Phosphorus budget and organic phosphorus fractions in response to long-term applications of chemical fertilisers and pig manure in a Mollisol.
Phosphorus (P) status and dynamics in soil are important for the sustainability of agricultural ecosystems. Phosphorus is a macronutrient and its addition to soil through fertilisation is needed to maintain an appropriate level of soil P. However, excess P in soil, resulting from continual applications of fertilisers, can have negative environmental impacts. Build-up of P in the topsoil is a major source of soluble and particulate P in surface runoff (Daniel et al. 1998), polluting freshwater and coastal marine ecosystems (Conley et al. 2009). Therefore, a balance of soil P under long-term fertilisation is essential for both high productivity and a healthy environment.
Phosphorus in soil is present in both inorganic and organic forms. Different forms of P exist in different amounts and ratios, depending on soil type and management. The pools are transformed under certain conditions (Sharpley 2000). Studies have been carried out to investigate the effects of chemical fertiliser and organic manure on P adsorption and desorption (Iyamuremye and Dick 1996; Song et al. 2007), P transformations in soil (Sharpley et al. 1984; Motavalli and Miles 2002; Han et al. 2005; Wang et al. 2007; Vu et al. 2010), and P availability and recovery by crops (Damodar Reddy et al. 1999b; Shen et al. 2004). Most of these studies focused on the change in availability of inorganic P.
Organically bound P ([P.sub.o]), however, forms a significant portion of total P, ranging from 15 to 80% in soils (Stevenson 1982). Several studies showed that [P.sub.o] plays a key role in P cycling and plant nutrition in both temperate (Sharpley 1985; Stewart and Tiessen 1987) and tropical soils (Adepetu and Corey 1976). The availability of soil P fractions for plant uptake varies with soil and plant types, and even the so-called recalcitrant P fraction can be depleted by cropping (Vu et al. 2008). The dynamics of the different [P.sub.o] fractions in soils are poorly understood because there is no universally accepted fractionation and characterisation scheme.
One approach to examining the availability of soil P is the use of chemical extraction procedures to fractionate P according to its solubility in specific reactants, which are assumed to approximate to the bio-available P pools (Chen et al. 2000). For example, bicarbonate-extractable P, the sum of inorganic P ([P.sub.i]) and [P.sub.o], has been found to be labile and related to plant-available P, and is a better indicator of plant response than [P.sub.i] alone (Bowman and Cole 1978a). In soils with low labile [P.sub.i] content, NaOH-extractable [P.sub.o] was found to be the source for microbial uptake (Chauhan et al. 1981). Fulvic-acid-associated P, which forms a large fraction of the [P.sub.o] in most soils (Krivonosova and Basevich 1980), may in part be derived from recent organic materials and plant litter (Grindel and Zyrin 1965) and is considered to be fairly labile (Bowman and Cole 1978b). Humic-acid-associated P turns over slowly and is less affected by short-term perturbations (Grindel and Zyrin 1965; Batsula and Krivonosova 1973; Bowman and Cole 1978b). Such extraction schemes, when combined with experiments, aim to determine the biological availability of the fractions extracted and have been used to examine the processes involved in transformation of soil P (Stewart and McKercher 1982). Fractionation schemes were developed further by Hedley et al. (1982), Sharpley and Smith (1985), and Ivanoff et al. (1998). For example, Ivanoff et al. (1998) proposed a comprehensive scheme to fractionate soil [P.sub.o] into labile, moderate labile, and non-labile pools.
Previous studies suggest that Mollisols in Northeast China have 42-69% of the total P present in the organic form (Song et al. 2007). Soil organic fractions can be altered by crop and soil management practices, especially through applications of organic amendments and inorganic P fertilisers. Although some laboratory incubation studies have shown that addition of organic manure can significantly increase the soil organic P pool (Zhang et al. 1994), little information about the dynamics of the different [P.sub.o] fractions in soils is available from continuously cropped soils.
This paper reports on the effects of regular application of chemical fertiliser P, alone or in combination with pig manure, on change in soil P status, crop P uptake, the soil P budget, and the change in size of the different [P.sub.o] fractions as identified using the scheme based on Ivanoffet al. (1998) following a 14-year maize-soybean-wheat rotation.
