrients (Małachowska, 2007) and consequently, the main source of nutrients for plant (Wall and Hytönen, 2005). The quantity and quality of the litter layer build-up on the forest floor and the rate of decomposition of organic matter are important factors for forest ecosystems (Rothe and Binkley, 2001). 4.3. Sequential organic P fractionation The amount of Po in the soil was higher in mixed forest stands than in monoculture and this could be due to the higher organic matter input (leaves, roots etc.). A study by Göransson et al. (2006) found that deciduous trees have relatively higher nutrient uptake capacity thus, confirming that root distribution may be an indicator of the distribution of nutrient uptake. In addition, Po accounted for more than 50% of STP in investigated forest stands. This suggests that the organic P form plays an essential role in P cycling and plant nutrient availability in sandy soil. The greater P preserved in organic forms suggests that P cycling is dependent on the decomposition of the organic matter (Xavier et al., 2009). Soil P content held in labile forms generally decrease with soil depth, partly as a result of a decrease in root density and soil microbial activity with increasing depth (Trasar-Cepeda and Gil-Sotres, 1987; Rothe and Binkley, 2001; Rosengren et al., 2005). The higher Po values under oak forest stand in comparison with pine monoculture forest concurred with similar results of higher labile–Po under oak than under pine forests reported by TrasarCepeda and Gil-Sotres (1987). Moreover, the highest Po values observed in the upper soil layer may be due to a decline in soil microbial activity with increasing soil depth. Labile-P forms are considered to be readily available to plants in the short term (Bowman and Cole, 1978b). Our results suggest that labile–Po increased with age of oak trees that are capable of maintaining larger fractions of available P under mixed forest stands. Additionally, the availability and transformation of soil P, which depends on the interactions among soil properties, plants and microbial community (Novais and Smyth, 1981), may be a plausible explanation for the higher amounts of available P under mixed pineoak forest stands than in monoculture. Our results on STP and available P are comparable to that reported by Smal and Olszewska (2008) in coniferous forest, with STP and available P values approximately 170 and 8 mg kg− 1, respectively. In soils with low labile–Pi content, NaOH–Po was found to be a source for microbial uptake (Saá et al., 1998; Kramer and Green, 1999). Our study revealed that the NaOH–Po was the largest fraction of Po in soil. Other studies have also shown NaOH–Po to be the largest fraction of Po in the soil (Roberts et al., 1985; McDowell and Stewart, 2006; Richter et al., 2006; Xavier et al., 2009). This fraction accounted for three folds the amount of labile–Po in all treatments. The percentage of NaOH–Po in relation to STP decreased with soil depth in all treatments, with the exception of P + O10, and increased significantly with increasing age of oak trees (P b 0.01, P b 0.05 for 0–10 and 10–20 cm layer, respectively). This can be explained by the fact that Pi is released from organic forms in biologically active surface
A. Slazak et al. / Geoderma 158 (2010) 156–162
horizons and by the accelerated decomposition of Po. This is probably due to greater litter fall inputs in forests where the oldest oaks exist (unpublished data). The HCl–Po increased with soil depth and age of oaks and accounted for 5 to 16% of STP at the 0–10 cm soil layer and 6 to 37% of STP in the 10– 20 cm soil layer. Similar trend was found in gray Luvisols under native aspen forest catena where HCl extractable P increases with increasing soil depth (Schoenau et al., 1989). The fulvic acid–Po represented 47 and 40% of TPo in the 0–10 and 10–20 cm soil layers, respectively. Fulvic acid associated P constitutes a large fraction of the Po in most soils, and can be attributed to recent organic inputs (Grindel and Zyrin, 1965). The soil P distribution during the forest development can be illustrated by the separation into organic and inorganic P forms. The difference between organic and inorganic forms was most pronounced in pine-oak forest stands. Beck and Elsenbeer (1999) found similar trend of increasing organically bound P with increasing age of beech forest as well as an increasing inorganic bound P and decreasing organic forms with soil depth.
4.4. Relationship among soil properties and Po fractions The significant positive correlation between NaOH–Pi and Fe- and Al-oxides (Table 5) indicate that this P form is geochemically fixed on the soil matrix, thus become less available for plants. This is in line with findings of Sharpley and Smith (1985) who studied highly weathered soils.
5. Conclusion The focus of this study was to investigate the effect of the transformation of dominant coniferous monocultures into more stable mixed pine–oak forest stand on the Po dynamics. Our hypothesis that P availability and organic P pool (sum of all organic fraction) should increase with age-old oak trees was proved. The forest transformation promoted an increase in the most labile P pool. Moreover, the content of STP, TPo and labile–P were higher in soils under mixed pine–oak forest stands than under a pure pine monoculture. Following the method of P fractionation we observed that the organic forms of P occurred in the topsoil and reached the highest value under the oldest pine–oak forest stand (P + O124). The amount of TPo in the soil increased with the conversion from monoculture into mixed pine–oak, suggesting that mixed forests have a positive influence on the dynamics of organic P in the sandy soil. In addition TPo accounted for more than 50% of STP in the investigated forest stands, indicating that the organic P form plays an essential role in the P cycling and plant nutrient in a nutrient poor sandy soil. Furthermore, the forms of labile available P increased with age of oak trees, which are capable of maintaining larger fractions of available P in mixed forest stands. Nevertheless, further research is necessary to better understand the interactions among different tree species in mixed forest stands and their effects on soil development and nutrient supply.
Acknowledgements We would like to thank the Forest Brandenburg for allowing us to use the forest plots, G. Franke and R. Müller for the technical assistance in the P analyses, E. Khumbah for the help in soil sampling and laboratory work, M. Elmer and U. Bachmann for their comments, C. Corfield and S. Nii-Annang for language correction, and anonymous reviewers for their precious suggestions. This research was partly supported by the International Graduate School (IGS) of Brandenburg University of Technology.
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