Materiais and methods
Experimental site and soil
A long-term fertilisation field trial (1994-2007) was conducted at the Hailun National Field Station, Chinese Academy of Sciences. The experimental site is in the central area of the black soil region in Northeast China (N 47[degrees]26', E 126[degrees]38'). The soil is classified as a Mollisol (USDA 1998). At the study site, the annual rainfall is 500-600 mm, with 88% of the rain falling between May and September. Average monthly temperatures range from -22 to -25[degrees]C in January to 20-22[degrees]C in July. The area was originally covered with native prairie species before the start of cropping more than 100 years ago. No fertiliser or manure was applied during the first 60 years after clearing. During the following 20-year period, the soils were fertilised with farmyard manures, followed by the application of nitrogen (N) fertilisers during the next 20 years before 1985, and then followed by the application of N and P fertilisers (Song et al. 2007) before the start of the trial in 1994. The area is mainly used for dryland farming. No irrigation was applied in this trial.
Fertilisation treatments and crop rotation
The experiment was designed according to the local field crop cultivation and management system, and consisted of three fertiliser treatments: (1) no fertiliser application (control, CK), (2) continuous application of chemical N and P fertilisers in the forms of urea and [(N[H.sub.4]).sub.2]HP[O.sub.4] (NP), and (3) chemical N and P fertilisers plus pig manure (NPM). The experiment was a randomised block design with four replicates. Each plot had an area of 60 [m.sup.2] (4 m by 15 m).
Maize (Zea mays L. cv. Haiyu 6), soybean (Glycine max (Merrill.) L. cv. Heinong 35), and wheat (Triticum aestivum L. cv. Long 4083) were grown in a rotation, with one crop per year in the sequence of maize-soybean-wheat in all treatments from 1994 to 2002. Maize was first sown in 1994. Since 2003, no wheat crop was sown; the rotation was changed to maize-soybean with one crop each year. This rotation is typical of the cropping system used in the region. Wheat, maize, and soybean were sown on 1-6 April, 1-5 May, and 2-6 May, respectively, depending on the weather and soil conditions. Crops were harvested at maturity, around 1-5 August, 1-5 October, and 1-5 October for wheat, maize, and soybean, respectively. All the stubbles were removed after harvest.
The amounts of fertilisers applied in treatments NP and NPM are summarised in Table 1. All of the chemical fertilisers were applied into 0.10-0.15 m depth with tillage on the crop sowing date, whereas the pig manure was broadcast on the soil surface after crop harvest each year (for the next season crop). The average total N, P, and potassium (K) concentrations of the pig manure were 22.1, 2.6, and 2.4 g/kg, respectively, with an average [P.sub.o] concentration of 2.3 g/kg (Song et al. 2007). Other nutrients including K were considered to be at adequate levels in the soil and therefore were not applied.
Crop yield, P uptake, and soil P balance
Biomass and grain yields were measured for each crop at harvest. Inputs of P were calculated based on the amounts of P in the chemical P fertilisers, and pig manure (with a P concentration of 2.6g/kg, on average; Song et al. 2007). Removal of P was estimated according to the grain yield and biomass production. The average P concentrations (g/kg) of grain and shoot of crops in different treatments were derived from data obtained in another experiment at the same experiment station where similar P additions were applied (Table 2).
The P balance in soil between 1993 and 2007 was calculated using soil bulk density (p, g/[cm.sup.3]) and total P ([P.sub.tot], mg/kg) content in 0-0.20 m soil layer according to the equation:
[P.sub.balance] = [[([rho][P.sub.tot]).sub.2007] - [([rho]V[P.sub.tot]).sub.1993]] x [10.sup.-9] (kg/ha)
where V is the soil volume of the 0-0.20 m soil layer per ha; [10.sup.-9] converts mg/ha to kg/ha.
Soil sampling and [P.sub.o] fractionation
Soil samples from the surface 0.20 m layer were collected in October 1993 and October 2007 after the crop was harvested. Soil bulk density was measured according to Lampurlanes and Cantero-Martinez (2003) using three soil cores (50 mm in height and 60 mm in diameter) taken from undisturbed soil from each plot at depths of 0-0.07, 0.07-0.14, 0.14-0.21 m. Another six soil cores were taken randomly from each plot at 0-0.20 m and bulked. A subsample of each soil sample was air-dried and screened through a 2-mm sieve before analysis. Soil organic matter was determined using wet oxidation with [K.sub.2][Cr.sub.2][O.sub.7] and concentrated [H.sub.2]S[O.sub.4] (Nelson and Sommers 1996). Soil total P and total [P.sub.o] were measured using the ignition method as described by Legg and Black (1955).
The fractionation scheme for [P.sub.o] applied to the soils was primarily based on the scheme of Ivanoff et al. (1998). We classified [P.sub.o] in Mollisols into labile, moderately labile, and nonlabile fractions. Labile [P.sub.o] and [P.sub.i] in soil solution and sorbed on the soil surface were removed by a 0.5 M NaHC[O.sub.3] extraction at pH 8.5. The residue of NaHC[O.sub.3] extraction was then extracted with 1 M HCl to completely remove any [P.sub.i] from the soil. The [P.sub.o] extracted in 1 M HCl was considered as part of the moderately labile [P.sub.o] pool. The bulk of the moderately labile [P.sub.o] and some of the non-labile [P.sub.o] (and non-labile [P.sub.i]) were then extracted with 0.5 M NaOH. To separate the moderately labile (fulvic acid) [P.sub.o] from the non-labile (humic acid) [P.sub.o] pool in the NaOH extract, an aliquot of the extract was acidified to pH 0.2 with concentrated HCl. At pH 0.2, humic acids are precipitated, whereas fulvic acids remain soluble. Finally, the highly resistant and non-labile fraction was determined by ashing the residue from the NaOH extraction at 550[degrees]C for 1 h, followed by shaking for 24h in 1M [H.sub.2]S[O.sub.4]. The P concentration in all extracts was estimated colourimetrically using the method of Murphy and Riley (1962). Acid or alkaline extracts were neutralised before P analysis. The [P.sub.o] in the sample solutions was calculated by subtracting [P.sub.i] from total P. Total P in all extracts was measured after the aliquots were digested with [K.sub.2][S.sub.2][O.sub.8]+ [H.sub.2]S[O.sub.4] (Rowland and Haygarth 1997), whereas the [P.sub.i] in all extracts was measured by the colourimetric method without digestion. The recovery of [P.sub.o] was measured as the ratio of the sum of labile, moderately labile, and non-labile [P.sub.o] concentration determined by [P.sub.o] fractionation scheme to that total [P.sub.o] determined by ignition method (Legg and Black 1955).
Experimental data were analysed using one-way ANOVA appropriate to a factorial randomised block design. Wherever appropriate, the treatment means were compared at P = 0.05 using least significant difference (1.s.d.).
Changes in soil organic matter and P status
Table 3 shows the soil organic matter content, bulk density, and concentrations of different forms of P in the surface 0-0.20 m soil layer in 1993 and 2007. After 14 years of maize-soybean-wheat cropping, the soil organic matter content decreased in the treatments without pig manure application (both CK and NP) compared with the values measured in 1993, whereas organic matter content increased in the treatment with chemical N and P fertilisers plus pig manure application (NPM). In the control (CK), total P and [P.sub.i] declined after 14 years of cropping. In contrast, soil total P and [P.sub.i] concentrations increased in the NP and NPM treatments. The percentage of [P.sub.i] increased and the percentage of [P.sub.o] in total P decreased with increased applications P fertiliser. Changes in soil [P.sub.o] concentrations suggest that neither chemical fertiliser (NP) nor manure plus chemical fertiliser (NPM) had a significant effect on [P.sub.o] concentrations (Table 3). However, the total amount of [P.sub.o] declined in the surface 0.20m of soil after 14 years of rotation because of the decline in soil bulk density (Table 3, Fig. 1).
[FIGURE 1 OMITTED]
Crop yield, P uptake, and P budget
Overall, biomass and grain yields of maize, soybean, and wheat increased following applications of fertilisers (Table 4). The highest yield was measured in treatment NPM, which received the largest annual application of P. Compared with CK, the NP treatment increased maize, soybean, and wheat yield by 29.3, 3.4, and 6.8%, respectively, whereas the increases were 35.5, 33.9, and 7.5%, respectively, in the NPM treatment.
Phosphorus uptake by different crops was increased following additions of N and P fertiliser alone or in combination with pig manure (Table 4). The highest P uptake was measured in the NPM treatment. The mean P uptake of maize was 25 and 30 kg P/ha x year higher in treatments NP and NPM than the control (CK), respectively. For soybean, treatments NP and NPM increased crop P uptake by 24 and 57%, respectively, compared with CK. Wheat P uptake followed a similar trend to that of maize and soybean, the NP and NPM treatments increased wheat P uptake by 25 and 40%, respectively.
Phosphorus inputs, P removal, and soil P balance for different treatments are shown in Table 5. During 1994-2007, the treatment without fertiliser application (CK) had no P input, whereas total P input was 388 kgP/ha for treatment NP. The P application for treatment NPM was 2.8 times greater than for treatment NP. Removal of P by crops was 1.7 and 2.0 times greater in treatments NP and NPM, respectively, compared with the CK (Table 5). Soil P balance calculated based on P inputs and crop removal (net P change, Table 5) corresponded well to the balance calculated using soil P measurements from 1993 and 2007 (soil P balance, Table 5). The cumulative changes in soil total P in the three treatments are given in Fig. 2, showing a progressive decline in soil P in treatment CK, a balance of inputs and crop removal in treatment NP, and a net P build-up in treatment NPM. After 14 years of cropping, treatment CK had a net P export of 233 [+ or -] 15 kg/ha, treatment NP a small net P export of 12 [+ or -] 47 kg/ha, and treatment NPM a net P gain of 635 [+ or -] 22 kg/ha (Fig. 2, Table 5).
[FIGURE 2 OMITTED]
Soil organic P fractionation
Concentrations of [P.sub.o] fractions, relative distributions, and the recovery of different [P.sub.o] fractions in the soil are presented in Table 6. The recovery of [P.sub.o] ranged from 96.1 to 99.5%, with an average of 97.4 [+ or -] 1.5%. This shows that total [P.sub.o] in soils was satisfactorily extracted by the extraction scheme used in this study. The [P.sub.o] of the experimental Mollisols mainly comprised moderately labile and non-labile [P.sub.o], ranging from 165 to 232 and 113 to 186 mg/kg, with relative contributions of 42.2-59.1% and 28.7-47.6% to total [P.sub.o], respectively. Labile [P.sub.o] ranged from 39.2 to 84.0 mg/kg with a relative contribution of 10.0-19.5%. In contrast, the main fraction in the moderately labile [P.sub.o] pool was fulvic acid-P, accounting for 36.7-52.9% of total [P.sub.o]. Another fraction of the moderately labile [P.sub.o], HCl-[P.sub.o], accounted for 5.1-11.7% of total [P.sub.o]. Non-labile [P.sub.o] consisted of humic acid-P and residual [P.sub.o], with relative contributions of 7.7-26.1% and 16.7-21.5% to total [P.sub.o]. In summary, labile and moderately labile [P.sub.o] contributed 52.2-71.3% of the total [P.sub.o] pool, i.e. >50% of total [P.sub.o] in Mollisols can be easily decomposed and then used by the plant.
After 14 years of cropping, the soil labile [P.sub.o] content in the control (CK) reduced by 18.3%, the moderately labile [P.sub.o] (which includes HCl-[P.sub.o] and fulvic acid-P) content was reduced by 29.0%, and the non-labile [P.sub.o] (including humic acid-P and residual [P.sub.o]) content was increased by 64.9% (Table 6). Fertiliser application increased labile [P.sub.o] content and nonlabile [P.sub.o] pool, but decreased the moderately labile [P.sub.o] pool after 14 years of cultivation. Compared with the pools measured in the control, NaHC[O.sub.3]-[P.sub.o], HCl-[P.sub.o], and fulvic acid-P contents were increased by 25.8, 95.0, and 8.28%, respectively, in treatment NP, whereas the humic acid-P and residual [P.sub.o] contents were decreased by 23.5 and 11.0%, respectively. Chemical fertiliser plus pig manure (NPM) further increased NaHC[O.sub.3]-[P.sub.o], HCl-[P.sub.o], and fulvic acid-P content by 88.5, 54.3, and 0.69%, whereas humic acid-P and residual [P.sub.o] content decreased by 12.3 and 3.61%, respectively, compared with treatment NP.
Phosphorus budget and soil fertility
At the study site, continuous cropping for 14 years without fertilisation significantly decreased the soil total P and [P.sub.i] content. This has led to deficiency of P in soil, consistent with the results of Song et al. (2007). Further, cropping without fertilisation also led to a decline in soil organic matter and soil organic P (Table 3, Fig. 1), suggesting declined soil fertility and decomposition of [P.sub.o] pools. Applications of chemical N and P fertiliser with and without pig manure (treatments NP and NPM) resulted in increases in total P and [P.sub.i] (Table 3), and applications of P fertiliser and pig manure led to a positive P balance (Table 6). The application of pig manure for 14 years doubled the [P.sub.i] fraction but slightly decreased the [P.sub.o] fraction in the soil (Fig. 1). It indicates that the added [P.sub.o] in the manures had been transformed into [P.sub.i], due to decomposition by soil microbes (Petersen et al. 1998) and root exudates (Dakora and Phillips 2002).
The increase in wheat biomass and soybean yields in the NPM treatment compared with the NP treatment (Table 4) suggests that the rates of N and P applied in the treatment NP may be inadequate to meet potential crop demand of wheat and soybean at this location. The yield increase in the NPM treatment may also be due to improved soil physical properties (as reflected in changes in bulk density) and other added nutrients. Further, the addition of manure may have stimulated plant growth by promoting bacterial activity with beneficial effects on plants. The significant increase in P uptake by maize and soybean in the treatment NPM was mainly due to the tripled P input compared with the treatment NP. Some of the increase could also be due to improved availability of P in soil owing to reduced P sorption by manure (Iyamuremye and Dick 1996; Holford et al. 1997; Song et al. 2007). However, detailed studies are needed to find out the optimal N and P fertiliser application rates for the soil and climate conditions at the site. The significant P accumulation in soil in the NPM treatment (Table 5, Fig. 2) implies excess P build-up in soil, which may lead to future environmental problems.
[P.sub.o] fractions and transformation
Fractionation of soil [P.sub.o] as used in this paper provides an effective means for detecting the [P.sub.o] availability compared with the soil analysis methods used to determine total [P.sub.o] without fractionation (Oehl et al. 2002; Song et al. 2007). Previous reports show that NaHC[O.sub.3]-extractable [P.sub.o] can serve as a sensitive indicator of 'readily mineralisable' soil P pools (Phiri et al. 2001). In that case, our results showed that long-term cropping without fertilisation reduced the readily mineralisable soil [P.sub.o], whereas supplementary P additions to the soil can preserve the soil labile [P.sub.o] content. Chemical P fertiliser, in combination with pig manure, improved soil labile [P.sub.o] content and its percentage in total [P.sub.o].
The HCl-[P.sub.o] had a similar trend of change to that of NaHC[O.sub.3]-[P.sub.o], but the content of soil fulvic acid-P in all three treatments decreased compared with the values in the 1993 soil sample. Therefore, the soil moderately labile [P.sub.o] content and percentages in total [P.sub.o] in all three treatments are lower than the initial values before the experiment. This suggests that moderately labile [P.sub.o] may be changed into labile [P.sub.o] and [P.sub.i], possibly due to long-term tillage and root activity (Zhang et al. 1994). The decrease in moderately labile [P.sub.o] may also be related to the decrease in soil organic matter in treatments CK and NP (Table 3). Chemical P fertiliser application (with or without pig manure) tended to stabilise the moderately labile [P.sub.o] to some extent, consistent with the results of Damodar Reddy et al. (1999a).
Compared with the 1993 soil sample, the soil humic acid-P content of the three treatments increased at different rates, whereas the soil residual [P.sub.o] content remained almost unchanged. This suggests an increase in soil non-labile [P.sub.o] content with time, implying that some active soil P fractions (include [P.sub.i] and [P.sub.o]) were immobilised during long-term cropping. The percentage data (Table 6) of [P.sub.o] fractions show that fertiliser application can alleviate soil P immobilisation. Furthermore, manure addition may lead to increased microbial activity that affects mineralisation and P availability to the crops, and this warrants further studies in this cool region.
Long-term applications of chemical P fertiliser with and without pig manure in the Molisols in Northeast China significantly increased soil total P, [P.sub.i], and labile [P.sub.o] and changed other soil properties such as soil bulk density. The P added by long-term fertilisation contributed to the increase in all [P.sub.i] fractions, but not to the [P.sub.o] fractions. The additional P in the pig manure resulted in substantial accumulation of P in the soil despite greater crop production. This suggests that the current P rates in the treatment with inorganic N and P fertilisers plus pig manure (NPM) should be reduced to avoid possible environmental problems. Comparison between the 1993 and 2007 soil samples indicates that the moderately labile [P.sub.o] content declined in all treatments, whereas the non-labile [P.sub.o] content increased. These results suggest that the moderately labile [P.sub.o] may be transformed into labile [P.sub.o] and [P.sub.i], and that some active P fractions were immobilised during long-term cultivation. Further studies are warranted to examine the mechanisms of P transformation and the accessibility of these P fractions by plants.
This research was supported by the National Natural Science Foundation of China (No. 40971152), National Basic Research Program of China (2010CB134509), and Regional Natural Science Foundation of Heilongjiang Province, China (ZD200904).
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Manuscript received 15 August 2010, accepted 8 October 2010
Chun Song (A,B,C), Xiaozeng Han (A,D), and Enli Wang (B)
(A) Key Laboratory of Mollisols Agroecology, National Observation Station of Hailun Agroecology System, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, China.
(B) CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia.
(C) Graduate University of Chinese Academy of Sciences, Beijing 100039, China.
(D) Corresponding author. Email: firstname.lastname@example.org
Table 1. Amounts (kg/ha.year) of fertiliser applied in treatments with nitrogen and phosphorus fertilisation (NP), and NP fertilisers plus pig manure (NPM) since 1994 Values in parentheses are N or P contents of the pig manure Year Nitrogen Phosphorus Maize Soybean Wheat Maize Soybean NP 1994-99 138 20 69 30 23 2000-03 150 30 75 33 20 2004-07 150 32 120 33 36 NPM 1994-99 138 (332) 20(332) 69(332) 30(39) 23 (39) 2000-03 150 (663) 30(332) 75 (332) 33 (78) 20(39) 2004-07 150 (663) 32 (332) 120 (332) 33 (78) 36(39) Values in parentheses are N or P contents of the pig manure Year Pig manure Wheat Maize Soybean Wheat 1994-99 20 -- -- -- 2000-03 20 -- -- -- 2004-07 24 -- -- -- 1994-99 20(39) 15000 15000 15000 2000-03 20(39) 30000 15000 15000 2004-07 24(39) 30000 15000 15000 Table 2. Phosphorus concentrations (g/kg) of grain and shoot in treatments without fertiliser application since 1994 (CK), with nitrogen and phosphorus fertilisation (NP), and NP fertilisers plus pig manure (NPM) Data are means [+ or -] standard errors Crop CK NP Grain P Shoot P Grain P Maize 2.96 [+ or -] 0.05 0.58 [+ or -] 0.16 4.00 [+ or -] 0.31 Soybean 5.11 [+ or -] 0.01 0.64 [+ or -] 0.01 6.11 [+ or -] 0.19 Wheat 3.23 [+ or -] 0.30 0.40 [+ or -] 0.12 3.77 [+ or -] 0.48 Data are means [+ or -] standard errors Crop NPM Shoot P Grain P Shoot P Maize 1.02 [+ or -] 0.25 4.15 [+ or -] 0.24 1.21 [+ or -] 0.07 Soybean 0.75 [+ or -] 0.05 6.11 [+ or -] 0.01 0.81 [+ or -] 0.01 Wheat 0.45 [+ or -] 0.15 3.99 [+ or -] 0.25 0.57 [+ or -] 0.08 Table 3. Soil organic matter (SOM), total phosphorus (TP), inorganic P ([P.sub.i], and organic P ([P.sub.o]) contents and soil bulk density measured in 1993 and 2007 in treatments without fertiliser application since 1994 (CK), with N and P fertilisation (NP), and NP fertilisers plus pig manure (NPM) Values in parentheses are the percentages of total P in soil; n.s., no significant difference between treatments SOM TP [P.sub.i] [P.sub.o] (%) (mg/kg) (mg/kg) (mg/kg) Initial value 5.85 736 327 (44.4) 409 (55.6) '(1993) CK (2007) 5.04 699 298 (42.6) 401 (57.4) NP (2007) 5.28 845 433 (51.2) 412 (48.8) NPM (2007) 6.38 1288 856 (66.5) 432 (33.5) l.s.d. (P=0.05) 0.51 35 43 n.s. Values in parentheses are the percentages of total P in soil; n.s., no significant difference between treatments Bulk density (g/[cm.sup.3]) Initial value 1.13 '(1993) CK (2007) 1.03 NP (2007) 0.97 NPM (2007) 0.90 l.s.d. (P=0.05) 0.07 Table 4. Effect of fertiliser application on crop grain yields (GY) and shoot biomass (SB) and phosphorus uptake in maize-soybean-wheat rotation Treatments without fertiliser application since 1994 (CK), with N and P fertilisation (NP), and NP fertilisers plus pig manure (NPM). Values shown are mean values of 6 years of maize, 5 years of soybean, and 3 years of wheat; n.s., no significant difference between treatments Crop yields (t/ha.year) Maize Soybean Wheat Treatment GY SB GY SB GY SB CK 6.65 9.44 1.77 1.79 2.81 3.09 NP 8.60 15.0 1.84 1.85 3.00 3.66 NPM 9.01 14.8 2.37 1.91 3.02 4.04 l.s.d. (P=0.05) 0.57 2.39 0.31 n.s. n.s. 0.36 P uptake (kg/ha.year) Treatment Maize Soybean Wheat CK 25.2 10.2 10.3 NP 49.7 12.6 12.9 NPM 55.3 16.0 14.4 l.s.d. (P=0.05) 4.14 2.03 3.37 Table 5. Total phosphorus input, estimated P removal by crops, and change in soil P during 1994-2007 without fertiliser application (CK), with N and P fertilisation (NP), and NP fertilisers plus pig manure (NPM) All values in kg/ha Treatments Net P Soil P P input P removal change balance (B) CK 0 233 [+ or -] 15 -233 [+ or -] 15 -223 NP 388 400 [+ or -] 47 -12 [+ or -] 47 -24 NPM 1090 455 [+ or -] 22 635 [+ or -] 22 655 (A) input--P removal. (B) Calculated using soil bulk density and total P in 0-0.20 m. Table 6. Concentrations, relative distributions, and recovery of different organic P ([P.sub.o]) fractions in soils before the trial began and after 14 years of cultivation [TEP.sub.o]: Total extracted [P.sub.o], the sum of labile, moderately labile, and non-labile [P.sub.o] concentrations determined by [P.sub.o] fractionation scheme. [TP.sub.o]: Total [P.sub.o], determined by ignition method (Legg and Black 1955) Moderately labile Labile [P.sub.o] [P.sub.o] NaHC[O.sub.3]- [P.sub.o] HCl-[P.sub.o] Fulvic acid-P (mg/kg) (%) (mg/kg) (%) (mg/kg) (%) 1993 sample 48.0 12.2 24.4 6.2 208 52.9 CK 39.2 10.0 20.1 5.1 145 37.1 NP 49.3 12.4 39.2 9.9 157 39.4 NPM 84.0 19.5 50.1 11.7 158 36.7 l.s.d. (P=0.05) 3.4 5.6 15 Non-labile [P.sub.o] Residual [TEP.sub.o] Humic acid-P [P.sub.o] [TP.sub.o] (mg/kg) (%) (mg/kg) (%) (mg/kg) (%) 1993 sample 30.2 7.68 82.7 21.0 393 409 CK 102 26.1 84.2 21.5 391 401 NP 78.0 19.6 74.9 18.8 398 412 NPM 65.5 15.2 71.9 16.7 430 432 l.s.d. (P=0.05) 10.8 6.6 Recovery (%) 1993 sample 96.1 CK 97.5 NP 96.6 NPM 99.5 l.s.d. (P=0.05)